# Venus Evolution Through Time: Key Science Questions, Selected Mission Concepts and Future Investigations

Thomas Widemann, Suzanne E Smrekar, James B Garvin, Anne Grete Straume-Lindner, Adriana C Ocampo, Mitchell D Schulte, Thomas Voirin, Scott Hensley, M Darby Dyar, Jennifer L Whitten, et al.

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## To cite this version:

Thomas Widemann, Suzanne E Smrekar, James B Garvin, Anne Grete Straume-Lindner, Adriana C Ocampo, et al. "Venus Evolution Through Time: Key Science Questions, Selected Mission Concepts and Future Investigations." Space Science Reviews, 2023, 219, pp.56. 10.1007/s11214-023-00992-w. insu-04231792

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# Venus Evolution Through Time: Key Science Questions, Selected Mission Concepts and Future Investigations

Thomas Widemann · Suzanne E. Smrekar · James B. Garvin · Anne Grete Straume-Lindner · Adriana C. Ocampo · Mitchell D. Schulte et al. [full author details at the end of the article]

Received: 28 September 2022 / Accepted: 7 August 2023
© The Author(s) 2023

## Abstract

In this work we discuss various selected mission concepts addressing Venus evolution through time. More specifically, we address investigations and payload instrument concepts supporting scientific goals and open questions presented in the companion articles of this volume. Also included are their related investigations (observations & modeling) and discussion of which measurements and future data products are needed to better constrain Venus' atmosphere, climate, surface, interior and habitability evolution through time. A new fleet of Venus missions has been selected, and new mission concepts will continue to be considered for future selections. Missions under development include radar-equipped ESA-led EnVision M5 orbiter mission (European Space Agency 2021), NASA-JPL's VERITAS orbiter mission (Smrekar et al. 2022a), NASA-GSFC's DAVINCI entry probe/flyby mission (Garvin et al. 2022a). The data acquired with the VERITAS, DAVINCI, and EnVision from the end of this decade will fundamentally improve our understanding of the planet's long term history, current activity and evolutionary path. We further describe future mission concepts and measurements beyond the current framework of selected missions, as well as the synergies between these mission concepts, ground-based and space-based observatories and facilities, laboratory measurements, and future algorithmic or modeling activities that pave the way for the development of a Venus program that extends into the 2040s (Wilson et al. 2022).

**Keywords** Venus · Planetary system formation · Geological processes · Atmospheric dynamics · Atmospheric chemistry · Space instrumentation · Surface processes · Interior structure · Thermal state · Synthetic aperture radar · Subsurface sounder · Radio-science · Multispectral imager · Ground and space-based observatories

## 1 Introduction

Each of the companion articles in this collection has identified key open questions about the evolution of Venus' atmosphere, climate, surface, interior and habitability through time, as well as the measurements or approaches that are needed to address them. To capture the

Venus: Evolution Through Time

Edited by Colin F. Wilson, Doris Breuer, Cédric Gillmann, Suzanne E. Smrekar, Tilman Spohn and Thomas Widemann
wide variety of scientific domains and fields covered in this collection, and before describ-
ing current and future investigations to address these questions, we provide a summary of
their conclusions as well as open questions regarding the dynamical properties and various
processes of the present-day atmosphere.

VERITAS (Smrekar et al. 2022a), DAVINCI (Garvin et al. 2022a), and EnVision (Euro-
pean Space Agency 2021) will greatly advance our understanding and lead to new questions
about the evolution of Venus through time. Key advances will come from new types of data
to better constrain the interior, such as improved crustal thickness and structure, mantle vis-
cosity/temperature from seismology, lithospheric thickness from electromagnetic sounding,
in-situ heat flow to constrain thermal lithospheric thickness and radiogenic heat budget and
distribution. Over the next 15 years, these three missions will work together to answer many
of the outstanding questions in Venus science and rocky planet evolution described above
(Fig. 2; Table 1).

In addition, several Venus missions are under consideration on in development: Rus-
sia's Venera-D orbiter, descent module and lander mission (Zasova et al. 2020); an Indian
radar-equipped orbiter, Shukrayaan-1 (Antonita 2022); a Chinese radar-equipped orbiter,
VOICE (Dong et al. 2023); Rocket Lab's private, low-cost "Morning Star" concept mission
to Venus (Seager et al. 2021). Their science observation strategy is under competitive study
or development. Furthermore, various mission concepts, whether from landers, from aerial
platforms or from orbit require further technology development beyond the current frame-
work of selected missions to enable long-term surface science (seismic, compositional, heat
flow investigations); missions that take advantage of mobility in the surface, near-surface,
and atmospheric environments; and collection and return of atmospheric samples to Earth
(Wilson et al. 2022; Limaye and Garvin 2023). Therefore, the current definition phase is
an ideal time to collate knowledge of Venus long-term evolution scenarios and the observa-
tions needed to distinguish between them. These questions are left for future investigators to
address through a wide range of research approaches, including Earth-based observations,
laboratory and modeling studies based on existing data, and future new spacecraft missions.

Section 2 presents an overview of conclusions and open questions from the companion
papers in the following order: (1) Comparison of Venus with exoplanets; (2) Venus initial
conditions; (3) Venus surface processes, surface age and evidence for volcanic and tectonic
activity; (4) Interior regime through history, water and other volatiles.

Sections 3-8 outline the science objectives of upcoming and future missions, in addition
to their observational strategy, including expectations for addressing the conclusions sum-
marized in Sect. 2. In June 2021, NASA selected two missions in its Discovery program:
VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) (Sm-
rekar et al. 2022a) and DAVINCI (Deep Atmosphere Venus Investigation of Noble Gases,
Chemistry, and Imaging), a descent chemistry/imaging probe coupled with a carrier, re-
lay and imaging spacecraft (Garvin et al. 2022a). EnVision has been selected as ESA's 5th
Medium-class mission in the agency's Cosmic Vision plan, and is targeted for launch in the
early 2030s. The mission is a partnership between ESA and NASA, with NASA providing
the Synthetic Aperture Radar (European Space Agency 2021). In this Section, mission de-
sign proposals Venera-D (Zasova et al. 2020) and Shukrayaan-1 (Antonita 2022) are also
described in some detail.

Sections 9-12 address future mission concepts and measurements that require further
technology development beyond the current framework of selected missions, future mission
concepts, the synergies between currently selected missions and future laboratory measure-
ments in different experimental setups, and expected modeling activities to address the evo-
lution of Venus' climate, surface, interior and habitability through time. Complementarity
with non-Venus missions (e.g., exoplanet observatories) is also addressed. The review con-
cludes with a Summary and Conclusions (Sect. 13), which discusses key questions about
Venus' evolution can be answered convincingly with the current and next-generation mis-
sion concepts, and which fundamental questions will remain open for future investigations.

**2 Open Science Questions and Required Investigations to Address Venus Evolution Through Time**

This section presents an overview of conclusions and open questions from the companion
articles, organized along the following science themes:

*Comparison of Venus with Exoplanets*, summarizes conclusions and open questions about how Venus' ancient evolution can inform exoplanet studies regarding the importance of primordial & basal magma oceans and their evolution toward habitability, and, conversely, how terrestrial exoplanet studies can inform Venus' evolutionary history. We summarize conclusions of Way et al. (2023, this collection) and Westall et al. (2023, this collection) regarding water inventory, early tectonics, and volatile cycling between the interior and atmosphere of Venus, and whether liquid water ever existed on the surface at temperatures conducive to the emergence of life. We explore the longevity of a habitable Venus, the divergent paths for planets in the Venus Zone (Kane et al. 2014), and the conditions of Venus evolution from a habitable to an inhospitable planet.

*Initial Conditions, Accretion, and Early Venus*, discusses modeling and observational constraints on early Venus based on different accretion scenarios. How did the accretion of Venus and Earth differ? Is Venus a more primordial or primitive body than Earth? Was there a late giant impact and to what extent it could have affected its initial thermal state and differentiation? What are the processes driving the thermal evolution of the latter and the concurrent early atmosphere formation, volatile trapping in the solidified mantle and water distribution? (Salvador et al. 2023, this collection). How does the elemental abundances and isotopic compositions of noble gases (He, Ne, Ar, Kr, Xe) and stable isotopes (H, C, N, O, S) constrain the budget of volatile elements outgassed in the atmosphere, the timing and mechanisms of volatile transport between planetary reservoirs, and the geodynamical history of Venus through time? (Avice et al. 2022, this collection).

*Surface Processes, Age of the Surface and Evidence for Current Activity*, addresses key open science questions about the resurfacing history and volcanic activity of Venus and their relationship to present-day volcanism and tectonism. Upcoming orbital missions will improve our understanding of the resurfacing history of Venus in crucial ways for a better understanding of the sequence of events that occurred in producing the geologic landscape: how are impact features and their associated deposits (ejecta, haloes, parabolas) altered over time, has the nature of volcanism and tectonics changed over time, and how does this compares with global resurfacing models and constrain the global evolution of Venus through time? (Herrick et al. 2023, this collection; Ghail et al. 2023, this collection). Sediments and sedimentary rocks are also critical to understanding surface modification processes and how Venus works today, but are also extremely important for determining how Venus's climate has changed through time and whether it was once a habitable planet (Carter et al. 2023, this collection). Furthermore, mineralogy of the Venus surface provides a critical record of geologic and climatologic history and the current chemical exchanges between the atmosphere and solid body (Gilmore et al. 2023, this collection).

*Interior Regime Throughout History, Water and Other Volatiles*, discusses open science questions regarding dynamics and evolution of Venus' mantle: how did mantle cooling
history control the state of Venus' core and the tectonic regime; and constraints on the vari-
ability of heat flow through time by Rolf et al. (2022, this collection); what was the evolution
of the atmosphere-interior of Venus, including its core (Gillmann et al. 2022, this collec-
tion)? What are the signatures and potential detectability of present-day volcanically emit-
ted material in the atmosphere of Venus by incoming Venus missions, is there a non-gaseous
component of volcanic plumes? Could these measurements shed light on the compositional
history of magmatic volatiles and their reservoirs? (Wilson et al. 2023, this collection).

In addition to open questions along the previous science themes, this section also ad-
dresses investigations and open questions based on the recent analysis and exploitation of
ESA's Venus Express and JAXA's Venus Climate Orbiter (Akatsuki) - in addition to recent
ground-based observations, on how to better constrain the dynamical variability and cou-
plings from surface to cloud tops in present day's atmosphere. Important variability on all
time scales, in latitude, in local time of the main dynamical and photochemical tracers at all
altitude levels, such as CO, SO or SO₂, variability of the cloud convective layer, atmospheric
structure and turbulent processes, and large bow-shaped topography-driven stationary waves
above the main equatorial highlands, all contribute to the study of the complex dynamical
structure and properties of the Venusian atmosphere, and their relation to the long-term evo-
lutionary path of Venus.

**2.1 Comparison of Venus with Exoplanets**

2.1.1 Synergies Between Venus and Exoplanetary Observations (Way et al. 2023, this collection)

a) *The importance of magma oceans*: Exoplanetary observations of planets in the Venus Zone (VZ), defined by Kane et al. (2014) as part of the Habitable Zone (HZ) in which an Earth-sized planet is more likely to be a Venus analog than an Earth analog, can help us to constrain the magma ocean (MO) lifetime of Venus. Constraining the magma ocean lifetime prior to solidification is extremely important in understanding the likelihood of water ever condensing on the surface of a Venus-like world. The reason lies in 1-D calculations by Hamano et al. (2013), who demonstrated that Venus may sit at a boundary between a world that receives so much solar insolation that the magma ocean lifetime is long (∼100 Myr), providing ample time for photodissociation and escape of the overly-ing steam atmosphere, and effectively drying out Venus (classified in Hamano et al. 2013 as a Type II world). On the other hand, Venus may lie on the other side of this boundary with a short-lived magma ocean with a steam atmosphere lifetime of ∼1 Myr (i.e., comparable to Earth's, see e.g., Salvador et al. 2017, 2023, this collection) that allows the planet to condense water on the surface (a Type I world). Work by Turbet et al. (2021) suggests that it is more likely that Venus ended up as a Type II world because their 3-D model generates clouds that appear to efficiently trap heat at the poles and night side. However, both the Hamano and Turbet models use CO₂-H₂O or N₂-H₂O atmospheres. It is not clear from recent work by Gaillard et al. (2022) and Bower et al. (2022) that these combinations of gases are adequate. For this reason, exoplanetary observations of young planets around G-stars in the Venus Zone will be critical, in addition to selected Venus missions, to discerning early Venus' history (see Sect. 11.2).

b) *Are there divergent evolutionary paths for exoplanets in the Venus Zone?* As indicated above, the magma ocean lifetime is critical to understanding the likelihood of water ever condensing on the surface of a world in the Venus Zone. At the same time, 3-D General Circulation Modeling (GCM) by Yang et al. (2014), Way et al. (2016) and Way and Del
Genio (2020) has demonstrated that slow rotation is key to keeping a planet temperate within the Venus Zone. Moving present-day Earth into the Venus Zone will rapidly move the planet into a moist and then runaway greenhouse state, as 1-D models have shown that Earth is already at the inner edge of the habitable zone (Kopparapu et al. 2013, Fig. 8). The 3-D GCM modeling studies have shown that if Earth were rotating as slowly as modern Venus does, an efficient, large-scale cloud-albedo feedback at the substellar point would generate high enough albedos for a significant portion of the incident solar radiation to be reflected back to space, keeping the surface of the planet temperate. Most observable habitable-zone exoplanets are terrestrial exoplanets orbiting M dwarfs, with rotation periods of order of 10-30 days; such planets close to their star are expected to become tidally despun into a synchronously rotating state; atmospheres of typical tidally locked terrestrial exoplanets are expected to superrotate (Imamura et al. 2020, and references therein). Examining planets in the Venus Zone of exoplanetary systems will determine whether these 3-D GCMs are correct, and whether Venus-like worlds ever have temperate surface conditions and the role that rotation rate may play. Even if water condenses early on the surface of a Venus-like world, its later evolution may diverge, depending upon its rotation rate.

c) What is the longevity of habitability of an Earth-sized planet in the Venus Zone? If Venus had a temperate period, its longevity may be difficult to constrain, but Earth-size worlds in their Venus Zone will help us to bound the problem. Conversely, new data from upcoming Venus missions should give us a constraint on the longevity of water on Venus and encourage the planetary and exoplanetary science communities to search for such worlds in exoplanet databases in the coming decades. Yet the latter is not unambiguous; for instance, key observations that the DAVINCI descent probe analytical instruments within and below the clouds (Sect. 5.3) and the EnVision VenSpec-H spectrometer (Sect. 6.3) are related to the D/H ratio and the heavy noble gas isotopes. Work by Avice et al. 2022 (this collection) demonstrates that the D/H ratio in itself is insufficient to determine when Venus lost its water and the time-scale of that loss, as implied in the published Pioneer Venus D/H measurements by Donahue et al. (1982). The heavy noble gas isotope measurements by DAVINCI will be crucial to understand the epoch and timescale of the loss on Venus (Garvin et al. 2022a; this review, Sect. 5.3). If Venus had a habitable period, what constraints can interior, tectonic, and atmospheric escape models provide to understand the likelihood of long-term volatile cycling? Here again, exoplanetary observations of planets in the Venus Zone will be a unique opportunity to test our models and their application to Venus' long-term evolution.

### 2.1.2 The Habitability of Venus (Westall et al. 2023, this collection)

a) What was Venus' water inventory and was there liquid water on its surface at temperatures conducive to the emergence of life? Did water condense after crystallization of the magma ocean? Given the lack of direct access to the ancient history of the planet, this question is best addressed by refining models and through eventual comparisons with exoplanets exhibiting characteristics such as rotation speed of the planet that may have similarities with early Venus. Important requirements for habitable conditions would be a slow rotation of early Venus and a corresponding weak Coriolis force to allow for a large and reflective cloud cover (e.g., Way and Del Genio 2020). Liquid water on the surface may allow silicate weathering and thereby maintain a low atmospheric pressure of CO₂, protecting a subaqueous habitable environment.
b) *If there was once water on the early planet's surface, how long was the transition from habitable to uninhabitable planet?* This question is closely related to the tectonic state of early Venus. With active plate tectonics, a carbonate-silicate cycle similar to that on Earth could have allowed for a substantial habitable period until the proposed resurfacing event accompanied by catastrophic mantle outgassing some several hundred million years ago (e.g., Way and Del Genio 2020; Krissansen-Totton et al. 2021). In contrast, without plate tectonics but with liquid surface water, recycling of carbonates into the mantle would have been rare, limiting the long-term habitability on Venus (Höning et al. 2021). Knowledge of Venus is currently insufficient to rule out any but the most extreme scenarios, but further observation should yield important evidence to constrain Venus' evolution. In particular, constraints on ancient plate tectonics, which could be derived from future seismic measurements as well as from a more detailed exploration of surface features such as the tesserae and their compositions, would shed light into the early habitable period of Venus.

c) *How habitable is the Venusian cloud environment, and are there signs that it was ever inhabited?* Conditions at today's 55-70 km altitude range are juxtaposed with the observed limits for terrestrial life. By these metrics, hypothetical life in Venusian aerosols may be within required bounds of temperature, pressure, and pH, and energy sources (Grinspoon and Bullock 2007; Nicholson et al. 2010; Limaye et al. 2021; Westall et al. 2023, this collection, Fig. 8); is the purported phosphine signature (Greaves et al. 2021; Encrenaz et al. 2020b; Villanueva et al. 2021) real and is it really a biosignature? The odds against there being life in the clouds of Venus today are high due to extreme conditions in terms of water activity (which takes extreme acidity and aridity into account), and the lack of permanent habitability (for Earth-like life in any case). Venusian life would have to be quite different from that on Earth to not just survive but thrive in its clouds. This does not mean that the hypothesis should be completely disregarded, but it remains just a hypothesis, awaiting further boundary conditions from missions including DAVINCI and EnVision. If Venusian cloud life exists, and if it is indeed very different from terrestrial life, then could it be identified as living and viable? If the answer to these speculations is yes, then this would answer our first question of Venus habitability. This question requires an in-depth characterization of the cloud-level environment: gas and cloud composition, available light levels, cloud droplet microphysics (droplet size, formation / precipitation cycles), UV & ionizing radiation levels. A detailed investigation of how these environmental factors vary with altitude, latitude and local time would require sustained measurements from an aerial platform such as a balloon or powered aircraft (Limaye and Garvin 2023).

## 2.2 Initial Conditions, Accretion, and Early Venus

### 2.2.1 The Accretion and Differentiation of Venus

Venus and Earth contain 41% and 51% of the remaining mass of the inner protoplanetary disk vs 3% and 5% for Mercury and Mars; based on this comparison, we might expect the smaller planets to be outliers and Venus and Earth to be similar and represent good averages of the composition of the inner solar system. Yet Earth and Venus appear fundamentally different. The dynamical causes and timing of this globally-important difference are a topic of active work and debate, with broad implications for planet accretion models, early solar system dynamical stability, volatile delivery to the terrestrial planet region, and the early impact rate throughout the solar system (e.g., Bottke et al. 2017).
a) **When, from where, and how many volatiles were accreted by Venus?** Venus and Earth are expected to have formed over several million years by accretion of planetesimals and planetary embryos originating from various heliocentric distances, with the majority coming from a narrow annulus near 1 AU (O'Brien et al. 2006). Did Venus accrete as an average body as the protoplanetary disk was rapidly cooling, or did proto-Venus components emerge from distinctive regions? The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk, are not known or strongly underconstrained (e.g., Spitzer et al. 2020; Morbidelli 2020). A variety of different processes such as thermal processing in the primordial atmosphere and atmospheric escape must have had a dramatic impact on the bulk and isotopic compositions of planetary embryos accreting to form Venus, being possibly a major factor in volatile depletion (Lammer et al. 2020).

b) **Was the accretion of Venus Earth-like with a late giant impact?** The final stages of planetary accretion involve collisions between the forming planet and leftover bodies such as large planetesimals or planetary embryos. Earth suffered from a final major collision at the end of its accretion and the Moon is the witness of this event. For Venus, it remains unknown if the planet ever suffered from an impact energetic enough to create a moon (Brooks and Jacobson 2019; Jacobson and Dobson 2022). Medium or large impactors on early Venus affect the primordial atmosphere through impact erosion and might trigger further degassing through energy deposition in the mantle and crust. High temperatures generated in the upper mantle and the spreading of the thermal anomaly lead to partial melting and the formation of new basaltic crust. Yet, giant impacts are not the only potential interactions. Alternative scenarios involving smaller and successive multiple impacts have also been proposed to explain the Moon's formation (Rufu et al. 2017). Whether the energy deposition was then sufficient to melt the entire mantle depends primarily on impact parameters (e.g., Nakajima et al. 2021) and on their frequency. Finally, the accretion sequence of the Earth and Venus may significantly differ, with Venus possibly experiencing more hit-an-run collisions (Emsenhuber et al. 2021). To what extent these alternatives apply to Venus and how they affected its initial thermal state and differentiation remains unclear. Is Venus therefore a more primordial or primitive body than Earth, if there was no giant impact in its early history? How might these be related to a plausible water-rich past?

### 2.2.2 Magma Ocean, Water, and the Early Atmosphere of Venus (Salvador et al. 2023, this collection)

a) **What proportion of initial mantle volatile inventory is outgassed and escaped?** Water and its distribution between the different planetary reservoirs are of fundamental importance in controlling the processes and feedbacks at play, from the deep interior to the upper atmosphere, during the entire evolution of the planet (e.g., Crowley et al. 2011; Tikoo and Elkins-Tanton 2017). Furthermore, surface conditions and thus the potential habitability of the planet are direct outcomes of the evolutionary pathways followed (e.g., Hamano et al. 2013).

The earliest stages of planetary evolution, and in particular the so-called magma ocean stage, are crucial in distributing water between the different reservoirs (e.g., Salvador et al. 2017; Nikolaou et al. 2019). During this phase, the surface is molten and the absence of a thick, long-lasting boundary between the molten mantle and the atmosphere
allows for free and extremely efficient thermal and chemical exchanges between the inte-
rior and the atmosphere. Volatile species initially dissolved within the molten mantle are
thought to concentrate readily into the melt during the crystallization of the magma ocean
until melt saturation is reached. At that point, volatiles in excess of saturation likely ex-
solve out of the melt, form gas bubbles, and rise up to burst at the surface and be expelled
out, forming the atmosphere. Magma ocean outgassing and therefore (secondary) atmo-
spheric formation have been thought to be efficient because of the vigorous convection
at play in the melt, believed to bring the entire melt volume close to the surface, where
outgassing occurs rapidly enough.

In part because of the extreme *P*, *T* conditions of the magma ocean, which are far
out of reach of current numerical models and experimental setups capabilities, many
processes affecting the thermo-chemical evolution of the magma ocean, such as the crys-
tallization scenario and the convection regime, remain highly unconstrained. In addition,
the initial state of the magma ocean itself is highly uncertain. For instance, the mantle re-
dox state or the initial volatile abundance and their evolution with time are still unsettled.
Yet these processes are thought to significantly affect the type and timing of outgassed
volatiles and thus their evolving distributions between the interior and the atmosphere.
Further investigations considering the range of uncertainties and the interplays between
these mechanisms are needed to draw a consistent picture of the early volatiles out-
gassing and escape processes and reconcile the early evolution with the current state of
Venus. Any measurable constraint on the timing and amount of early outgassing, volatile
loss, and on the amount and type of remaining volatiles in the present-day mantle could
be used in evolution models and would help in choosing among the different scenarios,
thereby improving our understanding of the processes at play.

b) *What is the timing of silicate / metal differentiation?* Several parameters of primary importance for the early evolution stages remain highly unconstrained, for instance including the initial water content, and the initial mantle state. These two aspects are tightly linked to the outcomes of the accretion sequence. While it seems reasonable to assume, given their vicinity, that the Earth and Venus experienced similar accretion histories with similar volatiles delivery, the absence of a moon and associated late Moon-forming giant impact challenges the assumptions regarding the timing and extent of large-scale melting episodes on early Venus. Indeed, a moon-forming impact on early Earth (e.g., Canup 2004) is thought to induce a global-scale mantle melting that is generally assumed to be the initial state of coupled magma ocean-atmosphere models. However, it is important to note that this event is not the only heat source that can support large-scale mantle melting. Other plausible scenarios than the single giant moon-forming impact hypothesis, i.e., smaller multiple-impact models, have been proposed and may better explain the compositional similarities of the Earth and Moon (Rufu et al. 2017). These scenarios would not ultimately discard the likelihood of deep and global magma oceans but constraining the timing and timescale of Venus formation/accretion would help clarify the initial state of planetary evolution. This could be achieved using the ¹⁸²Hf ¹⁸²W chronometer to constrain the timing of silicate/metal differentiation and thus core formation (e.g., Lee and Halliday 1995; Harper and Jacobsen 1996). This ultimately relates to the molten state of the mantle and thus to the timescale and intensity of the accretion phase (e.g., Zahnle et al. 2007). Knowing the amount of accretionary energy delivered within a constrained time frame would help test the global magma ocean hypothesis and provide clues for the initial state of the early evolution of Venus.

c) *Could the mantellic water content be constrained through the geodynamic regime (and present-day volcanic outgassing)?* Because of the influence of water on mantle melting
and viscosity (and thus rheology) (e.g., Lange 1994; Hirschmann 2006; Ohtani 2020), information on current mantle convective regime and dynamics might provide indirect clues about its current and therefore past water content. The mantle present-day outgassing rate and composition might be another way to sample the planetary interior state and water content and inform models of early and long-term planetary evolution. Estimates of the Venusian mantle water content would indeed provide constraints for evolutionary models to match with and thus help deciphering which paths and associated mechanisms are most likely to match with these constraints.

d) *Are there hints for the existence of liquid water in the past? Can they be inferred through Venus surface mineralogy, mantle present-day rheology, outgassing and composition?* Hints for the existence of liquid water in the past would provide significant constraints for the early evolution scenarios. If liquid water ever existed at the surface of Venus, it would be strong evidence that enough water has to be retained, either in the planetary interior or in the atmosphere, for a substantial amount of time and that temperate climates were plausible in Venus' past. It would discard all scenarios where water is lost in Venus' early history due to desiccation via a slowly (~100 Myr) cooling magma ocean (Hamano et al. 2013 type II planets), and can thus never sustain habitable conditions (Turbet et al. 2021). Conversely, it would favor scenarios where the early evolution of the magma ocean allows for a rapid (~1 Myr) solidification (type I planets according to Hamano et al. 2013 classification). For instance, this could either be due to an inefficient magma ocean outgassing (e.g., Ikoma et al. 2018; Salvador and Samuel 2023) and reduced atmospheric greenhouse effect, or due to mechanisms reducing the incoming solar flux and allowing for temperate climates at Venus orbital distance, such as the presence of highly reflecting clouds. In the latter case, temperate surface conditions would directly be inherited from the early magma ocean stage evolution while in the former, habitability would be related to the subsequent long-term evolution of Venus (e.g., Way and Del Genio 2020; see also Gillmann et al. 2022, this collection).

### 2.2.3 Volatiles and Noble Gas Isotopes (Avice et al. 2022, this collection)

The elemental and isotopic compositions of volatile elements (H, C, N, O, S, P and noble gasses) contained in the Venus atmosphere hold clues to the origin and evolution of the entire planet and can provide decisive answers to three major fundamental questions:

a) *Did Venus acquire its volatile elements from the protoplanetary solar nebula, asteroids, comets, or a mix of these sources?* Classical views, supported by geochemical constraints and outcomes of models of the formation of terrestrial planets, propose that Earth, and by extension Venus, accreted relatively dry and acquired most volatile elements later by bombardment of volatile-rich material (asteroids/comets) during the final stages of planetary formation (Marty 2012; Halliday 2013). These late events could also have delivered chemical elements and/or compounds conducive to the emergence and development of life. The detection of solar-derived gasses in the interior of Earth (e.g., Williams and Mukhopadhyay 2018) and on Mars (Swindle 2002) also leaves room for an early contribution of gasses from the solar nebula, or from solar wind implanted at the surface of grains (Péron et al. 2018), to the budget of volatile elements on terrestrial planets. Recent investigations propose that the Earth's building blocks did contain significant water (Piani et al. 2020). Estimating the delivery mix of volatile elements to Venus would thus help to constrain the timing of the formation of Venus and the building blocks of the planet, also contributing to placing Venus in the context of the formation of the entire Solar System. For example, the isotopic composition of Venus' atmospheric
xenon could carry the signature of a delivery of cometary material to the atmosphere of Venus (Avice et al. 2017; Marty et al. 2017). This cometary contribution is visible in the isotopic composition of Earth atmospheric xenon as a marked depletion in $^{134}$Xe and $^{136}$Xe isotopes relative to Solar or Meteoritic end-members (Avice et al. 2022, this collection, and references therein). In addition to Ar/Ne and $^{20}$Ne/$^{22}$Ne, detecting (or not) cometary xenon on Venus would thus be a key constraint for models attempting to understand the late delivery of volatile-rich bodies originally formed in the outer Solar System to the terrestrial planets. A list of key measurements of noble gases and of their associated maximal uncertainties required to answer the scientific questions is summarized in Avice et al. 2022, this collection, Table 1. DAVINCI’s Descent probe quadrupole mass spectrometer instrument VMS is described in Sect. 5, Sect. 5.3.1 (see also Garvin et al. 2022a, Table 3.2).

b) **How was Venus' atmosphere shaped by early impacts and atmospheric escape?** On Earth, the Moon-forming impact likely removed the primitive, possibly solar-derived, atmosphere and set the stage for the emergence of a secondary atmosphere. Loss of water has been a key driving mechanism for the evolution of Venus (Baines et al. 2013), but the extent of water loss and the history of atmospheric escape, including of other atmospheric species, remain largely unconstrained. Measurements of the isotopic composition of nitrogen and precise determinations of the elemental and isotopic compositions of noble gasses in the atmosphere of Venus would provide constraints on the presence or absence of remnants of primordial solar gasses, on the regime of atmospheric escape (thermal vs. non-thermal) but also on its timing in the planet's history. For example, elevated $^{38}$Ar/$^{36}$Ar and $^{15}$N/$^{14}$N ratios measured in Mars' atmosphere demonstrate that non-thermal escape processes have been active on Mars (Atreya et al. 2013). Knowing the abundance and isotopic composition of xenon in the atmosphere of Venus would also clarify if Venus suffered from joint hydrogen-xenon escape processes (Zahnle et al. 2019; Avice and Marty 2020) like Earth and Mars. Determining the $^{129}$Xe/$^{132}$Xe ratio, which might have recorded contributions of radiogenic $^{129}$Xe from the decay of now extinct $^{129}$I ($T_{1/2}$ = 16 Ma), would also help evaluate the relative timing of atmospheric escape and outgassing processes (see next paragraph). NASA's DAVINCI mission will address these issues directly via in situ sampling and measurements of Xe isotopes.

c) **What is the outgassing history of Venus?** Although Venus is currently in a quiet stagnant-lid regime, the planet is not “dead” and there is evidence for recent activity including recent hotspot volcanism (Smrekar et al. 2010a). Several models propose that Venus might have been in a much more active regime in the past and even that plate tectonics was active on ancient Venus. Radioactive decay of extinct ($^{129}$I, $^{244}$Pu) and extant ($^{238}$U, $^{40}$K) nuclides present in silicate portions of Venus have been producing excesses of radiogenic and fissiogenic isotopes of noble gasses (e.g., $^{40}$Ar, $^{129,131-136}$Xe) relative to primordial compositions. Given the wide range of half-lives of the parent nuclides (ranging from Ma to Ga), the relative proportions of these excesses measured in a reservoir should vary with time. Magmatic-driven outgassing contributes to the progressive degassing of these radiogenic isotopes from Venus' interior to its atmosphere. Measuring the elemental and isotopic composition of noble gasses in the atmosphere of Venus would thus help to refine current estimates on the relative proportions of radiogenic noble gasses degassed in the atmosphere versus those still retained in the planet's interior (Kaula 1999). Such measurements will also allow a coherent picture to be built of the geodynamical history of Venus through time.

Two broad types of science investigations are envisaged for gathering data on the elemental and isotopic compositions of noble gasses and stable isotopes (H, C, N, O, S) in
the atmosphere of Venus. One would be an in-situ mission carrying a scientific payload to sample and measure the abundances and isotope ratios of the chemical elements of interest below the homopause such as DAVINCI (Garvin et al. 2022a, see Sect. 5). Another would be a sample mission during which a portion of the Venus atmosphere would be sampled below the homopause, which corresponds to a pressure level of $10^6$ mbar (approximatively 135 km, see Mahieux et al. 2015), such as JPL Cupid’s Arrow concept (Sotin et al. 2018a,b; Rabinovitch et al. 2019), with collected sample(s) possibly returned to Earth for characterization with state-of-the-art technologies available in international laboratories (Shibata et al. 2017; see also Sect. 10, Sect. 10.3.2).

## 2.3 Surface Processes, Age of the Surface and Evidence for Current Activity

### 2.3.1 Resurfacing History and Volcanic Activity of Venus (Herrick et al. 2023, this collection)

Upcoming orbital missions will improve our understanding of the resurfacing history of Venus in crucial ways. We will have a better understanding of the sequence of events that occurred in producing the geologic landscape. Placement of craters within that sequence will provide a timeline for that sequence, and constraints on the current level of volcanic and tectonic activity will provide a present-day “boundary condition” on that history. Advances in understanding will be achieved by upcoming missions for three critical science questions, including the following:

a) *How are impact features and their associated deposits (ejecta, haloes, parabolas) altered over time?* Key to establishing the absolute timing of geologic events is evaluating whether impact craters largely postdate the volcanic and tectonic activity observed on the Venus surface, or whether they are a population of features that are in various stages of being obliterated. It is expected that improved resolution in imaging and topography, along with SAR polarimetry and imaging at multiple wavelengths (e.g., by the VERITAS and EnVision radar orbiter missions), will enable the processes altering the appearances of impact craters over time on the Venusian surface to be distinguished. If aeolian or chemical weathering processes are the dominant mechanisms for removing the emissivity and backscatter signatures associated with distal impact deposits such as dark haloes and parabolas, and sediment fill is responsible for creating low-backscatter floor deposits, then most of the craters can be viewed as being at the top of the stratigraphic column. In such a case, the surface would have formed from a relatively rapid sequence of events several hundred million years ago. If large portions of the craters have deposits that have been altered by one or tectonic or volcanic events, then the timeline of geologic activity spreads, and much of the surface, are probably younger than 100 My.

b) *Has the nature of volcanism and tectonics changed over time?* Improvements in imaging and topography from the upcoming missions will enable seeing key geologic contacts, individual volcanic flows, fault blocks, and other details of surface geology. Considerable advancements in our knowledge of compositional information will come from both infrared and SAR imaging. This information will allow us to build an understanding of the sequence of events on the surface and evaluate whether or not fundamental changes in the nature of geologic activity have occurred over the past several hundred million years.

c) *What is the current level of volcanic and tectonic activity?* Magellan images compared against changes observed during the upcoming orbital missions (VERITAS and EnVision), will constrain where and how much current geologic activity is occurring on the
surface. Most of this work will simply involve change detection among images taken at different times to search for new flows, landslides, new fractures or faults, etc. Repeat pass SAR interferometry will also provide information regarding cm-scale movement in tectonic zones, caldera inflation and deflation, and other active small-scale deformation. The near-infrared descent imaging by the DAVINCI probe will search for signs of mass wasting in 3D using very high resolution imaging and topography acquired under the clouds for a region within Alpha Regio to complement the orbital SAR observations.

## 2.3.2 Volcanic and Tectonic Constraints on the Evolution of Venus (Ghail et al. 2023, this collection)

a) What stresses and thermal, mechanical and geochemical parameters are responsible for the formation of Venus' extremely diverse volcanic features? Venus hosts an enormous diversity of volcanic features: direct analogs and those whose formation mechanisms are extremely challenging to understand, such as narrow lava channels that extend many 1000s of km. Large volcanic rises termed hotspots are directly linked to mantle plumes, probably arising at the core-mantle boundary. But where do smaller plumes that are likely to form at least some coronae originate? Why are coronae arguably unique to Venus? What processes are responsible for Venus' many enigmatic volcanic features? Are differences in features due to spatial or temporal differences in crust/lithosphere/mantle conditions? These questions will be addressed by NASA-JPL's VERITAS orbiter mission (Smrekar et al. 2022a), NASA-GSFC's DAVINCI mission (Garvin et al. 2022a) and by the ESA-led EnVision M5 orbiter mission (European Space Agency 2021), but better understanding of crust/mantle/core composition and rheology will be needed to take the next steps in understanding.

b) What are the driving forces and mechanisms for stress accommodation that produce the variable scales and apparent strains seen in Venus complex tectonic terrains? Ex-tensional and compressional features on all spatial scales dominate deformation, with limited evidence for strike-slip faulting. In some environments, the origin of stress is clearly linked to mantle plumes or volcanic processes. In most feature types, there are multiple possible origin hypotheses. For example, what causes the ~major 5000-10,000 km rifts? There is no apparent compressional zone of accommodation for the displaced, extended lithosphere. Local scale (<150 square km) studies using fine-scale topography from the DAVINCI probe's sub-cloud imaging will provide boundary conditions for strain within Alpha Regio at scales < 30 m.

c) What is the mechanism of tesserae formation? Tesserae are characterized by highly elevated topography, small-scale surface roughness and multiple sets of cross-cutting tectonic structures and appear to represent areas of intense, past tectonism. Tessera terrain covers about ~8% of the surface of Venus and is morphologically clearly distinct from the volcanic plains that dominate the remainder of the planet. Detailed study of the type, number, spacing, distribution, and stratigraphic position of tessera structures will yield insight into the geodynamics of Venus before the production of the plains. Are tessera structures compressional or extensional in nature? Does their formation require a different strain rate, heat flow, or composition than in the plains? Is there evidence of lateral accretion of materials to form tessera plateaus? Do the tesserae underlie the plains across Venus? Are the tesserae dynamically compensated? The formational models to explain such high, complex and strained terrain are still the subject of much debate and uncertainty: horizontal convergence, extension, mantle upwelling, sub-crustal flow, crustal underplating, sub-crustal rejuvenation, crustal plateau formation, diapiric
intrusion, gravitational sliding and relaxation, or all of these? (Hansen and Willis 1996; Ivanov and Head 2011).

d) *Absent Earth-like plate tectonics, what is Venus' overall geodynamic system that links mantle convection, surface deformation and volcanism, and volatile history?* Venus' interior heat engine provides ample energy to the geologic activity that creates Venus' young surface and massive atmosphere. But fundamental questions remain. Why does Venus lack a dynamo? How does Venus lose its heat? Have processes changed with time? What is the extent of lithospheric recycling? Is Venus, with its hot lithosphere, a good analog for Earth's Archean? Are current tectonic processes the precursors of plate tectonics and continent formation? Can up- and downwelling plumes produce all surface features? Numerous hypotheses have been put forward, but additional data are needed to discriminate between them.

### 2.3.3 Mineralogy of the Venus Surface (Gilmore et al. 2023, this collection)

The Venera, VeGa, and Magellan missions found that Venus is dominated by basaltic rocks associated with widespread volcanism (Basilevsky and Head 2003a). Near-infrared observations from Venus Express and Galileo first detected variations attributed to mineralogy. They suggest that there is diversity in the FeO content of materials, including relatively high FeO content consistent with less weathered rocks, and low FeO content consistent with differentiated, non-basaltic compositions.

The stratigraphically oldest material on Venus are found among the major tessera terrains. They record an extinct geodynamic regime and have a near-IR emissivity signature that is different from the basaltic plains. The nature of the tesserae is critical to our understanding of Venus prior to the emplacement of the plains. Several major questions about Venus history are recorded in tessera terrain. Near-IR observations from VERITAS, EnVision, and DAVINCI (including below the cloud deck at spatial scales < 100 m) will provide the first global assessment of surface composition, which is critical to addressing the following questions:

a) *What is the composition and diversity of Venus surface materials?* What are the primary rocks and minerals recorded on the surface of Venus? How do these compositions spatially correspond to morphological units? How do these units vary with time and location? What do these differences tell us about the ancient and modern geologic history of Venus?

b) *What is the style of weathering recorded in surface rocks over the history of Venus?* Thermodynamic models of Venus weathering make predictions about the products of surface-atmosphere weathering under current Venus conditions, but there is a lack of consensus over exactly which phases might form from bulk versus diffusion-constrained reactions between surface and atmosphere and the timing of their formation. Weathering reactions depend upon the composition and crystallinity of the surface rocks, which are unknown. Do we detect these predicted phases? Can the presence or absence of these phases be used to constrain the age of surface units? Do we see weathering products that are consistent with weathering under an extinct atmosphere? Can we constrain the composition of the high radar reflectivity materials found across Venus?

c) *Is there compositional evidence for aqueous or hydrous minerals?* Is the near-IR signature of the tesserae consistent with Fe-poor magmas, clay minerals, or primary sedimentary phases? Is there evidence for Fe-poor phases in other regions of Venus?

d) *Are the tesserae felsic?* The near-IR emissivity of the tesserae may be consistent with Fe-poor materials, which, if igneous, could be consistent with the production of felsic lavas
(Hashimoto et al. 2008; Mueller et al. 2008; Gilmore et al. 2015) or of granitic rocks. If confirmed, these would require a planet with abundant water and a plate-recycling mechanism. Such a discovery would be critical evidence of a once-habitable Venus and elevate the targeting of tessera for future *in situ* study.

e) *Are the tesserae compositionally, morphologically, and stratigraphically heterogeneous?* Is there compositional variation across and within the tesserae? What is the detailed stratigraphic relationship between the tesserae and the plains? Is there evidence of unrecognized craters in the tesserae? Is there evidence of sedimentary materials in the tesserae?

Each of these questions requires laboratory work to examine the near-IR signatures of rocks and minerals and their weathering products expected under Venus conditions (see also Sect. 12.1.2 below).

### 2.3.4 Sediments, Regolith / Sediment Supply, Evolution (Carter et al. 2023, this collection)

a) *What are the nature, distribution, and range of sedimentary surface modification processes?* How has the surface of Venus been modified since it was formed? In particular, what are the processes that have modified and partially filled impact craters? What are the causes of the lower emissivity material in some highlands, and what do they imply about weathering processes and possible volatile transport? On Venus, sediments are likely produced by impact cratering and weathering (Garvin 1990). Even if these are not volumetrically large, sediments are likely widespread, covering a large fraction of the surface. Thus aeolian erosion and deposition may be important processes at the Venus surface. Local-scale evidence from Venera lander panoramas indicates possible sedimentary processes that would need connection to regional and global scale models to place them in a proper context (Garvin et al. 1984).

b) *Is there evidence of active physical and chemical landscape change?* Landscape evolution refers to processes that modify the morphology of a planet's surface, such as gravity-driven mass-wasting landslides and slumps. Mass-wasting is a ubiquitous geomorphological process operating on any planetary body large enough to have gravity; such features are observed on Earth, the Moon, Mercury, Venus, Mars, icy satellites, comets, and asteroids. Magellan's low-resolution radar imagery provided the first evidence of mass movement on Venus in the form of large-scale slope failures: rock slumps, rock and/or block slides, rock avalanches, debris avalanches, and possibly debris flows are seen in areas of high relief and steep slope gradients (Malin 1992).

c) *Understanding the range and scope of mass-wasting processes.* Although impact cratering is likely the main process behind sediment production on Venus, the planet's hot, dense and highly oxidizing atmospheric conditions may cause chemical weathering of surface materials. In the absence of near-surface water which, on Earth, affects material bulk density, shear strength and pore-pressure, and thus lead to slope instability, the mechanisms of slope instability and failure on Venus are unclear, and it is likely that landslides require triggering by external forces, such as earthquakes. Magellan imagery (Malin 1992) revealed a very strong spatial relationship between the locations of large-scale mass-wasting features and steep slopes related to rift zones and volcanic edifices, which may in turn point to them being geodynamically active in the recent geological past. Wind-streaks and debris-fans (downwind of impact craters) are relatively large-scale features on Venus (kilometer to tens of kilometers in length) and are also commonly observed in Magellan images (Greeley et al. 1992, 1995; Kreslavsky and Bondarenko 2017; Neakrase et al. 2017).
## 2.4 Interior Regime Throughout History, Water and Other Volatiles

### 2.4.1 Dynamics and Evolution of Venus' Mantle Through Time (Rolf et al. 2022, this collection)

a) *How did mantle cooling control the state of Venus' core?* The state and size of Venus' core are largely controlled by the cooling efficiency of the mantle and thus provide constraints for the evolution of Venus' mantle convection and surface tectonics. Available observations ($MoI, k_2$) do not pin down these important characteristics of the core well enough (see Gillmann et al. 2022, this collection), though they will be addressed by the upcoming VERITAS and EnVision missions. A smaller Venus core implies a thicker mantle that experiences high pressures at the core-mantle boundary, possibly high enough for the occurrence of post-perovskite that is known to influence lowermost mantle and core dynamics on Earth. Current estimates of Venus' core size (Margot et al. 2021) seem inconsistent with the occurrence of this high-pressure phase, but further refinement is necessary. Next to core radius, the state of the core remains a fundamental unknown. A completely solidified core would imply efficient cooling through time and a cold state of the mantle. Compared to present-day, such a scenario would point to more efficient cooling during parts of Venus' evolution, possibly expressed as mobile lid tectonics with more large-scale horizontal surface motion than inferred for the present day. The opposing end member of a fully liquid core would in contrast point to smaller core cooling rates and reduced heat loss across the core-mantle boundary. In such a case, remnants of a basal magma ocean may be preserved inside Venus' mantle today, with implications for the planets' magnetic field history (O'Rourke 2020). The explained end members may be extreme scenarios, but they emphasize the importance of further pinning down the properties and state of Venus' core and deep mantle.

b) *How much heat does Venus lose today?* Heat loss from the mantle to the atmosphere and its efficiency through time are crucial for understanding Venus' interior evolution. Venus loses its interior heat via thermal conduction through the lithosphere and via volcanism, but how these fluxes vary through time and across Venus' surface and how they are linked to the various geological and tectonic features on Venus' surface remains to be established. Conductive heat flux has not been measured in-situ, but is indirectly determined from flexural modeling of elastic thickness. Strong lateral variations are indicated, yet insufficiently mapped out. Improving this knowledge gap could provide key information on local differences in lithospheric and crustal thickness as well as in the temperature of the uppermost mantle. The latter is important for the rheology of the uppermost mantle and lower crust, both of which are key aspects for interior-surface coupling on Venus (see Ghail et al. 2023, this collection).

c) *Has Venus' surface preserved anomalously old regions; are these felsic tessera?* The crucial question of Venus' mean surface age and its lateral uniformity links back to the debate of whether Venus' crust is renewed via catastrophic events or by more equilibrium processes. These different options have contrasting implications for the regime of mantle convection inside Venus. Venus' sparse crater distribution provides some constraints (Herrick et al. 2023, this collection), but the degree of age uniformity across the surface has been questioned. Refined surface age characteristics would further pin down the rates and scales with which tectonism and volcanism renew the surface. An important issue is how to link this to underlying mantle patterns, such as up- or downwellings? Tesserae may form some of the oldest regions on Venus, but whether they are substantially older-than-average as proposed by some and perhaps even predate a
phase of near-global resurfacing is challenging to answer without knowing their formation mechanism. This relates to their composition, which if felsic as indicated for some tessera (see Gilmore et al. 2023, this collection) would demand a relatively wet environment during formation and thus a tectonomagmatic regime that can maintain enough water in the shallowest interior.

## 2.4.2 Atmosphere-Interior Evolution of Venus and Evolution of the Core (Gillmann et al. 2022, this collection)

a) *Was there ever liquid water on the surface of Venus?* The presence or absence of water in any form, but especially liquid water on the surface of Venus, is a major unknown in the scenarios for the planet's evolution. Currently, investigated evolutionary pathways range from a Venus that desiccated early and never harbored any substantial water inventory after the magma ocean phase to possibly habitable scenarios until recently, with a full spectrum of intermediate cases. Water in the atmosphere or on the surface could potentially make a huge difference for the evolution of climate, volatile exchanges, and general planetary evolution. It affects weathering, surface reactions, outgassing conditions, atmosphere structure, escape mechanisms, and possibly volatile recycling and interior dynamics. Vague clues to the accretion of Venus and its very early history (about 4 Ga and older) come from comparisons with Earth, modeling, impact hypothesis (Gillmann and Tackley 2014; Gillmann et al. 2016) and noble gas measurements. We also have Venus' present/recent state, because the majority of its surface formed from ~0.3-1 Ga ago. However, no data points lie in between to identify a most likely scenario, define an intermediate state in the planetary evolution (ideally, with time constraints), or help find criteria to refine scenarios. Knowing if liquid water was present on Venus at any point in its history would be an important starting point, followed by estimating the amount of water necessary to explain those observations. Having an idea of the time period for that wet past then defines a basic succession of eras. Definite proof of a liquid water surface could finally constrain a tighter set of evolution scenarios. Lack of proof could mean that Venus was dry or that evidence was just lost. If ancient material is still present at the surface of Venus, it could offer a window into the planet's past. Likewise, the presence and abundance of granite-like rocks (see Gilmore et al. 2023, this collection) could yield information on the conditions during their formation and on the availability of liquid water. VERITAS will distinguish unequivocally the difference between granite and basalt based on their distinctive emissivity signatures near 1 micrometer, resolving this controversy at last. Additionally, new km-scale topography of Alpha Regio developed from analysis of Arecibo polarimetric imaging and radargram-based reanalysis of Magellan altimetry showcases possibilities of stream networks in this region when compared to Earth (Garvin et al. 2022b).

b) *When was the thick CO₂ atmosphere formed?* Another way to look at the situation is to understand when the current state of Venus' atmosphere came to be. The distinctive 90 bar CO₂ atmosphere is in itself an interesting limitation for many processes, from outgassing to climate modeling. Based on the lack of small craters on the surface of Venus, the atmospheric pressure has likely been high at least since the formation of the current surface. Little is known for sure beyond that. Radiogenic argon suggests that Venus is less outgassed than Earth, but re-constructing the entire outgassing history relies on under-constrained models. The timing and means of the CO₂ atmosphere formation can inform evolution models. Is it a recent feature (formed just before the change to present-day conditions, 500-1000 Myr ago), a relic from the magma ocean freezing, or the result
of a long term build-up? If it is a recent feature, there would be strong implications for the planet's past climate, under a thin atmosphere. A catastrophic transition could point either to the destabilization of carbonate deposits or mantle-related massive volcanic outgassing. The mineralogy of the surface can help us understand if the CO₂ in the current atmosphere of Venus is buffered by solid-gas reactions.

c) **What is the dominant volatile exchange process on the surface (buffering reactions, outgassing, oxidation...)?** A key to understanding the past is to understand how volatile species are exchanged between the interior and the atmosphere of the planet, and how they can be trapped or recycled. Recent work has shown that quantifying the volatile exchanges could provide invaluable insights into the evolution of the planet, and help understand how various processes and feedback mechanisms have shaped Venus. However, such investigations rely on the ability to constrain volatile flux reasonably well and identify all the processes involved. Because the planet is a complex system that changes with time, the relative importance and even the existence of those processes will also vary, depending on specific conditions such as climate affecting surface reactions for example, or liquid water affecting recycling, thus mantle conditions and finally outgassing. This makes the modeling of fully consistent evolution scenarios a daunting task, with widely different mechanisms operating under different states of the planet on a variety of time scales and domains (from the core to the upper atmosphere). We still do not know if a single exchange process dominates the others at the surface of Venus or if its atmosphere is close to a steady state. Is it slowly evolving due to volcanic outgassing or are surface-gas reactions buffering the atmosphere to a stable level? It has been suspected that water cannot be outgassed beyond marginal concentrations due to the surface pressure (Gaillard and Scaillet 2014): do the observations support this idea? What does it mean for surface lava flow composition? The volatile species may be trapped in the flow during the ascent of the magma and its extrusion. Does this imply high gas fractions and generate explosive volcanism for which there is only very limited evidence on recent Venus? How would it have changed with time? Does it mean that volcanic activity actually is a trap for water due to oxidation processes, rather than being a source?

d) **Where is the oxygen?** This question is perhaps the corollary to all the previous ones. If Venus had water at any time (and it did, at least initially), where did its constituents (two hydrogen and one oxygen atoms) go? Oxygen is especially critical because hydrogen could be lost to space and removed from the atmosphere efficiently, while oxygen mostly remains. Atmospheric escape of oxygen species has been recently suggested to be extremely low, which indicates that either some other process has been involved or that there was very little oxygen in the atmosphere in the first place. Can that be verified with recent instruments that have proved themselves on Mars, with the MAVEN mission? Does it mean the O is in the solid planet instead? If so, is it in the mantle (either not outgassed or recycled), or in the crust? Both confirmation of atmosphere measurements and surface investigation will be needed. NASA's DAVINCI mission will address the oxygen within the clouds and deep atmosphere directly via its VfOx student collaboration experiment (see Sect. 5, Sect. 5.3.5), and the detailed measurements of oxygen-bearing trace gas species by its Venus Mass Spectrometer and Venus Tunable Laser Spectrometer (Garvin et al. 2022a).

### 2.4.3 Magmatic Volatiles and Effects on the Modern Atmosphere (Wilson et al. 2023, this collection)

a) **What is the rate and style of volcanic outgassing in the present era?** Were the radically different evolutionary paths of Earth and Venus driven solely by distance from the Sun,
or did internal dynamics, geological activity, volcanic outgassing, and weathering also played an important part? What types of volcanism may be expected on Venus in light of its unique interior-surface coupling history described in companion articles (Avice et al. 2022; Gillmann et al. 2022; Ghail et al. 2023; Gilmore et al. 2023; Herrick et al. 2023, this collection)? To understand the long-term climate evolution of Venus, we need to establish (1) whether there is any morphological and compositional evidence of an epoch with abundant liquid water on the surface; (2) whether Venus is geologically active now, and whether this is a continuous or episodic style, to constrain interior-atmosphere exchange throughout history; (3) if there is atmospheric evidence of present day volatile sources and sinks at the atmosphere of Venus, including potential active volcanic sources; (4) whether and how sulfur- and water-related volatiles are transported through the atmosphere and how they interact with cloud layers (Bullock and Grinspoon 2001; Wilson et al. 2008; Titov et al. 2018).

b) *How are tropospheric and geological processes coupled on Venus?* Do exchanges take place from direct outgassing of volatiles into the lowermost atmosphere, buffering of atmospheric species with surface reservoirs, or aeolian/chemical alteration of surface minerals? The high surface pressure of Venus is maintained through surface-atmosphere chemical buffering reactions that are, as yet, unidentified. Buffer systems proposed have included calcite-anhydrite (Fegley and Treimann 1992; Fegley et al. 1992, 1997) and pyrite-magnetite (Hashimoto et al. 1997; Hashimoto and Abe 2005) systems, but there is little evidence constraining these claims because several of the relevant minerals including pyrite are unstable in Venus surface conditions (Hashimoto and Abe 2005). Latitudinal variability of minor species in the troposphere is thought to arise either from non-uniform vertical profiles, or from planetary-scale meridional transport through global convection cells, not restricted to cloud layers where most of solar energy is deposited, but extending deeply into the troposphere where latitudinal contrasts are observed. Because latitudinal gradients have already been observed in CO and OCS, these species act as tracers for the meridional circulation and provide glimpses into some of the chemical cycles of the troposphere. The water vapor vertical gradient in the deep atmosphere is not known and may be a steep gradient due to surface-atmosphere reactions (Ignatiev et al. 1997). Studying how trace gas abundances change over terrain of different compositions and/or elevations may yield insights into the surface-atmosphere exchanges and coupling (Zolotov 2018; Zolotov and Garvin 2020; Garvin et al. 2022a).

c) *Does present day atmospheric chemistry involve volcanic trace gasses?* Sulfur dioxide variations in the present day mesosphere have been used as possible evidence of volcanic activity (Esposito 1984). The proximate cause for these variations is related to spatial and temporal fluctuations of the SO₂ supply through vertical mixing within the cloud region. However, the origin of these vertical mixing fluctuations is barely understood. Purely atmospheric phenomena such as momentum deposition from upward propagating atmospheric gravity waves induced by topography (Kouyama et al. 2019; Kitahara et al. 2019) or diurnal variations of cloud top convection through solar absorption certainly play a role. Thermal destabilization of the atmospheric column through hot volcanic outgassing has also been suggested (Esposito 1984). SO₂ exhibits the most dramatic variations at Venus' cloud top, both spatially and temporally (Esposito 1984; Esposito et al. 1988; Marcq et al. 2013; Vandaele et al. 2017a,b, Encrenaz et al. 2016, 2019, 2020a), spanning more than two orders of magnitude on timescales ranging from a few days up to several decades. The greater range of SO₂ variations compared to H₂O is currently explained by the fast photochemical destruction of SO₂ by UV sunlight at cloud top level, making this species a much more sensitive tracer of the atmospheric circulation and vertical mixing
between its lower atmospheric source and its cloud top photochemical sink. The varia-
tions of the vertical mixing are poorly understood. Conditions at which a buoyant plume
could reach an altitude of 40-50 km are very narrow and require volatile-rich eruptions
at higher elevation, their signature depending on the flux of volcanic gases, mixing rate
with the air through eddy diffusion and turbulence (Lefèvre et al. 2018, 2020; Morellina
and Bellan 2022), and wind velocities at different altitudes (Lorenz 2016; Bengtsson et
al. 2012). Deep atmosphere gradients (i.e., every 140-200 m) associated with trace gas
species involving SO₂ and other sulfur-bearing species will be obtained by the DAVINCI
mission together with altitude resolved measurements of pressure, temperature (p, T) to
address this issues regionally (Garvin et al. 2022a).

## 2.5 Global and Mesoscale Atmospheric Processes: Short Term and Long-Term Variability

a) *How to better constrain dynamical variability and couplings from surface to cloud tops?* The present-day surface of Venus is permanently obscured by a layer of optically thick clouds between 45 and 70 km altitude. The atmosphere at cloud level is in retrograde superrotation with a period of 4-7 Earth days in the zonal direction, i.e., parallel to the equator, relative to the solid surface, a motion driven by its thermal structure (Grassi et al. 2010, 2014). The sidereal day of Venus, by comparison, is 243.02 Earth days, also retrograde. The clouds have been studied *in situ* and are mostly composed of sulfuric acid droplets described as populations with lognormal size distributions with mode diameters of 0.15, 1.0, 1.3, 3.4 mm, called mode 1, 2, 2', and 3 (Knollenberg and Hunten 1980; Ragent et al. 1985). Several structures visible in UV and IR wavelengths at different altitudes indicate a dynamically active atmosphere as well as a significant geodynamic coupling with the surface: huge bow-shaped structures extending from northern to southern latitudes have been detected by the Longwave Infrared Camera (LIR) and the Ultraviolet Imager (UVI) on board JAXA's Akatsuki (Fukuhara et al. 2017, and references therein). The extension of wave trains in both upstream and downstream directions was also observed (Fukuya et al. 2022). Vertical wind oscillations attributed to topographic gravity waves have also been observed by the VeGa balloons over Aphrodite Terra, which has a top height of 3-4 km (Blamont et al. 1986).

One of the remaining questions about the dynamics of the Venusian atmosphere is
how the convective cloud layer and topographically generated waves mix momentum,
heat, and chemical species (Lefèvre et al. 2022). Radio occultation can be used to mon-
itor the main cloud constituent, H₂SO₄, in both vapor and liquid form, near the cloud
base, providing clues to cloud formation and convection processes. Using direct imaging
or high frequency (HF) radar, electromagnetic signatures of lightning could be consid-
ered (Lorenz 2018), an investigation that would also contribute to the understanding of
chemical and microphysical processes at work in the cloud layer. It should also be noted
that the complex dynamics of the Venusian atmosphere produce a periodic mass redis-
tribution pattern that generates a time-varying modulation of the Venusian gravitational
field (Cascioli et al. 2023).

b) **How variable is the upper atmosphere?** The Venusian mesosphere (65-120 km) is a transition region between the lower atmosphere (from the surface to the cloud layer near 60 km), where the circulation is mainly zonal, and the upper thermosphere (above 120 km), where the wind pattern is mainly driven by diurnal pressure contrasts and flows from the sub-solar to the anti-solar point (the so-called SSAS flow). The zonal component decreases with altitude above the clouds, probably due to the deceleration caused by
momentum transport by atmospheric waves (Sánchez-Lavega et al. 2017). Global structure of thermal tides in the upper cloud layer has been studied by the LIR camera on board Akatsuki (Kouyama et al. 2019). Monitoring of thermal profiles and winds in the mesosphere has shown that this transition region, where the two types of circulation coexist, is also characterized by important temporal variability. Large-scale images from Venus orbiters (e.g., VMC/VEx, UVI/Akatsuki) lacked spectroscopic capabilities, while orbiter-borne spectrographs (e.g., VIRTIS-H/VEx, SPICAV-UV/VEx) lacked extensive spatial coverage. Because mesospheric composition varies on time scales ranging from hours to years, measurements over a wide range of latitudes, local times, longitudes, and time scales are needed, including atmospheric airglow at 1.27 µm (Hueso et al. 2008; Soret et al. 2014), measurements of the spatial and temporal variability of albedo and UV absorber (Lee et al. 2019), ionospheric electron density, and temperature, pressure, and density of the neutral atmosphere by radio-occultation (Peter et al. 2023). HF sounding radar can be used to constrain the ionosphere, as routinely performed by MEx/MARSIS (Picardi et al. 2005). The DAVINCI mission will carry a technology demonstration instrument (CUVIS) that will acquire UV (200-400 nm) spectra at 0.2 nm spectral resolution on two Venus dayside flybys in 2030 with up to one million new spectra of the upper clouds, as well as a near UV frame imaging camera (VISOR UV) that will acquire “movies” during these periods to quantify cloud feature motions (Garvin et al. 2022a).

Venus is particularly important to our understanding of terrestrial planets' habitability, providing a natural laboratory to understand its evolution in time. Many significant questions remain on the current state of Venus, suggesting major gaps in our understanding of how Venus's evolutionary pathway diverged from Earth's (Fig. 1). Venus is the only spatially resolvable, Earth-sized world other than the Earth, where a diversity of geophysical envelopes, their interactions and evolution at several time scales and spatial scales, may be monitored from a variety of mission and instrumental concepts and support long-term evolutionary models of Earth-sized planets. Venus exploration offers therefore unique opportunities to answer fundamental questions about the evolution of terrestrial planets and the habitability within our own solar system.

Comparing the interior, surface and atmosphere evolution of Earth and Venus is essential to understanding what processes have shaped our planet, and is particularly relevant in an era where we expect thousands of terrestrial exoplanets to be discovered. Compelling recent insights, and the planet's relevance to exoplanetary systems (Kane 2022), have opened up new questions about the evolution and dynamic nature of Venus and its atmosphere. In Sects. 3-8, we explore how recently selected investigations and payload instrument concepts support the scientific goals and open questions presented in the accompanying papers of this collection. VERITAS (Smrekar et al. 2022a), DAVINCI (Garvin et al. 2022a), and EnVision (European Space Agency 2021) will greatly advance our understanding as well as lead to new questions about geologic evolution. In Sects. 9-12, we also address what key advances would come from new types of data to better constrain the interior, such as improved crustal thickness and structure, mantle viscosity/temperature from seismology, lithospheric thickness from electromagnetic sounding, in-situ heat flow to constrain thermal lithospheric thickness and radiogenic heat budget and distribution. In-situ geochemistry would help constrain rock rheology, mantle conditions, accretion considerations, and the source and history of specific rock types. Information on age of specific features will be very valuable; the detailed composition of weathering products will tightly constrain weathering rates and, if feasible, rock age dating would provide valuable absolute age information. Section 10 addresses, in particular, key areas and investigations at Venus not covered by the fleet of missions under development described in previous sections.
<figure><img src="image_3.png" /></figure>

**Fig. 1** Venus is essential to our understanding of the habitability of terrestrial planets, providing a natural laboratory for understanding their evolution throughout the history of our solar system. This clockwise helicoidal projection illustrates the parallel growth of an Earth-sized planet's interior, surface, and atmosphere (Gaillard and Scaillet 2014). In companion papers, Way et al. (2023), this collection, and Westall et al. (2023), this collection, discuss ongoing studies of the habitability of Earth-size planets, how they can inform the evolutionary history of Venus, and conversely, how the ancient evolution of Venus can serve as ground truth for studies of the divergent paths for Earth-size planets and the longevity of a habitable Venus-size or Earth-size body. The initial thermal state of the planet, which is closely related to its accretion sequence, determines the amount of energy the planet will dissipate over its history and the initial mantle inventory to be outgassed (Avice et al. 2022; Salvador et al. 2023; this collection), while later stages affect the extent, nature, and distribution of surface modification processes (Herrick et al. 2023, this collection; Ghail et al. 2023, this collection; Gilmore et al. 2023, this collection; Carter et al. 2023, this collection) and interior, surface, and atmosphere exchanges over time (Rolf et al. 2022, this collection; Gillmann et al. 2022, this collection; Wilson et al. 2023, this collection)

# 3 Overview of Mission Concepts and Observation Strategies to Address Venus Evolution Through Time

## 3.1 Selected Missions in 2021

On June 2, 2021, NASA announced the selection of two new missions to Venus, VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy, Sect. 4) and DAVINCI (Deep Atmosphere Venus Investigation of Noble gasses, Chemistry, and Imaging, Sect. 5) as part of the Agency's Discovery 2019 competition. On June 10, 2021, the European Space Agency (ESA) announced the selection of EnVision as its newest Medium-Class science mission in the framework of ESA's Cosmic Vision program (Sect. 6). VERITAS, DAVINCI and EnVision will greatly advance our understanding as well as lead to new questions about Venus evolution through time. Key advances will come from new types of data to better constrain the interior, such as improved crustal thickness and structure, mantle viscosity/temperature from seismology, lithospheric thickness from electromagnetic sounding, in-situ heat flow to constrain thermal lithospheric thickness and radiogenic heat budget and
<figure><img src="image_4.png" /><figcaption>**Fig. 2** A new fleet of elements of a Venus exploration program is now under development. These include two radar-equipped orbiters (ESA-led EnVision orbiter and NASA-JPL's VERITAS orbiter), and NASA-GSFC's DAVINCI entry probe with flyby remote sensing mission. Credit NASA / ESA / JAXA / Paris Observatory / VR2Planets</figcaption></figure>

distribution. Over the next 15 years, these three missions will work together to answer many of the outstanding questions of Venus science and rocky planet evolution described above (Fig. 2; Table 1).

NASA's VERITAS orbiter will map the topography of the planet, create a compositional map, search for surface properties indicative of formation during a water-rich period, search for thermal and chemical signatures of volcanic activity, constrain interior structure, and look for surface change using a combination of radar imagery and differential interferometric techniques (Smrekar et al. 2022a). NASA's DAVINCI probe/flyby mission will measure atmospheric chemistry (noble gas isotopic abundances to constrain Venus' formation and early evolution) and physical properties (including wind speed, pressure and lapse rate) down to the surface and collect multi-scale near-infrared (sub-cloud deck) images of composition and topography at scales from ~200 m down to ~25 cm of the ancient tessera terrain where an oceanic past may be recorded (Garvin et al. 2022a). ESA's EnVision orbital mission will follow with a range of observation modes including multi-resolution radar imaging, radar polarimetry and radiometry, a suite of infrared and UV spectrometers for sensing the atmosphere and surface, and conduct the subsurface sounding (European Space Agency 2021). EnVision's combination of surface and atmospheric measurements will characterize ongoing volcanic processes though an extended-timeline, search for their thermal, morphologic, and gaseous signatures, while also tracing key volatile species from the surface up to the mesosphere (Table 1). EnVision is an ESA-led mission in partnership with NASA, providing its Synthetic Aperture Radar instrument, VenSAR and Deep Space Network support for critical mission phases.

Both NASA and ESA orbital missions will provide global context to understand DAVINCI's local, high-resolution near-IR imaging (with topography and band-ratio composition) of its entry ellipse descent corridor within western Alpha Regio (Sect. 5.3.4), which in turn provides 'ground truth' for radar and near-IR VERITAS and EnVision data interpretation (Sects. 5.5 & 6.4). The scientific results of these three synergistic missions will
<table><caption>**Table 1** A comparison of missions VERITAS, DAVINCI and EnVision payload instruments as developed in the following Sects. 4-6, their synergies and complementarity; mission design and driving requirements at time of publication (Smrekar et al. 2022a; Garvin et al. 2022a; European Space Agency 2021). VISAR, Venus Interferometric Synthetic Aperture Radar (§4.3.1); VEM: Venus Emissivity Mapper (§4.3.2); VMS: Venus Mass Spectrometer (§5.3.1); VTLS: Venus Tunable Laser Spectrometer (§5.3.2); VenDI: Venus De-scent Imager (§5.3.4); VISOR UV: Venus Imaging System for Observational Reconnaissance (§5.4.1-5.4.2); VISOR near-IR: Venus Imaging System for Observational Reconnaissance (§5.4.1); CUVIS: Compact Ultra-violet Imaging System (§5.4.2); VenSAR, Venus Synthetic Aperture Radar (§6.3.1); SRS: Subsurface Radar Sounder (§6.3.2); VenSpec-H, -M, -U: Venus Spectrometer Suite (§6.3.3)</caption><thead><tr><th>Parameter</th><th>VERITAS (Section 4)</th><th>DAVINCI (Section 5)</th><th>EnVision (Section 6)</th></tr></thead><tbody><tr><td>Launch date / Duration (at time of publication)</td><td>Not earlier than 2031 / 4 cycles (2.7 years beginning ~2 yr after launch) + extended mission opportunities</td><td>June 2029: 2 flybys (Jan. 2030, Nov. 2030) + 1 entry, descent, science for chemistry, imaging & physical properties (June 2031)</td><td>Dec 2031, May 2032, or Nov 2032 / 6 cycles (4 years beginning ~4 yr after launch) + extended mission opportunities</td></tr><tr><td>Orbit / Inclination</td><td>180 x 255 km / 85.5 deg</td><td>Flybys from 180,000 km to 5,000 km; optional post-probe Venus Orbit Insertion (Jan 2032)</td><td>220 x 520 km / ~88 deg</td></tr><tr><td>Radar Imagery</td><td>VISAR: Global X-band (7.9 GHz) Imaging: 30 m; 15 m 25% of surface</td><td>N/A</td><td>VenSAR: S-band (3.2 GHz) Imaging: 30 m ~30% of surface; 10 m ~2% of surface</td></tr><tr><td>Topography at high resolution</td><td>VISAR: Single Pass Interferometry Global Topography; 250 m horiz., 5 m vertical</td><td>VenDI: Sub-cloud near-IR Structure-from-Motion processed images from 60 m at &lt; 4 m vertical to 25 m at &lt; 3 m vertical; other options include ~90 m at &lt; 7 m vertical & 5 m at 1 m vertical</td><td>VenSAR: Stereo, 25% surface, 300 m horiz. 30-50 m vertical Radar Altimetry, 75% surface, 3 km footprint, 2 m vertical precision; SRS Altimetry, 68% of surface</td></tr><tr><td>Subsurface boundaries</td><td>VISAR: indirect inference from synthetic aperture radar</td><td>N/A</td><td>SRS: penetration depth up to 1000 m; 20 m resolution</td></tr><tr><td>Microwave Radiometer brightness temperature and emissivity maps</td><td>N/A</td><td>N/A</td><td>VenSAR: Dual Polarization HH-VV 7% of surface nadir and off-nadir microwave radiometry, 75% surface. Short-term Temp. var. &lt; 1K</td></tr><tr><td>Repeat Pass Interferometry for Deformation</td><td>VISAR: Targeted 12-18 200 x 200 km sites; 1.5 cm deformation accuracy</td><td>N/A</td><td>Not in current baseline; may recover some opportunistic capability depending on ΔV</td></tr><tr><td>k<sub>2</sub> tidal Love number and Moment of Inertia Factor (MoIF)</td><td>k<sub>2</sub> accuracy 0.2% at 3-sigma, moment of inertia at 0.3% at 3-sigma using two-way Ka-band Doppler tracking and radar tie points</td><td>N/A</td><td>RSE: k<sub>2</sub> accuracy 1.2% at 3-sigma, moment of inertia at 1.2% at 3-sigma w/capability to combine Doppler tracking and radar tie points</td></tr><tr><td>Gravity field</td><td>Two-way Ka/Ka-band and X/X-band tracking; gravity field resolution &lt; 105 km globally with &lt;4 mGal RMS accuracy</td><td>N/A</td><td>RSE: Two-ways X / X-Ka band tracking; gravity field resolution &lt; 200 km globally and &lt; 170 km on at least 40% of planet with &lt; 10mGal RMS accuracy</td></tr><tr><td>Noble gases, trace gases & their isotopic ratios; loss mechanisms and past outgassing</td><td>N/A</td><td>VMS; VTLS: in-situ probe-based analytical chemistry of S, O species and He isotopes Isotopic ratios for Ne, Ar, Kr, Xe H, S, C, and O & their isotopic ratios including D/H</td><td>VenSpec-H: D/H in tropospheric H<sub>2</sub>O, HDO</td></tr><tr><td>Detect current and recent surface volcanism</td><td>VEM: thermal signature 1020 nm, unweathered flows: 6 near IR bands 860 to 1180 nm. VISAR: Surface changes within mission and between Magellan and EnVision</td><td>N/A</td><td>VEM: thermal signature 1020 nm, unweathered flows: 6 near IR bands 860 to 1180 nm. VenSAR: Surface changes vs Magellan, within mission, vs. VERITAS</td></tr><tr><td>Surface composition</td><td>VEM: 6 near IR bands 860 to 1180 nm</td><td>VenDI: sub-cloud near-IR band-ratios at scales from 150 m down to 12 m (740 to 1030 nm)</td><td>VenSpec-M: 6 near IR bands 860 to 1180 nm</td></tr><tr><td>Current volcanism using water vapor, S-bearing gas detection, and cloud top UV absorber</td><td>VEM: water vapor bands at 960 and 1160 nm; cloud bands at 1195, 1310, 1510 nm</td><td>CUVIS: hyperspectral UV spectroscopy (0.2 nm resolution) on two dayside flybys; VISOR: near IR emissivity mapping of Maat, Alpha, Ovda at 2 nightside flybys VMS; VTLS: Probe-based analytical chemistry of S, O species and He isotopes</td><td>VenSpec-M: water vapor bands 960 &amp; 1160 nm; cloud bands at 1195, 1310, 1510 nm VenSpec-H: 1000-2700 nm: H<sub>2</sub>O, HDO, CO and SO<sub>2</sub> VenSpec-U: SO, SO<sub>2</sub> variations 190-380 nm</td></tr><tr><td>p, T vertical structure of atmosphere</td><td>X/X+Ka/Ka-band radio occultation sounding, SO<sub>2</sub> and H<sub>2</sub>SO<sub>4</sub> liquid and gaseous content at 45-55 km, p, T profiles at 45-90 km</td><td>p, T, winds, accelerations every 15-50 m from 70 km - surface; for lapse rate (T vs. z). Trace gases every 140 m from 40 km - surface for masses from 8 to 272 Da with gradients in key species (SO<sub>2</sub>, OCS, H<sub>2</sub>O, CO<sub>2</sub>)</td><td>RSE: One-way X-Ka band radio occultation sounding, SO<sub>2</sub> and H<sub>2</sub>SO<sub>4</sub> liquid and gaseous content at 45-55 km, SRS/IR at 35-90 km, at all local time, lat./long., 4 soundings / day</td></tr></tbody></table>
answer fundamental questions about the early period of Venus history, how the transition
to its current forbidding environmental state came about, where volcanic eruptions are re-
shaping the surface today, and what the differences from Earth can tell us about possible
pathways of extrasolar rocky planet evolution.

**3.2 Missions Considered for Selection**

In addition to the three selected missions, several international space agencies have devel-
oped mission concepts. This collection is based on the papers presented at the Space Sci-
ence Series of the International Space Science Institute (ISSI) workshop ‘Venus: Evolution
through Time’ held in Bern, Switzerland, on September 13-17, 2021. It therefore captures
the discussions that took place among the participants during the workshop. At that time, two
large mission concepts were discussed for launch before the end of the current decade and
possibly before the planned launch period of the currently selected VERITAS, DAVINCI,
and EnVision missions: Venera-D (Zasova et al. 2020) and Shukrayaan-1 (Antonita et al.
2022). We therefore decided to describe extensively these two mission concepts in Sects. 7
and 8.

In conclusion to Sect. 3, we add a brief description of the Chinese radar-equipped VOICE orbiter mission proposal (Dong et al. 2023); and of Venus Life Finder “Morning Star”, Rocket Lab’s private low-cost concept mission to Venus (Seager et al. 2021; Limaye and Garvin 2023).

Venus Volcano Imaging and Climate Explorer (VOICE) is a Chinese orbiting mis-
sion to investigate Venusian geological evolution, atmospheric thermal-chemical processes,
surface-atmosphere interactions, and habitable environment and life in the clouds. Three
state-of-the-art scientific payloads, the Polarimetric Synthetic Aperture Radar (PolSAR), the
Microwave Radiometric Sounder (MWRS) and the Ultraviolet-Visible-Near Infrared Multi-
Spectral Imager (UVN-MSI), will be flown on a polar-circular orbit of ~350 km. VOICE
is currently proposed to Strategic Priority Program (SPP) on Space Science of the Chinese
Academy of Sciences (Dong et al. 2023).

Venus Life Finder “Morning Star” Mission, Rocket Lab’s low-cost, small entry probe mission to Venus is intended to be the first in a series (Seager et al. 2021). After the cruise phase, the Photon platform, designed for launch on the Electron small launch vehicle, will target an entry interface to deploy a small (~20 kg) probe directly into the atmosphere with a flight path angle (EFPA) between -10 and -30 degrees, communicating direct-to-Earth through an S-band antenna, containing up to 1-kg of science payload (French et al. 2022; Seager et al. 2022).

4 Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS)

The Venus Emissivity, Radio Science, InSAR, Topography And Spectroscopy (VERITAS) mission will address key science objectives about the geologic and volcanic history of Venus; it will elucidate how Venus' evolution differs from Earth's with three overarching Science Goals: 1) constrain Venus' geologic evolution; 2) determine which geologic processes are active; and 3) search for evidence of past and present water. VERITAS is a partnership between scientists and engineers at NASA/JPL and with the German, Italian and French Space Agencies. DLR provides the Venus Emissivity Mapper (VEM) instrument and VISAR interferometric processing support, ASI, which provides the Integrated Deep Space
Transponder, the lower power RF portion of the VISAR radar and the high gain telecommunications antenna, and CNES, which provides VEM optics and filter array subsystems, and the Ka-band TWTA. Equipped with an interferometric synthetic aperture radar and infrared imaging spectrometer VERITAS will globally map the surface of Venus producing imagery, rock type, topographic, and gravity maps. Following its selection by NASA, and according to the best information available at the time the manuscript is revised, VERITAS is programmed for launch no earlier than 2031 (Table 1).

# 4.1 VERITAS Science Objectives

## 4.1.1 Overview

VERITAS is structured to answer three essential science questions about the processes that have shaped and continue to shape the surface today:

1. What processes shape rocky planet evolution?

2. What geologic processes are currently active?

3. Is there evidence of past and present interior water?

These questions are organized into a series of specific investigations that can be addressed with two instruments and a radio science investigation.

VERITAS intends to definitively answer whether volcanism has been steady or catastrophic, why it lacks terrestrial-style plate tectonics, how it loses its heat, and if its plateaus are analogs of Earth's continents. It will also extend our knowledge of Venus with numerous firsts, including constraints on the core size and state (relevant to dynamo formation), high resolution topography and radar imaging, and a search for active surface deformation and active or recent volcanism.

Without an understanding of Venus' geologic evolution through time, we cannot fully test hypotheses for how Earth and other terrestrial planets evolve. Our knowledge of Earth in particular forms the basis for understanding habitability (Way et al. 2023, this collection; Westall et al. 2023, this collection). One of the central issues regarding the potential habitability of extrasolar planets is the extent to which plate tectonics is required to maintain habitability (e.g., Southam et al. 2015). A planet's interior is the engine for geologic activity, which drives climate and atmospheric evolution, thus setting the conditions for its long-term habitability. Because exoplanet surfaces cannot be spatially resolved, only indirect information on planetary radius, interior density, and atmospheric composition is available to predict habitability, in addition to radiative constraints to allow water to condensate in liquid phase (Hays et al. 2015; Meadows et al. 2018). Knowledge of the interior, including the size and state of the core, is critical to understanding how a planet loses its heat and evolves. Habitability models, such as that of Tosi et al. (2017), cannot predict the circumstances leading to habitability of a Venus-like planet, due to a lack of knowledge of Venus' geologic history and evolution.

Plate tectonics is Earth's defining geologic process. For billions of years, distinct lithospheric plates have moved above the upper mantle as they spread apart at undersea volcanic regions (the mid-ocean ridges), sink at subduction zones, and slide past each other at transform faults. Continental crust formed largely via melting of subducting slabs in the presence of water (Campbell and Taylor 1983; Arndt 2013). Earth's continents drift apart and crash together, but are never dragged into the interior because their Si-rich composition is lower in density.

Many new hypotheses linking plate tectonics to Earth's habitability are emerging, including the origin of the great oxygenation event (Duncan and Dasgupta 2017). Processes
**Fig. 3** VEM's 6 channel, high signal-to-noise ratio (SNR) spectra will determine definitively if tessera plateaus are felsic (low iron) like Earth's continents. The lower 1000 nm VIRTIS emissivity for Alpha Regio (color overlay) suggests low iron content (Gilmore et al. 2015; Dyar et al. 2020; Helbert et al. 2021). An example of the complex deformation that characterizes tesserae is shown in the Magellan SAR image inset

<figure><img src="image_5.png" /></figure>

that link volcanism (that releases volatiles such as H₂O, CO, and SO₂ from the interior to the atmosphere) and subduction (that cycles volatiles back into the mantle) help maintain a stable climate and hydrosphere (Kasting and Catling 2003; Driscoll and Bercovici 2013). In fact, the vast majority of Earth's present-day oceans and atmosphere came from interior degassing (Pearson et al. 2014; Marty et al. 2016, 2017). Interior heat loss maintains our planet's magnetic dynamo, protecting complex organic compounds from radiation damage by magnetically deflecting the solar wind. On Earth, plate tectonics is the dominant heat loss mechanism. Is there evidence of these processes on Venus? VERITAS looks for evidence of "continents," past geologic processes, volcanic history, and subduction—the first step in initiating plate tectonics.

### 4.1.2 What Processes Shape Rocky Planet Evolution?

**Global Rock Type, Terrains, and Tesserae.** - Venus surface geology is controversial and geochemistry is largely unconstrained. What little we know about surface composition comes from a handful of Venera and VeGa geochemical analyses (Grimm and Hess 1997; Treiman 2007), laboratory experiments (Shellnutt 2013), thermodynamic modeling (Tef-feteller et al. 2019), and conclusions drawn from Magellan radar and limited (single-band) emissivity data from VEX (Mueller et al. 2008; Helbert et al. 2008; Smrekar et al. 2010a; Gilmore et al. 2015; Mueller et al. 2020). Landed measurements suggest geochemically distinct volcanic plains units, but the large uncertainties preclude definitive interpretations (Gilmore et al. 2017). New Magellan emissivity analysis suggests variability among tessera plateaus (Gilmore et al. 2019).

The composition of tessera terrain provides critical constraints on Venus geochemistry, geodynamics and the history of water on the planet. Containing what may be the oldest surface rocks on Venus (Ivanov and Head 1996), they are extremely complex geologic terrains (Figs. 3 and 4). Tesserae occur as both large plateaus with diameters of 1000–4000 km, and as smaller, isolated 100s-km-scale areas embayed by plains volcanism. They have been proposed to be continent-like based on their morphology, gravity signature, and inferred low-Fe, high-Si composition (Hashimoto et al. 2008).

Surface emissivity data from the VIRTIS on VEX, as well as from Galileo, suggest that tesserae are more felsic (lower in iron) than basaltic plains (Hashimoto et al. 2008; Helbert et al. 2008; Gilmore et al. 2015; Helbert et al. 2021) and thus possible analogs of terrestrial continents. Tessera composition and formation are key to assessing the role that volatiles play in shaping Venus' evolution. Large uncertainty in Magellan topographic height (Fig. 4)
**Fig. 4** Constraining tesserae composition requires both VEM data and the VERITAS digital elevation model (DEM) to provide the elevation-dependent temperature correction. Tesserae heights are not fully resolved in either the spatially limited Magellan stereo DEM data (black) or Magellan global altimetry resolution (blue)

<figure><img src="image_6.png" /></figure>

and thus in the altitude-dependent temperature correction for emissivity, as well as lack of spectral data, together preclude an answer to whether Venus has continents.

Two main hypotheses have been proposed for the origin of tesserae: downwelling and upwelling mantle flow. Deciding between these has significant implications for the crustal strength, volatile content, thermal gradient and evolution, and formation timescale (Lenardic and Kaula 1995; Ghent et al. 2005). According to the downwelling scenario, plateaus form as the crust thickens above regions where the cold mantle sinks (Bindschadler and Parmentier 1990; Bindschadler et al. 1992). Downwelling and remelting of basaltic crust could produce more Si-rich compositions, similar to those produced at some of Earth's subduction zones (Elkins-Tanton et al. 2007; Romeo and Turcotte 2008; Gilmore et al. 2017). Alternatively, plateaus may have formed over upwelling mantle plumes, with crustal thickening caused by profuse mantle melting (Phillips and Hansen 1994) that should produce a basaltic composition. Discriminating between these formation models requires distinguishing between Fe-rich, Si-poor basaltic rocks and more Si-rich continental rock types, as well as ascertaining the detailed shape of the small-scale graben that are diagnostic of specific deformation sequences (Bindschadler and Parmentier 1990; Bindschadler et al. 1992; Hansen and Willis 1998). If intermediate rock types are present, they are expected to have intermediate emissivities.

The primary mountain belts on Venus encircle Lakshmi Planum, and may be similar in composition to tesserae. However, there are no VIRTIS emissivity or landed data in that location. If the plains' compositional differences (tholeiitic and highly alkaline basalts) suggested by landed data can be validated by VERITAS' near-global rock type mapping, they would provide evidence of varying depths of melting and fractionation and thus geologic setting (Gilmore et al. 2017).

In addition, gravity data also inform our understanding of rock types and global terrain. Global gravity and topography data can be used to estimate the elastic thickness/thermal gradient, which constrains mechanisms of formation, and can identify processes such as subduction, localized delamination/ upwellings, and fossilized rifts. Other fundamental questions also remain: Did all tesserae form the same way? Are they all the same composition?

**Prior Geologic Regimes: Buried Features.** - Global high-resolution surface topography, images, and gravity data can provide critical windows into the past. On Mars, high-resolution topography shows subtle signatures of buried impact craters, only revealed by MGS' Mars Orbiter Laser Altimeter (MOLA) instrument (Frey et al. 2002; Buczkowski and McGill 2002; Frey 2006). These demonstrate that an ancient cratered surface was buried
<figure><img src="image_7.png" /><figcaption>**Fig. 5** Bouguer gravity anomalies and gravity gradients reveal a pattern of narrow linear anomalies that border Procellarum associated with buried lunar subsurface structures (a, b; data from the GRAIL mission). VERITAS' high-resolution gravity can reveal such subsurface structures. Gravity gradient maps at VERITAS' resolution (c) would reveal buried rifts (outlined in dots); map at Magellan resolution (d) is insensitive to these features. Figure after Andrews-Hanna et al. 2014; 1 Eotvos (E) = $10^{-9}$ Gal cm$^{-1}$ = $10^{-9}$ s$^{-2}$)</figcaption></figure>

beneath plains material, disproving theories of resurfacing by plate tectonics and support-
ive of lowlands formation via a giant impact (Andrews-Hanna et al. 2008). MESSENGER
images of Mercury show volcanically flooded ghost craters (Head et al. 2011). On the
Moon, GRAIL gravity data reveal ancient rifts for which no surface signature exists (Fig. 5,
Andrews-Hanna et al. 2014), as well as buried impact craters (Evans et al. 2016; Sood et al.
2017).

What lies beneath Venus' volcanic plains? Evidence for buried impact craters would shed new light on the history of volcanism and age of the underlying terrain. Venus may have had plate tectonics earlier in its history, perhaps continuing until the most recent resurfacing event (Armann and Tackley 2012). Interconnected lineations in the gravity or topography could reveal the sites of past spreading centers or subduction zones. Alternatively, tesserae may underlie the plains.

Tesserae inliers are common and could be the gravitationally relaxed remnants of ancient tesserae plateaus (Ivanov and Head 1996; Hansen and López 2010). The topographic or gravity signature of tesserae under the plains would show that tesserae were widespread, and may have formed during a prolonged wet period on Venus.

**Impact and Volcanic History.** - Venus' impact crater population holds the key to the planet's integrated time history of volcanism. The small number of craters (less than 1000 observed) implies a young surface age, but there is uncertainty in the number of modified craters. The "Catastrophic Resurfacing" hypothesis postulates that a huge pulse of volcanism rapidly covered Venus ~1 Ga ago, followed by limited activity. This theory was based on two observations: 1) The distribution of craters cannot be distinguished from a random one; and 2) few craters appear to be modified by volcanism or tectonism (Schaber et al. 1992; Strom et al. 1994).

Numerous models derive a resurfacing rate by comparing the rates of crater formation and removal or modification. Many models are more consistent with an equilibrium rather than catastrophic resurfacing rate (Basilevsky 1993; Bjonnes et al. 2012; O'Rourke et al. 2014; King 2018). In all cases, the critical parameter is how many craters are modified. The two primary modes of crater modification are volcanic infilling and weathering removal of distal crater ejecta (halos and parabolas). In the catastrophic resurfacing model, major crustal resurfacing is confined to a discrete pulse, and consequently only a small number of craters, preserved from this time, should show volcanic modification. The equilibrium theory suggests resurfacing processes occur continuously, and the majority of craters should display some form of modification, although the degree of modification should vary regionally (Herrick and Rumpf 2011).
Studies investigating crater modification have found evidence of craters with varying levels of modification, however, these results remain controversial. If all dark-floored craters (~80% of craters) have been partially flooded, then volcanism has persisted over 100s of millions of years (Herrick and Sharpton 2000; Herrick and Rumpf 2011). However, the resolution of the Magellan stereo data (~100 m) casts the robustness of this interpretation into doubt.

The study of fine-grained distal crater ejecta (halos and parabolas) argues against catastrophic resurfacing. Analysis of variations in halo retention combined with regional crater density, suggest that the surface of Venus is divisible into three major age groupings (Basilevsky et al. 2003b). These include those superposed on wrinkle ridges on regional plains, those superposed on units younger than regional plains, and “other.” However, their study relies on the assumption that the processes removing the extended ejecta deposits operate at a uniform rate across the planet, despite no direct observations of the actual mechanisms, and the natural difficulty in mapping the morphology of the dark, diffuse deposits.

Radically different implications follow from the various resurfacing models. The “catas-
trophic” resurfacing hypothesis suggests that planets may behave episodically, with cycles of
plate tectonics or mantle overturn and massive melting (Parmentier and Hess 1992; Reese
1999; Armann and Tackley 2012) separated by periods with no plate motion (a “stagnant
lid” regime). These models predict the spike in volcanism implied by catastrophic resur-
facing. Volatiles outgassed by volcanism in a catastrophic event could have a significant
effect on climate, perhaps changing temperature enough to cause detectable thermal expan-
sion of the surface (Anderson and Smrekar 1999). In contrast, the equilibrium resurfacing
model implies more Earth-like rates of volcanism without a need for episodic plate tectonics
(O’Rourke et al. 2014).

All resurfacing models are constrained by the number of modified craters and extended ejecta deposits, which remains controversial. Thus, the distribution of volcanism in space and time, the relative age of different locations, and implications for the history of Venus' evolution are uncertain. One method to assess resurfacing history is to model the gradual removal of fine-grained crater ejecta (Phillips and Izenberg 1995; Ghail et al. 2023, this collection). Relatively old regions, without any volcanism for an extended time, would have both high crater density and relatively few halos, since aeolian or chemical weathering can also remove halos and not high-standing crater rims or rocky ejecta. A high-resolution Digital Elevation Model (DEM), gravity measurements, and surface imagery are needed to discriminate between models: to characterize subduction and extension zones, fault patterns, flexed terrains and their margins, elastic thickness anomalies, and the distribution, nature and extent of impact crater modification (Herrick et al. 2023, this collection). VERITAS will provide a new and detailed view of global tectonics, potentially revealing strike-slip faults, extensional zones and other deformation features (Fig. 6).

*What Are the Major Tectonic Processes? Is Subduction Currently Active?* - Venus does not currently have recognizable Earth-like plates bounded by spreading centers, subduction zones, and transform faults (Solomon et al. 1991). On Earth, plate formation at spreading centers and subduction of old plates dominate heat loss and drive geologic activity. Without global plate tectonics, how does Venus lose its heat?

One clue is that Venus' internal heat engine has created abundant tectonic deformations including >40,000 km of fractured troughs, possible subduction zones, and huge mountain belts. Among the terrestrial planets, only Earth and Venus have such massive deformation zones. Could Venus represent a transitional tectonic state between active (plate tectonic) and stagnant (no plate motion) regimes? Although not globally interconnected, Venus may have key elements of plate tectonics: subduction, major extensional zones, and even transform faults.
<figure><img src="image_8.png" /><figcaption>**Fig. 6** VERITAS' Digital Elevation Model (DEM) has the needed resolution to discover narrow deformation zones such as strike-slip plate boundaries not apparent in image data. Shuttle Radar Topography Mission (SRTM, Rodriguez et al. 2005; Farr et al. 2007) Earth's topography data (blue = low, purple = high), reduced to VERITAS resolution (left, 240 m horizontal, 5 m vertical noise) clearly show major tectonic boundaries. Such faults are invisible at Magellan resolution (right, 15 km horizontal, 100-m noise)</figcaption></figure>

Subduction is a necessary first step in initiating plate tectonics. Whether it occurs on Venus is vigorously debated (Hansen and Phillips 1993; Sandwell and Schubert 1992a,b; Hansen and Olive 2010; Baes et al. 2016; Crameri and Tackley 2016). Venus' huge trough systems (termed chasmata) are typically interpreted as extensional zones, based on numerous long, linear fractures that can sometimes be identified as graben. However, some chasmata have characteristics of subduction. Specifically, some troughs have the same asymmetric shape, with a ridge on the concave side and a trough on the convex side, as observed at some arcuate terrestrial oceanic subduction zones including the South Sandwich Islands trench (Sandwell and Schubert 1992a; Fig. 7).

Because proposed Venusian subduction zones have estimated elastic thicknesses and bending moments similar to their terrestrial analogs (Sandwell and Schubert 1992a,b), their mechanical behavior should also be similar. If Venus' asymmetric chasmata are produced by subduction, their gravity signature should be asymmetric, in contrast to the symmetric signature of extension (Smrekar et al. 2010b).

A recent theoretical development (Bercovici and Ricard 2014) assesses the likelihood of plate tectonics on exoplanets: subduction evolves into plate tectonics when reductions in lithospheric strength from breakage (due to microcracks and grain-size dependent deformation) dominate over lithospheric healing via grain growth. This suggests that Venus may have a hot lithosphere that anneals too rapidly to allow subduction to develop into full plate tectonics. Global estimates of elastic thickness will allow us to assess whether it indeed has a hot, thin lithosphere, similar to early Earth's. Finding unequivocal evidence for subduction on Venus would elucidate the conditions and mechanisms for subduction initiation on terrestrial planets. Evidence of other major tectonic boundaries (extensional zones like mid-ocean spreading centers or major transform faults) would suggest that Venus could even transition to an Earth-like plate tectonic regime, as predicted by modeling (Armann and Tackley 2012).

**Interior Structure and Thermal State.** - Our knowledge of the internal structure of Venus is based on limited data for mass, radius, gravity, and topography. One direct existing constraint is provided by the tidal Love number $k_2$, or gravitational potential modification due to the tidal deformation of the planet.

Lack of a magnetic field at this stage of Venus' evolution does not provide any constraint (Stevenson 2003). Thus, interior structure models use a core size that is simply rescaled from Earth. Published temperature profiles for Venus' interior differ by up to 1000 K and 500 K in the lower and upper mantle, respectively (Steinberger et al. 2010; Armann and
<figure><img src="image_9.png" /><figcaption>**Fig. 7** VERITAS determines whether Venus has subduction. The surface of Venus hosts a variety of different features - volcanoes, rifts, mountain belts - that are typically on the scale of hundreds of kilometers, set within globally extensive regional lowland plains. Many features resemble Earth's arc-shaped oceanic subduction zones, such as the trenches at Artemis Corona, shown in this overlay of Magellan SAR data on altimetry (vertical scale at lower left): troughs have the same asymmetric shape, with a ridge on the concave side and a trough on the convex side (after Davaille et al. 2017)</figcaption></figure>

Tackley 2012) with different implications for its cooling and volcanic history. Gravity and topography (Konopliv and Yoder 1996; Steinberger et al. 2010; Huang et al. 2013; Rolf et al. 2018) are linked at long wavelengths to the structure and dynamics of the sub-lithospheric mantle.

Precise measurements of the Moment of Inertia factor (MOIF) through the pole precession rate, the tidal Love number, $k_2$ and the tidal phase lag, *e*, will permit the first useful comparisons between the interior of Venus and other terrestrial planets (see also this Section, Sect. 4.3.3). The state of the core, its size (if liquid), and the tidal phase lag have already been determined by space missions with different accuracy for Mercury, Mars, and Moon. These previous studies demonstrate that knowledge of Venus' interior structure and therefore constraints on its long term evolution, can be derived from the moment of inertia, Love number $k_2$, and phase lag *e*.

As an example of results obtained for other Solar System bodies, Moment of Inertia factor constrains the core size of Mercury (Genova et al. 2019) and Mars (Smrekar et al. 2019), and the measured Love number $k_2$ is indicative of a liquid (or partially liquid) core for the Moon (Williams et al. 2014), Mercury (Margot et al. 2018), and Mars (Yoder et al. 2003). $k_2$ for Venus has been estimated from Doppler tracking of Magellan and Pioneer Venus Orbiter ($k_2$ = 0.295 ± 0.066; Konopliv and Yoder 1996), but the large uncertainty does not allow distinguishing between a liquid vs. solid core (Dumoulin et al. 2017). Use of
the Martian phase lag (Plesa et al. 2018) constrains core size and volatile components such as sulfur (Rivoldini et al. 2011), as well as mantle viscosity.

### 4.1.3 What Geological Processes Are Currently Active?

Multiple observations suggest that Venus is geologically active. Magellan topography and gravity data analysis indicate the likely presence of low density, hot mantle plumes under large volcanic rises, or “hotspots,” similar to those on Hawaii (Smrekar 1994). Atmospheric sulfur variations imply active volcanic outgassing (Marcq and Lebonnois 2013). The average sulfur content corroborates ongoing volcanism because SO₂ breaks down over time (Fegley and Prinn 1989; Fegley et al. 1997). Campbell et al. (2017) presented evidence for relatively recent pyroclastic volcanic flows based on their radar properties and lack of erosion. Finally, the 2011 Planetary Science Decadal Survey named the case for recent volcanism (Smrekar et al. 2010a) from near-IR emissivity anomalies to be one of the ten major discoveries of the last decade. Enhanced 1020 nm emissivity correlates with stratigraphically young flows at three areas classified as hotspots based on their broad topographic rises and large positive gravity anomalies (interpreted as mantle plumes). The emissivity increase between these areas and the surrounding plains is consistent with that between unweathered and weathered basalt, implying recent volcanism on timescales of years to decades, based on new laboratory data (Knafelc et al. 2019; Berger et al. 2019). But VEx VIRTIS data cover only the southern hemisphere with only a single band.

Are the six hotspots not observed by VIRTIS also active? Is there evidence for recent volcanism in other geologic settings? If present-day volcanism is restricted to hotspots, this may imply a shift from radiogenic internal heating to basal heating at the core-mantle boundary, allowing hotspot formation (Choblet and Parmentier 2009). Alternatively, restriction of volcanism to hotspots could arise from upper mantle heating under an insulating stagnant lid, leading to shutdown of widespread plains volcanism through desiccation of the upper mantle. In this scenario, hotspot volcanism occurs where water-bearing material from the lower mantle melts (Smrekar and Sotin 2012). Identifying the origin of current volcanism will dramatically change our understanding of interior dynamics.

If Venus remains volcanically active, it is also deforming. For the first time on another planet, we will have the ability to detect active tectonic deformation through repeat pass interferometry. Finding active deformation, and identifying the mechanism(s) responsible, would provide unprecedented insights into the geologic evolution of Venus, and by extension to terrestrial planets.

### 4.1.4 Is There Evidence of Past or Present Water?

*Geochemical Fingerprints of Past Water.* - Venus may have once had a shallow ocean's worth of water at the surface, based on the D/H ratio of the atmosphere (Donahue et al. 1982; Kumar and Taylor 1985; de Bergh et al. 1991). Its interior may still hold ~75% of its original volatiles (O'Rourke and Korenaga 2015). Its surface composition could retain a fingerprint of past water. Recent VIRTIS emissivity data at 1020 nm from a limited number of tesserae suggest a composition lower in iron than the plains, implying they formed in the presence of water (Mueller et al. 2008; Gilmore et al. 2015; Campbell et al. 2017). With an additional five emissivity bands and more accurate altimetry data to correct for altitude-dependent temperature (as well as detailed T vs z lapse rate boundary conditions from DAVINCI), VERITAS can fully confirm or refute this.

VERITAS uses VEM to map the distribution of FeO contents and relates them to geologic features. If continental-scale low-Fe silicic crustal regions on Venus are confirmed, this will
indicate past hydrated source regions. Determining that tesserae are in fact continent-scale silica-rich regions would illuminate the processes that shaped our home planet.

*Is Interior Water Being Volcanically Outgassed Today?* - Pioneer Venus and VEx (Marcq et al. 2013; Esposito 1984; Esposito et al. 1988) observed strong SO₂ variability at the cloud tops, suggestive of plumes of rising gas from active volcanoes. Recent ground-based observations using Infrared thermal mapping have continuously evidenced such rapid variations (Encrenaz et al. 2013-2020a, see also Sect. 11, Sect. 11.1.3). Plumes must include substantially lighter elements than SO₂, including ~2–5% H₂O, to have sufficient buoyancy to reach heights of tens of kilometers (Airey 2015). Terrestrial volcanoes emit large quantities of H₂O, CO, and SO₂ (e.g., Gerlach 1980). Thus, volcanically outgassed water will only be observed on Venus if there are large, Earth-like concentrations remaining in the interior. Radar-derived evidence for relatively recent pyroclastic flows implies significant outgassing (Campbell et al. 2017). VERITAS observes near-surface water vapor through near-IR atmospheric absorption bands. Unlike higher altitude VEx H₂O measurements, the association of near surface water vapor with volcanism can be confirmed with additional surface observations.

## 4.2 VERITAS Mission Overview

The VERITAS mission is an orbiter carrying an X-band Interferometric SAR and the VEM instrument to perform global SAR mapping at 30 m resolution; and to acquire topography, gravity, INSAR, and NIR emissivity data. Its indicative mass budget including all margins is 1450 kg (dry mass). After nominal launch VERITAS has a ~7-month cruise to Venus. Following Venus Orbit Insertion (VOI), VERITAS performs a maneuver to reduce the orbit period from 120 hrs to 13 hrs. Aerobraking is then used to place the flight system in its initial and final science orbits. VERITAS has two planned science phases: Science Phase I (SP1) and Science Phase II (SP2). Science Phase I uses a 6.1-hour, highly elliptical orbit, and Science Phase II has a 91-minute orbit period at a mean orbit altitude of 217 km. Both have a nearly an orbit inclination of 85.5°.

The planned aerobraking campaign begins about two weeks after the Period Reduction Maneuver. The aerobraking campaign is divided into two segments: Aerobraking I (AB1, 7 months) and Aerobraking II (AB2, 9 months), with the 4-month SP1 embedded between them. During aerobraking, the spacecraft lowers periapsis into the upper atmosphere, using the additional drag to reduce its apoapsis altitude from over 40,000 km to 400 km. In SP2, which consists of four Venus cycles (roughly equal to a sidereal day of 243 Earth days), all instruments, VEM and VISAR, as well as the gravity science observations are conducting observations. Science Phase II begins with a 60-day VISAR calibration phase after the post-Aerobraking Exit maneuver (post-ABX) 200 × 400 km orbit is reshaped into the final science orbit. Venus rotates so slowly that the ground track moves only 10 km per orbit at the equator. Venus is so spherical, with an equatorial bulge three orders of magnitude smaller than Earth, that the spacecraft's ascending node precesses extremely slowly. These effects create a ground-track repeat cycle slightly longer than the 243-day Venus sidereal day; this causes the orbit to experience the same gravitational perturbations repeatedly over every orbit, which leads to significant eccentricity vector evolution, similar to that experienced by low lunar orbiters such as GRAIL. The mean inclination of 85.5° has been selected to cause this evolution to bend back upon itself, yielding a near-frozen orbit with altitude variation of 182.6 km to 252.5 km that repeats with the topography. A small radial Eccentricity Control Maneuver (ECM) at the end of each cycle corrects the slight mismatch.

Repeat-Pass Interferometry (RPI) enables cm-scale change detection of the surface and requires that the spacecraft fly within 160 m of its previous path over the targeted site. In
<figure><img src="image_10.png" /><figcaption>**Fig. 8** Map of cumulative 30-meter VISAR radar images over the 4-cycle VERITAS ~3.5 Earth years mission Science Phase II (SP2). SP2 starts after further aerobraking has placed VERITAS in a near-polar, circular, low-altitude orbit. For 57% of each 224-day Venus year, science mapping will cover the entire orbit</figcaption></figure>

addition to maneuver execution errors and differential solar tidal torques, the uncertainty in the rotation period of Venus affects repeatability. To compensate, VERITAS will process radar tie points on the ground, as demonstrated with Magellan data, to improve orbit reconstruction (Chodas et al. 1992, 1993), and modeled for VERITAS (Cascioli et al. 2021).

The VERITAS mission design for SP2 enables a flexible orbit plan that balances 11 science mapping orbits with five downlink orbits each day. For 57% of each 224-day Venus year, science mapping will encompass the entire orbit. In the remainder, eclipses are long enough that additional power management is required. VISAR mapping occurs on a fraction of these orbits selected to optimize coverage within available power balance. Figure 8 shows the build of the 30 m radar imagery over the 4 cycle mission.

## 4.3 VERITAS Science Payload

VERITAS's payload is composed of two instruments crafted to study Venus' surface coupled with a radio science investigation to measure the gravity field. The two instruments are an X-band interferometric synthetic aperture, VISAR, and a fourteen-band infrared spectrometer, VEM.

### 4.3.1 Venus Interferometric Synthetic Aperture Radar (VISAR)

Persistent optically opaque cloud cover of Venus necessitates the use of synthetic aperture radar techniques to obtain high resolution imagery and topography of the surface. The Venus Interferometric Synthetic Aperture Radar instrument (VISAR) instrument is an X-band single pass radar interferometer designed to acquire high resolution imagery and topography of Venus as well as to make repeat pass interferometric measurements of surface deformation.

**VISAR Specifications.** - VERITAS requires a Digital Elevation Model (DEM) with 300 m horizontal postings over 90% of the Venus surface, with height accuracy ≤10 m for
<table><caption>**Table 2** Key VISAR radar design and performance parameters</caption><thead><tr><th colspan="2">VISAR Parameters</th></tr><tr><th>Instrument Parameters</th><th>Value</th></tr></thead><tbody><tr><td>Platform Altitude Range (km)</td><td>182–252</td></tr><tr><td>Polarization</td><td>VV</td></tr><tr><td>Peak RF transmit power (dBW)</td><td>26.0</td></tr><tr><td>X-band Wavelength (m)</td><td>0.038</td></tr><tr><td>Antenna azimuth × elevation dimensions (m)</td><td>3.9 × 0.65</td></tr><tr><td>Range bandwidth (MHz)</td><td>20</td></tr><tr><td>Slant Range/Azimuth Resolution (m)</td><td>7.5, 2.3</td></tr><tr><td>Incidence Angle at swath center (°)</td><td>32</td></tr><tr><td>Pulse Repetition Frequency (PRF) (Hz)</td><td>5500</td></tr><tr><td>Pulse length (µs)</td><td>35</td></tr><tr><td>Baseline length (m), and orientation angle (°)</td><td>3.1, 30</td></tr><tr><td>Range, Azimuth Ambiguities (dB)</td><td>−36, −25</td></tr><tr><td>Atmospheric Losses (dB)</td><td>−9.5 dB</td></tr></tbody></table>

95% of the mapped surface. Global imagery with resolution less than 30 m is also required with radiometric resolution better than 3 dB.

**VISAR Design Considerations.** - VISAR operates at a center frequency of 7.9 GHz (0.038 m wavelength), which optimizes topographic mapping accuracy by balancing the effects of atmospheric attenuation with baseline and antenna size constraints to fit within the spacecraft fairing. A 20 MHz bandwidth provides ~15 m ground resolution at the VISAR 30° angle of incidence. The radar must map a swath width greater than 14 km, spanning the 10 km of surface rotation at the equator during an orbital period with 2 km overlap with adjacent orbits. Table 2 lists key radar design and performance parameters.

**VISAR Onboard Processing.** - VISAR would include an On-board Processing (OBP) element to meet downlink constraints. Key processing steps in the OBP data flow are range compression, motion compensation, azimuth compression, interferogram formation and look averaging of imagery and interferograms.

**VISAR Modes of Operation.** - The radar has one science-mapping mode with several data downlink options that would accommodate different interferometric and imagery resolutions. For nominal science operations during SP2, we would upload a command table twice weekly specifying, as a function of S/C clock time, the radar parameters needing adjustment.

**VISAR Calibration.** - Preflight VISAR instrument calibration activities would include measurements of the Solid State Power Amplifier (SSPA) power output, pulse shape, receiver gain, ADC characteristics, Tx-cal-loop phase and amplitude over temperature, and antenna composite waveguides phase and amplitude variation over temperature. For in-flight radar calibration the radar would collect raw data from the two antennas to be downlinked for ground analysis. These data would be used to update calibration parameters needed for the proper collection and onboard processing of the radar data. Primarily, these data would be residual differential time delay between the two radar receive channels any yaw or pitch angle bias adjustments needed for the S/C pointing control to achieve zero-Doppler steering.

**VISAR Data Acquisition.** - Topography data would be acquired on ascending and descending passes with at least two observations (also called revisits) for 95% of Venus' surface, with more than 80% acquired 3–6 times. Revisits would provide the opportunity to detect surface changes. During descending passes for the VISAR left-looking sensor, matching
the dominant East-Looking data acquired by Magellan, data would be acquired to obtain a combination of MedRes (30 m resolution) imagery for nearly 100% coverage and HiRes (15 m resolution) imagery with 27% coverage. VERITAS would be capable of targeting between 12 and 17 200 × 200 km sites with RPI, covering approximately 0.1% of the surface, acquiring each site at least twice to form a repeat pass interferogram. The deformation accuracy including atmospheric variations, mostly due to SO₂ variations, at 50 m posting is about 1.5 cm. These data would be the first deformation SAR interferometry at another planet (Hensley et al. 2022). Potential surface activity areas included those that are likely to have recent volcanism, possible subduction zones and areas of gravitational relaxation; If active regions of Venus experience similar levels of activity as Earth analogs the VERITAS project predicts detection of 3–7 events on a total of 17 RPI acquisitions.

**VISAR Expected Performance.** - We developed a comprehensive model to evaluate radar performance at Venus including imaging, radar stereo, single and repeat pass interferometric modes (Hensley 2009; Hensley et al. 2018). The radar performance model elements specify the observing geometry and scenario, the instrument configuration and product specification parameters, propagation and scattering parameters that are used to determine radar performance depending on mode, time and location of measurement. Backscatter information is derived from Magellan S-band data using a physical scattering model to convert S-band backscatter measurements to the desired radar frequency and incidence angle. The impact of atmospheric attenuation as a function of terrain height is derived from a model described in Duan et al. (2010). Two-way losses at X-band as a function of elevation (in km) relative to the 6051 km reference sphere is roughly –9.5 dB. In assessing the interferometrically derived height accuracy we have assumed a “bundle adjustment” procedure to remove residual cross-track tilts due to baseline and phase errors. Bundle adjustment uses tie points between adjacent orbits and between crossing ascending and descending passes in a least squares procedure to estimate cross-track tilt and elevation bias between the swaths.

The expected elevation mapping accuracy of the VISAR instrument is shown in Fig. 9. Backscatter contributions to SNR and attenuation losses are factored in the overall performance. Elevation accuracy is computed every 10 km based on the orbital geometry. Phase noise limited elevation accuracy (in green in Fig. 9c) is compared to elevation accuracy before and after bundle adjustment (red and blue in Fig. 9c). The cumulative elevation accuracy is shown in Fig. 9d and shows that 95% of the surface is mapped with elevation accuracy better than 5.9 m.

### 4.3.2 Venus Emissivity Mapper (VEM)

The permanent cloud cover of Venus prohibits observations of the surface with traditional imaging techniques over much of the electromagnetic (EM) spectral range. Therefore, it was once thought that information about the surface composition of Venus could only be derived from lander missions. Given the harsh environmental conditions on the surface, any type of landed mission will have high complexity and therefore a higher associated risk than orbiting missions. In addition, mission concepts for Venus landers typically focus on one landing site instead of a global reconnaissance, forcing difficult choices to be made between different types of surface units.

The mapping of the southern hemisphere of Venus with VIRTIS instrument on Venus Express using the 1.02-µm thermal emission band can be viewed as a proof-of-concept for an orbital remote sensing approach to surface composition and weathering studies for Venus (Mueller et al. 2008; Helbert et al. 2008; Smrekar et al. 2010a; Gilmore et al. 2015). Thermal emission from the surface is observed on the night side at spatial scales > 50 km.
<figure><img src="image_11.png" /><figcaption>**Fig. 9** (a, b) Map of the VISAR elevation mapping backscatter/accuracy, (c) histograms of elevation mapping accuracy with the bundle adjustment and (d) cumulative elevation accuracy showing a 5.9 m 95% accuracy level</figcaption></figure>

Recent advances in high-temperature laboratory spectroscopy at the Planetary Spectroscopy Laboratory at DLR show that the atmospheric windows in the CO₂ clouds of the Venus atmosphere, ranging from 0.86 µm to 1.18 µm, are highly diagnostic for surface composition (Dyar et al. 2020, 2021; Helbert et al. 2021). Night-side observations at shorter wavelengths (<0.80 µm) from the Wide-Field Imager WISPR instrument on board Parker Solar Probe (PSP), allowed to extend measurements of this thermal emission into the optical regime (Wood et al. 2021).

The Venus Emissivity Mapper (Helbert et al. 2016, 2020) builds on these recent advances. It is the first flight instrument specially designed with a focus on mapping the surface of Venus using the narrow atmospheric windows around 1 µm. By observing with six surface bands centered at 0.86 µm, 0.91 µm, 0.99 µm, 1.02 µm, 1.11 µm, 1.18 µm coupled with 8 atmospheric and calibration bands, VEM will provide a global map of surface composition (Dyar et al. 2020, 2021, Table 3 and Fig. 10). Continuous observation of Venus' thermal emission would also provide tight constraints on current day volcanic activity (Smrekar et al. 2010a; Mueller et al. 2017). Measurements of atmospheric water vapor abundance as well as cloud microphysics and dynamics would permit accurate correction of atmospheric interference.

VEM is a pushbroom multispectral imaging system. The telecentric optics images the scene onto a filter array, and the image is relayed by a three-lens objective onto the detector. VEM's optical sub-system sits on top of the electronics compartment and the power supply. A two-stage baffle protects VEM from scattered light. A 45° FOV yields a swath width of 207 km at an altitude of 250 km, providing a thorough sampling of surface emissivity and orbit-orbit repeat coverage.

Scattering at the cloud particles limits the achievable spatial resolution at the surface to approximately 50-100 km (Moroz 2002, 1990; Hashimoto 2003). The VEM optical system has a theoretical on-ground resolution of 300 m from a 250-km orbit. Using digital TDI, the data are reprocessed in the instrument at a spatial resolution of 1 km, providing a significant
<table><caption>**Table 3** The 14 bands of the identical VEM instrument on VERITAS and the VenSpec-M instrument on En-Vision, fall into four categories depending on the altitude range from which the near-IR radiation originates (Helbert et al. 2016, 2020, 2021). (1) Radiation for the surface bands at 0.86, 0.91, 0.99, 1.02, 1.11, 1.18 µm originates at the surface. Surface bands are used to determine rock types and to monitor the thermal signature of active volcanism. (2) Radiation in the water vapor bands originates in a layer near the surface (0.96, 1.15 µm) and is sensitive to the abundance of water vapor that may be produced by active volcanic plumes. (3) In the cloud bands at 1.195, 1.31, and 1.51 µm, the radiation originates in an atmospheric layer above the surface but below the clouds. Because the signal in the cloud bands has no surface or water vapor contributions, measurements in these bands can be used to remove cloud-induced contrast variability from the other bands. (4) The background bands at 0.79, 1.06, and 1.37 µm correspond to an atmosphere that is opaque, allowing the removal of background signal on the detector. The spectral widths of the bands, approximately ±10 to ±20 nm, are optimized to cover the full range of atmospheric windows based on radiative transfer modeling while minimizing out-of-band radiation</caption><thead><tr><th></th><th>Mineralogy & active volcanism</th><th>Clouds</th><th>Water</th><th>Background</th></tr></thead><tbody><tr><td>Central wavelength (µm)</td><td>0.86 0.91 0.99 1.02 1.11 1.18</td><td>1.19 1.31 1.51</td><td>0.96 1.15</td><td>0.79 1.06 1.37</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 10** The projected VEM performance in all surface bands far exceeds the 4% requirement and will enable creation of the first global map of composition on the surface of Venus, and distinguish between felsic and mafic rocks (Dyar et al. 2020, 2021). The 14 bands of VEM fall in four categories depending on the altitude range from which the near-IR radiation is originating. Bands at 0.86, 0.91, 0.99, 1.02, 1.11, 1.18 µm originate at the rocky surface (Helbert et al. 2016, 2020, 2021). See Table 3</figcaption></figure>

gain in signal-to-noise ratio (SNR). Due to the low orbit required for the radar, the wide field of view of the VEM instrument would allow every spot on the surface to be viewed between 5 and 10 times in consecutive orbits. This would allow short-term variability in the
atmosphere of Venus to be accounted for. To distinguish between surface and atmospheric contributions, VEM would use an updated version of the extensively tested data pipeline developed to process VIRTIS surface data (Helbert et al. 2008), combined with a radiative transfer model (RTM) (Kappel et al. 2012; Kappel 2014; Kappel et al. 2016). Data would be processed at 10 km spatial resolution and the data from consecutive orbits would be stacked. Both provide an additional increase in the SNR.

Of VEM’s total of 14 bands, six would see the surface through all Venus atmospheric windows; three compensate for stray light; three measure cloud transparency; and two measure water abundance. The water vapor and cloud opacity channels would be used as RTM inputs to constrain near-surface water vapor abundance and cloud particle distributions. Observations at 1.16 µm have sufficient accuracy and precision to enable a search for active volcanic outgassing from retrievals of the water concentration in the atmosphere. Multiple observations over the duration of the mission would be used to account for additional unknown atmospheric variability not accounted for in the RTM. This would reduce both atmospheric and instrument noise by averaging image swaths acquired at different times. Applying an updated analysis (Dyar et al. 2020) of atmospheric error for VEM parameters, and taking multiple look averaging into account, our capability for emissivity precision is between 0.3 and 1.2%.

### 4.3.3 Radio Science / Gravity Experiment

The gravity investigation of VERITAS aims to fill the large knowledge gap on the internal structure of Venus as compared to the other terrestrial planets and the Moon. This investigation addresses Venus evolution science goals involving interior structure of thermal state (Sect. 4.1.2). Geophysical models of the interior are based on limited data for mass, radius, gravity, and topography. Work by Dumoulin et al. (2017) indicates that the state of the core and its size, as well as the viscous response of the interior, can be well constrained by the VERITAS requirements, that is a determination of $k_2$ to $\pm0.01$ and of $e$ to $0.25^\circ$ accuracies. These accuracies will be further improved through the use of radar tie points from VISAR (Cascioli et al. 2021).

As introduced in Sect. 4.1.2, precise measurements of the Moment of Inertia factor (MOIF) through the pole precession rate, the tidal Love number, $k_2$ and the tidal phase lag, $e$, will permit the first useful comparisons between the interior of Venus and other terrestrial planets. VERITAS will measure the precession rate with an accuracy of 50 arcsec/cycle (1 cycle equals 1 Venus sidereal day or 243.02 Earth days). Information derived from the moment of inertia is crucial to model the thermochemical evolution of Venus’ interior including differentiation, as well as key surface processes; e.g., core size which is a key parameter in predicting vigor of mantle convection and the size and number of hot mantle plumes. Recent determinations of the pole precession and the Moment of Inertia factor (MOIF) at $0.337 \pm 0.024$, using Earth-based observations of radar speckles tied to the rotation of Venus in 2006–2020, find a core radius of approximately 3500 km (58% of the planetary radius) with large ($>500$ km) uncertainties due to both model limitations and current uncertainties on normalized moment of inertia $C/MR^2$ (Margot et al. 2021). VERITAS will determine core to $\pm 20$ km using both the gravity field and radar tie points (Cascioli et al. 2021). Additionally, loading of the surface by the atmospheric thermal tides can be extracted from the gravity field, providing further constraints on both interior structure and atmospheric circulation (Cascioli et al. 2023).

The knowledge of crustal processes, essential to determine why and how Venus and Earth diverged, will greatly benefit from high fidelity mapping of the gravity field. As discussed in
Sect. 4.1, global gravity and topography data are linked at long wavelengths to the structure and dynamics of the sub-lithospheric mantle, and therefore can be used to estimate the elastic thickness/thermal gradient, which constrains mechanisms of formation and how different Venus' evolution processes are from Earth's (Mazarico et al. 2023). Doppler measurements are the primary observables for reconstructing the orbit of the spacecraft and recovering the gravity field of a planet. These measurements are collected by recording the Doppler shift of a radio signal sent from the ground station to the spacecraft, which then coherently retransmits it back to the Earth by means of an onboard transponder. The estimation of Venus' gravity field and VERITAS orbit will rely on the use of advanced orbit determination codes built on accurate mathematical models of the solar system dynamics and of the observables. The 2-way Ka-band radio tracking data are analyzed to reconstruct the VERITAS trajectory and estimate model parameters. The gravity field harmonic coefficients up to an average degree strength of 130 (~145 km) are generated together with corrections to the spin rate and to the pole right ascension and declination.

By combining global high-resolution gravity and topography VERITAS would look for possible subduction, buried features (e.g., as observed on Mars and the Moon) and unrecognized deformation. In addition, the improved uniformity of the gravity field knowledge would provide precise estimates globally of the elastic thickness, a proxy for heat flow.

## 4.4 Summary / Outcomes Revealing Venus Evolution

NASA's VERITAS mission is designed to study the geologic evolution of Venus and the processes that affect the habitability of terrestrial planets. Venus most likely had elements essential for habitability because its present conditions can be seen as a geodynamic analog to early Earth, when the lithosphere was hotter and thinner, plate tectonics and continents began to form, and life emerged.

VERITAS will carry two instruments: VISAR and VEM. The Venus Interferometric Synthetic Aperture Radar (VISAR) X-band measurements will provide: 1) a global digital elevation model (DEM) with 250 m postings, 5 m height accuracy, 2) Synthetic aperture radar (SAR) imaging at 30 m horizontal resolution globally, 3) SAR imaging at 15 m resolution for targeted areas, and 4) surface deformation from RPI at 2-centimeter vertical precision for >12 200 × 200 km potentially active area targets. Community input would be solicited for both RPI and high-resolution imaging targets. VEM will produce surface coverage of most of the surface in 6 near-IR bands located within five atmospheric windows and of eight atmospheric bands for calibration and water vapor measurements. VERITAS will also conduct radio science. Magellan spherical harmonic gravity field has an average resolution of only 550 km. Rigorous modeling shows that VERITAS data, with an average resolution of 155 km, would enable estimation of elastic thickness-a proxy for thermal gradient and resolution of specific geologic processes. Measurements of the moment of inertia factor and $k_2$ will constrain core size and state (Cascioli et al. 2021). The reader may refer to Smrekar et al. (2022a) for a full mission overview.

The VERITAS mission profile consists of two phases. Science Phase I (SP1) occurs while aerobraking is paused, about 6 months after insertion into a polar elliptical orbit. Science Phase II (SP2) starts after further aerobraking has placed VERITAS in a near-polar, circular, low-altitude orbit that allows global observations throughout the mission. Over ~3.5 Earth years, the mission will return synergistic, global datasets with unprecedented coverage, resolution, and accuracy to meet its science goals: high resolution topography, X-band radar imagery, targeted surface deformation, near-IR spectroscopy and gravity. The VERITAS spacecraft downlinks a total of 20.9 terabits of data to the DSN stations using
CCSDS packets. The total volume of science data products to be archived (raw, reduced, and derived) is estimated to be 134 terabytes. These rich datasets will allow VERITAS to meet key required investigations and science objectives.

VERITAS' rich global datasets will provide an invaluable resource for a new generation of Earth, planetary, and exoplanet scientists, and reveal the truth about how Earthlike Venus really is. These datasets are highly synergistic with DAVINCI (Sect. 5) and EnVision (Sect. 6), providing information about Alpha Regio and identifying key targets for EnVision exploration. These missions also enrich VERITAS contributions by, for example, providing high resolution visual and near-IR images of Alpha Regio (i.e., DAVINCI flyby near-IR emissivity and sub-cloud near-IR imaging and high-resolution local topography before the VERITAS launch) and continuing the search for volcanic activity (EnVision).

# 5 Deep Atmosphere Venus Investigation of Noble Gasses, Chemistry, and Imaging (DAVINCI)

DAVINCI inherits the legacy of the successes of previous atmospheric probe missions, connecting definitive analytical chemistry of Venus' deepest and bulk atmosphere with new surface compositional constraints linked to the history of water. It provides critical measurements about the evolution of the atmosphere via noble gas isotopes, isotopes of hydrogen (D/H), and other species all in a highly detailed physical context while also imaging the surface in the near-infrared from under the clouds at spatial scales not possible from orbital altitudes.

## 5.1 DAVINCI Science Objectives

Compelling recent insights (e.g., Garvin et al. 2022a), and the planet's relevance to exoplanetary systems (Kane 2022; Way et al. 2023, this collection), have raised new questions about Venus' atmosphere, climate, and habitability evolution through time. Since 1978, six orbiting missions have comprehensively mapped Venus' surface and upper atmosphere. Despite the success of these reconnaissance missions and prior missions to the surface (Venera and VeGa landers), significant knowledge gaps about Venus' early state and overall evolution remain that can only be addressed through state-of-the-art *in situ* analytical and flyby remote sensing measurements. The international scientific community is now asking key questions about a possible long-lived oceanic state and considering past and present life on Venus, posing new hypotheses that are directly testable by measurements of the local surface and atmosphere.

### 5.1.1 Overview

The Deep Atmosphere Venus Investigation of Noble Gasses, Chemistry, and Imaging (DAVINCI) mission is one of several needed next steps in Venus' exploration, originally suggested by Morrison and Hinners (1983) in their summary of solar system exploration priorities. DAVINCI will be the first mission to Venus to incorporate science-driven Venus dayside and nightside flybys and an instrumented descent sphere (DS) into a unified architecture. The mission will deliver both a deep atmosphere probe and a flyby remote-sensing carrier relay imaging spacecraft (CRIS) to Venus; it will assess the habitability of Venus over time, establishing how the planet evolved from upper atmosphere to the surface. Its complement of *in situ* and remote sensing measurements will reveal the processes that may
<table><caption>**Table 4** The DAVINCI mission traces its primary science goals and objectives through the operation of its 7 instruments to specific outcomes related to Venus' evolution and connections to exoplanets like Venus. Each of the Key Science questions (Left column) is linked to Planetary Decadal Survey questions including # 3, 6, 10, and 11 (NASEM 2022) with specific measurement strategies listed (middle column). Color codes map instruments shown at far right (column) to key questions at the far left. The CUVIS technology demonstration instrument will provide 0.2 nm UV (200-400 nm) spectral observations of the Venus dayside at favorable solar phase angles on both flybys (January, November 2030), providing raw spectra as well as AI/Machine Learning analysis on-board (see Sect. 5.4.2)</caption><thead><tr><th colspan="2">DAVINCI Goals Address Key Questions</th><th rowspan="2">VMS</th><th rowspan="2">VTLS</th><th rowspan="2">VASI</th><th rowspan="2">VFOx</th><th rowspan="2">VenDI</th><th rowspan="2">VISOR</th><th rowspan="2">CUVIS</th></tr><tr><th>Key Science Questions</th><th>Traceability of DAVINCI Measurements</th></tr></thead><tbody><tr><td>What is the origin of the Venus atmosphere and how has it evolved?<br>How and why is Venus different from Earth and Mars, and how does it compare to Earth-sized exoplanets?</td><td>VMS: noble gas abundance and isotope ratios to test current hypotheses of origin and evolution<br>VMS &amp; VTLS: atmospheric and isotopic composition, search for exotic chemistry, constrain mineralogy by constraining surface-atmosphere exchange<br>VFOx: oxygen abundance near the surface<br>VISOR &amp; CUVIS: UV absorbers in the upper clouds and dynamics from flybys</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Was there an early ocean on Venus?<br>If so, when and where did it go?<br>What is the rate of volcanic activity on Venus?</td><td>VMS, VTLS, &amp; VASI: D/H and other key trace gases above and below the clouds down to the surface; history of water<br>VMS: radioactive decay products *Ar and *He to determine long-term and recent volcanism rate<br>VenDI &amp; VISOR: compositional insights into past water-rock interaction</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>What exactly are the tesserae highlands? What is their origin and history? How do they compare with major highlands?</td><td>VenDI: high-resolution morphology, composition, and role of crustal water in igneous rock formation and erosional processes at Alpha Regio<br>VFOx: constrain surface redox state through oxygen measurements near the surface-atmosphere interface<br>VISOR &amp; VenDI: IR emissivity for composition at scales of 5-200 m (VenDI) and ~70 km (VISOR) to constrain regional composition of mountains on Venus</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_13.png" /></figure>

have allowed surface water to persist and then dissipate. DAVINCI will further establish new bounds on planetary habitability and enable-improved interpretation of biosignatures in our solar system and beyond (Garvin et al. 2022a; VEXAG 2019; U.S. National Academies of Sciences, Engineering, and Medicine/NASEM 2022).

DAVINCI fully addresses its three overarching science goals (Table 4), which are described in the next three subsections: (1) Atmospheric origin and evolution (Sect. 5.1.2); (2) Atmospheric composition and surface interaction (Sect. 5.1.3); (3) Venus surface properties (Sect. 5.1.4). The results will further catalyze years of productive follow-on scientific analysis that will quantitatively connect Venus to exoplanets and place its evolution into the context of recently selected orbiter missions VERITAS (Sect. 4) and EnVision (Sect. 6).

### 5.1.2 What Is the Origin of Venus' Atmosphere and How Has It Evolved?

**Atmospheric Origin and Evolution.** - DAVINCI answers questions about atmospheric formation and evolution of habitable zone planets, including the timing and rate of volcanic outgassing in the past and present that can only be addressed through *in situ* measurements of key noble gasses (including Xe) and nitrogen, never before adequately measured for Venus. DAVINCI unambiguously quantifies atmospheric noble gasses deeply enough (below ~60 km) to avoid strong compositional dependencies on time of day, latitude, and molecular mass prevalent higher in the atmosphere. This information will help us understand similar evolutionary processes for exoplanets of various ages, many of which are expected to be Venus-like.

Enticing Earth-based remote sensing data have suggested the presence of phosphine, a possible biosignature, in the atmosphere of Venus (Greaves et al. 2021), an evidence later questioned (Encrenaz et al. 2020b; Villanueva et al. 2021). Modern life detection science strategies summarized in framework reports by the USA National Academies of Sciences requires that putative biosignatures be evaluated within the systems-level chemical context of the environment – presently poorly constrained at Venus. Investigating its past and present
habitability through a definitive analysis of chemical reservoirs and cycles would revolutionize our understanding of Venus, its place in our solar system, and its prospects as a future astrobiology target (Limaye and Garvin 2023).

### 5.1.3 Was There an Early Ocean on Venus?

**Atmospheric Composition and Surface Interaction.** - DAVINCI tests hypotheses of when and how Venus lost its putative early oceans, plus chemical processes in the cloud and sub-cloud atmosphere down to the surface. The descent profile enables vertically resolved (i.e., at altitude scales as fine as 100-200 m) measurements of chemical species across a broad mass range combined with high-precision abundances and isotopes of targeted trace gasses. The highest cadence measurements focus on the deep atmosphere (<16 km), which contains 66% of the atmospheric mass, where no definitive *in situ* data exist and orbiting remote sensing techniques are largely blind. Such precision and broadband analysis is critical to reveal unknown chemical cycles. Cross-calibrated descent and flyby surface emissivity mapping in the near-infrared (~1 µm) will connect the deep atmosphere chemistry measurements to compositional maps of Alpha Regio and other tesserae.

### 5.1.4 What Are the Tesserae Highlands, Their Origin and History?

**Surface Properties.** - Analysis of the near-IR radiance of Alpha Regio tessera using VEx VIRTIS data shows that the tessera material differs from the plains materials in a manner that is consistent with a lower FeO (more felsic) content (Gilmore et al. 2015), corroborating earlier measurements by Galileo NIMS during its Venus flyby (Hashimoto et al. 2008). During the descent above western Alpha Regio, which is also the largest (1600 x 1300 km) known exposure of tessera terrain, the DAVINCI probe distinguishes felsic rock (i.e., formed in association with water) from others, such as primitive basalt, at new spatial scales (<100 m) not accessible from orbit via multi-band near-IR descent imaging in 3D context. Alpha Regio is considered an ideal, representative example of tesserae terrain unique to Venus. Compositional constraints from the high-sensitivity 3D views produced by the near-IR descent imaging system will be developed at unprecedented resolution (5-200 m) to connect with ~60-km scale emissivity mapping from the DAVINCI CRIS remote sensing flybys, as well as previous (VEx) and future orbital data (e.g., from Venus emissivity mapping to be accomplished by VERITAS and EnVision in the 2030's). Existing orbital radar (SAR) and near-IR emissivity data have insufficient spatial resolution to characterize such geomorphology definitively without ground-truth from DAVINCI from beneath the clouds, including meter-vertical resolution imaging and derived topography.

## 5.2 DAVINCI Mission Overview

DAVINCI inherits the legacy of the successes of previous missions, connecting definitive analytical chemistry of Venus' deepest atmosphere with new surface compositional constraints at regional scales from flyby near-infrared remote sensing. DAVINCI probes the composition and physical structure of Venus' atmosphere from an altitude of ~67 km (in the upper clouds) to the surface, addressing terrestrial planet formation, evolution, and the boundaries of habitability (Table 4). During its robust operational phase, including two science-guided Venus flybys, a ~1-hour descent probe *in situ* investigation, and options for extended mission operations (Fig. 11), the DAVINCI mission acquires up to 500 Gbits of uncompressed data about Venus in <2.1 years.
<figure><img src="image_14.png" /><figcaption>**Fig. 11** DAVINCI offers definitive measurements from the top of the Venus cloud deck to the surface, while observing cloud dynamics and regional composition of key highlands including Alpha Regio. The Figure shows the nominal launch in June 2029; after a ~6-month cruise, the spacecraft would fly by Venus in January 2030 for initial remote sensing in the UV and near-IR, then the trajectory returns 9 months later for a second flyby in November 2030. Both flybys will include dayside UV imaging and spectroscopy, as well as night side near-IR surface emissivity mapping of multiple tesserae including Alpha Regio. After an additional 7-month cruise, the flight system will deliver the probe in June 2031 for its entry, descent, and science campaign above western Alpha Regio at very high solar illumination conditions</figcaption></figure>

DAVINCI nominally launches in June 2029 (Garvin et al. 2022a). After a ~6-month cruise, the Carrier Relay Imaging spacecraft (CRIS) flies by Venus for unique remote sensing science (i.e., near UV cloud motion videos, near-IR surface emissivity of tesserae and volcanic centers), before setting the spacecraft on a trajectory to return for a second science flyby, followed by delivery of the *in situ* probe to Alpha Regio, with favorable solar illumination (Figs. 11, 12). DAVINCI’s imaging target area within western Alpha Regio has been comprehensively mapped by prior missions (and Arecibo radiotelescope, see Sect. 11, Sect. 11.1.1) and is large enough to avoid complex controlled descent. DAVINCI’s entry-descent-touchdown ellipse (~348 × 160 km) fits within this area with large margin and high-resolution near-IR descent images will assess its relevance to the history of Venus water in association with rock units and geomorphology.

In June 2031, two days before arrival at Venus, the Probe Flight System (PFS) is released. The Carrier-Relay Imaging Spacecraft (CRIS) observes PFS release (via imaging using the VISOR camera system) then conducts a divert maneuver to communicate with the descent sphere (DS) throughout the *in situ* science mission (i.e., by flying overhead with its 2 m HGA for two-way S-band telecommunications). The DAVINCI descent sphere is a hermetically sealed titanium pressure vessel with dimensions (1.1 m × 0.85 m; 250 kg) similar to the Pioneer-Venus Large Probe (PVLP). After DS atmospheric entry and parachute deployment (~70 km altitude), the heat shield is released and the DS-based instruments begin to collect and transmit altitude-resolved, high-fidelity measurements of noble, trace gas, and isotopic abundances; atmospheric temperature, pressure, and winds; and high-resolution broadband and ~1 µm narrow-band images (Fig. 12). Although not required to function following touchdown, the DS has sufficient resources to conduct science and relay data for an additional ~18 minutes from the surface, if it survives the ~13 m/s touchdown. After the CRIS spacecraft has recorded the required probe *in situ* data, it turns toward Earth and transmits those data to the DSN. Via an optional extended science mission, six months after the DS entry-descent-science phase, the CRIS spacecraft conducts a Venus orbit-insertion (VOI) maneuver and enters a 5-day, 60-degree inclined science orbit for most of a Venus year (~6 months), mapping dayside cloud dynamics and nightside near-IR surface emissivity potentially at the same time as VERITAS observes the surface with its powerful payload to evaluate synergies including those associated with different observational times.

DAVINCI delivers definitive atmospheric chemistry measurements, coupled to unprecedented 3D views of ancient tesserae at local scales (5-60 m horizontally) that will transform
<figure><img src="image_15.png" /><figcaption>**Fig. 12** DAVINCI delivers critical science using five descent sphere (DS)-based instruments, and two remote-sensing instruments on the carrier relay imaging spacecraft (CRIS) (Figure from Garvin et al. 2022a). The probe instruments will operate during its ~59 minute long descent from the upper clouds (~67 km) to the surface over an entry-descent-science corridor with a landing error ellipse located within western Alpha Regio (see Fig. 13)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 13** DAVINCI entry error ellipse with Magellan S-band SAR mosaic draped over new 1 km scale Digital Elevation Model (DEM) of Alpha Regio based on combined Arecibo polarimetric radar & Magellan radar altimeter datasets (Garvin et al. 2022b, 2023, in preparation)</figcaption></figure>

our understanding of the planet next door and serve as the foundation for future exploration. Given the recent unexpected but contentious discovery of phosphine at Venus (Greaves et al. 2021; Encrenaz et al. 2020b; Villanueva et al. 2021), *in situ* measurements are required to uncover new chemical cycles, including those involving oxygen, sulfur, phosphorus, and others. Enabled by DAVINCI's quantitative investigation of Venus' lower atmosphere and its pathfinding high-resolution near-infrared views of enigmatic tesserae, future missions can follow to accomplish additional 2022 Planetary/Astrobiology Decadal Survey objectives (NASEM 2022). DAVINCI's measurements can also be tied to JWST investigation of Venus analogue exoplanets around M-dwarf stars as “planetary atmosphere ground truth”, which is underway at the time of this writing by the JWST observatory (see also Sect. 11, Sect. 11.2).
## 5.3 DAVINCI Descent Probe Instruments

Five descent probe instruments and two carrier-relay-imaging spacecraft instruments leverage highly successful partnerships between NASA/Goddard Space Flight Center (GSFC), NASA/Caltech Jet Propulsion Laboratory (JPL), NASA/Johns Hopkins University Applied Physics Laboratory (JHU/APL), and Malin Space Science Systems (MSSS), see Table 4; Fig. 12. These include the pairing of a quadrupole mass spectrometer and tunable laser spectrometer evolved from the SAM (Sample Analysis at Mars) suite on MSL/Curiosity rover now in its eleventh year of operation on Mars in Gale Crater. A flyby camera suite based upon imaging systems on the OSIRIS-REx mission provides sensitive nightside NEAR-IR mapping and dayside UV cloud imaging (with movies) from new vantage points over Venus. The *in situ* and flyby observations combine to define the DAVINCI mission baseline.

### 5.3.1 Venus Mass Spectrometer (VMS)

DAVINCI's Venus Mass Spectrometer (VMS) provides the first comprehensive survey of Venus' noble gasses, as well as detailed analysis of trace gas species – both those expected and those yet to be discovered. Employing mature, tested operational modes, VMS definitionally measures isotope ratios for Ne, Ar, Kr, and Xe, and collects hundreds of measurements of each trace species to constrain fine variations with altitude. The VMS is a Quadrupole Mass Spectrometer (QMS) with a gas-enrichment system and pumping system that will provide a comprehensive *in situ* survey of the planet's noble gases. It has significant heritage from the Mars Science Laboratory/MSL (Curiosity) Sample Analysis at Mars/SAM QMS (Webster and Mahaffy 2011; Atreya et al. 2013) and with a broad mass range from 2 to 550 Dalton (Da), VMS has the capability to discover new trace gas species (Garvin et al. 2022a, Table 3.2) within the deep atmosphere where non-equilibrium chemistry is suspected.

VMS measurements will occur every ~200 m or better below 61 km, particularly in the lowest 30 km of the atmosphere (Fig. 12), where it will probe the supercritical $CO_2$ boundary and properties of the $CO_2/N_2$ mixture in the temperature and pressure conditions of the deep atmosphere of Venus (Lebonnois & Schubert 2017), and profile new species, including CHNOPS-bearing molecules (including, potentially, $P_4O_6$ and $PH_3$) and those trace gases expected to be tied to surface mineralogy and the thermochemical cycle of sulfur-bearing species.

### 5.3.2 Venus Tunable Laser Spectrometer (VTLS)

DAVINCI's Venus Tunable Laser Spectrometer (VTLS) answers major questions about the Venus atmosphere by providing the first precise abundance and isotopic measurements of key gasses containing hydrogen, sulfur, carbon, oxygen, and potentially phosphorus. VTLS provides a series of ten definitive measurements of the D/H ratio in water vapor throughout the atmosphere (i.e., from 67 km to ~2 km), critical to understanding the longevity, and loss mechanisms of past oceans (Way et al. 2023; Salvador et al. 2023, this collection). The instrument consists of a multipass Herriott cell with three laser channels at 2.64, 4.8, and 7.4 µm, specifically targeting key science questions that discriminate chemical processes in the upper clouds and near-surface environment. VTLS draws heritage from the MSL/SAM tunable laser spectrometer (e.g., Webster and Mahaffy 2011; Pla-Garcia et al. 2019).

VTLS is specifically tailored to answer critical questions about the long-term evolution of Venus' atmosphere by providing the first highly sensitive *in situ* measurements of key gas species containing H, S, C, and O, as well as their high-precision isotope ratios including
D/H (Garvin et al. 2022a, Table 3.2). It should be noted that VTLS offers the possibility to directly measure trace species at different heights in the clouds with a sensitivity of ~1 ppbv, allowing to set new upper limits for PH₃ at the 1 ppbv level discussed in Villanueva et al. (2021) or Encrenaz et al. (2020b).

### 5.3.3 Venus Atmospheric Structure Investigation (VASI)

DAVINCI's Venus Atmospheric Structure Investigation (VASI) characterizes the fine-scale vertical structure and dynamics of the Venus atmosphere during descent, including wind speed, pressure, and the first detailed profile of the deep atmosphere temperature (e.g., the lapse rate, dT/dz). The instrument consists in a suite of sensors that measure atmospheric pressure, temperature, and dynamics. Internally mounted accelerometers and gyroscopes combined with Doppler tracking via the spacecraft-to-DS communications link enables detailed reconstruction of the descent probe trajectory. VASI provides thermodynamic context for the composition measurements and enables reconstruction of the detailed descent profile and precise landing position, with most measurements every 15-50 m, as well as a final measurement set within ~100 m of the local surface.

### 5.3.4 Venus Descent Imager (VenDI)

DAVINCI's Venus Descent Imager (VenDI) is a near-IR descent-imaging system with a nadir orientation. It will deliver clear, high contrast, high SNR images (>100:1), providing the first geologic constraints on the highland surface environment at 2-200 m length scales from reflectance imaging below the cloud-deck (and sub-cloud hazes). A narrow-band, near-IR channel delivers 1.02 µm albedo maps with sensitivity to felsic rocks or alteration products when ratioed against broadband images (0.74 to 1.02 µm). Topography can be derived using machine-vision algorithms via Structure-from-Motion (SfM), an expansion of Scale-Invariant Feature Transform (SIFT) algorithm to construct a Digital Elevation Model (DEM) from multiple overlapping images with varying vertical and horizontal baselines (Garvin et al. 2018, 2022a). SfM processing of bundles of descent images produces first 5-60 m scale topography of tesserae and establishes boundary conditions for tectonic and erosional models. Final imaging scales from VenDI below ~1.5 km produce unblurred images at scales finer than 1 m, permitting feature-identification resolution of key indicators of sedimentary processes at scales that connect to those observed by prior Venus landers (e.g., Garvin et al. 1984). Evaluation of Earth-based analogue datasets (Pilbara, Zagros mountains) that emulate VenDI near-IR bandpasses and spatial scales have demonstrated discrimination of felsic surfaces at <100 m (down to 5-10 m) providing confidence that sub-cloud descent imaging will complement 50-100 km scale orbital near-IR emissivity mapping by multiple missions (see Sects. 4.3.2; 6.3.3).

### 5.3.5 Student Collaboration Experiment VfOx; DAVINCI's Engineering Science Investigation (ESI)

The oxygen cycle on Venus, like those involving sulfur, hydrogen, phosphorus, and carbon, is incompletely resolved on the basis of current data and DAVINCI's quadrupole mass spectrometer (VMS) and tunable laser spectrometer (VTLS) measurements of altitude-resolved species will extend beyond extrapolated equilibrium models of likely chemistry to measured abundances across the deep atmosphere all the way to the surface just about the complex ridged terrain in Alpha Regio.
Student Collaboration Experiment VfOx obtains independent measurements of the oxygen fugacity ($fO_2$) in the lowermost scale height of the Venus atmosphere to compare with indirect (and independent) measurements of oxygen-species retrieved by VTLS. DAVINCI's Student Collaboration Experiment partners with Johns Hopkins University (and others), engaging students to implement an *in situ* sensor that measures oxygen partial pressure, also known as fugacity ($VfOx$).

DAVINCI's Engineering Science Investigation (ESI) meets high priority NASA measurement objectives for Venus entry with measurements tied to improving future Venus Entry, Descent and Landing (EDL) activities. Measurements that document the entry conditions after Atmospheric-Entry-Interface below 140 km will be obtained via support from NASA's Space Technology Mission Directorate in partnership with the Science Mission Directorate. Final instrument selection is in progress as DAVINCI advances toward its mission confirmation by NASA.

## 5.4 DAVINCI Carrier/Flyby Instruments

### 5.4.1 Venus Imaging System for Orbital Reconnaissance (VISOR)

VISOR is an integrated system of four cameras that provides global dayside coverage of Venus in the UV and nightside coverage in the near-IR (0.93–1.03 µm). Each of the VISOR cameras has a field of view of 11.3 degrees by 8.9 degrees which can be converted to a spatial sampling scale as a function of distance to target. Three cameras image night-side Venus in three independent near-IR bands, from 930–938 nm, 947–964 nm, and 990–1030 nm. They will deliver night-side surface emissivity mapping (three near-IR bands to properly characterize clouds and scattered light) to constrain regional composition at ~60 km scales, unveiling new regional patterns associated with highlands during 2030 flybys prior to global mapping by two future orbiters.

DAVINCI first and second Venus gravity-assist flybys, with a closest approach on the night-side hemisphere near Equator at 00:00 LT, are scheduled on January 30 and November 15, 2030 for the planned June 2029 Launch Readiness Date (Table 1). Thousands of images are acquired during the two flybys, potentially identifying felsic regions, “calibrated” by local-scale, sub-cloud VenDI band-ratio mapping at Alpha Regio, as well as with VASI lapse rate information. The fourth VISOR camera will provide global, dayside coverage of Venus in the unknown UV absorber band (355–375 nm). Dayside UV imaging (single band) will measure cloud dynamics as the spacecraft approaches and recedes from pericenter, allowing ultraviolet feature tracking at 355-375 nm at a frequency that exceeds any existing Venus orbital imaging dataset.

### 5.4.2 CUVIS (Compact Ultraviolet Imaging System)

Technology Demonstration Opportunity (TDO) experiment CUVIS (Compact Ultraviolet Imaging System) acquires 0.2 nm resolution spectra and hyper-cubes of images from 0.2 to 0.4 µm, concurrent with VISOR UV imaging on the dayside flybys of Venus, in a technology demonstration of a new class of small planetary instruments. This UV hyperspectral sensor (CUVIS) will perform upper atmosphere $SO_2$ and SO chemistry and gather spectral information on the unknown UV absorbing species at a spectral resolution of 0.2 nm in the UV from 0.20 µm to 0.40 µm (Pollack et al. 1980; Wilson et al. 2023, this collection). CUVIS can be accommodated on the CRIS spacecraft and is implemented in a fully separable, *do no harm* fashion. It will further employ Machine Learning to identify key species and
test approaches to accommodate effective data transmission for rich datasets such as those delivered by CUVIS. All of its observations will be coupled to VISOR near UV dayside observations which will provide wider field-of-view context (and multi-frame “movies”).

## 5.5 Summary / Outcomes Revealing Venus Evolution

The overall DAVINCI mission, scheduled for launch in June 2029, will provide up to 500 Gbits (uncompressed) new data about the atmosphere and near surface, as well as the first unique characterization of the deep atmospheric environment and chemistry. DAVINCI returns to the Venus atmosphere at a time of heightened interest in understanding terrestrial planet evolution, habitability, and astrobiology in our solar system and beyond (NASEM 2022; Garvin et al. 2022a). Understanding the evolutionary pathways of Venus necessarily involves the interplay between the time-variable atmosphere-climate system, the lithosphere, and the interior. The NASA DAVINCI mission addresses several questions about such components of evolution as they relate to five of the key priorities (as questions) recently published in the US National Academies Planetary and Astrobiology Decadal Survey including for example numbers 3, 4, 5, 6, 10, and 12 (NASEM 2022).

DAVINCI's *in situ* analytical chemistry measurements of noble gasses in the bulk atmosphere will distinguish between current models by resolving the isotopic ratios of xenon, which is currently unmeasured. As for Mars, the full suite of noble gasses and their isotopes will provide chemistry boundary conditions for models that range from early impact blow-off of an initial atmosphere to the consequences of catastrophic volcanic resurfacing on the evolved atmosphere, as well as others. Coupled to these noble gas measurements are ten altitude-resolved observations of D/H in water from as high as 67 km down to the near surface at ~2 km altitude.

A suite of 10 such measurements by means of DAVINCI's tunable laser spectrometer will expand upon Pioneer Venus Large Probe based measurements in the cloud deck (above 38 km) and those from remote sensing retrievals from ESA's Venus Express in the upper atmosphere, and connect to the state of D/H in the deep atmosphere, where surface-atmosphere interactions may have affected the history of water. Near surface quantification of the oxygen fugacity by means of multiple experiments (i.e., including the DAVINCI Student Collaboration experiment "VfOx", as well as VTLS and VMS) will resolve the state of oxygen species in the atmosphere and mineral stability near the surface in the tesserae highlands where DAVINCI will come to rest after its atmospheric transect. The oxygen cycle on Venus, like those involving sulfur, hydrogen, phosphorus, and carbon, is incompletely resolved on the basis of current data and DAVINCI's quadrupole mass spectrometer (VMS) and tunable laser spectrometer (VTLS) measurements of altitude-resolved species will extend beyond extrapolated equilibrium models of likely chemistry to measured abundances across the deep atmosphere all the way to the surface just above the complex ridged terrain in Alpha Regio.

Mineral stability assessments associated with rocks containing Fe, S, and other elements will be conducted to infer possible weathering pathways within ~2 km of the surface, with direct connections to near-infrared band-ratio descent imaging in the 740 to 1200 nm spectral region at scales as fine as a few meters for potential identification of water-related rock units. Connections between the near-IR descent imaging of possible felsic rock compositions at scales from 100 m down to a few meters and the analytical chemistry of trace gas species potentially relevant to rock formation or modification processes will provide local ground-truth for global assessments of rock unit compositional patterns at 50-100 km scales across Venus. DAVINCI will further address aspects of evolution of the Venus deep atmosphere over time by directly measuring gradients in key species involving S, O, H, P, and
C as often as every 150-200 m in the deepest atmosphere where connections to local rock compositions can be made.

By the time DAVINCI completes its *in situ* transect of the atmosphere (late June 2031), new information of the vertical stratification of the atmosphere from 67 km to the surface as well as a resolved lapse rate (temperature vs altitude at 0.1 K precision every 15-50 m down to the surface) will enable systems-level modeling of Venus evolution that connect the history of the atmosphere to that of the lithosphere, with linkages to Venus tectonic evolution via connections with VERITAS observations. Ultimately the possible evolutionary signatures of water in the Venus system over time will be resolved, preparing the way for future landed experiments that make use of mineralogical signatures in the context of the massive Venus atmosphere that is clearly a major factor in Venus evolutionary divergence from Earth (i.e., see Kane 2022). For further details, see the mission overview in Garvin et al. (2022a).

# 6 EnVision: Understanding Why Earth's Closest Neighbor Is so Different

On June 10, 2021, the European Space Agency (ESA) announced the selection of EnVision as its 5th Medium-class science mission, targeting a launch in the early 2030s. EnVision is an ESA-led mission in partnership with NASA, providing its Synthetic Aperture Radar instrument, VenSAR and Deep Space Network support for critical mission phases. EnVision will use an array of payload instruments to perform holistic observations of Venus from its inner core to upper atmosphere to better understand how Earth's closest neighbor in the Solar System evolved so differently (European Space Agency 2021).

EnVision's overarching science questions are to explore the full range of geoscientific processes operating on Venus. It will investigate Venus from its inner core to its atmosphere at high resolution, characterizing the interior, signs of past geologic processes, and looking for evidence of past liquid water. As developed in companion articles, recent modeling studies strongly suggest that the evolution of the atmosphere and interior of Venus are coupled at all stages of the planet's long-term evolution (Way and Del Genio 2020; Weller and Kiefer 2020), emphasizing the need to study the atmosphere, surface, and interior of Venus as a system. EnVision's combination of surface and atmospheric measurements will characterize ongoing volcanic processes through an extended-timeline, search for their thermal, morphologic, and gaseous signatures, while also tracing key volatile species from the surface up to the mesosphere.

The mission is scheduled for launch in the fourth quarter of 2031 (see Table 1); the final schedule will be agreed between ESA and NASA at Mission Adoption in January 2024, with back-up launch readiness dates every 6 months in 2032 and 2033, on Ariane 62. Following orbit insertion and periapsis walk-down, orbit circularization will be achieved by aerobraking over a period of several months, followed by a nominal science phase lasting at least 6 Venus sidereal days (4 Earth years). The EnVision payload consists of five instruments provided by European and US institutions (Fig. 14). The five instruments comprise a comprehensive measurement suite spanning infrared, ultraviolet-visible, microwave and high frequency wavelengths. This suite is complemented by the Radio Science investigation exploiting the spacecraft Telemetry, Tracking and Command (TT&C) system. All instruments in the payload have substantial heritage and robust margins relative to the requirements with designs suitable for operation in the Venus environment. This suite of instruments has been selected to meet the wide range of measurement requirements in support of EnVision science investigations.
<figure><img src="image_17.png" /><figcaption>**Fig. 14** Rendering of the generic concept EnVision spacecraft orbiting Venus, with the SRS, VenSAR feeder and reflectarray antennas deployed. Credit ESA / NASA / Paris Observatory / VR2Planets</figcaption></figure>

# 6.1 EnVision Science Objectives

EnVision will deliver new insights into geological history through complementary imagery, polarimetry, radiometry and spectroscopy of the surface coupled with subsurface sounding and gravity mapping. It will search for thermal, morphological, and gaseous signs of volcanic and other geological activity; and it will trace the development and transport of key volatile species from their sources and sinks at the surface through the clouds up to the mesosphere. Following the same approach through which our understanding of Earth and Mars has been developed, EnVision will combine global observations at low or moderate spatial resolution (e.g., surface emissivity and atmosphere composition) with regionally targeted observations of higher spatial resolutions from a dual polarization S-band synthetic aperture radar (SAR) and subsurface sounding radar profiles.

## 6.1.1 Overview

EnVision will investigate both present and past geological activity on Venus, and how its atmospheric, surface and interior processes are linked. The background for EnVision's scientific investigations and strategic knowledge gaps is presented in this Section following the lines of three top-level science questions:

1. History - How have the surface and interior of Venus evolved?

2. Activity - How geologically active is Venus?

3. Climate - How are Venus' atmosphere & climate shaped by geological processes?

## 6.1.2 How Have the Surface and Interior of Venus Evolved?

**Geologic Mapping of Volcanic Features and Their Surface Morphology.** - Geologic mapping of volcanic features and their surface morphology and dielectric constant is a cornerstone of Magellan data interpretation (Campbell and Campbell 1992; Campbell 1994). There is a need to carry this work to finer spatial scales and into the subsurface to answer fundamental questions of localized stratigraphy (from subsurface profiles and geologic mapping from images), magma composition (from morphology, roughness, and dielectric properties), surface mineralogy, order-of-magnitude eruption rates and volumes (from morphologic features and subsurface profiles), and post-emplacement weathering (from morphologic features and dielectric properties). In a complementary approach to VERITAS, EnVision will accomplish this objective in part through SAR imaging at 10 m resolution and polarimetric imaging at 30 m resolution (Fig. 15), along with VenSpec-M surface investigations. EnVision 30-m SAR imagery will dramatically enhance our understanding of volcanic surface
<figure><img src="image_18.png" /><figcaption>**Fig. 15** Map of geological terrains and named landmarks covered by the Regions of Interest (ROIs) defined in EnVision's Science Operations Reference Scenario. The ROIs are chosen to include representative samples of all major geological terrain and feature types. These different features are not distributed at random but are in specific, known locations. EnVision's approach is to define roughly thousand-kilometer square ROIs covering most of the highlands and a representative selection of the lowland features. This strategy allows to progressively build up along the 6 cycles the required global and targeted measurements dataset, in particular over all pre-selected regions of interest, which represent a fraction of about 30% of Venus surface. Definitions of geological terrain types are as mapped by Ivanov and Head (2015).</figcaption></figure>

features. At the >120 m resolution of Magellan (120 m azimuth resolution and 93 m best case range resolution), features like flow channels are visible only where they are at the highest end of those typically seen in terrestrial flow fields, vent locations and associated ash or rugged clinkers are too small to observe, and collapsed tubes or skylights are unseen. Within any single major shield volcano, there are often a wide range of features indicative of magma storage beneath calderas, rapid eruptions that form rugged, channelized flows, fine-grained pyroclastic ash from volatile-rich eruptions, and steep-sided constructs linked with higher-viscosity magma (Campbell and Rogers 1994). Targeted observations at 10 m resolution will bring out crucial details in the stratigraphic relationship between flows, their likely thickness, and the range of scales in flow fields (i.e., short high-volume eruptions or long-term, tube-fed complexes).

**Variations in Morphologic Characteristics, Stratigraphic Relationships, and Dielectric Properties of Plains.** - The volcanic plains cover around 80% of Venus. Far from being uniform, they exhibit signs of extensive geological activity, from volcanic and tectonic to aeolian and weathering processes. Did the plains form rapidly, with few flow boundaries (like lunar mare) or are they constantly reformed by small-scale volcanism, below the resolution of Magellan? Understanding and mapping stratigraphic boundaries is important in distinguishing geologically old and young units, and between directional and equilibrium surface histories.

The Subsurface Radar Sounder (SRS) will be used to look for layering in the plains and elsewhere on Venus, as has been successfully done on both the Moon and Mars (Fig. 16, and Sect. 6.3.2 below). Analyses of this type enable a far better understanding of Venus's recent geological past and reveal vital information about the character, thickness and mode of resurfacing on Venus. For example, catastrophic resurfacing models for Venus (Strom
<figure><img src="image_19.png" /><figcaption>**Fig. 16** (a, left): A SHARAD radargram showing layering about 100 meters thick in Amazonis Planitia warped by wrinkle ridges. The image is 400 km across (Campbell et al. 2008). (b, right): Schematic representation of the three potential scenarios of subsurface stratigraphy interpreted from SHARAD radar sounding of volcanic layering in the Arisia Mons caldera (A - stacked lava flows with vesiculated and less dense flows overlying very dense lava, B - less dense lava-flow and a thick tephra deposit overlying denser bedrock, C - pyroclastic or other low-density material deposited over dense lava-flows in the southern part of the caldera, adjacent to the wall (Ganesh et al. 2020; Watters et al. 2006)</figcaption></figure>

et al. 1994) predict that the plains were resurfaced in a brief epoch several hundred million years ago. In such a model, there might not be sufficient time between lava flows to develop thick weathering layers that would produce discrete layered returns in SRS data. If SRS does detect clear layering in the plains, it would tend to favor more gradual resurfacing models for Venus.

**Mapping of Tectonic Structures.** - Magellan observations provide a valuable overview of tectonic processes on Venus (Solomon et al. 1991), but are limited by the resolution of the radar images and especially the topography (10-30 km). Complementing observations by VERITAS, EnVision's much higher horizontal resolution: 10-30 m imagery, 300 m horizontal resolution of the SAR stereo Digital Elevation Model (DEM) will enable much clearer definition of the styles of tectonic deformation and of the superposition and cross-cutting relationships used by geologists to map the sequence of deformation in a given region.

High resolution radar data are particularly essential in understanding the tesserae of Venus, which contain fine-scale, complex patterns of deformation. We expect that tesserae represent the oldest terrain, locally, but they may not have all formed at the same time; better understanding of their structure and arrangement, their relationship with volcanic terrains and their correlation from one place to another would help to unravel these temporal and structural conundrums. Magellan imagery revealed very varied tesserae interiors often with complex arrangement of solid and deformed rocks, blanketed by finer grained or smoother materials (Hansen and Willis 1996; Ivanov and Head 2011) but without greater spatial resolution and better topographic detail, the nature of the materials and their origins could not be resolved. Multi-polarimetry observations (HH and HV) are needed to better understand their surface textures and physical structures, to reveal emissivity variations of solid lithologies and to discriminate them from unconsolidated materials.

**Impact Crater Modification.** - The only method for determining the absolute age of a surface, in the absence of measurement of radioactive isotopes, is through the use of crater counts. Because Venus has so few craters it is difficult, or impossible, to distinguish the age of different geological units using craters alone. However, craters on Venus are modified to varying degrees, first by loss of radar-dark halo, and then by infilling, causing dark floors. Some are also modified volcanically or tectonically (Izenberg et al. 1994). Because initial crater depth depends on crater diameter, the extent to which a particular crater deviates from the expected depth-diameter relationship provides a guide to post-impact infilling by lava or sediments at that crater. The height of the crater's rim above the surrounding terrain similarly provides a guide to the thickness of post-impact fill in the crater's ejecta blanket. Initial estimates of crater fill with Magellan data (Herrick and Rumpf 2011) were limited
by the accuracy of the available stereo topography digital elevation model. In contrast, Venus SAR observations will be optimized to produce high resolution DEMs and nadir altimetry profiling will provide global topographic data. Possible direct measurements of crater infilling with sounding radar will be complementary to topography-based estimates of crater fill thickness. Craters are globally distributed, so such measurements can provide important new information about the global resurfacing history of Venus.

**The Heat Engine in Venus' Interior: What Are the Driving Forces for Volcanism and Tectonism?** - While Venus and Earth have similar bulk geophysical properties, they have clearly followed divergent geodynamic paths – the former apparently characterized by a strong continuous lithosphere and stagnant lid convection, and the latter characterized by plate tectonic recycling of the lithosphere (Rolf et al. 2022, this collection; Gillmann et al. 2022, this collection; Herrick et al. 2023, this collection). At the root of these distinctions, it is interior dynamics that essentially governs the cooling of a planet. Stagnant lid convection represents a heat transport mechanism much less effective than mobile lid convection. It shows different tectonic characteristics than the plate tectonic regime on Earth. Internal dynamics can therefore cause surface stresses and thus tectonic structures on the planetary surface. Different convection regimes will lead to different tectonic characteristics. As a result, it is expected that planetary surfaces reflect their inner dynamics: a number of features on Venus are tantalizingly similar to structures on Earth, including continent-like tesserae plateaus, chasmata interpreted as rift zones, and some coronae that are surrounded by troughs resembling subduction zones.

### 6.1.3 How Geologically Active Is Venus?

**Detecting Volcanic Activity in Repeated SAR Images.** - Detecting and characterizing of relatively large eruptions over the past 40 years will come from three sources in the SAR image data: i) any new, large lava flows (>200 m wide and 100s m long) erupted since the Magellan mission and within EnVision's mapped area will be revealed in the imaging cycles of the EnVision mission; ii) any large scale changes in the morphology of volcanic edifices will also be revealed within EnVision cycles; and iii) any new, small lava flows (>60 m wide and at least a few hundred meters long) erupted in the 4-year duration of the EnVision mission. Detected changes (or non-detection) will be used to place bounds on the volcanic activity rate as described in Lorenz (2015).

**Searching for Surface and Near-Surface Temperature Changes.** - In addition to SAR imaging, temperature signatures associated with volcanic activity from both hot lava and hot volatile gasses will be detected and monitored in the infrared (IR) and microwave domains. Temperatures associated with volcanic eruptions can range from only 500 °C for low viscosity carbonatite lava to well over 1000 °C for ultramafic lavas. Such young, hot lavas will be directly detectable by their signature in IR emissivity data, (provided lava outflows cover an area of at least 0.1 km³). Cooling rates at the surface are estimated to be on the order of hours (Mueller et al. 2017), but microwaves offer the prospect of sensing the shallow subsurface and thus may detect warmth from old lava flows, i.e., lava flows which have cooled at the surface possibly years ago and thus have no more IR emission signature but are still hundreds of K above ambient at depth (Lorenz et al. 2016). Polarimetric radiometry measurements (used to determine whether candidate areas have anomalous emissivity rather than high physical temperature) and a better knowledge of the topography (and therefore of the altitude-dependence of the surface physical temperature) will greatly enhance the reliability of the volcanic detection and monitoring.

**Understanding the Range and Scope of Mass-Wasting Processes (Landslides).** - Though Magellan imagery showed us evidence of mass-wasting and aeolian features, it