was not able to reveal their temporal changes during the mission's lifetime, so their geomorphological and temporal properties remain unknown, and we have almost no information about weathering, surface alteration or other aeolian processes. Since there is currently no constraint on the mechanisms and rates at which these processes might be occurring, better topography and nested imaging at multiple resolutions, and repeated imaging during the mission, are needed.

Landscape evolution refers to processes that modify the morphology of a planet's surface, in particular gravity-driven mass-wasting processes such as landslides and slumps. Mass-wasting is a ubiquitous geomorphological process operating on any planetary body with gravity (such as those observed on Earth, the Moon, Mercury, Venus, Mars, icy satellites, comets and asteroids). Malin (1992) provided the first evidence of mass movement on Venus in the form of large-scale slope failures. Magellan's imagery also provided evidence for two dune fields (Greeley et al. 1992, 1995) and indirect evidence for putative 'micro-dunes' (Weitz et al. 1994) that were not resolved by the 100 - 200 m spatial resolution of Magellan's imagery. The surface winds evidenced by these dune fields and by wind streaks and debris fans (downwind of impact craters) are likely to be important agents of aeolian geomorphological change, but data of higher spatial and temporal resolution, and the ability to distinguish loose from consolidated surface materials, are needed to characterize them. Higher resolution, VenSAR observations, with consistent geometry, should reveal many smaller features and better resolve the morphology of features that were not resolved by Magellan. Repeated observations of regions expected to be active, e.g., along rifts, will help to characterize processes operating at decadal (Magellan-VERITAS-EnVision comparison over 40 yrs) and yearly (EnVision inter-cycle comparison) time scales. Local scale DEM's at 5-60 m spatial sampling within the Alpha Regio tesserae will complement the EnVision measurements at scales as wide at 150 km².

In the absence of near-surface water which, on Earth, affects material bulk density, shear strength and pore-pressure, and thus leads 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 revealed a very strong spatial relationships 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. EnVision's proposed Regions of Interest and higher resolution imaging offer excellent coverage of known mass-wasting features and increase the likelihood of imaging new or previously undetected smaller features. The planned VenSAR investigations will include detailed characterization of mass-wasting geomorphological properties and features with stereo imagery, and of their surface conditions with multi-polarimetry.

### 6.1.4 How Are Venus' Atmosphere & Climate Shaped by Geological Processes?

**Detection of Volcanogenic Gas and Particulate Plumes.** - Sulfur dioxide variations in the mesosphere have been attributed as possible evidence of volcanic activity (Esposito 1984), but they also could be due to intrinsic dynamic variability of the atmosphere, associated with temporal changes in transport of SO₂ from troposphere (where it is highly abundant) to mesosphere (where it is detected). On the other hand, volcanic gas plumes in the troposphere (below the clouds) would have quite a distinct signature, with distinct plumes advecting with the prevailing East-to-West winds (Fig. 17). Water vapor is likely to be a better tracer of volcanic activity than sulfur dioxide, because it is less abundant in the Venus atmosphere than SO₂, and because it can be mapped at three different altitudes in the troposphere using different spectral bands on the nightside. Analyses of Venus Express data found no evidence of
<figure><img src="image_20.png" /><figcaption>**Fig. 17** Simulated advection of a volatile gas plume emitted from Imdr Regio. Black contours represent topography. Colors show excess water vapor (in arbitrary units) after 72 hours of outgassing, at (a, left:) 10 km altitude and (b, center:) 35 km altitude. (Wilson and Lefèvre 2020). (c, right): Variations of water vapor at 40 km altitude (Tsang et al. 2010). This result was later found to be potentially attributed to degeneracies between cloud and water vapor retrieval. The higher spectral resolution of VenSpec-H, compared to VIRTIS-M, will enable unambiguous disentangling of these signals</figcaption></figure>

tropospheric water vapor variations (Bézard et al. 2009, 2011), but these analyses represent data only from a few days and, due to low spectral resolution, could only determine water vapor to a fairly wide range of 25 – 40 ppmv.

The nominal column mass of volcanic gasses in the Venus atmosphere, integrated from surface to space, is ~200 kg m⁻² for SO₂, ~10 kg m⁻² for H₂O and ~0.1 kg m⁻² for HDO. If the composition of Venus volcanic gasses is the same as on Earth - provided that plume dispersion does not exceed 10 km, the limiting spatial resolution induced by cloud scattering - then a large, Pinatubo-size eruption would change H₂O abundance, D/H ratio, and SO₂ abundance, respectively, by ~ +30%, -30%, and +1%. The latter effect may be underestimated with respect to the others, both because the Venusian interior may be much drier than Earth's, and because the outgassed SO₂/H₂O ratio is expected to be higher for a given magma volatile content due to Venus' high atmospheric pressure (Gaillard and Scaillet 2014). The frequency of occurrence, and the ratio of gasses and particulates in any volcanic plumes detected would provide constraints on the upper mantle properties.

**Explore the Main Constituent of the Cloud, H₂SO₄, in Both Vapor and Liquid Form.** - The main constituent of the clouds, H₂SO₄, in both vapor and liquid form, can be monitored near the cloud base altitude, yielding clues as to cloud formation and convection processes. Geological activity can affect clouds in several ways: (1) volcanic ash can contribute to cloud and haze layers; (2) volcanic sulfur dioxide emissions can contribute to formation of sulfate cloud & haze layers and to the as-yet unidentified UV absorber seen at cloud-tops; (3) volcanically emitted volatiles can form condensate layers; (4) heat from volcanic activity can cause changes in atmospheric circulation (Esposito 1984); (5) near-surface winds in Venus' dense atmosphere can lift dust & other particulates from the surface into airborne suspension. Understanding the dependence of the cloud layer on outgassed mantle volatiles is critical for understanding the long-term climate evolution of the planet. All of these effects can be studied by monitoring the spatial and temporal variations of clouds and hazes. Characteristic timescales of cloud formation and dissipation are expected of the order of hours to days, therefore observations on such timescales are properly addressed from a Venus low polar orbit.

## 6.2 EnVision Mission Overview

EnVision will be launched on an Ariane 62 in the fourth quarter of 2031 (current working assumption, the final schedule will be agreed together with NASA at Mission Adoption),
with back-up launch dates every 6 months until mid 2033. Indicative mass budget including all margins is 1350 kg (dry mass); estimated total wet mass including launch adapter at the time of mission selection is 2500 kg. An interplanetary cruise of 15 months (to be confirmed and pending final launch date) is followed by orbit insertion and then circularization by aerobraking over a period of about 16 months to achieve the nominal science orbit, a low quasi-polar Venus orbit with inclination between 87 and 89 deg, altitudes varying from 220 to 510 km and orbital period of about 92 min. The nominal science phase of the mission will last six Venus sidereal days (four Earth years). The choice of science orbit around Venus is mostly driven by a need for global VenSpec, SRS, and VenSAR altimeter and radiometer coverage, stereo topography, polarimetric and repeated VenSAR imaging, and for high-resolution gravity mapping. The spacecraft is approximately rectangular, 3 m in height × 2 m in depth and width in stowed configuration, with chemical propulsion and powered by two deployable solar arrays. EnVision will downlink ~210 Tbits of science data, using a Ka-/X-band comms system with a fixed high-gain antenna (HGA) of diameter > 2.5 m.

The communication subsystems relies on X-band uplink for simultaneous telecommand and ranging reception, on X-band downlink for simultaneous spacecraft telemetry and ranging transmission, and Ka-band (32 GHz) downlink for high data rate transmission of science data or alternatively for ranging. The HGA is the primary antenna used for spacecraft communication in X and Ka-band, and is completed by several Low Gain Antennas (LGA) used for X-band communications only, during Launch and Early Operations Phase (LEOP) and spacecraft safe modes. To maximize the data return, the Ka-band communications subsystem relies on a powerful Travel Waveguide Tube Amplifier (TWTA) with a radio frequency power output of 120 W. This architecture, together with daily communication passes with 35 m Deep Space Antennas of 9.3 hours in average, allow to downlink the required science data return whatever the Earth to Venus distance.

## 6.3 EnVision Science Payload

EnVision's science payload consists of VenSAR, a dual polarization S-band radar also operating as microwave radiometer, three spectrometers VenSpec-M, VenSpec-U and VenSpec-H designed to observe the surface and atmosphere of Venus, and the Subsurface Radar Sounder (SRS), a High Frequency (HF) sounding radar to probe the subsurface. These are complemented by a radio science investigation which achieves gravity mapping and radio occultation of the atmosphere, for a comprehensive investigation of the Venusian surface, interior and atmosphere and their interactions. This extensive suite of instrumentation and experiments work together to comprehensively assess surface and subsurface geological processes, interior geophysics and geodynamics, and atmospheric pathways of key volcanic gasses, which together illuminate how and why Venus turned out so differently to Earth. The synergistic and holistic way in which the payload instruments collaborate to investigate processes at different altitudes, depths and spatial scales is characteristic of the EnVision mission (European Space Agency 2021).

### 6.3.1 VenSAR on EnVision

A Synthetic Aperture Radar, VenSAR, will image pre-selected regions of interest at a resolution of 30 m/pixel, and subregions at 10 m/pixel. An order of magnitude better than Magellan and with a better sensitivity, these images are the key to understanding geological processes from local to global scale, discriminating relationships between units of different
age, and identifying the changes caused by geological activity. Topographic information at 300 m spatial and 20 m vertical resolution across these regions, derived from stereo imaging at two different incidence angles, is complemented by a global network of altimetry mode tracks with a vertical resolution of 2.5 m. This enables to map the surface at a better resolution than any previous dataset, essential for resolving the geometry of faults, folds and other features, and enabling the quantitative analysis of geological processes. Surface properties such as roughness will be derived from active imaging in both HH and HV polarizations – a first for a Venus orbiter - and passive radiometry at a range of angles, which also permits the detection of surface temperature anomalies. Repeated observations and comparisons with Magellan imagery allow for the detection of volcanic, tectonic and geomorphic changes over periods of months, years and decades.

EnVision will acquire dual-polarization SAR imagery at 30 m resolution for about 7% of the surface after 6 cycles, which aids surface characterization by exploiting the polarimetric reflection properties of the surface. SAR polarimetry is essential for differentiation of surface types and properties, because it is sensitive to surface roughness and structure (e.g., consolidated vs fine-grained material). VenSAR employs a dual polarization mode (transmitting H and recording H and V polarizations) to enable differentiation between terrain types and first-order surface properties characterization. Dual polarization was chosen for data rate and swath width considerations and H polarization to match the Magellan data enhancing change detection studies. Passive radiometry will be carried out in a near-nadir (with an incidence angle of 14°) or nadir viewing geometry, in parallel with other EnVision instruments. The surface microwave brightness temperature will be recorded globally (>75% of the surface) with repeated observations (at least 2 times) and a final resolution likely better than 10 km when using all overlapping near-nadir observations.

**Surface Topography.** - Surface topography is integral to many of the EnVision science investigations, either as the primary data source for inferring the type and magnitude of geologic processes that shape the surface, or as ancillary data necessary for proper interpretation of other data. The resolution and vertical accuracy required depends on the investigation and varies from several kilometer scale resolution to roughly quarter kilometer with vertical accuracy of 10s of meters. Magellan global topographic data with its 15-20 km resolution and vertical accuracy of 50-100 m is insufficient to support these investigations (Ford et al. 1992). Quantitative modeling of faulting and folding requires knowledge of topography with a vertical resolution of 25-50 m. Such models can constrain the physical processes that produce the observed tectonic landforms, the magnitude of the deformation, and the mechanical structure of the crust and lithosphere in the vicinity of the tectonic feature.

Topography from SAR stereo data for impact craters, at horizontal resolutions less than a quarter to a third of a crater diameter, i.e., less than 10 km, and vertical resolutions better than 20 m, will enable the measurement of the thickness of post-impact crater fill. Still finer spatial- and vertical-accuracy topography measurements will reduce the uncertainty in crater depth-diameter measurements and more accurate crater fill thickness estimates. Moreover, the plains of Venus are under-represented in the ROIs, and a globally distributed set of topographic measurements will be particularly important for understanding the plains resurfacing history.

Topography data are also needed for investigations other than those of the SAR. The Sub-surface Radar Sounder (SRS) requires topographic information to identify likely off-nadir echoes (“clutter”) that may confuse subsurface feature identification. Knowledge of the absolute surface temperature is needed for calculation of the absolute surface emissivity from near-IR nightside observations. The variation of surface temperature is primarily dependent on surface altitude; reducing the accuracy of the surface altitude determination to ≤10 m also reduces the uncertainty in the absolute determination of surface emissivity.
**Surface Properties: Nadir and Near-Nadir Radiometry, Surface Polarimetry, Microwave Emissivity.** - Used in passive radiometry mode, EnVision SAR will map the thermal emission emanating from Venus surface with significantly better precision and accuracy than the Magellan radar (0.7 K against 1-2 K and 1.7 K against 15 K, respectively). Emission maps, in the form of surface brightness temperature maps, will then be used to search for thermal anomalies or, if the surface temperature is known, to map the emissivity of the surface which, in turn, provides insight into its composition (through the dielectric constant) and physical properties (roughness, density).

Passive nadir and near-nadir radiometry SAR modes are primarily designed for the search of thermal anomalies but will also be used, based on assumptions on the physical temperature, to build a mosaic of the surface emissivity at 9.5-cm by dividing the measured brightness temperatures by an estimate of the surface temperature. At nadir or near-nadir the microwave emissivity of a surface is largely controlled by its dielectric constant and the surface roughness only has a second order effect. In turn, the dielectric constant is related to the bulk composition and density of the surface material and the dielectric map inferred from radiometry measurements will be used to distinguish surface units. More specifically, for dry materials, the relationship between dielectric constant and the density is generally well described by a power-law function and, with some assumptions, the dielectric map can be readily converted into a global near-surface density map (Campbell and Campbell 1992; Campbell and Rogers 1994).

In addition to near-nadir and nadir observations, polarized radiometry measurements will be acquired in an off-nadir geometry (with a viewing angle of 25-30°) in selected regions. As aforementioned, the main advantage of nadir radiometry is to be less sensitive to roughness than off-nadir radiometry. However, the average of two orthogonally polarized emissivity values (or the polarization ratio) is also less sensitive to roughness than either individual component and can be used to provide an even more reliable estimate of the dielectric constant, requiring no assumption on the physical temperature. Such measurements will be primarily performed in Venus highlands to confirm or inform their unusually high dielectric constant and put new constraints on their composition candidates. Recording of both H and V polarization in an off-nadir geometry will distinguish between the effects of dielectric constant and roughness/volume scattering, thus offering an additional powerful tool for surface characterization (European Space Agency 2021).

By collecting microwave emissivity data at a higher resolution than the radar of Magellan, with better precision and especially accuracy (by a factor ~10) and geometries (targeted off-nadir polarized measurements) relevant to the science objectives, the EnVision radar operating as a radiometer combined with the instrument high-resolution topography and polarimetric imaging will refine the mapping of Venus surface in terms of composition and physical properties. It will thus provide key information to retrieve the geological history and age of its terrains. In particular, it will help unravel the nature and rate of alteration in Venus high-altitude low-emissivity regions, investigate impact modification in crater ejectas and maybe unveil deeply weathered regions, thick sedimentary layers or signatures of recent resurfacing. By the end of the EnVision mission (6 cycles) we should be able to produce a radiometry map of >90% of the surface, with a resolution of about 10 km using all overlapping measurements.

### 6.3.2 EnVision Subsurface Radar (SRS)

A Subsurface Sounder, SRS, will characterize the vertical structure and stratigraphy of geological units including volcanic flows. Geological inferences from Magellan data point to
<figure><img src="image_21.png" /><figcaption>**Fig. 18** (a, left): Typical range of subsurface sounding depths in which different geological targets of the EnVision Subsurface Radar (SRS) can be identified; (b, right:) SRS average penetration depth calculated for different Venus-like samples (from measurements on Moon and Earth analogue materials at Venus temperature) in the SRS bandwidth.</figcaption></figure>

a range of subsurface structures and geometries that are as yet unquantified. The SRS provides a unique opportunity to sound the great variety in geologic and geomorphic units. It will also provide unprecedented information on the surface in terms of roughness, composition and permittivity (dielectric) properties at wavelengths completely different from those of VenSAR, thus allowing a better understanding of the surface properties (Fig. 18). SRS observation will also result in altimetry measurements by providing low-resolution profiles of the topography that can be integrated with the altimetric data of VenSAR.

EnVision is the first mission to Venus with a confirmed sounding instrument (ISRO's proposed Venus mission is also considering a sounder, see Sect. 8.3.2) that will allow for the direct measurement of subsurface features. Despite some geological surface investigations that provide hints about possible existence and nature of subsurface structures, no direct measures exist. In this context the Subsurface Radar Sounder (SRS) onboard EnVision mission represents a unique opportunity to sound the great variety of geologic and geomorphic units. SRS will investigate stratigraphic and structural patterns, to test hypotheses related to the origin of structures at the surface and in the shallow subsurface and their relationships. This will enable investigation of interaction processes between surface and subsurface structures as well as subsurface structures not directly linked with surface ones.

There are many geological investigations for which the detection of subsurface boundaries may provide invaluable constraints. They include impact craters and their infilling, buried craters, tesserae and their edges, plains, lava flows and their edges, and tectonic as well as volcanic features. For those features subsurface characteristics are crucial for: the relative dating of surfaces by the analysis of stratigraphic relationships, the modeling of three-dimensional structure, the identification of boundaries between units/edges. The subsurface material boundary delineation by sounding will improve the understanding of Venus resurfacing history and geologic evolution.

These investigations will be performed Venus wide (with an average observation density of 2 per degree of longitude at Equator) and on selected RoIs which include the mentioned features (with an average observation density of 10 per degree of longitude at Equator). The scientific investigations call for a penetration down to a few hundreds of meters (up to 1000 m) and about 20 meters of vertical resolution. The typical depth needed for sounding of different subsurface feature types is shown in Fig. 18a. Calculations of SRS penetration depth, shown in Fig. 18b, determine that the SRS will be able to investigate a wide variety of geological targets. The SRS penetration depth has been calculated using a large variety
<figure><img src="image_22.png" /></figure>

**Fig. 19** Airborne radar sounder profile at 40 MHz central frequency (more than four times higher than the SRS one) over the Dolomieu Crater on the top of the Fournaise Volcano in the reunion Island in the Indian Ocean. Fournaise is a hot spot effusive volcano with geomorphological features and magma dynamic very similar to several Venusian volcanoes (Anderson 2005). The radargram crossing from South to North the main crater on the top of the volcano show the fractured areas (white areas before and after the crater) that are materialized by the strong signal scattering resulting from the fractures. Inside the crater the radargrams shows the layering that is on the crater northern wall arising from the succession of debris flowing from the collapsing northern part. The crater depth is approximately 100 m and its width 1 km. The lava temperature ranges from 40 C at the surface to 600 C beyond the 10 m level, demonstrating the viability of HF sounding through rocks at these elevated temperatures

of different rock types and surface topologies; for a quick demonstration of the viability of HF subsurface sounding through rocks at Venus temperatures, Fig. 19 shows an example of sounding through lava at >600 °C of a volcanic crater floor on Earth.

### 6.3.3 EnVision Spectrometer Suite (VenSpec)

A Spectrometer suite, VenSpec, will obtain global maps of surface emissivity in six wavelength bands using five near-infrared spectral transparency windows in the nightside atmosphere, to constrain surface mineralogy and inform evolutionary scenarios; and measure variations of SO₂, SO and linked gasses in the mesosphere on the dayside, to link these variations to tropospheric variations and volcanism. In combination of its three instruments, detailed below, VenSpec will provide unprecedented insights into the current state of Venus and its past evolution. VenSpec will perform a comprehensive search for volcanic activity by targeting atmospheric signatures, thermal signatures and compositional signatures, as well as a global map of surface composition.

VenSpec-M, like the identical VEM instrument on-board VERITAS (Sect. 4.3.2), is a pushbroom multispectral imager optimized to map thermal emission from Venus' surface using six narrow bands ranging from 0.86 to 1.18 µm, and three bands to study cloud microphysics and dynamics. VenSpec-M will provide near-global compositional data on rock types, weathering, and crustal evolution by mapping the Venus surface in five atmospheric windows. VenSpec-M will use the methodology pioneered by VIRTIS on Venus Express but with more and wider spectral bands, the NASA VERITAS VISAR and EnVision VenSAR-derived Digital Elevation Models (DEMs) and EnVision's lower orbit compared to Venus Express to deliver near-global multichannel spectroscopy with wider spectral coverage and an order of magnitude improvement in sensitivity. It will obtain repeated imagery of surface thermal emission, constraining current rates of volcanic activity following earlier observations from Venus Express (Smrekar et al. 2010a; Mueller et al. 2017). In combination with the observations provided by the identical VEM instrument on the NASA VERITAS mission VenSpec-M will provide more than a decade of monitoring for volcanic activity, as well as search for surface changes (Fig. 10).
VenSpec-M uses the same 14 bands filter array as VEM on board VERITAS (Helbert et al. 2016, 2020, see also Table 3 and Fig. 10). Those 14 bands fall in four categories depending on where the radiation is originating. The radiation for the six 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 (Dyar et al. 2020, 2021; Helbert et al. 2021) as well as monitor for the thermal signature of active volcanism. The radiation in the two water vapor bands originates in a layer close to the surface and is sensitive to the abundance of water vapor which may see changes due to volcanic exhalations, complementing the H₂O and HDO measurements by VenSpec-H in the middle atmosphere. In the three cloud bands, radiation originates at an atmospheric layer above the surface but below the clouds. Because the signal in the cloud bands has no surface or water vapor contributions, the measurements in these bands can be used to remove cloud-induced contrast variability from the other bands. Finally, the three background bands are sensitive in spectral regions where the atmosphere is opaque, thus allowing the removal of background signal on the detector. The high density of cloud particles results in multiple scattering of the radiation, reducing the spatial resolution to 50–100 km.

VenSpec-H is dedicated to high spectral resolution atmospheric measurements in the near-infrared. It will focus on the volcanic and cloud forming gases and search for composition anomalies potentially related to the volcanic activity. The instrument, designed to measure H₂O, HDO, CO, OCS, and SO₂ on both the night and day side, is a nadir-pointing, high-resolution (R~8000) infrared spectrometer that will perform observations in different near-IR spectral windows between 1 and 2.5 µm. Spectra in these bands will be recorded sequentially as the EnVision spacecraft moves along its quasi-polar orbit, and will allow the sounding of different layers in the Venusian atmosphere: close to the surface (1.17 µm), 15-30 km (1.7 µm), 30-40 km (2.4 µm) and above the clouds (1.38 & 2.4 µm). Two additional polarization filters will be used during dayside observations to better characterize the clouds' properties.

The instrument will include a total of four spectral bands: 1.165 - 1.180 µm (B#1), 2.34 - 2.48 µm (B#2), 1.72 - 1.75 µm (B#3) and 1.37 - 1.39 µm (B#4). B#2 is further divided into two ranges: 2.34 - 2.42 µm (2a) and 2.45 - 2.48 µm (2b). Bands 1, 2a, 2b and 3 are observed on the night side, bands 2a, 2b and 4 on the day side. In this near-IR region, the high spectral resolution combined with the high sensitivity of the instrument will allow to clearly identify the absorption features of the targeted species. Spectral band selection is performed in part by a filter wheel mechanism with stringent lifetime requirements and a filter-slit-assembly that allows sequential measurements in the 4 spectral bands of interest. Design measures are taken to make the VenSpec-H observations insensitive to polarization, while exploiting the polarization information contained in the light reflected from Venus.

VenSpec-U, a dual-channel ultraviolet spectrometer, will monitor minor sulfur species (mainly SO and SO₂) and investigate the complex and highly variable upper atmosphere and its relationship with the lower atmosphere. VenSpec-U will search for atmospheric effects of geological activity, in order to determine how much outgassing is occurring, and how the atmospheric chemistry is coupled with surface/subsurface geochemistry and weathering cycles; study how mesospheric gas variations are linked to volcanism, in order to identify the causes of variability in the mesospheric sulfured gases (SO, SO₂); and finally how cloud and particulate variability is linked to volcanism, in order to detect plumes of volcanic ash or sulphate clouds caused by volcanism, and to understand any link between the Venus sulfuric acid clouds and volcanism.

Observations can be conducted in a strict nadir geometry (null emission angle), or in near-nadir geometry (emission angle < 30°) thanks to a UV imaging spectrometer operating
in the 190 – 380 nm spectral range. Spectral resolutions shall be better than 0.3 nm in the 205 – 235 nm range (typical SNR of 100 at 220 nm) in order to distinguish SO and SO₂ spectral lines, and better than 2 nm in the 190 – 380 nm range (typical SNR of 200 at 220 nm) which encompasses the unknown UV absorber peaking near 365 nm. Spatial sampling shall range from 3 km to 24 km, depending on spectral resolution and orbiter altitude. The narrow-slit axis of the instrument contains the spectral information, whereas the long-slit axis contains the spatial information along the 20° field of view. The remaining spatial direction is provided through orbital scrolling.

### 6.3.4 EnVision Radio Science / Gravity Experiment (RSE)

A Radio Science Experiment uses the spacecraft-Earth radio link for gravity mapping and atmospheric profiling. Measurements of the lateral variations in the strength of a planet's gravity field is an important tool in probing the subsurface structure of a planet. Regional differences in elevation can be supported by differences in crustal thickness, by flexure of the elastic lithosphere, or by convective flow in the mantle. These mechanisms can in turn be distinguished by their expected gravity signatures, resulting in estimates of the thickness of the crust and lithosphere in different regions of Venus. As discussed previously (Ghail et al. 2023; Herrick et al. 2023; Gilmore et al. 2023, this collection), we need to understand whether the tesserae, which contain fine-scale, complex patterns of deformation, represent thick, ancient remnants of deformed and deep-rooted continental crust. The tesserae may also hold clues to the nature of past resurfacing; particularly whether there have been periods of enhanced crustal mobility, or whether Venus has been in its current state for most of its history.

Crustal thickness affects the stratification of mechanical strength in the lithosphere and thus can also affect the style of tectonic deformation (Dumoulin et al. 2017). Magellan gravity data are consistent with an organized pattern of mantle convection broadly similar to Earth; but it lacks the resolution necessary to understand its connection with geological-scale features, such as individual coronae or mountain belts. EnVision can measure the integrated amount of volcanism over time and thus provide tests of thermo-chemical evolution models, but can also sometimes be the product of extensional or compressional tectonism. Determining lithospheric thickness with gravity data is particularly sensitive to data with wavelengths less than 500 km. Higher spatial resolution than the Magellan solution of the Venus gravity field is required to better constrain the crustal and lithospheric structure variations. Combining Magellan and EnVision gravity data will allow determination of the gravity field over at least 95% of the planet, with an average spatial resolution better than 200 km, and an accuracy better than 20 mGal (Rosenblatt et al. 2021; European Space Agency 2021).

Venus' moment of inertia, Love number, and tide-induced phase lag also characterizing the signature of the internal structure in the gravity field will be extensively constrained during the six cycles of the EnVision mission (one cycle equals one Venus sidereal day or 243.02 Earth days). EnVision will constrain the size of the main internal layers crust, lithosphere, mantle and core, and whether or not the core is fluid, will help to understand fundamental differences or even possible similarities between Venus and Earth. Indeed, the overall size of the chemical reservoirs (crust, mantle, core) gives information about the composition of Venus; the average thickness of the crust about the rate of magmatism; the average thickness of the lithosphere about the mechanisms of heat transfer at the surface; and the state of the core about the long-term cooling rates. The EnVision spacecraft-Earth radio link will measure the gravity field of the planet with spatial resolution better than 270 km, and accuracy of <0.2 mm/s² globally, with an improved higher spatial resolution of <200 km
and accuracy of <0.1 mm/s² in most of the Southern hemisphere (40% of the planet) and k₂-Love number with an accuracy of ±0.01. As discussed before, the potential Love number helps to determine the state of the core and, in the case of a liquid core, also its size. A Love number k₂ lower than 0.27 would indicate the presence of a fully solid iron core, while for larger values, solutions with an entirely or partially liquid core are possible (Dumoulin et al. 2017).

Furthermore, the EnVision radio-occultation experiment aims at sounding of the temperature structure of the Venus atmosphere in the altitude range 90-35 km and abundance of sulfuric acid in gaseous and particulate phases. The experiment relies on the observation of the radio-link propagation (frequency and amplitude) through the atmosphere of Venus during radio-occultation. The radio ray path changes in the ionosphere and neutral atmosphere are induced by a change in the refractivity profile. This leads to a shift in the measured frequency at the ground station. These frequency changes can be used to retrieve the neutral number density, temperature and pressure profiles as a function of the planetary radius at a high vertical resolution. Thanks to the use of the dual X-Ka band, the cloud contents in both gaseous and liquid phase of sulfuric acid, and its spatial and temporal variability, will be estimated for the first time at 35-55 km, with an accuracy of 1 mg/m³ (liquid) and 1 ppm (gaseous) on time scales from hours to years, with vertical resolution of ~100 m. (European Space Agency 2021). In addition to H₂SO₄ content in both gaseous and liquid phase within and below the clouds, static stability profiles retrieved from temperature, pressure and number density profiles (35-90 km), provide valuable information about small-scale fluctuations in the thermal profiles and the latitudinal dependence of gravity wave activity. Understanding the dependence of the cloud layer on outgassed mantle volatiles is critical for understanding the long-term climate evolution of the planet, and Venus would be indeed the first planet beyond the Earth where we could relate the dynamics of gravity waves and small-scale turbulence and temperature fluctuations, and the cloud composition.

Both VERITAS and EnVision VISAR and VenSAR will produce repeated imaging of surface features throughout their mapping cycles, allowing to create radar tie points, thus tying the inertial position of the probe to the planetary body-fixed frame. Leveraging on the combination of tracking data and radar tie points, both VERITAS and EnVision will be able to measure the precession and monitor the variable spin rate of the planet with a much-improved precision.

## 6.4 Summary / Outcomes Revealing Venus Evolution

EnVision was selected as ESA's 5th M-class mission, targeting a launch in the early 2030s. The mission is a partnership between ESA and NASA, where NASA provides the Synthetic Aperture Radar payload. The scientific objective of EnVision is to provide a holistic view of the planet from its inner core to its upper atmosphere.

The mission is scheduled for launch on an Ariane 62 in the fourth quarter of 2031 (current working assumption, the final schedule will be agreed together with NASA at mission adoption), with backup launch dates every 6 months until mid-2033. It will provide new insights into geologic history through complementary imaging, polarimetry, radiometry, and spectroscopy of the surface, coupled with subsurface sounding and gravity mapping; search for thermal, morphological, and gaseous signs of volcanic and other geologic activity; and follow the fate of key volatile species from their sources and sinks at the surface through the clouds to the mesosphere. Following the same approach that has advanced our understanding of Earth and Mars, EnVision will combine global observations at low or medium spatial resolution (e.g., surface emissivity & atmospheric composition) with regionally focused observations at higher spatial resolution.
VenSAR, a dual-polarization S-band radar that also operates as a microwave radiometer, builds on NASA-JPL's experience with planetary radars since the Magellan mission; the Subsurface Radar Sounder (SRS), a high-frequency (HF) sounding radar to probe the subsurface, inherits from the RIME instrument on JUICE. These will be complemented by a radio science investigation that will provide gravity mapping and radio occultation of the atmosphere. The three VenSpec spectrometers build on the heritage of ESA's suite of planetary missions, in particular ESA's Venus Express from 2006 to 2014, will also highly benefit from complementarity with DAVINCI and VERITAS measurements.

To achieve its science objectives, EnVision must return 210 Tbits (26.25 Terabytes) of science data to Earth, using a Ka/X-band comms system with a fixed diameter hight-gain antenna, with a large dynamic range of distance to Earth (from 0.3 to 1.7 AU), from a low Venus polar orbit, in the hot Venus environment (exacerbated by the operation of highly dissipative units), while operating three spectrometers in a near cryogenic environment. Achieving the science objectives under these multiple constraints without oversizing the spacecraft requires careful planning of the science operations, making the science planning strategy a critical driver in the overall mission design against which the spacecraft and ground segment are then sized (Sect. 6.2 and Fig. 15).

EnVision science operations strategy is to obtain the widest range of data types that enables us to put the highest resolution datasets into regional and global context. characterize the sequence of events that generated the regional and global surface features of Venus, determine crustal support mechanisms, mantle and core properties, and the geodynamics framework that controls the release of internal heat over Venus history, by determining the styles of volcanic processes which have occurred on Venus, studying the sources, emplacement styles, magma properties and relative ages of different volcanic flows; assessing the styles of tectonic deformation that have operated on Venus by studying their surface expression and gravity signatures, and determining their role in planetary heat loss. It will also characterize surface modification processes - such as impact crater modification, low emissivity/radar bright highlands - to improve our understanding of Venus geochronology and constrain Venus' internal structure, through measurements of gravity field and tidal response, to study the properties and thicknesses of Venus' crust, mantle and core. It will better assess whether Venus once had condensed liquid water on its surface and was thus perhaps hospitable for life in its early history, and therefore fully support the scientific goals and open questions presented in the companion papers of this collection.

# 7 Venera-D: A Comprehensive Exploration of Venus' Atmosphere, Surface, Interior and Plasma Environment

Since the discovery of Venus' atmosphere by Mikhail Lomonosov in 1761 (Marov 2005), and further observations of the Venus transits, Venus has always been a celestial object of interest among Russian astronomers. It is not surprising that the multistage studies of Venus became the central and most successful part of the Soviet robotic space program. was the site of the first entry probe in any solar system atmosphere in 1967 (Venera-4), first soft landing in Dec. 1970 (Venera-7), first image from the surface of another planet in 1975 (Venera-9). The Soviet series of Venera & VeGa missions were phenomenally successful, not only in their technologically advanced landers which returned color pictures from Venus and successfully analyzed drill samples, but also successfully deployed balloons in the atmosphere in 1985. The Venera-D concept is the logical next step in the highly successful series of Venera and VeGa missions of the 1970s and 1980s (Marov et al. 1973; Marov 1978; Avduevskii
et al. 1977; Florensky et al. 1977; Barsukov et al. 1982, 1986; Garvin et al. 1984; Surkov et al. 1984; Moroz 1990; Moroz et al. 1985, 1996; Sagdeev et al. 1986a, 1992).

The Russian Venera-D (Венера-Д) flagship mission concept has been under develop-
ment with the goal of advancing the investigation of Venus' atmosphere, surface, and inte-
rior and the processes that link them as a system. Intense discussions about Venus began in
2013 as part of the Joint Science Definition Team (JSDT) established by NASA and Roscos-
mos with the goal of shaping a collaborative project. The JSDT developed a full Venera-D
mission scenario (Venera-D Joint Science Definition Team 2019; Venera-D Venus Modeling
Workshop proceedings 2018; Glaze et al. 2018; Zasova et al. 2020), but as of 2021, for a
variety of (mostly non-scientific) reasons, Venera-D has been developed as a national pro-
gram for Venus. Science objectives of the Venera-D mission concept currently address key
questions about atmospheric dynamics, emphasizing atmospheric superrotation and radia-
tive balance; the processes that have formed and modified Venus' surface, highlighting the
mineralogical and elemental composition of surface materials; and the chemical processes
occurring at the interface of the surface and the atmosphere. The Venera-D lander would
not only perform descent phase measurements but would also analyze surface composition.
This would of course provide invaluable "ground truth" for VERITAS, DAVINCI and EnVi-
sion's surface composition mapping, as well as contributing to understanding of geophysical
evolution (Venera-D Joint Science Definition Team 2019).

## 7.1 Venera-D Science Objectives

Venera-D is designed to study the atmosphere, surface, internal structure and properties of plasma surrounding Venus at new scientific and technological levels. As we discussed in Sect. 2, Venus is an Earth-sized terrestrial planet that has taken a different evolutionary and habitability path. To examine the reasons for this difference is very important to understand the divergent Earth and Venus evolutionary pathways. This is of particular relevance for the study of exoplanets and conditions for their habitability (O'Rourke et al. 2023; Way et al. 2023; Westall et al. 2023; Gillmann et al. 2022, this collection). Among these key aspects and objectives of the Venera-D mission, we can formulate those high-level objectives that directly address the long-term history and evolution of Venus through time, as well as those that are not addressed by currently selected missions:

**Coupling Between Geologic and Climate History.** - Current and past rates of volcanic outgassing are unknown, as is an understanding of how volcanoes have affected the atmosphere and climate. More fundamentally, the role of water in geodynamics and petrogenesis must be constrained. As on Earth, the geology and climate of Venus are linked (Bullock and Grinspoon 2001). The causes and effects of rapid changes in geologic expression can be studied in detail with a capable surface payload and remote sensing techniques (Helbert et al. 2008; Mueller et al. 2008; Gilmore et al. 2015). To address key geologic questions, it is necessary to characterize the geochemistry, mineralogy, emplacement, sediment supply, and petrology of surface features and terrains (Herrick et al. 2023; Gilmore et al. 2023; Carter et al. 2023, this collection); obtaining this information for the tesserae would provide insight into the oldest exposed rocks. These data will allow us to constrain the history of volatiles, especially water, on Venus and provide a basis for direct comparison of crustal evolution on Earth and Mars. In addition, isotopic measurements of the composition of the Venusian atmosphere and an improved understanding of atmosphere-surface interactions will help constrain the outgassing history, in particular the current and past volcanic outgassing rates (Avice et al. 2022, this collection).
**Abundance of Light Elements, Rare Elements and Their Isotopes.** - To characterize Venus' origin and evolution through time, accurate assessment of the composition of the atmosphere's composition is essential. Like Earth and Mars, the atmosphere of Venus seems to have substantially evolved from its original composition. Whether the major processes that shaped the atmospheres of Earth and Mars—such as impacts of large bolides and significant solar wind erosion—also occurred on Venus is largely unknown. Detailed chemical measurements of the composition of the atmosphere—in particular, the noble gases and their isotopes along with light elements and isotopes—will aid in understanding if the modern (secondary) atmosphere is a result of degassing from the interior or if it formed from comet or asteroid impacts (Avice et al. 2022; Salvador et al. 2023, this collection). Likewise, it is imperative to determine how the atmospheric abundances of water, sulfur dioxide, and carbon dioxide change under the influence of the exospheric escape of hydrogen, outgassing from the interior, and heterogeneous reactions with surface minerals.

**Venus Surface Geochemistry.** - The only means by which the geochemical data from Venus' surface can be obtained are the landers. Several landers visited the planet in a period from 1972 (Venera-8) to 1985 (VeGa-1 and 2) and reported the only data on the chemical composition of soils on the surface of Venus. Chemical measurements were made at seven sites that are concentrated in the Beta-Phoebe region and in Rusalka Planitia to the north of Aphrodite Terra. Selection of the landing sites were based purely on the interplanetary ballistic constraints because no knowledge on the surface geology existed when the Venera-VeGa missions were implemented. At four landing sites (Venera-8, -9, -10, and VeGa-1), concentrations of the three major thermal-generating components, K, Th, and U, were determined by gamma spectrometry (Surkov 1997). The mean values of their concentrations on Venus are well within the range that is typical of terrestrial basalts (Kargel et al. 1993; Nikolaeva 1995, 1997). However, enhanced concentrations of K, Th, and U in soils at the Venera-8 landing site raises the possibility for the presence of a non-basaltic material on Venus (Nikolaeva 1990). In two landing sites (Venera-13, and -14), the concentrations of major oxides (without Na₂O) were measured by the X-ray fluorescence (XRF) method. At the VeGa-2 site, both methods (gamma spectrometry and XRF) were used separately and the concentrations of the thermal-generating elements and major oxides were measured (Surkov 1997). The XRF data also suggested that rocks of basaltic composition make up the landing sites (Surkov et al. 1984, 1986; Kargel et al. 1993). Two important factors, unfortunately, strongly limit the value of the Venera and VeGa data and prevent their robust interpretation: (1) we do not know the exact position of the landers. All stations landed somewhere within their own landing circle, which is ~300 km in diameter and usually embraces terrains of different origin and age; (2) past accuracy of measurements. A new generation of lander instruments (Sect. 11.1.4) will determine the mineralogy and chemistry of terrain to ascertain rock type, and look for evidence of past water.

**Role of Solar Absorbers and Near-IR Opacity Sources in Venusian Clouds.** - The Venusian disk in reflected light is practically featureless in the visible and near-IR spectral regions (contrasts maximum 2 to 3%), but in the UV they reach or exceed 30% at 365 nm. The albedo of Venus decreases from a value of ~0.8 at wavelengths >550 nm to as low as 0.3 at UV wavelengths. Cloud contrast peaks at 365 nm. UV contrasts observed between 0.33 µm and 0.5 µm are the result of absorption by a species of unknown origin. UV absorption at 0.32 to 0.5 µm was observed to disappear below 58 to 60 km by the Pioneer Venus spacecraft (Tomasko et al. 1985). Thus, absorption by the UV-absorbing species of Venus was primarily associated with the upper clouds. However, measurements taken by the VeGa lander during descent show that absorption of UV radiation (220 to 400 nm) occurs down to 47 km altitude, indicating the presence of absorbers whose identities are still unknown (Bertaux et
al. 1996). Spatial variations in this absorption produce contrasts in daytime images and are a means of inferring bulk motions in the cloud top atmosphere. Measurements of small-scale feature motions over latitude and longitude provide information about the superrotation of the Venusian atmosphere at the level of cloud contrasts. Both the vertical distribution and the composition of UV-absorbing species are poorly known. The few available profiles of Venus' UV flux obtained between the cloud top and the surface indicate that the UV absorber is present in the middle and upper clouds (between ~47 and 72 ± 2 km), but may be occasionally found in the upper haze (~70 and 80 km) (Lee et al. 2015). It is currently unclear whether the cloud-level abundance of the absorber is solely the result of material upwelling from below, or whether it depends on chemical reactions between upwelling and downwelling species (see also Sect. 7.3.2).

**Solar Wind-Venus Interaction and Venus Magnetosphere.** - Plasma and magnetic field experiments on Venera-9 and Venera-10 in the 1970s provided the first data on the solar wind interaction and magnetosphere formation of Venus (Vaisberg et al. 1976). Important subsequent studies of the magnetic barrier and tail were performed by the Pioneer Venus Orbiter (Russell and Vaisberg 1983). Based on these experimental data, a model of the induced magnetosphere was developed (Vaisberg and Zelenyi 1984; Zelenyi and Vaisberg 1985). To further advance the field, Venus Express also performed investigations of the solar wind interaction with Venus (Barabash et al. 2007; Futaana et al. 2017). The discovery of the comet-like planetary plasma interaction and the processes leading to atmospheric losses allowed us to estimate how these losses vary with solar and interplanetary conditions, and their potential to cause significant changes in the chemical composition of the Venusian atmosphere over time. Despite the significant progress made by previous Venus missions, there are still outstanding problems in the study of the solar wind-Venus interaction and Venusian escape processes on recent and geological time scales.

Many of outstanding questions in Venus exploration are therefore both synergistic and complementary to the new generation of missions discussed above. Venera-D consists of a VeGa-like lander targeting the plains and an orbiter observing the atmosphere at several wavelengths, including near-IR. Beyond its surface science capabilities, the descent module is synergistic and complementary to the currently selected missions EnVision, DAVINCI and VERITAS because it will be conducting similar investigations on a different terrain type—the Venusian plains (Venera-D Joint Science Definition Team 2019). The combination of chemical and mineralogical data from both the plains and tesserae would significantly advance our understanding of the Venusian crust, mantle and igneous processes, the evolution of volcanism with time and the range of surface-atmosphere interactions on modern Venus. Atmospheric measurements from the Venera-D orbiter would complement the volatile mapping carried out by EnVision, in complement to the geophysical studies and atmospheric descent probe measurements which are at the heart of EnVision, DAVINCI and VERITAS science questions (Sects. 4-6).

## 7.2 Venera-D Mission Overview

The Venera-D mission architecture is composed of orbiting, landing and atmospheric modules. This mission structure provides the opportunity to make measurements in the Venus-induced magnetosphere, in the planetary atmosphere, and at the planetary surface. Figure 20 shows a general view of the mission.

### 7.2.1 Mission Requirements and Design Drivers

Mass budget of the spacecraft includes: 4800 kg total mass, which includes 1920 kg orbital module (OM), 2660 kg descent module (DM) and 50 kg OM/DM adapter. In addition, the
<figure><img src="image_23.png" /><figcaption>**Fig. 20** Venera-D composite spacecraft consists of orbiting, landing and atmospheric modules. The orbiter module (OM) is designed for Earth-Venus transit, delivery of the lander and payload equipment (PE) to Venus, functioning while in orbit, collecting data, transmitting collected data to Earth and data from the lander (Sect. 7.2.2); The descent module (DM) is designed for performing scientific measurements during the descent and on Venus' surface (Sect. 7.2.3)</figcaption></figure>

mass of the OM/upper stage adapter is 70 kg. The launch vehicle currently planned for the Venera-D mission is the new Angara-A5 LV launch vehicle with the DM-03 upper stage.

## 7.2.2 Orbiter Module (OM)

The Orbiter Module (OM) will carry out its science program from a highly elliptical polar orbit with a pericenter of 500 km, located above the Southern Pole, and an apocenter of 69,000 km, with an orbital period of 24 hours. Compared to Venus Express' pericenter of 250 km above the Northern Pole, and apocenter of 60,000 km, the Venera-D Orbiter Module is, essentially, symmetrical to Venus Express' orbit. The expected lifetime of the OM is 7 to 8 years. Figure 21 shows a general view of the orbiter module with a few selected instruments and subsystems. The OM will carry a complex set of science instruments to study the atmosphere of Venus from the surface to the ionosphere. Orbital studies will clarify the climate history of Venus and hopefully reveal the hidden mechanisms of water escape and the extreme greenhouse effect.

The set of imagers operating from UV to longwave IR will monitor dynamics of the atmosphere in the cloud layer on various levels. A combination of a Fourier spectrometer, a mm-radiometer, a long-IR imager and a radio science experiment will allow the construction of a three-dimensional thermal map of Venus atmosphere and the monitoring of its temporal variations. The suite of UV-to-IR spectrometers will provide new insights into minor species of the atmosphere and cloud aerosol properties.

The highly elliptical orbit of OM provides very good opportunities for plasma science. Venus has an extended magnetotail produced by the trapping of planetary ions at interplanetary magnetic field lines. The details of the interactions between the solar wind and the planetary ionosphere, and the corresponding plasma wave excitations, will hopefully be resolved by the charged particle and electromagnetic wave instruments of OM.

## 7.2.3 Descent Module (DM)

The Descent Module includes

- a ~800-kg Lander Module (LM)

- a ~420 kg Aerial Platform (AP).
<figure><img src="image_24.png" /><figcaption>**Fig. 21** Venera-D Orbiter Module (OM). OM will carry out its science program from a highly elliptical polar orbit with a pericenter of 500 km and an apocenter of 69,000 km. The orbital period is 24 hours</figcaption></figure>

Landers have always been a key component of Russian missions to Venus. The Venera-D lander will be the first since the successful descent and landing of the Soviet Vега spacecraft in 1986. The fleet of recently selected Venus missions, described in Sects. 4-6, do not include specific surface components.

**Lander Module (LM).** - The Venera-D Lander Module (LM) concept resembles in shape the Soviet landers that successfully operated on the surface for 1.5-2 hours, equipped with a modern scientific payload and a much more powerful data transfer system. The design of the lander includes a titanium structure developed for accommodation of onboard avionics equipment (OE); instrument container with OE, including a temperature-resistant cover that allows the LM to operate on Venus' surface for not less than 3 hr; the landing device with the damper, designed to absorb vibrations of the LM during atmospheric entry and to land on the surface; and a separable structure with a parachute system, designed to aerobrake the lander.

High resolution camera system TVS-VD (Table 6) will provide panoramic stereo imaging of the surface around the landing site with the best possible quality. The cameras will use a visible range color imaging system consisting of one landing, four to five panoramic and one close range cameras, providing detailed stereo imaging of the surface with the spatial resolution better than 0.2 mm. See also Sect. 7.3.2 - Lander science, and Fig. 24.

Harsh conditions at the surface of Venus require significant resources to extend the expected lifetime of spacecraft after landing. This lifetime should be sufficient to conduct all planned experiments and upload the obtained data to the Orbiter. As our analysis has shown, the optimum duration of the surface operations in this case is at least 2, maximum 3 hours.
Lander descent time in the atmosphere: ~50 min (from 125 km to the surface).
<figure><img src="image_25.png" /><figcaption>**Fig. 22** Venera-D Lander Module conceptual design</figcaption></figure>

Lander instrumentation will be operative during the descent and will collect both meteorological data and information about composition of atmospheric gasses and cloud aerosols. The ISKRA-V spectrometer (see Sect. 7.3.2, and Table 6) will study chemical composition of the atmosphere including abundances of gases SO₂, CO, COS, H₂O, NO₂, HCl, and HF, and their isotopologues and isotopic ratios D/H, ¹³C/¹²C, ¹⁸O/¹⁷O/¹⁶O, and ³⁴S/³³S/³²S during descent from 65 km and after landing; a suite of sensors (MTK-V) will determine temperature, pressure, wind speed, temperature gradient, acceleration from 120 km altitude to the surface and at the surface; at the surface, the package measures chemical composition of rocky sample (which must be delivered inside the lander) and continues measurements of chemical composition of the atmosphere: abundance and isotopic ratio of noble gases in the atmosphere (Sect. 7.3.2, and Table 6). General view of the Venera-D Lander is shown in Fig. 22.

An important event for the planning of the Venera-D mission was the International Landing Site Selection Workshop held in Moscow in October 2019. After intense debate, the collective wisdom of 43 participants prioritized terrain types based on scientific importance and landing safety (see also Ivanov et al. 2017a,b). The results of this analysis are presented in Fig. 23. Regional plains (rp1) were identified as the currently preferred terrain type, as they are considered representative (~30% of the Venus surface), morphologically uniform, and may represent a good sample of the fertile/depleted upper mantle. Last, but not least, is that the regional plains are relatively smooth, which increases the chances of a safe landing. The baseline mission scenario (Eismont et al. 2020), with direct DM insertion, offers a limited choice of accessible landing sites. Alternatively, landing can be achieved virtually anywhere, using gravity assist and resonant trajectories (Eismont et al. 2021a,b), at the cost of a longer transfer.

**Aerial Platform (AP).** - Another sub-system of the Venera-D descent module is an aerial platform, the design of which is currently less advanced than the other mission elements. The aerial platform (balloon) will have operational altitudes of 54-58 km and will offer very
<figure><img src="image_26.png" /><figcaption>Fig. 23 Potential landing sites for the Venera-D Lander</figcaption></figure>

significant opportunities to make long-term continuous measurements in the planetary atmosphere and in particular in the cloud layer. The preliminary estimate of the mass allocated to the scientific payload is about 30 kg and the set of instruments for the balloon is currently under study.

## 7.3 Venera-D Science Payload

### 7.3.1 Orbiter Science

The Orbital Module (OM) investigations aim to clarify the climate history of Venus, the mechanisms of water escape and the characteristics of the greenhouse effect. The potential suite of instruments under study includes 16 scientific instruments to perform studies of Venus atmosphere, ionosphere and magnetosphere, among which a dedicated suite of 7 instruments to analyse plasma spectra and electromagnetic field fluctuations (Table 5).

A set of monitoring cameras (VMC, LIR, IR2R) will allow to follow the dynamic trends of the Venus atmosphere and to improve the general atmospheric circulation models. The synergy of the long-wavelength IR camera (LIR), the Fourier IR thermal spectrometer (SVET) and the radiometer (MM-R) will provide an unprecedented 3D model of the thermal structure of the atmosphere. The heterodyne spectrometer IVOLGA will measure the mesosphere structure and winds, resolving CO₂ lines. UV and IR spectrometers (VOLNA, SVET, VENIS and VIKA) operating both in nadir and solar occultation modes will explore the Venus mesosphere, the cloud layer and possibly identify as yet unknown minor constituents. VIKA includes a nadir channel for the 1.05-1.65 µm range to access the atmosphere below the clouds.

The orbital experiments target to answer many outstanding questions of Venus climate evolution including the role of Solar wind direct interactions with its atmosphere. The OM instrument list is developed in Table 5.
<table><caption>**Table 5** Table of Venera-D Orbital Module (OM) instruments under study. Notes: (1) instrument provisionally provided by INAF/ASI; (2) instruments provisionally provided by ISAS/JAXA. A dedicated suite of 7 instruments for measurements of particles (neutral and charged) and electromagnetic fields</caption><thead><tr><th colspan="3">Venera-D Orbiter Module (OM)</th></tr><tr><th>Instrument</th><th>Description</th><th>Scientific goals</th></tr></thead><tbody><tr><td>SVET</td><td>Thermal IR Fourier transform spectrometer 5-40 µm (2000-250 cm-1, Δν=1 cm-1)</td><td>3-D thermal structure 55-100 km, SO2 and H2O and clouds composition 55-75 km, dynamics, thermal tides, thermal balance</td></tr><tr><td>VOLNA</td><td>UV mapping nadir and limb spectrometer (190-590 nm, Δλ=0.2-0.3 nm)</td><td>UV absorber, SO, SO2, cloud composition, dynamics, night glow emissions</td></tr><tr><td>VENIS (1)</td><td>IR grating spectrometer 2-5 µm and Imager, 2 channels, Δλ=13 nm</td><td>Atmospheric structure, composition, dynamics</td></tr><tr><td>VIKA</td><td>IR spectrometer suite 1.05-1.65 µm (λ/Δλ=20 000, nadir and occultations), 2.3-4.3 µm (λ/Δλ=40 000, occultations)</td><td>Atmospheric composition, thermal structure (vertical), aerosols</td></tr><tr><td>IVOLGA</td><td>Hi-res heterodyne spectrometer in 1.58 and 2.05 µm CO2 lines (solar occultation)</td><td>Dynamics above clouds, meso- and thermosphere structure up to 180 km</td></tr><tr><td>MM-radiometer</td><td>5-channel radiometer (90, 50, 30, 20, 10 GHz); scanning antennae</td><td>Thermal structure 0-50 km, and H2SO4, SO2 etc. below the clouds</td></tr><tr><td>VMC</td><td>4 coaxial monitoring cameras: 365, 513, 965, 1000 nm</td><td>Dynamics at day side at different altitudes in upper clouds, UV absorber distribution, aerosol, surface emission at 1 µm</td></tr><tr><td>LIR (2)</td><td>Thermal IR camera in 8-12 µm</td><td>Thermal structure and dynamics in upper cloud layer</td></tr><tr><td>IR2R (2)</td><td>IR camera in 1.74, 2.02, 2.26 and 2.35 µm</td><td>Dynamics and morphologies of middle-to-lower atmosphere, variations of CO</td></tr><tr><td>X-range radio occultation</td><td>Radio occultation atmospheric experiment</td><td>Thermal structure and dynamics of the atmosphere, structure of the ionosphere</td></tr><tr><td>Plasma & EM Field complex</td><td>7 instruments to analyze plasma spectra and EM field fluctuations</td><td>Magnetosphere, ionosphere, escape rate, D/H, lightning</td></tr><tr><td>ARIES-V</td><td>Wide angle ion spectrometer 10 eV-30 k</td><td>Composition and intensity of escape ions; ion capture by solar wind; ionospheric clouds; influence of solar activity on Venus plasma; transfer of momentum and energy from solar wind to ionosphere; interplanetary ion influx</td></tr><tr><td>ELSPEC</td><td>Electron spectrometer 10 eV-10 keV</td><td></td></tr><tr><td>NPD</td><td>Neutral particles detector 50 eV-3 keV</td><td></td></tr><tr><td>ASPECT-V</td><td>Ion and electron energy spectrometer</td><td>Magnetospheric structure; influence of solar activity on magnetosphere</td></tr><tr><td>BMSV-V</td><td>Plasma spectrometer 100 eV-5 keV</td><td>Monitoring of solar wind, its interaction with magnetosphere; turbulence of interplanetary medium at high frequencies</td></tr><tr><td>FM-V</td><td>Magnetometer ±1000 nT, up to 32 Hz</td><td>Interaction of solar wind with magnetosphere; non-thermal atmospheric dissipation; magnetic properties of ionosphere; lightning activity</td></tr><tr><td>GROZA</td><td>Radiowave analyzer 15-50 MHz</td><td>Detection of radiowave events from lightning activity</td></tr></tbody></table>

Operating orbit of the Venera-D orbiter was designed to satisfy interests and requirements expressed by the atmospheric and plasma science communities. A set of instruments (ARIES-V, ASPECT-V, ELSPEC) for charged particle measurements together with the neutral particle detector (NPD) will be capable to determine the effects of solar activity on Venus' atmosphere and ionosphere, characteristics of the loading of Solar wind plasma stream by planetary ions and parameters of multiscale ionospheric structures. Upstream information on SW parameters will be provided by the BMSV-V instrument and magnetic field variations, produced by various plasma interactions, will be analyzed by the FM-V
<table><caption>**Table 6** Table of Venera-D Lander Module (LM) instruments under study. Table of scientific payload (phase A stage). (*): S- instrument requires surface sample; A- instrument requires atmospheric sample; ¹- instrument provisionally provided by Germany</caption><thead><tr><th colspan="4">Venera-D Lander Module (LM)</th></tr><tr><th>Instrument</th><th>Description</th><th>Scientific goals</th><th>Sample (S = Surface; A = Atm.)</th></tr></thead><tbody><tr><td>TVS-VD</td><td>Panoramic (5), descent (1) and microscopic (1) cameras</td><td>Characterization of local landforms</td><td>None</td></tr><tr><td>Surface Sampling System</td><td>Drill and vacuum pump system to acquire and deliver surface samples to XRD/XRF, MIMOS II, APXS-V,LMS</td><td></td><td>None</td></tr><tr><td>XRD/XRF</td><td>X-Ray diffraction and fluorescence spectrometer</td><td>Elemental and mineral composition of surface materials; possible detection of bounded water in the surface minerals</td><td>S</td></tr><tr><td>MIMOS II (1)</td><td>Mössbauer spectrometer</td><td>Surface mineralogical composition, iron phases</td><td>S</td></tr><tr><td>APXS-V</td><td>Analysis of scattering of α-particles from Cm-224 radiotopic source on atmospheric molecules and surface material</td><td>Elemental composition of the atmosphere and surface</td><td>S</td></tr><tr><td>LMS</td><td>Laser mass spectrometer for surface samples</td><td>Surface elemental and isotope composition, local geological processes, surface-atmosphere interaction</td><td>S</td></tr><tr><td>AGNESSA</td><td>Active gamma and neutron spectrometer</td><td>Radioisotopes (K, U, Th), surface elemental composition to 0.5 m depth (Al, Mg, Fe, O, Na, Si, C)</td><td></td></tr><tr><td>VCS</td><td>Gas chromatograph & mass spectrometer for atmospheric samples</td><td>Vertical profile (70-0 km) of atmospheric composition, trace & noble gasses, aerosol composition</td><td>A</td></tr><tr><td>ISKRA-V</td><td>IR multi-channel tunable laser absorption spectrometer, resolution 10-3 cm-1</td><td>Vertical profile (70-0 km) of major atmospheric constituents [SO2 CO CO2 OCS H2O PH3 ] and their isotopes [C, O, S, D/H]</td><td>A</td></tr><tr><td>DAVUS</td><td>Descent UV spectrometer 250-400 nm (Δλ=0.15 nm) for in situ atmosphere analysis</td><td>Vertical structure of SO2, SO from 70 to 0 km; unknown UV absorbers; ClO; aerosol extinction in clouds/hazes</td><td>None</td></tr><tr><td>MTK-V</td><td>Suite of sensors for key atmospheric parameters</td><td>Vertical profile (70-0 km) of main meteo parameters</td><td>None</td></tr><tr><td>VERBA</td><td>Actinometric spectroradiometer in VIS and IR (0.4-1.1, 1-1.8, 2.4, 3.7, 6.2, 8.7 µm)</td><td>Thermal balance below clouds, optical properties of aerosol, profile of H2O</td><td>None</td></tr></tbody></table>

magnetometer. Venus is expected to display potential lightning activity (e.g., Lorenz 2018) which will be studied on board by the GROZA radio wave analyzer.

### 7.3.2 Lander Science

A list of Lander Module science instruments under study is presented in Table 6. The key element of this package is the surface sampling system that will drill the surface to a certain depth and then, using the vacuum pump device, deliver the acquired samples to four analytical instruments:

- X-ray diffraction and fluorescence spectrometer (XRD/XRF) and a Mössbauer spectrometer (MIMOS II) for structural analysis;

- laser mass spectrometer (LMS) and an active APXS-V alpha-particle experiment for elemental analysis.

Gamma and neutron spectrometer with neutron activation AGNESSA will measure the naturally radioactive elements (K, U, Th) and the main rock-forming elements (Al, Mg, Fe, O, Na, Si, Ca) in the activation mode down to ~0.5 m depth without requiring sampling.

Gas chromatograph (VCS) and Laser absorption spectrometer (ISKRA-V) will study atmospheric samples acquired during the Lander descent (~50 min) and at the surface, already mentioned in Sect. 7.2.3. These two instruments feature their own sample preparation system and will use the pre-vacuumized Lander as a dump volume. The UV-spectrometer (DAVUS) will continuously study the composition of the atmosphere during the descent down to ~10 km using an optical cell open to the atmosphere.
<figure><img src="image_27.png" /><figcaption>**Fig. 24** Concept of the Venera-D Lander Module (LM) imaging system (5 + 2 units): FOV: 90° × 90°, frame size: 2048 × 2048 pix, angular resolution-2.5', Linear resolution @ 2.5 m is 2 mm (see Table 6)</figcaption></figure>

The camera system (TVS-VD) of the Venera-D Lander will provide a series of synoptic images from the descent camera (resolution from a few tens to meters per pixel), complete 360° surface panoramas (Fig. 24). A close-up microscopic camera imaging with resolution of ~100 microns per pixel aims at observing the sampling spot at sub-millimeter scale to characterize its color, texture, grain sizes, traces of weathering, before and after its brushing/sawing/drilling.

## 7.4 Summary and Conclusions

The Venera-D mission is currently at the Phase A development stage and some mission parameters as well as characteristics of scientific instruments are to be clarified at the subsequent stages. The Aerial Platform will definitely be an important part of the Venera-D mission, currently its scientific payload as well as operation scenarios are being discussed. At the time of publication, the Venera-D launch is planned for the 2029 launch window, although the funding situation may result in a shift to the early part of the next decade.

The Venera-D science team looks forward to productive cooperation (including coordination between the different missions) with its VERITAS, DAVINCI and EnVision colleagues. Certainly, such cooperation could begin even earlier for the Indian Shukrayaan-1 mission to Venus (see Sect. 8), which will carry Russian instruments. The VeSCoor activity in development by ESA, NASA and other international partners could enhance the synergies between Venera-D and the other missions mentioned above.

## 8 Shukrayaan-1

Capitalizing on the successes of the Moon and Mars missions, with scientific payloads and instruments such as Synthetic Aperture Radar aboard Chandrayaan-1, 2, Mangalyaan-1 and numerous Earth orbiters, ISRO is considering to take a step towards exploring Venus (Antonita et al. 2022). ISRO has been a leader in the development of Synthetic Aperture Radar (SAR) instruments capable of polarimetric measurements. The hybrid-polarization mode in C-band SAR, RISAT-1 and full-polarimetric mode in Dual Frequency SAR in Chandrayaan-2 were implemented for the first time in any mission for observing Earth and Moon respectively.

The main goal of the planned Venus Orbiter Mission or Shukrayaan-1 is to undertake global mapping of the Venusian surface with a polarimetric SAR and to conduct the first penetration radar experiment to access shallow subsurface stratigraphy. The mission also plans to carry multiple instruments targeting the structure, composition, and dynamics of
the atmosphere, and for investigating solar-wind interaction with the Venusian ionosphere. The final schedule and the science payload of the mission will be announced following approval by the Union Government of India.

A non-exhaustive list of Shukrayaan-1 contributions to Venus' evolution study includes the global characterization of current geologic activity, insights to the past by assessing vertical structure and stratigraphy of geological units including buried lava flows, global distribution of craters at better spatial resolution and including buried craters. The mission promises long-term monitoring of the atmosphere and clouds, potentially documenting their response to volcanic events, and detailed characterization of the contemporary escape.

# 8.1 Shukrayaan-1 Science Objectives

## 8.1.1 Investigation of the Surface Processes and Shallow Subsurface Stratigraphy

The surface and subsurface observations by Shukrayaan-1 will help in understanding geologic and resurfacing history, aeolian features on the surface, impact processes including detection of buried impact craters, vertical structure and stratigraphy of geological units including active volcanic hotspots and lava flows. The proposed VSAR instrument will carry out full polarimetric SAR observations of the surface of Venus which will provide highly accurate estimates of dielectric permittivity (Fung et al. 2010) and surface feature classifications (Ainsworth et al. 2009; Xie et al. 2015).

The first science objective aims to answer several outstanding questions by characterizing the contemporary and past Venus. The global characterization of volcanic and tectonic landforms will help in understanding how active Venus is at present, and assess its current geologic activity. The global mapping at better surface resolution will deliver a refined distribution of impact craters, quantifying their floor, rim morphology, and the parabola deposits. Characterization of prominent features like tessera terrain, unique geologic landforms such as Coronae and anomalous radar-bright regions found from Magellan using polarimetric surface decomposition techniques will be done.

Potentially, active volcanic hotspots and lava flows will be detected via observations of thermal emission in the near-IR atmospheric windows and brightness temperature measurements by SAR instrument in the radiometer mode. Also, variations of emissivity can be used to assess broad-scale surface composition of tessera terrains.

Venus' past will be addressed by stratigraphy of geological units, potential detection of buried impact craters and larger impact basins, particularly within the older tessera terrains. The subsurface observations will allow estimation of buried lava flows thickness and volume at different times.

## 8.1.2 Studying the Structure, Composition and Dynamics of the Atmosphere

This goal includes cloud dynamics and morphology, variability of SO₂, studying the super-rotation, detection and understanding the role of lightning in the Venusian atmosphere.

By observing the clouds from UV to thermal IR, the mission will address the correlation of the cloud-top altitude with particles' microphysical properties, monitor the spatial and temporal variations of the planet's albedo, particularly in the UV-blue domain, use the cloud opacity to detect changes over various time scales. In the mesosphere, the aerosol will be characterized spectrally, in vertical and horizontal dimensions, including thin clouds and detached layers.
The SO₂ cycle, closely linked to primarily H₂SO₄ clouds and hazes, will be studied via IR occultation spectrometry and the cloud monitoring UV camera. The measurements will provide important insight into the ongoing chemical evolution, atmospheric dynamics, and as an indicator of possible geological activity.

The super-rotation, its long-term trends, the role of wave dynamics, associated planetary-scale cloud structures (the Y-shape) will be studied via the cloud features tracking in the UV and characterizing the 3D thermal structure and wind field using finely-resolved near-IR CO₂ lines.

Assessing the lightning in Venus' atmosphere is important to understand its rate, strength and spread as well as its possible role in the cloud or atmosphere chemistry. Can lightning be hazardous for atmospheric probes? The detection is planned by analyzing the Hz - kHz electromagnetic spectrum, using the magnetometer and visible imaging data.

### 8.1.3 Investigation of Solar Wind Interaction with Venusian Ionosphere

This goal includes studying ionospheric dynamics and plasma waves, interaction with the Solar wind, detection of oxygen airglow and understanding its role, to study the upper atmosphere/ionosphere dynamics. Specific questions to address within this goal are: What makes the Venus ionosphere (V-1 and V-3) layers so elusive? What causes ion-holes in the Venus nighttime ionosphere? The measurements are to decipher how the ionospheric parameters depend on the Solar cycle and Solar Zenith Angle (SZA), detect the occasional meteor layer, determine spatio-temporal variation of the neutral composition and density of the upper thermosphere and exosphere, detect the ionosphere plasma waves and characterize escape rates of the Venusian upper atmosphere.

Detecting and observing the oxygen green (557.7 nm) and red (630 nm) lines would help to assess the day-to-night transport in the upper atmosphere, detect signatures of the atmospheric gravity waves. It would probe the atmosphere and ionosphere response to solar disturbances, characterize the sustenance of the nightside ionosphere, and detect large-scale ionospheric structures.

The outstanding questions to address for the Solar wind interaction goal are the loss of the upper atmosphere/ionosphere, the role of different escape mechanisms, characterization of plasma boundaries, assessing the variability of magnetosphere domains and processes responsible for atmospheric sputtering using Energetic Neutral Atom (ENA) measurements.

### 8.1.4 Astrobiology

The goal of indirect detection of possible bio molecules and life forms in the cloud deck is recognized. The measurement available is the potential PH₃ detection using the IR solar occultation spectrometer.

## 8.2 Shukrayaan-1 Mission Overview

Shukrayaan-1 is a highly-sophisticated Venus orbiter carrying up to 100 kg of science instruments and delivering up to 500 W of electric power. Shukrayaan-I will be launched on either GSLV Mk II or GSLV Mk III. ISRO has backup launch dates in 2026 and 2028 should it miss the 2024 opportunity. At the time of this publication, ISRO has not yet received approval from the Indian government for the Venus mission and that, as a result, the mission may be delayed until 2031.
<table><caption>**Table 7** Key VSAR radar design and performance parameters</caption><thead><tr><th>Shukrayaan-1 VSAR Parameters</th><th>Value</th></tr></thead><tbody><tr><td>Instrument Parameters</td><td></td></tr><tr><td>Platform Altitude Range (km)</td><td>200-600</td></tr><tr><td>Frequency (GHz)</td><td>3.2</td></tr><tr><td>Antenna azimuth × elevation dimensions (m)</td><td>5.8 × 2</td></tr><tr><td>Atmospheric Losses (dB)</td><td>-1.6</td></tr><tr><td>Polarization modes</td><td>Tri-Pol (HH, VV, VH) &amp; Hybrid Pol</td></tr><tr><td>Mapping modes RFBW</td><td>7.5 MHz, 40 m with 15-20 km swath</td></tr><tr><td>Number of looks</td><td>6, 13</td></tr><tr><td>Radiometric resolution at 10 dB, SNR (dB)</td><td>1.2, 1.6</td></tr><tr><td>Peak power (W)</td><td>300</td></tr><tr><td>NES0 (dB)</td><td>≤ -25</td></tr><tr><td>Swath (km)</td><td>15-20</td></tr></tbody></table>

After the Earth-to-Venus cruise and the heliocentric phase, the spacecraft will perform trajectory maneuvers and final braking to reach intermediate Venus 500 × 60,000 km near-polar orbit. A lower orbit, suitable for SAR mapping, with the pericenter at 200 km and the apocenter at 500–600 km, will be achieved by aerobraking. The aerobraking phase will take 6–8 months with controlled pericenter altitude down to 130–140 km in the atmosphere.

For optimized science operations, the spacecraft features 2D orientable (0°–360°; ±90°) solar panels and a 2D gimbal high-gain antenna (Nigar 2022).

## 8.3 Shukrayaan-1 Science Payload

The Shukrayaan-1 orbiter will carry the VSAR instrument, for mapping the surface. Up to seventeen further potential science instruments have been shortlisted, including a subsurface sounder and a number of instruments targeting the atmosphere, clouds and the plasma environment (Nigar 2019).

### 8.3.1 VSAR on Shukrayaan-1

Global mapping by Magellan at resolution of 100–200 m was performed in single polarization (HH). Complete polarimetric data of selected sites are only available from earth-based radar experiments. The main instrument onboard, the S-Band Synthetic Aperture Radar (VSAR) targeting high-resolution (40 m), fully-polarimetric global mapping, would provide a major step in Venus surface studies, complementary to the VISAR and VenSAR operations and performance (Table 7). VSAR will be capable of polarimetric decomposition by retrieval of the complete scattering matrix (all four Stokes parameters). It is well known that the cross-pol channels (HV or VH) are more sensitive towards surface roughness and volumetric inhomogeneities within the medium and are not optimally used for dielectric constant estimation (Fung et al. 2010). Thus, co-polarization channels (HH&VV) will estimate surface dielectric properties with the highest accuracy.

Full polarimetric mode is challenging in terms of handling the radar ambiguities, power requirements and has double penalty in terms of radar coverage due to high data rate and reduced PRI. The radar ambiguities are controlled by partially processing azimuth bandwidth
against a trade-off of a number of looks (Shah and Seth 2022). The wide antenna area of 5.8 m × 2 m is optimized to reduce the transmit power requirement which aids in reducing the volume and thermal management requirements of the payload. A trade-off of having antenna width of 2 m is that maximum possible swath would be 15-20 km, which either way was restricted due to the doubled PRF in full pol mode. However due to the slow rotation of Venus consecutive orbit passes are 11 km apart at the equator and will ensure continuity in coverage.

The possibility of cycle-to-cycle interferometry is foreseen. In the radiometer mode, SAR will estimate the brightness temperature with an accuracy better than 2 K. The science objectives of Shukrayaan-1 significantly differ and complement those of EnVision and VERITAS, and therefore the overall mission operations and VSAR capabilities, operations and performances are different when compared to VISAR and VenSAR. For each of the three orbiter missions, payload instruments were chosen to meet the broad spectrum of measurement requirements needed to support mission science investigations. Combining the data from all three missions would provide us with deep scientific insights into the geology and evolution of Venus with topographic, polarimetric and global information available at a high resolution.

## 8.3.2 Subsurface/Surface IR Emissivity

A low frequency (9–30 MHz) radar VARTISS will provide direct measurement of subsurface features. It will provide a vertical resolution of 10–25 m down to a depth of ~1 km, in a similar technique as the subsurface radar (SRS) currently developed for the EnVision mission (see Sect. 6.3.2 and European Space Agency 2021). VARTISS utilizes three-wire 25-m dipole antennas. VSEAM (Surface Emissivity) instrument to provide planetary temperature and radiation emission data in the 1-µm atmospheric transparency window is also considered.

## 8.3.3 Atmosphere/Clouds Observations

The atmospheric package includes the Venus Thermal Camera (VTC), UV and visible camera (VCMC) for cloud Monitoring. Together with VSEAM, the imagers will provide cloud tracking at different altitude levels to measure the zonal and meridional wind fields, estimate the horizontal scale of the atmospheric waves.

The near-IR spectro-polarimeter (VASP) aims at cloud microphysics and measuring the cloud-top altitude.

The solar occultation package includes an occultation photometer (SPAV) in the near-IR and visible ranges targeting sub-micron aerosol vertical distribution in the upper clouds. Solar occultation IR spectrometers target detecting trace gasses and studying the mesosphere dynamics through vertical profiling of atmospheric composition and wind speed. Selected as two separate spectrometers, the high-resolution echelle (2.3–4.4 µm) Venus Infrared Atmospheric gas Linker (VIRAL) and a laser heterodyne (1.6 µm) IVOLGA, they are combined into a single two-channel Roscosmos-provided VIRAL. A very high-resolution IR spectrograph ($\lambda/\Delta\lambda > 20,000$), VIRAL will measure trace gases at the limb during solar occultations, improving performances of the VEx/SOIR spectrometer by a factor of 10. It will therefore be possible to gain 1 to 2 atmospheric scale heights in depth to explore the upper cloud region and contribute to study the trace species photochemistry at all latitudes at 6 AM and 6 PM local time.

The lightning sensor analyzing the Hz-kHz spectrum (LIVE) will detect lightning activity in coordination with VCMC camera and magnetometer observations.
### 8.3.4 Measurements Related to Solar Wind/Ionosphere Interactions

The measurements dedicated to Venus' plasma environment will target ionospheric parameters measurements, characterization of neutral and plasma particles, monitoring of radiation environment and solar X-rays. The planned instruments are VEDA (electron density) and RPA (Retarding Potential Analyser), to measure electron and ion density, composition and temperature, VISWAS for mass spectrometry of energetic neutral atoms and ions, VIPER to measure plasma-wave parameters using Langmuir probe, electric field analyzer and magnetometer.

To study the region connecting the neural and ionized atmosphere, a radio-occultation experiment RAVI with German participation and a high-throughput airglow imager NAVA to detect oxygen green and red lines are planned.

To characterize the environment and solar wind, two instruments, VREM to monitor the charged particles (100 keV–100 MeV) and SSXS spectrometer to measure Solar soft X-rays (1–15 keV) are foreseen.

Finally, VODEX, Venus Orbiter Dust Experiment, to detect interplanetary dust during the cruise phase and at Venus orbits, is being considered.

## 8.4 Summary and Conclusions

Solar system studies have seen a remarkable growth in the last few decades, due to advances in space technology, observational capabilities and computational technologies. This has enhanced our knowledge and understanding of the diversity of complex processes across the Solar system. It is quite interesting to find clues as to how the planetary systems might have originated and evolved, and how they are different and similar to each other. In this context, the first ever planet explored by humankind for the search of life in the solar system is Venus, our nearest neighbor. Despite many missions including flybys, orbiter and lander, large gaps remain today in our understanding of Venus on its formation, spin, surface evolution, runaway greenhouse phenomenon, super rotation of its atmosphere, its evolution and interaction with solar wind, etc. In this context an orbiter mission to Venus is being considered by ISRO, with the above science objectives, to further improve our understanding of resurfacing processes, neutral atmospheric and ionospheric processes, influence of Sun on Venus atmosphere and ionosphere in particular to produce highest resolution topography, to map sub-surface of Venus, to detect lightning and airglow with better techniques and improved resolutions.

# 9 Overview of Future Mission Concepts and Science Investigations
## Addressing Venus Evolution Through Time

In the following Sects. 10-12, we address future mission concepts and measurements that require further technology development 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; Limaye and Garvin 2023). Continued development of planetary radar technologies, while applicable to many planetary missions, is particularly important for Venus because of the opacity of Venus' cloudy atmosphere to optical imagers. We also discuss complementarities between Venus mission concepts that target specific outcomes
that reveal Venus evolution through time, as summarized in Sect. 2, and with non-Venus missions (e.g., exoplanet observatories).

Section 10, Future Mission Concepts and Measurements, addresses key areas and investigations at Venus not covered by the fleet of missions under development described in Sects. 4-8. They include:

**Future Surface Investigations.** - Since all previous probes have landed in basaltic plains, a new lander in the tesserae highlands, thought to represent the oldest terrains on Venus, is needed to perform a detailed, comprehensive analysis of Venusian surface materials to understand Venus' geologic history, tectonic style, mineralogy, and composition (Sect. 10.1). Meteorology and seismometry, on the other hand, require measurements over months or years. Rather than try to use silicon electronics with associated cooling and power systems, such long-duration lander measurements can be implemented using high- temperature electronics of silicon carbide, gallium nitride or other wide band-gap materials (Wilson et al. 2022). As well as their own scientific investigations, such long-term landers would serve as technology demonstrators developing technology in preparation for mobile surface exploration, which is a post-2050 Venus exploration goal. Note that the DAVINCI probe will touch-down in tesserae highlands providing ground-truth to enable future landings in complex ridged terrains.

**Aerial Platforms; Long-Term Atmospheric in-situ Measurements.** - Longer duration measurements in the cloud layer could be achieved by using balloons, whether these are constant- or variable-altitude balloons (Sect. 10.2). Balloons offer a lifetime of weeks in arguably the most habitable environment found outside Earth, where temperature is around 20 degrees C, pressure is 0.5 bar, and there is ample sunlight and liquid water in the clouds (albeit mixed with sulfuric acid). Such a mission would explore the coupled chemical, dynamical, and radiative processes at work in this critical part of the long-term evolution of the Venusian atmosphere, as well as further investigations of past and present habitability and astrobiological aspects of the cloud layers. As summarized in Sect. 2 (Gillmann et al. 2022, this collection; Avice et al. 2022, this collection; Westall et al. 2023, this collection) volatile elements have a strong influence on the evolutionary pathways of rocky bodies and are critical for understanding the evolution of the Solar System. Because Venus experienced a different volatile element history than Earth, it provides the only accessible example of an end state for habitable Earth-size planets.

**Future Orbital or Suborbital Investigations.** - Future orbital investigations include longer-term observations of variable phenomena such as the atmospheric signature of surface events (e.g., large Venusian quakes and their potential airglow signature) and the collection of data on the sources, propagation, and dissipation of gravity waves in the Venusian atmosphere, which will require high-frequency imaging of the entire disk (Sect. 10.3). In addition, various concepts for skimmer probes of the upper atmosphere, diving below the Venusian homopause, and returning samples to Earth have been proposed recently (Shibata et al. 2017; Sotin et al. 2018a). Extremely high RF bandwidth radars operating at S or L-bands, could achieve meter-scale polarimetric SAR imaging of key regions after VERITAS and EnVision, building off of planned L-band systems such as NASA-ISRO's NISAR mission and planned Mars-orbital L-band measurements (JAXA, CSA, ASI, and others).

Sections 11 and 12 address the importance of ground-based, Earth orbit-based, observations and laboratory and modeling activities in support of Venus' evolution through time. They include:

**Ground-Based and Space-Based Observatories.** - Ground-based observations, or observations from Earth orbit by space-based observatories, complement the space-based instrumentation at Venus by providing long temporal baselines of variable phenomena of interest
to the long-term evolution of Venus, such as ground-based radar images of the Venusian surface (Campbell et al. 2017), thermal infrared imaging spectroscopy to monitor SO₂ and H₂O variability possibly related to present-day volcanic activity (Encrenaz et al. 2019), millimeter and submillimeter wave observations, observations of key trace gases, or extended series of ground-based dynamical observations of wind speeds and gravity waves (generated by both convection and topography) that support Venusian general circulation models and present-day dynamical coupling between the lower atmosphere and the surface (Sect. 11.1). Section 11.2 focuses on space-based platforms and the synergies between Venus and exoplanetary observations such as TESS, JWST, CHEOPS, and PLATO, as well as next-generation ground-based facilities such as the ELT (Way et al. 2023, this collection).

**Laboratory and Modeling Efforts.** - Experimental facilities on Venus are critical to advancing our understanding of this planet in the coming decades (Glaze et al. 2018; Santos et al. 2021, see Sect. 12). Several scientific investigations that address the previously discussed scientific themes and knowledge gaps related to how and when Venus' evolutionary path diverged from Earth are enabled by experimental facilities. Section 12.1 highlights a selection of current facilities and experiments that are particularly relevant to understanding ancient Venus and the long-term evolution of the planet, such as geochemical experiments (Sect. 12.1.1), mission calibration libraries (Sect. 12.1.2), rheology experiments (Sect. 12.1.3). Section 12.2 focuses more on progress and future directions in modeling efforts, particularly in the rapidly expanding capabilities for characterizing the chemical and dynamical interactions between the surface and the atmosphere (which are key to understanding the processes driving the long-term evolution of the spin rate); radiative and radiative-convective models for determining the long-term climate evolution. Newly proposed laboratory capabilities in the USA and Europe (2023 and beyond) could facilitate studies that refine the upcoming observations to improve interpretations.

Finally, Sect. 13, Summary and Conclusions, summarizes the key findings of Sects. 3-12, highlighting which key questions about Venusian evolution through time will be answered convincingly by current and next-generation mission concepts and future investigations; how current limitations in our knowledge of Venus affect current and future exoplanetary science and modeling of the evolution of rocky, Earth-sized exoplanets; and how the study of Venus' evolutionary history informs its habitability history through time.

# 10 Future Mission Concepts and Measurements

The data acquired with the VERITAS, DAVINCI, and EnVision missions from the end of this decade, in addition to other missions concepts described in Sects. 7-8, will fundamentally improve our understanding of the planet's long term history, current activity and evolutionary path. Yet even with the discoveries that await those missions, compelling science questions will likely remain—such as those regarding the chemical and physical cycles of the atmosphere, the interactions between that atmosphere and the surface, and the make-up and structure of the planet itself. Although new missions may be proposed in the near-term using capabilities currently in development, investments in future technologies and new techniques will enable both long-term surface science and missions that take advantage of mobility in the surface, near-surface, and atmospheric environments (Fig. 25).

## 10.1 Future Surface Investigations

One of the fundamental measurements that is needed is the bulk elemental composition and mineralogy of the surface from key locations, especially tesserae. Tesserae are thought to be
<figure><img src="image_28.png" /><figcaption>**Fig. 25** The Venus *In Situ* Transfer and Analysis mission concept (VISTA) provides an opportunity to obtain measurements that cannot be obtained by a simple, short-term mission to Venus (Izenberg et al. 2023; see also Sect. 10.2.1). This illustration is provided as an example. Data acquired with the VERITAS, DAVINCI, and EnVision missions from the end of this decade will fundamentally improve our understanding of the planet's long-term history, current activity and evolutionary path (top left). Although new missions may be proposed in the near-term using capabilities currently in development, investments in future technologies and new techniques will enable both long-term surface science and missions that take advantage of mobility in the surface, near-surface, and atmospheric environments. Additional future Venus exploration vehicles may include: (1) orbital platforms such as geophysics orbiters for atmospheric remote sensing, and in-situ orbital sensing; aerial or cloud-level platforms; and (3) surface platforms including short-lived landers, long-lived landers, and, ultimately, mobile surface assets. credit: Keck Institute for Space Studies/Noam Izenberg/Chuck Carter</figcaption></figure>

older than the regional plains, and as such may retain evidence of an earlier epoch prior to volcanic resurfacing, including evidence of different climate and weathering (Gilmore et al. 2023, this collection). The ability to perform detailed, comprehensive analysis of Venus surface materials would be game changing. Of course, landers (Fig. 25) must cope with the incredibly harsh conditions of ~470 °C at the surface of Venus. The Venera landers used only thermal inertia and thermal insulation to keep a central electronics compartment cool that, together with limited radio relay capabilities, only allowed for operation times of order an hour (see e.g., Vorontsov et al. 2011). Modern landers using the same, brute-force thermal approach could last for several hours but certainly not more than, say, one Earth day. Even so, a huge amount of surface mineralogical and petrological science could be achieved in that time.
### 10.1.1 Surface Images and Mineralogy

The principal science payload of such a lander would include surface imagers and non-contact chemical and mineralogical sensors such as a gamma-ray spectrometer with neutron activation, capable of measuring elemental abundances of U, Th, K, Si, Fe, Al, Ca, Mg, Mn, Cl (Mitrofanov et al. 2010) and/or Raman/LIBS (Clegg et al. 2011). Although the extreme Venus surface temperature (740 K) and atmospheric pressure (93 atm) provide a challenging environment for surface geochemical and mineralogical investigations, laboratory Raman experiments have been conducted under supercritical CO₂ Venus surface conditions, involving single-mineral and mixed-mineral samples (Clegg et al. 2016). The inclusion of surface sample ingestion via a drill/grinder/scoop would enable additional analysis techniques such as mass spectroscopy and X-ray fluorescence (XRF) spectroscopy, but would equally require important, continued technology development and verification.

Gamma- and XRF-based spectroscopic measurements were acquired by the Venera and VeGa landers, but modern equivalents of these instruments would provide much improved measurement accuracy. Moreover, carrying out compositional analyses in a tessera region (none of which has been sampled before) would reveal whether these terrains are chemically distinct from the lava plains where previous analyses have been conducted (e.g., Gilmore et al. 2023, this collection).

Collecting imaging data from the surface is not straightforward (e.g., Garvin et al. 1984), as silicon-based detectors typically used in virtually all modern imagers will not work at Venus surface temperatures. For short-lived landers, one can use silicon imaging chips inside insulated housings (e.g., Kremic et al. 2020). For prolonged operation at the surface, however, imagers capable of sustained operations at high temperatures are needed. Such imaging capability would be essential not only for surface science but for eventual mobile platforms (e.g., rover missions).

### 10.1.2 Surface Geophysical and Meteorological Data

Long-lived landers would prove valuable in numerous other ways, as well. For instance, long-duration surface operations would allow acquiring essential seismological, heat flow, and meteorological data. But the technological barriers to such long-lived surface missions are twofold, and formidable: (1) the high-temperature environment of the Venus surface, which is too hot for silicon electronics; and (2) the lack of sunlight at the surface, making solar-based power systems difficult of even unviable. As has been discussed by Kremic et al. (2020), Kremic and Hunter (2021), and Wilson et al. (2022), recent advances in high-temperature electronics have made long-duration uncooled landers an exciting possibility that can be explored in the coming decades—but doing so requires continued investment in the development of the electronics, in their packaging, and in their environmental qualification under Venus conditions. Power for long-duration landers on the surface could come from primary molten salt batteries, for at least a first generation of such long-lived landers. Second-generation long-life landers could be powered by RTGs or even wind power—again, both of which would require technology development. Yet, long-lived landers would provide not only essential meteorological and geophysical measurements but also would serve as essential precursors for post-2050 surface missions including more capable seismometry stations but also for eventual surface rovers—a technology that we envisage as being tractable beyond 2050.

Of these measurements, meteorological measurements are the most straightforward to make—indeed, Soviet Venera & VeGa landers made measurements of pressure, temperature,
and even wind speed during their brief operation at the surface. However, long-duration op-
erations, i.e., those lasting for at least one Venus solar day (118 Earth days) would be needed
in order to understand the temporal evolution of surface winds. Recent modeling suggests
that the deep atmosphere of Venus, far from being still like the bottom of a pond, exhibits
complex diurnal patterns, mesoscale circulations, and slope winds, all driven by the small
day-night temperature difference estimated to be of order 1–2 K (Lebonnois et al. 2018;
Lefèvre 2022; Lefèvre et al. 2022). Investigating this deep atmosphere circulation from the
surface is important for understanding the evolution of Venus; to give two examples, deep
circulation governs the transport of magmatic volatiles (or volcanic ash) into the atmosphere
(Wilson et al. 2022, this collection) as well as the aeolian and sedimentation processes that
shape the surface of Venus (Carter et al. 2023, this collection).

Long-term meteorological measurements on the surface would require sensors capable of operating at ambient temperatures, such as those proposed by Kremic et al. (2020), Kremic and Hunter (2021). As well as the direct relevance to Venus evolution as described here, such meteorological measurement would provide vital information for the next generation of long-lived surface stations: the viability of wind power for surface stations depends, for example, on knowing the reliability of wind speed and direction. The viability of seismom-etry at the surface also requires characterization of atmospheric turbulence at the surface so that atmospheric perturbations can be removed from seismic data (Spiga et al. 2018).

10.1.3 Surface Heat Flow Measurements

Surface heat flow—that is, the energy flow from the interior to and across the lithosphere—is an important geophysical parameter that can be used to place constraints on estimates of the chemistry of the interior as well as models of the global tectonics (compare Rolf et al. 2022; Gillmann et al. 2022, this collection; and e.g., Breuer et al. 2022). If we assume that Venus formed with the same concentration of heat producing elements as Earth, and further that it has lost heat at a similar rate, an estimate can be made using the Earth’s value which is found to be 45–47 TW (Davies and Davies 2010; Jaupart et al. 2015). Given the mass ratio of Venus and Earth, the corresponding global value range for Venus is 37–38 TW or 82 mW/m². However, this scaling argument is overly simplified: numerous estimates of heat flow from surface deformation suggest that the global average is higher than this value; O’Rourke and Smrekar (2018) derived elastic lithosphere thicknesses from Magellan data at coronae to estimate heat flow values of up to 95 mW/m² or greater Ruiz (2007); and more recently, Maia and Wieczorek (2022) estimate several 100 mW/m² for tessera terrains at the time of formation. New elastic thickness estimates from flexural bending (Smrekar et al. 2022b) correlate well with global elastic thickness estimates from admittance (Anderson and Smrekar 2006). This correlation indicates that ~40% of Venus has high heat flow today, with more low-moderate values elsewhere. Very high values are consistent with active regions (Smrekar et al. 2022b). Since such estimates have significant errors, and require assumed or under-constrained parameters, in-situ heat flow would be a very valuable ground truth.

However, measuring the heat flow *in situ* remains a long-term challenge. On Earth and Mars, daily and seasonal temperature perturbations penetrate for meters into the near-surface and disturb the diurnal / seasonal thermal gradient from which the heat flow is usually calcu-lated (if, in addition, the thermal conductivity is measured, see Grott et al. 2021). On Venus, diurnal temperature variations are expected to be small in amplitude (of order 1 K: Lebon-nois et al. 2018) but even this small diurnal temperature difference can lead to heat flow disturbances of the order of 1 W/m², an order of magnitude larger than the ~100 mW/m² internal heat fluxes when assuming the thermal properties of basalt. The thermal skin depth
through which the diurnal perturbation decreases by 1/e can then still be a few meters, depending on the thermal diffusivity. At 60 mW/m², the temperature difference per unit value of thermal conductivity (W/m K) per m is 60 mK, and thus within the accessible range of space-qualified temperature sensors, assuming that such technology is otherwise inured to the harsh thermal environment at Venus' surface. Although there is a case for sediments on Venus (Carter et al. 2023, this collection), current radar data indicate rock surfaces over most of the planet.

Heat flow is typically measured in a borehole, as done on Earth; and as was done using holes drilled by Apollo astronauts on the Moon's landing sites. Technologies to install sensors at depth include drilling (e.g., Vago et al. 2017), or penetrators or hammering mechanisms such as on the Rosetta or InSight missions (Spohn et al. 2007, 2018, 2022); or pneumatically, as has been studied for the Moon (Zacny et al. 2013). However, drilling into basalt for constraining the heat flow value at Venus is very challenging. Alternatively, the temperature gradient and thermal conductivity could be measured with the “thermal blanket” technique used for heat flow studies at ocean bottoms (e.g., Johnson et al. 2010), which has been considered for Venus by Smrekar et al. (2014) and Kremic et al. (2020). This technique would see a low-thermal-conductivity blanket or plate, equipped with temperature sensors to measure the warming due to the heat flow from below, placed directly onto the ground. The device would need to be large enough to avoid edge effects from lateral heat conduction, and would be soft or flexible enough to smoothly adapt to uneven ground. On Earth, those blankets typically cover a square meter, and are particularly useful in rocky areas lacking a regolith coverage where a drill or penetrator could not be relied upon to work.

As a further alternative to measuring the thermal conductivity and temperature gradient a miniaturized heat flow probe has recently been proposed (Dominguez et al. 2020). It measures the heat flow across the cm-scale probe after thermal equilibrium is attained. The probe still needs to be emplaced below the thermal skin depth but that might be less demanding than installing a suit of temperature sensors.

### 10.1.4 Seismic Investigations

Seismology is the preeminent methodology for studying the structure and composition of a planet's interior. The behavior of seismic waves traveling through planetary bodies provides constraints on interior composition, compositional boundaries and transitions, and the state of interior materials (e.g., fluid versus solid, hot versus cold, porous or not). Seismology enables the study of several high-scientific priority questions of Venus. The similar size of Venus and Earth, their similar overall surface ages, and recent evidence of ongoing volcanic activity on Venus (Herrick and Hensley 2023) suggest that it should have a level of seismic activity comparable to Earth's, or at least much higher than the Moon or Mars. The nature of Venusian seismic activity, its level, and where earthquakes are occurring on Venus could enable us to distinguish between big-picture geodynamic hypotheses that predict differing versions of current geological activity. Constraints on models for ancient plate tectonics, which could be derived from future seismic measurements in combination with a more detailed exploration of surface features such as the tesserae and their compositions, would shed light into whether Venus had an early habitable period.

A workshop in 2014 (Stevenson et al. 2015) brought together several seismologists to discuss Venus seismicity. Some participants felt that the overall higher temperatures of the uppermost crust on Venus might make most fault movement aseismic, although the consensus at the workshop was that these elevated surface temperatures probably play a minimal role in affecting overall seismicity. The absence of water in the near surface of Venus could
also potentially affect the nature of seismicity, as could the higher surface air pressure, but at present these are mostly unstudied and poorly understand aspect of Venus seismicity.

Because of those high surface temperatures on Venus and the limited solar energy reaching the surface relative to the power needed to transmit data, the option of placing solar-powered seismometers on the surface for an extended period—as has been done on Mars and the Moon—is not currently a viable option for Venus. The longest that a surface lander with conventional electronics has lasted on the Venusian surface is about two hours. However, to accomplish even the most basic goal of determining seismicity levels requires the ability to operate over a period of at least several Earth days. Although nuclear power might be viable for either active cooling or simply as a long-lived power source, regulatory and cost considerations are such that they are not likely to be used for seismology on the Venus surface for the foreseeable future (Venus Exploration Analysis Group/VEXAG 2019).

Over the past several years, research and development of high-temperature electronics has advanced to the point where a seismometer that can operate under Venus ambient conditions using battery power has become technically feasible (Kremic et al. 2020). Even so, the constraints on operation of a first-generation seismometer on Venus will be severe. The battery will likely be capable of enabling the seismometer to operate for a period of a handful of months (Glass et al. 2020; Kremic et al. 2020), but transmission of data from the surface to a orbiting relay spacecraft will be power intensive, such that less than ten hours of data will likely be able to be transmitted from the surface. Because data transmission rate depends in part on transmitter power, it may be the case that the frequency and dynamic range of the instrument will be limited to less than its intrinsic capability. Furthermore, although limited computer memory is being developed for high-temperature devices, power consumption will be high, such that storing even small amounts of data for later transmission may not be possible for such first-generation long-endurance landers. A primary mitigation strategy will involve designing a low-memory algorithm that triggers transmission during earthquakes and avoids transmission during wind and other noise events (Tian et al. 2023).

## 10.2 Future Aerial Investigations

Venus' clouds represent an important exploration target for understanding Venus evolution, whether for characterizing past and present habitability (Westall et al. 2023, this collection), for placing bounds on tectonic and volcanic activity through infrasound measurements (Ghail et al. 2023, this collection), for searching for atmospheric signs of volcanic activity (Wilson et al. 2023, this collection), for conducting long-duration measurements of noble gas isotopic abundances (Avice et al. 2022, this collection), or for mapping felsic surface composition at higher resolution than is possible from orbit (Gilmore et al. 2023, this collection). Some of these goals can be addressed by descent probe missions: notably, the last three of the above five points will be addressed by DAVINCI (2029 launch) and Venera-D during their descent phases, as has been discussed in Sects. 5 and 7 above. The Venus Life Finder Mission "Morning Star" (Seager et al. 2021) considers a small entry probe that would use an ultraviolet autofluorescence backscatter nephelometer to characterize cloud particles and search for biosignatures as it passes through the clouds.

### 10.2.1 Aerial Platform Technologies and Operations

Descent probes offer an atmospheric investigation time of, at most, 10-100 of minutes. This short duration can be extended to weeks or months, and to a wide range of latitude, longitudes, altitudes and solar times, by using balloons. The deployment of two small balloons
at 55 km altitude, in the heart of the main convective cloud layer, was successfully demonstrated by the Soviet VeGa mission in 1985 (Sagdeev et al. 1986a,b; Linkin et al. 1986; Blamont et al. 1986; Preston et al. 1986). At this altitude, the ambient temperature is a comfortable 20 °C and the pressure is 0.5 atm. The main environmental hazard is the concentrated sulfuric acid that makes up the cloud particles (O'Rourke et al. 2023, this collection; Wilson et al. 2023, this collection); however, this environmental threat can be mitigated by choosing appropriate acid-resistant materials for external surfaces. Balloon-borne platforms at this altitude can take advantage of the fast, super-rotating winds that will carry the spacecraft all the way around the planet in a week or less (depending on latitude and altitude), negating the need for horizontal propulsion with motors. A cloud-level aerobot is an ideal platform for studying interlinked dynamical chemical and radiative cloud-level processes (Fig. 25). It also offers a thermally stable, long-lived platform from which measurements of noble gas abundances and isotopic ratios can be carefully carried out and repeated if necessary—in contrast to a descent probe, which offers one chance for making this measurement, in a rapidly changing thermal environment, at a single location, and at a single time.

By the use of a pumping system to alter a balloon's internal pressure, aerial platforms can explore a range of altitudes: (1) One key altitude range to target is the so-called 'habitable zone' of 54–58 km, corresponding approximately to temperatures of 0–40 °C. This region is not only of greatest astrobiological interest but is also at the heart of the 50–60 km convective cloud zone, and also offers the most benign conditions from the standpoint of safely operating a Venus aerial platform (see Sect. 10.2.2). (2) Operation above 60 km, in the convectively stable upper clouds, would be optimal for photochemical processes and identification of the UV absorber, but the low atmospheric density leads to a relatively small mass fraction for scientific payload. (3) Operation below the main cloud deck has been proposed by Japanese researchers with a primary goal of establishing wind fields below the clouds, but temperatures here exceed 100 °C. But operating an aerial platform between 52 and 62 km would enable detailed investigations of the physical and chemical cycles operating in both the convective cloud and the upper (convectively stable) clouds, as well as how volatiles (and even perhaps ash from volcanic eruptions) are transported to, and through, these parts of the atmosphere.

Balloon-borne aerial platforms could also plausibly be used to image the surface, if they are able to descend—or deploy an imaging system—to below the global cloud layer, at an altitude of about 38 km. This capability would be restricted to near IR imagery, since the atmosphere would be optically thick with respect to Rayleigh scattering at shorter wavelengths. Once again, this environment poses considerable thermal challenges to an aerial platform or instrument, and requires continued work to mature technologies capable of functioning at those sub-cloud altitudes. An intriguing means for revealing winds in the lower atmosphere is to use passive balloons, reflective to radio waves, which could be tracked by radar from orbit. It also bears noting that balloons are not the only type of aerial-based platform that could be utilized at Venus: studies of powered-flight vehicles such as Northrop Grumman's Venus Atmospheric Maneuverable Platform (VAMP) concept is one such example of a non-balloon-based Venus aerial platform (Lee et al. 2015; Warwick et al. 2017).

## 10.2.2 Aeronomy and Surface-Atmosphere Interactions

By being carried passively along with the prevailing winds at a given altitude, a balloon-borne instrument suite could traverse the entire planet longitudinally in less than one Earth week. There is, at present, no prospect for any remotely comparable level of mobility for a surface-based platform.
As discussed in Sect. 10.2.1, balloons are ideally suited for exploring Venus because they can operate at altitudes where pressures and temperatures are far more benign than at the surface. An airborne instrumental payload operating at ~55 km can take advantage of the fast super-rotating winds which will carry the balloon all the way around the planet in a week or less, depending on latitude and altitude (Limaye and Garvin 2023). A cloud-level balloon is an ideal platform for studying interlinked dynamical chemical and radiative cloud-level processes. It also offers a thermally stable long-lived platform from which measurements of noble gas abundances and isotopic ratios can be carefully carried out and repeated if necessary.

The mechanical couplings between the solid and atmosphere parts of the planet are sixty times better than on Earth (Garcia et al. 2005), with almost 6% of the energy of a quake radiated in the atmosphere (Lognonné et al. 2015). It has previously been hypothesized that ground motion on Venus could be detected and characterized using infrasonic waves (or infrasound, pressure waves with a frequency less than 20 Hz) generated by quakes and volcanic activity through coupling between the solid planet and the atmosphere. Infrasound is known to travel large distances from the originating event and could be characterized using barometers suspended from balloons at approximately 60-km altitude on Venus, where the temperature and pressure are more Earthlike, and much longer mission lifetimes compared to surface missions can be achieved. Work has already been undertaken to explore the prospect of recording acoustic infrasound with balloons that is generated by ground movements. For example, various low-altitude experiments have been conducted with pressure and accelerometer sensors using active sources (Krishnamoorthy et al. 2018, 2019; Garcia et al. 2021), demonstrating that pressure sensors and accelerometers are capable of detecting the acoustic waves generated by ground movements above the seismic source, but also those generated by seismic surface waves propagating below the balloon.

Further, it was demonstrated recently that pressure records in the Earth's stratosphere can record seismic surface waves from small- and large-magnitude quakes (Brissaud et al. 2021; Garcia et al. 2022). These studies clearly show that the dispersion of seismic surface waves can be observed in pressure records, and that key quake parameters (magnitude, distance, etc.) can be recovered. These data also show that the response of the balloon system to acoustic forcing has an imprint on the pressure records that should be better modeled and understood for application to Venus (Bowman and Krishnamoorthy 2021), as well as Earth and even other atmosphere-bearing worlds such as Mars and Titan. Besides the detection of seismic events, the measurements of acoustic waves in Venus atmosphere can also be used to investigate volcanic eruptions (Byrne and Krishnamoorthy 2020), bolide events, infrasound from interactions between wind and topography (Hupe 2018; Poler et al. 2020), and even potential thunder signals.

The discrete investigations described above could be combined on a single aerial platform, given that the middle atmosphere offers a unique vantage point for exploring both the surface and the upper atmosphere and its interaction with the space environment. Indeed, a combined focus on atmospheric chemistry and physics, aeronomy, and surface-atmosphere interactions forms the basis of Phantom, a mission concept under development for the NASA New Frontiers 5 competition (Byrne 2022).

## 10.3 Future Orbital or Sub-Orbital Investigations

### 10.3.1 High-Frequency Imaging of the Atmosphere and Thermosphere

Atmospheric response to geological and atmospheric events (e.g., volcano eruption, quake, storm) can be of short duration and require a large field of view and a high sampling rate
to be measured accurately. The VAMOS (Venus Airglow Measurements and Orbiter for Seismicity) mission concept was designed to cover this need (Didion et al. 2018; Sutin et al. 2018). External events created by solar energy injection in the Venus system also have a short duration. A mission capable of performing high-frequency imaging of the thermosphere would allow recovery of these events, which may play an important part in the atmospheric escape of Venus. In addition, the detection of meteor entry tracks from such high rates of imaging would allow us to infer the meteor flux in the inner Solar System, and the seeding of Venus by external sources. Finally, high-frequency imaging of the atmosphere/thermosphere is required to better understand the sources, propagation, and dissipation of gravity waves in the Venus atmosphere. These waves may be key to explaining the long-term evolution of Venus' rotation dynamics by providing a way to transfer mechanical energy from the solid surface to the atmosphere.

All these science objectives require an imaging of the planet full disk with a horizontal resolution on the order of 5–10 km and a sampling rate around 1 second. Due to the large amount of data generated, onboard data processing and selection must be implemented, possibly requiring machine-learning methods trained on the ground but applied onboard. Other orbital observations potentially involving atmospheric lidar remote sensing (e.g., as in CALIPSO/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations in Earth orbit: e.g., Winker et al. 2008, 2010) could add to these measurements with both spectroscopy but also in terms of vertical structure.

### 10.3.2 Collection and Return to Earth of an Atmospheric Sample

The elemental and isotopic compositions of noble gasses and stable isotopes in the atmosphere of Venus hold clues to the origin and evolution of the entire planet. Past space missions managed to get a first glance of the composition of volatile elements in the atmosphere, and planned investigations (DAVINCI, Venera-D) will greatly improve our knowledge (see this review, Sect. 2, Sect. 2.2.3; Sect. 5, Sect. 5.1; Sect. 7, Sect. 7.1; see also review by Avice et al. 2022, this collection). However, some rare isotopes of noble gasses are important for identifying the source of volatile elements to Venus but are extremely challenging to detect and measure (e.g., $^{78,80}$Kr or $^{124,126}$Xe). Measurements of other, more abundant isotopes are also greatly complicated by the presence of isobaric interferences (e.g., $CO_2^{++}$ ions interfering with $^{22}$Ne$^+$ ions). A direct way to draw up the inventory of volatile elements in the atmosphere of Venus would be to send a probe, skimming through the atmosphere of Venus below the homopause < 120 km (Mahieux et al. 2012; Sutin et al. 2018a,b; Rabinovitch et al. 2019) to collect atmospheric samples, before possibly returning them back to Earth (e.g., Shibata et al. 2017). Such samples would be measured with state-of-the-art instruments in laboratories on Earth, enabling us to characterize the composition of the Venus atmosphere to unprecedented accuracy and precision. Although such investigation will likely allow comparative planetology of Earth, Venus, and Mars on a similar basis, sampling and returning to Earth an atmospheric sample of Venus remains challenging. In the configuration of a skimmer probe, gas would be sampled at high velocity (> 10 km/s), behind a shock wave and the extent of modifications of the molecular, elemental, and isotopic composition of the atmospheric sample remains to be fully studied. After returning the sample to Earth, innovative curating techniques will have to be developed to preserve those invaluable samples for extended periods of time, in the manner of samples from the lunar Apollo program that continue to be assessed to this day.
### 10.3.3 Venus Lagrange Points Mission

A Venus Lagrange Points (LP) Mission, in which two light spacecraft using heritage, technological readiness & development of Akatsuki, Hayabusa-2 and Destiny+ spacecraft systems are placed in orbit around the Sun-Venus Lagrange points, is under consideration by the Japanese Space Agency's Institute of Space and Astronautical Science (ISAS/JAXA) as one of possible follow-ups of the Akatsuki Mission (Yamashiro et al. 2022). A Venus Lagrange Points Mission, able to continuously and simultaneously monitor the entire planet's atmospheric activity from a distance of about 1 million kilometers, is able to capture phenomena such as atmospheric dissipation and lightning on a global scale and continuous basis.

### 10.3.4 Cubesat Concepts

Several future projects involve deployment of cubesats in Venus' orbit to monitor atmospheric processes and their evolution: CUVE (CubeSat UV Experiment), a mapping spectrometer concept carrying a multi-spectral UV imager (320-570 nm) complemented by an imaging UV camera (see CUVIS on DAVINCI, Sect. 5.4.2) to study Venus cloud chemical, dynamical properties and radiative balance (Cottini et al. 2018) and TERACUBE, an instrument concept in the Terahertz frequency range. The high spectral resolution (e.g., 100 KHz) and sensitivity of such a heterodyne receiver will allow the spatial and temporal mapping of Doppler lineshifts winds, abundance of minor species (e.g., CO (5-4) at 576 GHz, H₂O 110-101 at 557 GHz) down to a few ppb, and atmospheric temperature, in the altitude range of 70-120 km (Moreno et al. 2020).

## 10.4 Future Directions

The technology for orbital, aerial, and short-duration surface missions to Venus exists today (Limaye and Garvin 2023). Continued work will naturally lead to ever longer-duration spacecraft, such that—with continued investment in and testing of a variety of technologies from materials to electronics to navigation software—we might reasonably see weeks-long surface operations and perhaps even years-long aerial platform missions in the coming decades.

For the foreseeable future, covering vast distances on Venus will be the exclusive purview of aerial platforms (excluding orbiters, of course); cloud-deck level balloons are to Venus as rovers are to Mars. But the scientific motivation for conducting *in situ*, detailed chemical and geophysical analyses at Venus is at least as compelling as it is at Mars. And so future technology development should take, as its long-term objective, the ability to move along the Venus surface itself (or relocated via hops)—that is, the design and flight of Venus rovers. Leveraging high-temperature electronics designed for long-life landers, rovers (Sect. 10.1.2-10.1.4) could be deployed to numerous terrain types including sites otherwise deemed unsafe for landers such as tesserae. This capability is because of the thick Venus atmosphere: using a delivery system similar in concept to the Skycrane powered descent vehicle that safely placed the Curiosity and Perseverance rovers onto Mars, a Venus rover delivery vehicle could take advantage of the planet's thick atmosphere to target high-standing lower-temperature (<715 K) terrains, in contrast to the Red Planet where low elevations and high surface atmospheric pressures are preferred for safe landing, to slow from hypersonic atmospheric entry speeds.

There are likely no technological showstoppers to exploring, at least with robots, the surface of the second planet, such as the Hybrid Automaton Rover-Venus (HAR-V) studied
at the Jet Propulsion Laboratory (Sauder et al. 2019, and references therein). But if we
are to see rovers traverse Venus' hostile surface later this century, the foundations for that
technology must be laid now.

# 11 Ground-Based and Space-Based Observatories

Ground-based observations of Venus are complementary to spacecraft observations in a number of fields, from geology to atmospheric composition and dynamics. Venus is distant by a fraction of an Astronomical Unit near its inferior conjunction every 584 terrestrial days (synodic period), and periodically very well suited to observation from Earth (or from near-Earth observatories in space).

The highest resolution ground-based radar images of Venus, from the Arecibo observatory, reached spatial resolutions of 1–2 km; while order of magnitude poorer than Magellan radar images (Campbell et al. 2017) the long temporal baseline offered by decades of observation allows a search for temporal changes on these timescales. In addition, ground-based images include polarimetric information not captured by Magellan allowing constraints on surface properties. Earth-based radar can also be used for monitoring of Venus’ spin (Margot et al. 2021, Sect. 11.1.1).

Ground-based observatory facilities and their instrumentation may obtain simultaneous measurements sampling a large range of altitudes, using wavelengths and/or spectral resolutions not available among spaceborne / onboard instruments – for studying the atmosphere, ground based telescopes have the advantage that they can be equipped sophisticated spectrometers far too massive and complex to deploy on a spacecraft; they can thus be sensitive to trace constituents, or faint motions of the atmosphere in a way which is complementary to spacecraft observations; and improve the latitude, longitude and local time coverage, and temporal baseline of rapidly variable phenomena. They can also provide monitoring of properties over decade-long periods of time, particularly times when no spacecraft were at Venus, and expand temporal baseline of rapidly variable phenomena (Lellouch et al. 2007; Lellouch and Witasse 2008; Widemann et al. 2008).

Sulfur dioxide and water vapor are key species in the complex photochemical cycles taking place in the troposphere and mesosphere of Venus, and have been extensively observed from Earth in the past decades (Mills et al. 2021, and references therein). They are also the most variable species in the atmosphere of Venus, and can be observed in millimeter wave (Sect. 11.1.2) or in the near-IR and mid-IR (Sect. 11.1.3). Both play a crucial role in determining climate on Venus, as key volatile species to constrain the rate and style of volcanic outgassing in the present era (see Sect. 2.4.3, Sects. 4.1.3-4.1.4, Sects. 5.3.1-5.3.3, Sects. 6.1.4, 6.3.3-6.3.4, and references therein). Amongst all trace constituents, SO₂ exhibits the most dramatic variations at Venus’ cloud top, both spatially and temporally (Esposito 1984; Esposito et al. 1988; Marcq et al. 2013, 2020; Vandaele et al. 2017a,b, Encrenaz et al. 2012, 2016, 2019, 2020a, 2023) and so require continuous observations and long temporal baselines to support our understanding of long term climate evolution. Ground-based observations will continue over the decades ahead as newly selected or proposed missions described in Sects. 3-8 prepare a new era of Venus exploration.

The past years have seen an extraordinary growth in our knowledge of planetary systems around other stars, as well as the remarkable diversity and abundance of exoplanets (Sect. 11.2). These advances have been supported by several ground-based and orbital facilities, such as TESS and JWST, and a progression from planet detection to detailed study and characterization of individual planets. The prevalence of Venus analogs will continue to
be relevant to Venus. Such observations capable of identifying key atmospheric abundances for terrestrial planets will face the challenge of distinguishing between possible Venus and Earth-like surface conditions.

# 11.1 Venus Observations from Ground-Based and Earth Orbiting Facilities

## 11.1.1 Earth-Based Radar Mapping and Speckle Tracking

Geologic mapping of volcanic features, their surface morphology and dielectric permittivity is a cornerstone of Magellan data interpretation (Campbell and Campbell 1992; Campbell 1994). Earth-based radar mapping can achieve 1–2 km spatial resolution and measure echoes in both the opposite-sense (OC) circular polarization and the same-sense circular (SC) mode. The OC echoes are very similar to Magellan measurements and are strongly modulated by slopes that face toward the radar. The SC echoes are much more sensitive to small-scale surface roughness than to topographic slopes. We can also form the circular polarization ratio (CPR = SC/OC), which allows for simple comparisons with rough surfaces on the Earth. The utility of these data was demonstrated in mapping of fine debris in the Venus highlands (Campbell and Rogers 1994; Whitten and Campbell 2016). Earth-based polarimetric mapping using the Arecibo radar shows that information on small-scale roughness correlates Venus lava flows with those in terrestrial settings (Campbell and Campbell 1992), and may reveal deposits formed during recent, volatile-rich eruptions (Campbell et al. 2017).

New methods for combining Earth-based hybrid astronomical radar polarimetry with refined Magellan radar measurements can extend the scientific value of existing datasets while we await the arrival of the next wave of orbital radar mapping missions, VERITAS and EnVision, as well as other radar missions considered for selection described in Sect. 3. Recent work by Garvin et al. (2022b, 2023 in prep.) demonstrated how multiple viewpoint Arecibo polarimetric radar data could be used via Shape-from-Shading together with re-analyzed Magellan radar altimeter radagrams to produce km-scale topography of Alpha Regio as a test-case. Improved methods building on these efforts could map regions on Venus in advance of the upcoming orbiters with advanced computational tools.

Earth-based radar can also be used to monitor Venus' spin state and moment of inertia (Margot et al. 2021). High-precision measurements of the instantaneous spin state of Venus may be obtained with a radar speckle tracking technique that requires two telescopes and does not involve imaging. Margot et al. used the 70 m antenna (DSS-14) at Goldstone, California (35.24°N, 116.89°E) and transmitted a circularly polarized, monochromatic signal at a frequency of 8560 MHz ($\lambda$ = 3.5 cm) and power of ~200-400 kW. Radar echoes were recorded at DSS-14 and also at the 100 m Green Bank Telescope (GBT) in West Virginia (38.24°N, -79.84°E) with fast sampling systems. Despite results not yet sufficient to rule out certain classes of interior models (Dumoulin et al. 2017), the best-fit value of the moment of inertia factor combined with knowledge of the bulk density enable a crude estimate of the size of the core of Venus (Margot et al. 2021). Improved determinations of the spin axis orientation, precession rate, and spin period form the basis of a recommended orientation model for Venus (see also Sects. 4.3.3 and 6.3.4).

## 11.1.2 Earth-Based Millimeter Wave Observations

Observing Venus at millimeter and submillimeter wavelengths with heterodyne spectroscopy provides unique means to probe the upper mesosphere of Venus (70-120 km). Heterodyne spectroscopy measurements have been performed with single dish antennas for
decades in the millimeter range for CO, HDO and H₂O (e.g., Encrenaz et al. 1991, 1995), and in the submillimeter range for CO, SO₂, SO, H₂O and HDO (e.g., Sandor et al. 2010, 2012) using the James Clerk Maxwell Telescope (JCMT) on Maunakea, Hawaii. Altitude resolution is derived from the shape of pressure-broadened spectroscopic lines and the exponential variation of pressure with altitude. This region is dynamically a transition region between the retrograde super-rotation characterizing the lower atmosphere and the day-to-night flow regime prevailing in the thermosphere. Rotational lines of minor species such as carbon monoxide CO and isotopic ¹³CO are formed at altitudes ranging from 70 km to 110 km, depending upon the strength of the transition used. Millimeter and Submillimeter observations complement the altitudes probed with ground-based IR observations, which investigate atmosphere levels within and below the clouds, in the millimeter-wave range. The Atacama Large Millimeter Array (ALMA) also offered a unique opportunity to probe the upper mesosphere (60–120 km) and monitoring minor species, winds and the thermal structure, targeting CO, SO, HDO and SO₂ transitions in the submillimeter range to derive 3D maps of mesospheric temperatures and minor species in the altitude range 70–105 km (Encrenaz et al. 2015; Piccialli et al. 2017).

### 11.1.3 Mid-IR, Near-IR and Visible Observations: Chemistry and Dynamics

**CO, CO₂, H₂O and SO₂ in the Mid-Infrared 4.3–19 µm Range.** - At thermal wavelengths (~4–50 µm), the spectrum of the planet is close to that of a blackbody at the cloud top temperatures with spectral features mainly belonging to mesospheric CO₂, H₂O, SO₂, and other gasses that absorb at levels within and above the clouds as well as broad signatures of sulfuric acid aerosols. Encrenaz et al. (2012, 2013, 2016, 2019, 2020a, 2023) have performed study of the SO₂ over nearly a decade, using the TEXES high-resolution imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF), also on Maunakea, Hawaii. Maps recorded around 1345 cm⁻¹ (7.4 microns, z = 62 km), where SO₂, CO₂ and HDO are observed, and around 530 cm⁻¹ (19 µm, z = 57 km) where SO₂ and CO₂ are observed, as well as around 1162 cm⁻¹ (8.6 µm, z = 66 km) where CO₂ lines are observed. Mixing ratios are estimated from HDO/CO₂ and SO₂/CO₂ line depth ratios, using weak neighboring transitions of comparable depths. An anti-correlation has been evidenced in the long-term variations of H₂O and SO₂ at the cloud top, a long-term decrease of H₂O associated with a long-term increase of SO₂, as well as a planetocentric distribution of the SO₂ volume mixing ratio enhancement between 120 and 200 East longitude at Venus (Encrenaz et al. 2020a). High-resolution spectroscopic observations of both day and night sides of Venus were also acquired using the CSHELL spectrometer at NASA IRTF between 4.53 and 4.54 µm, to investigate the effect of the decrease of SO₂ (from 2007 onward) at the cloud top level on the spatial distribution of CO, since both species are involved in the mesospheric photochemical cycles (Marcq et al. 2015).

**Near-Infrared Windows: H₂O, HCl, CO, OCS, SO₂ in the 1.18, 1.74, 2.32 and 2.46 µm Windows.** - Infrared windows at shorter wavelengths in the near-IR probe deeper regions in the atmosphere (Allen and Crawford 1984). Several key gasses can be mapped below the cloud deck, at 0-50 km altitude: water vapor (H₂O and HDO) (Bézard et al. 2009; Arney et al. 2014), sulfur compounds (SO₂, OCS) and carbon monoxide (CO) (Marcq et al. 2006, 2021; Iwagami et al. 2010; Arney et al. 2014) - all potential volcanic volatile gasses or involved in long-term surface-atmosphere exchanges. In particular, discovering spatial variability of the D/H ratio - whether associated with volcanic plumes or other fractionating processes - would be fundamental for understanding the history of the water on Venus. The atmosphere is known to be variable on a range of time scales from minutes to years, so
measurements over a wide range of timescales are still required. A plan to monitor Venus'
atmosphere using an Earth-orbiting cubesat, CLOVE (Chasing the Long-term Variability of
Our Nearest Neighbor Planet Venus), is currently under study by the Institute for Basic Sci-
ence (IBS) of South Korea to perform observations from Earth's orbit between 320 nm and
the near-infrared (Lee et al. 2022).

**Atmospheric Circulation in the Visible and Near-IR.** - The measurement of wind regimes in support of Venus General Circulation Models is achieved by two means: (1) directly, by Doppler or image correlation velocimetry using cloud features at different altitudes within the cloud layer, and (2) indirectly, using thermal and cyclostrophic wind balance relations to calculate equilibrium wind fields from measured temperature and pressure fields. In complement to cloud tracking in images taken at different wavelengths, which has proved an invaluable tool for extracting wind speeds at distinct vertical levels in the cloud region (Sánchez-Lavega et al. 2008, 2016; Hueso et al. 2012, 2015; Khatuntsev et al. 2013; Titov et al. 2018; Horinouchi et al. 2018; Fujisawa et al. 2022), wind speeds measured using Doppler-shifted spectroscopy, provided signal-to-noise limited precision of ~5 m/s using scattered solar Fraunhofer and CO₂ lines in the visible (dayside), sounding cloud tops (70 km) and a few kilometers above, using ESPaDOnS high-resolution spectrograph at Canada-France-Hawaii telescope (Widemann et al. 2007, 2008; Machado et al. 2012, 2014, 2017, 2021). Cloud-tracked winds may also be observed in the near-IR 2.26 µm window (Peralta et al. 2016, and references therein).

**Solar Transits of Venus in 2004 and 2012.** - A rare 2004 Venus transit imaging observing campaign with NASA's Transition Region and Coronal Explorer (TRACE) demonstrated the ability of Earth-orbiting observatories to constrain the properties of the upper atmosphere of Venus as a model for a transiting exoplanetary atmosphere (Ehrenreich et al. 2011; Pasachoff et al. 2011; Tanga et al. 2012). A follow-up ground-based campaign was organized in 2012 in coordination with Venus Express/SOIR and the HMI instrument aboard the Solar Dynamic Observatory (SDO) at the time of the 2012 transit to observe the refracted sunlight light curve as a probe of the thermal structure and composition of the upper atmosphere near the terminator (Widemann et al. 2012; Pere et al. 2016; Machado et al. 2023).

11.1.4 Ultraviolet Observations: Albedo Variations

Hubble Space Telescope Imaging Spectrograph (HST/STIS) UV observations of Venus' up-
per cloud tops have been used in coordination with VMC on board ESA's Venus Express,
JAXA's Akatsuki UVI images, and NASA MESSENGER/MASCS UV spectral data to mon-
itor the sulfur cycle and long-term UV albedo variations (Jessup et al. 2015; Lee et al. 2019).
Lee et al. discuss the decadal variation of Venus's 365 nm albedo between 2006 and 2017;
Solar EUV radiation might affect photochemical reactions involving SO₂ that are necessary
for aerosol formation on Venus (Mills et al. 2007; Parkinson et al. 2015). Further studies are
required to explore the role of the solar activity cycle on the Venusian upper atmosphere, as
many intervening factors that may act in combination to produce the observed albedo vari-
ations: the chemical composition and reaction rate of the unknown absorber, its interaction
with or dependency on the chemical state of other atmospheric constituents, such as sulfur
species SO and SO₂, and the variability of the cloud and haze structure as a function of time
(Lee et al. 2019).

11.2 Exoplanets Detection and Characterization

The planetary systems that have been discovered - as exemplified by the 7-planet system
around TRAPPIST-1 (Gillon et al. 2016, 2017) only about 12 pc away - are extremely di-
verse, and study of the demographics of large numbers of exoplanets has led to several
advances in understanding, including the recognition that many small-mass planets possess hydrogen atmospheres. Progress in observational technology will enable discovery of an even greater number of systems, and much expanded characterization of individual exoplanets by, e.g., the James Webb Space Telescope. Overall, the ability to image exoplanets both when they are forming and in their mature stage, the ability to characterize these exoplanets and their atmospheres, is providing us with new opportunities to understand planetary systems in the universe and to compare them with the long-term evolution of solar system planets.

Before the discovery of exoplanets, planet formation theories were limited to explaining the Solar system and thus, were unintentionally biased (e.g., Scora et al. 2020). Now, with thousands of extrasolar planetary systems, there is a diverse set of data to test against formation theories. Since planets at the inner edge of can sustain several very different possible atmospheres, depending on the initial water inventory and the water loss time-scales (see e.g., Turbet et al. 2020; Fauchez et al. 2022; Kaltenegger et al. 2023; Barrientos et al. 2023), exoplanetary observations of young planets around G-stars in the Venus Zone will be critical to understanding Venus' long term evolution through time. Determining that a planet resides in the Venus Zone provides a first-order estimate about the potential environment on that planet and criteria for its long term evolution.

Three next generation ground-based (>20 m in diameter) observatories are currently under construction or likely to be built in the near future: the European led Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT). The former two are currently under construction in Chile while the TMT is proposed for the northern hemisphere, although the exact location remains uncertain. With increasing advances in adaptive optics, they will afford new opportunities to explore the atmospheres of nearby exo-Venuses, as they are discovered by space observatories devoted to detecting such systems via the transit method (e.g., Kepler, TESS, CHEOPS, PLATO).

In space, JWST has already demonstrated how it can detect atmospheres around a few nearby terrestrial planets in systems such as Trappist-1 (Fig. 26), although such observations continue to be challenging, as discussed by (Way et al. 2023, this collection and references therein). The ARIEL mission (Tinetti et al. 2021), led by ESA, is also scheduled to be launched by the end of the decade with the ambition to measure the chemical fingerprints of ~1000 exoplanetary atmospheres.

# 12 Laboratory and Modeling Efforts

## 12.1 Laboratory Experiments and Measurements

There are many areas where new laboratory work is needed to support our understanding of the Venus system. Experimental facilities can replicate the pressure, temperature, and chemical conditions of various layers of the planet and their interfaces; they also can be used to develop, test, and prove technologies to explore Venus. Thus, Venus experimental facilities are critical to moving forward our understanding of this planet in the next decades (Glaze et al. 2018; Santos et al. 2021). In this section we highlight a small selection of current facilities and experiments which the authors consider particularly relevant to the theme of understanding ancient Venus and the long-term evolution of the planet.
<figure><img src="image_29.png" /><figcaption>**Fig. 26** The Trappist-1 system planets (Gillon et al. 2016, 2017) are among an abundance of Venus Zone planets which are promising candidates for follow-up ground- and space-based observatories, such as JWST and the TESS missions. Of these candidates, the TRAPPIST-1 planets in the Venus Zone are especially intriguing, and observational constraints on their atmospheres will provide an opportunity to compare the differences between Earth and Venus to planets receiving similar insolation flux (Way et al. 2023, this collection)</figcaption></figure>

### 12.1.1 Geochemical Experiments

Geochemical experiments focus on the stability of minerals in the Venus surface environment and the transfer of elements or isotopes that take place during mineral-mineral or mineral-gas chemical reactions. These studies are important to ancient Venus and long-term planetary evolution because the preservation of mineralogical evidence of ancient planetary processes depends on mineral stability/weathering over geologic time. Additionally, secondary minerals produced by reactions of the atmosphere with the surface of a planet are a major sink of atmospheric gasses and can therefore strongly impact its long-term climate. Many aspects of mineral stability can be assessed using thermodynamic calculations, however major outstanding questions remain in the field of reaction kinetics and mechanisms that are crucial to resolve to understand how mineral-gas reactions unfold over time. Experimental studies that provide relevant reaction kinetic information will also aid in constructing models of surface weathering that include factors such as weathering rates (Gilmore et al. 2017; Santos et al. 2021; Reid 2021; Gilmore et al. 2023, this collection). There are many experimental approaches that can be taken to address these questions, several of them available in typical high temperature lab setups, but here we will highlight one facility that can accommodate both geochemical experiments and exploration technology testing. The reader is referred to many of the other current Venus weathering studies, and references therein, to understand the breadth of approaches in this field (e.g., Berger et al. 2019; Port et al. 2020; Port and Chevrier 2020; Radoman-Shaw 2019; Reid 2021; Teffeteller 2020).

The Glenn Extreme Environments Rig (GEER), located at NASA's Glenn Research Center, is capable of reproducing Venusian temperature, pressure, and complex atmospheric chemistry (CO₂, N₂, traces of SO₂, H₂O, CO, OCS, HCl, HF, and H₂S) for long durations (see experiment description in Radoman-Shaw 2019). The GEER pressure vessel is an
~800 L cylinder made of 304 stainless steel, and this volume allows the accommodation of mission hardware for development and testing. Along with its size, one of the key capabilities of this facility is its precision gas mixing and control system that is capable of making compositional corrections at high pressure and temperature. The gas composition is monitored using an external gas chromatograph. The gas boosting capability is useful for mineral reaction experiments because it can keep the gas composition within a specified range, as opposed to a typical batch reactor where the gas composition is permanently changed by reaction with the samples. This facility has been used for mission and technology development (e.g., Neudeck et al. 2018), materials science investigations (e.g., Costa et al. 2018; Lukco et al. 2018, 2020), and mineral weathering experiments (Radoman-Shaw 2019; Santos et al. 2023).

### 12.1.2 Mission Calibration Libraries: Planetary Spectroscopy Laboratory (PSL)

The unique environmental conditions on Venus can interfere with some of our traditional remote sensing techniques, for example, the thick atmosphere prevents visible light imaging or mapping of the planet's surface from orbit. Venus-specific exploration methodologies have to be developed and tested as a result. It was determined from data returned by the Galileo mission that there are windows in the CO₂ spectrum around 1 µm that allow us to see surface near-IR emissivity (Carlson et al. 1991), however spectral libraries built from analyses of a large number of geologic materials need to be developed to maximize the return on this type of data (Hashimoto et al. 2008; Mueller et al. 2008). In response to this challenge, the Planetary Spectroscopy Laboratory (PSL) at the German Aerospace Center (DLR) has designed a chamber to demonstrate and calibrate near-IR spectroscopy for Venus (Helbert et al. 2016, 2018). This data can be used as a reference to compare with surface emissivity spectra obtained by a future Venus orbiter. Furthermore, the emissivity chamber has an near-IR transparent window allowing mounting of near-IR spectrometers built for future Venus orbiter missions to take measurements at Venus conditions, for instrument calibration and performance study. A number of sample preparation and analysis tools and experiment sub-systems are available to the facility: a collection of hundreds of rocks and minerals, synthetic minerals, an Apollo 16 lunar sample, several meteorites, set of sample holders for reflectance (plastic, aluminum or stainless steel), various sets of sieves, grinders, mortars, saw, scales, microscope, an oven (20° to 300 °C), ultra-pure water, wet chemistry materials, a second ovens (30° to 3000 °C) for sample treatments, a press to produce pellets (10-mm or 20-mm diameter), purge gas generator for water and CO₂ free air, liquid-nitrogen tank, an ultrasonic cleaning unit and 2 microscopes. When enough sample material is available, the typical grain size separates that are produced for spectral measurements are <25 µm, 25-63 µm, 63-125 µm, 125-250 µm. Larger separates as well as slabs are also routinely measured. Models of anhydrite and hematite coatings on basalt mixtures suggest that changes in emissivity spectra due to chemical weathering can result in shifts in total emissivity, usefully constrain rock types and surface composition based on transition metal contents, but also provide local scale assessments of fresh versus mature lava flows (Dyar et al. 2021).

New High-temperature Dielectric Permittivity Laboratory Measurements relevant to future Venus radar mapping are underway at NASA's JPL (Barmatz et al. 2022) to further extend the value of SAR-based observations and enhance retrievals of surface electrical properties for the upcoming era of Venus radar mapping by VERITAS and EnVision. Measuring the complex dielectric permittivity of Venus analogue rocks and fines, as well as their intrinsic dielectric anisotropy, is important as new radars and radar sounders measure Venus
from orbit, and eventually from cloud-deck altitudes (e.g., with balloon-born micro-SAR instruments).

### 12.1.3 Rheology Experiments: Plume-Induced Subduction on Venus

Venus relatively young surface points towards either a quite recent catastrophic renewal of the whole planet surface, or the continuous renewal of small areas. Geophysical observations suggest that the large coronae are due to the impact of hot plumes rising in the Venusian mantle. Sandwell and Schubert (1992a) had proposed that these plumes could induce subduction, but this hypothesis had remained in the schematic state since then. One challenge for evaluating the plume-induced subduction mechanism is the difficulty of simulating the brittle viscosity, and history-dependent lithospheric rheology in three-dimensional (3D) numerical models, which still cannot fully model deformation on a wide range of scales. Laboratory experiments using complex rheology fluids such as colloidal dispersions provide a means to bridge this gap (Davaille et al. 2017).

A more detailed analysis of Magellan data (radar, topography and gravimetry), where the resolution is sufficient, confirms the existence of the plume-subduction association on the largest corona, Artemis (2300 km diameter), and on Quetzalpetlatl (800 km diameter). In both cases, the proven subduction does not describe a complete circle but only an arc. Furthermore, high emissivities have been measured on Quetzalpetlatl (Smrekar et al. 2010a), and interpreted as a signature of recent volcanism. This suggests that the plume beneath Quetzalpetlatl is still hot and active, which implies that the subduction around it must also be currently active. Larger and probably at a later stage of its evolution, Artemis also shows a large oceanic ridge-like structure inside the corona, where new plate is created by upwelling of hot magma. Different corona structures may represent not only different styles of plume-lithosphere interactions but also different stages in evolution (Smrekar and Stofan 1997; O'Rourke and Smrekar 2018; Smrekar et al. 2018; Gülcher et al. 2020).

Laboratory experiment mechanisms predict the asymmetric, arcuate trenches, and the extensional fractures that radiate outward from the trench, observed at Artemis and Quetzalpetlatl coronae, as well as at other coronae on Venus. Davaille et al. (2017) compare laboratory experiments of plume-induced subduction in a colloidal solution of nanoparticles to observations of proposed subduction sites on Venus. The experimental fluids are heated from below to produce upwelling plumes, which in turn produce tensile fractures in the lithosphere-like skin that forms on the upper surface. Plume material upwells through the fractures and spreads above the skin, analogous to volcanic flooding, and leads to bending and eventual subduction of the skin along arcuate segments. In this unique experiment, the tank is dried from above and uniformly heated from below, allowing for the development of both a gravitationally unstable skin, the experimental 'lithosphere', and several hot upwelling plumes below this skin (Fig. 27). Both processes are due to convection, either solutal or thermal, respectively. In both cases, the intensity of convection is in the range of a planetary mantle. The laboratory experiment mechanisms predict the asymmetric, arcuate trenches, and the extensional fractures that radiate outward from the trench, observed at Artemis and Quetzalpetlatl coronae, as well as at other coronae on Venus. The gravity data are also consistent with the thickness, lengths and dips of those observed in experiments.

Further laboratory experiments are needed to bridge the gap between interior evolution models and surface observations of deformation structures, topography, and volcanism.
<figure><img src="image_30.png" /><figcaption>**Fig. 27** Experimental facilities can replicate the pressure, temperature, and chemical conditions of different layers of the planet and their interfaces. Here, a laboratory experiment uses an aqueous colloidal dispersion dried from above and heated from below to simulate the plume-induced subduction mechanism in the Venusian mantle (Davaille et al. 2017; see Sect. 12.1.3)</figcaption></figure>

## 12.2 Numerical Modeling

Modeling studies strongly suggest that the evolution of the atmosphere and interior of Venus are coupled (Way and Del Genio 2020; Weller and Kiefer 2020), emphasizing the need to study the atmosphere, surface, and interior of Venus as a system.

The interaction between the surface and the atmosphere is a key to understanding the processes driving the dynamics of both the atmosphere and the solid planet. The exchanges of heat and angular momentum drive the temperature and wind structure in the deepest layers of Venus's atmosphere. Chemical and dynamical modeling of interactions between the lower atmosphere and the surface, at the inner edge of the habitable zone, must take into account the variety of properties, trace atmospheric compounds and their reaction rates, local circulation and energy balance at the surface (e.g., Leconte et al. 2013; Lebonnois et al. 2018).

1-D radiative and radiative-convective models for the determination of climate are suddenly widespread as researchers worldwide attempt to determine the likely climate of exoplanets (Turbet et al. 2019, 2020; Fauchez et al. 2019, 2022; Way and Del Genio 2020; Bower et al. 2022; Wolf et al. 2019, 2022; Kaltenegger et al. 2023; Barrientos et al. 2023; Way et al. 2023, this collection, and references therein). Venus offers a proving ground for these models much closer to home, one where the conditions are much better known than on exoplanets. Radiative transfer calculations on Venus are difficult: uncertainties in the radiative transfer properties of carbon dioxide at high temperatures and pressures are the main unknown, particularly in the middle- and far- infrared where there are no spectral window regions to allow empirical correction. As on Earth, clouds play an important role, reflecting away sunlight but also trapping upwelling infrared radiation. The state-of-the art Venus radiative balance are still mainly 1-D models representing an average over the whole planet. However, we now know that the clouds are very variable; the vertically integrated optical thickness (as measured at 0.63 µm) can vary by up to 100% (Barstow et al. 2012) and the vertical structure of clouds varies strongly with latitude. In-situ measurements of cloud properties with co-located radiative flux measurements are also needed to determine the diversity of cloud effects on the global radiative balance.

## 12.3 Summary / Outcomes Revealing Venus Evolution

Future geochemical modeling and experiments will greatly benefit from better constraints on near-surface atmospheric composition and the composition and mineralogy of the solid
surface. In fact, the composition of the solid planet (in terms of chemistry, mineralogy, and isotopic values) is a major knowledge gap that is relevant to almost all areas of Venus science. Obtaining better data in this area will also enable other kinds of experiments to be conducted, such as those in the field of experimental petrology, which have provided significant insight into the evolution of other planetary bodies, but cannot be as rigorously applied to Venus due to the current state of our petrologic data from the planet.

## 13 Summary and Conclusions

The discoveries of many exoplanets, including terrestrial exoplanets perhaps similar to Venus, due to increasingly sensitive methods of discovery and characterization, make exchange between exoplanetary and planetary scientific communities increasingly necessary. The search for exoplanets is largely motivated by the answers to the questions: Is our solar system common and is there life outside our solar system? Answering these questions requires also understanding the habitability of a planet, i.e., the potential of a planet to develop and maintain a living environment. Venus and Earth formed under very similar conditions and were probably supplied with water in the same way. At some point in their history, the evolution of their surfaces and atmospheres diverged dramatically. Venus could be the type of planet that has changed from a habitable and Earth-like state to an uninhabitable one (Way and Del Genio 2020).

Thus, Venus is particularly important to our understanding of terrestrial planets' habitability, providing a natural laboratory to understand its evolution in time. Venus exploration offers therefore unique opportunities to answer fundamental questions about the evolution of terrestrial planets and the habitability within our own solar system. Venus' enhanced D/H ratio suggests that it has lost large amounts (possibly several terrestrial oceans) of water, but it is not clear whether it condensed (as happened on Earth) or whether this water was lost in the steam atmosphere phase; if it had a liquid water ocean phase, Venus may have been habitable for billions of years. There is no consensus on how much water there is in Venus' interior, and how much of this water has been outgassed, a question which has important implications for Venus' atmospheric water and in turn for its habitability through time. Exoplanet transit detection surveys have a bias to detecting exoplanets close to their parent stars: the growing number of such Venus-like exoplanets discoveries emphasizes the relevance of Venus in the search for habitable exoplanets.

Venus has been an object of fascination throughout the space age. It was the site of the first planetary flyby in 1962 (Mariner-2), first entry probe in 1967 (Venera-4), first soft landing in Dec. 1970 (Venera-7), first image from the surface of another planet in 1975 (Venera-9), first orbiter and radar in 1978 (Pioneer). The Soviet series of Venera & VeGa missions were phenomenally successful, not only in their technologically advanced landers which returned colour pictures from Venus and analyzed drill samples despite 450 °C heat, but also successfully deployed balloons in the atmosphere in 1985 (Sagdeev et al. 1986a, 1986b; Linkin et al. 1986; Blamont et al. 1986; Pieters et al. 1986).

Now is a pivotal time in Venus exploration. Since NASA-JPL's Magellan orbiter provided initial global radar imaging and altimetry (1989–1994), and USSR's Venera landers measured major and heat-producing elements in several locations (1975–1985), there have been considerable advances relevant to understanding Venus' evolution. ESA's Venus Express (2006-2014) and JAXA's Akatsuki (2010-present) orbiters focused primarily on atmospheric science. Both revealed many secrets of Venus' atmosphere, but have also left many
questions unanswered. There have been new ground-based observations of surface and atmospheric properties e.g., from Arecibo and NASA's Infrared Telescope Facility; new analyses of existing data, in particular surface emissivity from Venus Express; new hypotheses for the origin of plate tectonics; advances in the numerical tools and laboratory simulation of interior convection; and new modeling of the evolution of rocky earth-sized exoplanets, in need of observational constraint using Venus as a reference point. The difficulties in modeling the atmospheric superrotation of Venus meet new developments as the atmospheres of typical tidally locked terrestrial exoplanets are expected to superrotate (Imamura et al. 2020, and references therein).

Many important questions about the current state of Venus remain unanswered, suggesting that there are major gaps in our understanding of how and when Venus's evolutionary pathway diverged from Earth's (Morrison and Hinners 1983). As we developed in this final review, a new fleet of Venus missions is currently in development. These include radar-equipped orbiters (such as the ESA-led EnVision M5 orbiter and NASA-JPL's VERITAS orbiter missions), entry probes/landers/flybys (such as NASA GSFC's DAVINCI mission). Further Venus missions are also considered with Russia's Venera-D orbiter, aerial platform and lander mission and India's ISRO/Shukrayaan-1 orbiter mission. Japan and China have also announced a likely orbiter proposed for launch before the end of this decade. Furthermore, various concepts to detect seismic activity, whether from landers, from balloons or from orbit are also under study (Limaye and Garvin 2023).

The science strategy for all of these missions is in development now and in the coming few years; therefore, now is an ideal time to collate knowledge of Venus evolution scenarios and the observations needed to distinguish between them. Sects. 4-6 captured the considerable advances that the three newly selected missions VERITAS (Sect. 4), DAVINCI (Sect. 5) and EnVision (Sect. 6) will bring to these science goals. These rich, highly synergistic datasets will provide an invaluable resource to assess the long-term history, stability of water reservoirs in the mantle and atmosphere, current levels of activity, divergent pathways and evolution toward habitability. Together they will reveal whether Venus-like and Earth-like planets can potentially transition into one another over time, which would imply that Earth-like exoplanets may be common among Earth-sized exoplanets.

List of Acronyms and Glossary

<table><tr><td>ABX</td><td>Aerobraking Exit Maneuver</td></tr><tr><td>ADC</td><td>Analog to Digital Converter</td></tr><tr><td>AFN</td><td>Autofluorescing Nephelometer (Venus Life Finder Science Payload)</td></tr><tr><td>AP</td><td>Aerial Platform (Venera-D mission)</td></tr><tr><td>AU</td><td>Astronomical Unit</td></tr><tr><td>CC</td><td>Carbonaceous</td></tr><tr><td>CLOVE</td><td>Chasing the Long-term Variability of Our Nearest Neighbor Planet Venus, an Institute for Basic Science (IBS, South Korea) cubesat project to perform observations from 320 nm to the near-infrared</td></tr><tr><td>CNES</td><td>Centre National d'Études Spatiales</td></tr><tr><td>CPR</td><td>Circular Polarization Ratio</td></tr><tr><td>CRIS</td><td>Carrier Relay Imaging Spacecraft (DAVINCI mission)</td></tr><tr><td>CUVIS</td><td>Compact Ultraviolet Imaging System (DAVINCI science payload)</td></tr><tr><td>D/H</td><td>Deuterium to Hydrogen isotopic ratio</td></tr><tr><td>DAVINCI</td><td>Deep Atmosphere Venus Investigation of Noble Gases, Chemistry, and Imaging</td></tr></table>
<table><tr><td>DM</td><td>Descent Module (Venera-D mission)</td></tr><tr><td>DS</td><td>Descent sphere (DAVINCI mission)</td></tr><tr><td>DSN</td><td>Deep Space Network</td></tr><tr><td>DV</td><td>Delta Velocity</td></tr><tr><td>ECM</td><td>Eccentricity Control Maneuver</td></tr><tr><td>EDL</td><td>Entry, Descent and Landing</td></tr><tr><td>ELT</td><td>39-m Extremely Large Telescope, European Southern Observatory</td></tr><tr><td>EM</td><td>Electromagnetic</td></tr><tr><td>ENA</td><td>Energetic Neutral Atom</td></tr><tr><td>Eotvos</td><td>A non-SI unit of acceleration divided by distance;<br/>1 Eotvos (E) = 10<sup>-9</sup> galileos cm<sup>-1</sup>; in SI, 1 E = 10<sup>-9</sup> s<sup>-2</sup>; after Loránd Eötvös (1848-1919)</td></tr><tr><td>ESA</td><td>European Space Agency</td></tr><tr><td>ESI</td><td>Engineering Science Investigation (DAVINCI)</td></tr><tr><td>Ga</td><td>Gigayear, one billion years</td></tr><tr><td>GBT</td><td>Green Bank Telescope, West Virginia</td></tr><tr><td>GCM</td><td>General Circulation Model</td></tr><tr><td>GMT</td><td>Greenwich Mean Time</td></tr><tr><td>GMT</td><td>25-m Giant Magellan Telescope, Las Campanas Observatory, Chile</td></tr><tr><td>GRAIL</td><td>Gravity Recovery and Interior Laboratory</td></tr><tr><td>GROZA</td><td>Radio Wave Analyzer 15-50 Mhz (Venera-D Science Payload)</td></tr><tr><td>GSFC</td><td>Goddard Space Flight Center</td></tr><tr><td>GSLV</td><td>Geosynchronous Satellite Launch Vehicle</td></tr><tr><td>Hadean</td><td>Geologic eon extending -4.6 to -4 Ga preceding earliest known minerals on Earth</td></tr><tr><td>HF</td><td>High Frequency, a range of radio frequencies extending from 3 MHz to 30 MHz i.e., from 10 to 100 m in wavelength.</td></tr><tr><td>HGA</td><td>High Gain Antenna</td></tr><tr><td>HH and HV</td><td>Horizontal and Vertical Polarization (conventional imaging radar systems)</td></tr><tr><td>HZ</td><td>Habitable Zone, a range of distances around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure, and thus provide conditions for the emergence of life, or its precursors. By extension, range of altitudes within the atmosphere or the liquid layers of a planetary or natural satellite interior with similar properties.</td></tr><tr><td>INSAR</td><td>Interferometric Synthetic Aperture Radar</td></tr><tr><td>ISRO</td><td>Indian Space Research Organisation</td></tr><tr><td>IVOLGA</td><td>Infrared Heterodyne Fiber Analyzer/Spectrometer (Venera-D Science Payload)</td></tr><tr><td>JAXA</td><td>Japan Aerospace Exploration Agency</td></tr><tr><td>JCMT</td><td>James Clerk Maxwell Telescope, Maunakea, Hawaii, a ground-based millimeter-submillimeter wave telescope facility</td></tr><tr><td>JPL</td><td>Jet Propulsion Laboratory</td></tr><tr><td>JSDT</td><td>Venera-D Joint Science Definition Team</td></tr><tr><td>k<sub>2</sub></td><td>Tidal Love number, gravitational potential modification due to the tidal deformation of a planet. After Augustus E. H. Love (1863-1840)</td></tr></table>
<table>
  <tr>
    <td>Ka-band</td>
    <td>a nominal frequency range, from 26 to 40 GHz (0.8-1.1 cm in wavelength) within the microwave portion of the electromagnetic spectrum</td>
  </tr>
  <tr>
    <td>LGA</td>
    <td>Low Gain Antenna</td>
  </tr>
  <tr>
    <td>LIDAR</td>
    <td>Light Detection And Ranging</td>
  </tr>
  <tr>
    <td>LIR</td>
    <td>Longwave Infrared Camera (Venera-D Science Payload)</td>
  </tr>
  <tr>
    <td>LIVE</td>
    <td>Lightning Sensor (Shukrayaan-1 Science Payload)</td>
  </tr>
  <tr>
    <td>LM</td>
    <td>Lander Module (Venera-D mission)</td>
  </tr>
  <tr>
    <td>LOD</td>
    <td>Length of Day</td>
  </tr>
  <tr>
    <td>LOS</td>
    <td>Line of Sight</td>
  </tr>
  <tr>
    <td>LT</td>
    <td>Local (solar) time, hour angle of the Sun as observed from a given point on Venus</td>
  </tr>
  <tr>
    <td>LWIR (or Thermal IR)</td>
    <td>Long Wavelength Infrared radiation, 8 – 15 µm in wavelength, within the infrared portion of the electromagnetic spectrum</td>
  </tr>
  <tr>
    <td>Magellan</td>
    <td>NASA Venus Orbiter Mission 1990-1994</td>
  </tr>
  <tr>
    <td>MARSIS</td>
    <td>Mars Advanced Radar for Subsurface and Ionosphere Sounding</td>
  </tr>
  <tr>
    <td>MAVEN</td>
    <td>Mars Atmosphere and Volatile Evolution</td>
  </tr>
  <tr>
    <td>Meridian</td>
    <td>Any great circle joining the North and South poles of a planet</td>
  </tr>
  <tr>
    <td>MERTIS</td>
    <td>Mercury Radiometer and Thermal Infrared Spectrometer (BepiColombo science payload)</td>
  </tr>
  <tr>
    <td>MESSENGER</td>
    <td>Mercury Surface, Space Environment, Geochemistry, and Ranging</td>
  </tr>
  <tr>
    <td>Mid IR or MWIR</td>
    <td>Mid Infrared (Wavelength) radiation, 3 – 8 µm in wavelength, within the infrared portion of the electromagnetic spectrum</td>
  </tr>
  <tr>
    <td>Millimeter wave</td>
    <td>Range of electromagnetic spectrum between 10 millimeters (30 GHz) and 1 millimeter (300 GHz), also known as the extremely high frequency (EHF) band.</td>
  </tr>
  <tr>
    <td>(volume) Mixing ratio</td>
    <td>Amount of an atmospheric constituent (in moles) divided by the total (in moles) of all other atmospheric constituents. For minor species, it is usually expressed in parts per million (ppm) or parts per billion (ppb)</td>
  </tr>
  <tr>
    <td>MGS</td>
    <td>Mars Global Surveyor</td>
  </tr>
  <tr>
    <td>MM-R</td>
    <td>Millimeter Wave Radiometer</td>
  </tr>
  <tr>
    <td>MO</td>
    <td>Magma Ocean</td>
  </tr>
  <tr>
    <td>MoI</td>
    <td>Moment of Inertia</td>
  </tr>
  <tr>
    <td>MOIF</td>
    <td>Moment of Inertia Factor</td>
  </tr>
  <tr>
    <td>MOLA</td>
    <td>Mars Orbiter Laser Altimeter (MGS science payload)</td>
  </tr>
  <tr>
    <td>MSL</td>
    <td>Mars Science Laboratory / Curiosity</td>
  </tr>
  <tr>
    <td>MWRS</td>
    <td>Microwave Radiometric Sounder (VOICE Science Payload)</td>
  </tr>
  <tr>
    <td>Myr</td>
    <td>Megayear = Million years</td>
  </tr>
  <tr>
    <td>N/A</td>
    <td>Not applicable</td>
  </tr>
  <tr>
    <td>Nadir</td>
    <td>Direction pointing directly below a particular location. The radar nadir refers to the downward-facing viewing geometry of an orbiting radar.</td>
  </tr>
  <tr>
    <td>NASA</td>
    <td>National Aeronautics and Space Agency</td>
  </tr>
  <tr>
    <td>NASEM</td>
    <td>National Academies of Sciences and Engineering</td>
  </tr>
  <tr>
    <td>NC</td>
    <td>Non-carbonaceous</td>
  </tr>
  <tr>
    <td>Near IR or nIR</td>
    <td>Near Infrared radiation, 0.75 – 1.4 µm in wavelength, within the infrared portion of the electromagnetic spectrum (from the approximate end of the response of the human eye to that of silicon)</td>
  </tr>
</table>
<table>
  <tr>
    <td>NES0</td>
    <td>Noise Equivalent Sigma Zero, a measure of SAR sensitivity, usually expressed in dB</td>
  </tr>
  <tr>
    <td>OBP</td>
    <td>On-board Processing</td>
  </tr>
  <tr>
    <td>OC</td>
    <td>Opposite Sense, circular polarization</td>
  </tr>
  <tr>
    <td>SC</td>
    <td>Same Sense, circular polarization</td>
  </tr>
  <tr>
    <td>OM</td>
    <td>Orbiter Module (Venera-D mission)</td>
  </tr>
  <tr>
    <td>p, T</td>
    <td>Pressure, temperature</td>
  </tr>
  <tr>
    <td>PFS</td>
    <td>Probe Flight System (DAVINCI mission)</td>
  </tr>
  <tr>
    <td>Polarization</td>
    <td>orientation of the electric field vector in an electromagnetic wave, "horizontal" (H) or "vertical" (V) in conventional imaging radar systems.</td>
  </tr>
  <tr>
    <td>PolSAR</td>
    <td>Polarimetric Synthetic Aperture Radar (VOICE Science Payload)</td>
  </tr>
  <tr>
    <td>QMS</td>
    <td>Quadrupole Mass Spectrometer</td>
  </tr>
  <tr>
    <td>Radar</td>
    <td>RAdio Detection And Ranging</td>
  </tr>
  <tr>
    <td>R-LIBS</td>
    <td>Raman-Laser Induced Breakdown Spectroscopy</td>
  </tr>
  <tr>
    <td>RAVI</td>
    <td>(Shukrayaan-1 Science Payload)</td>
  </tr>
  <tr>
    <td>RPI</td>
    <td>Repeat-Pass Interferometry</td>
  </tr>
  <tr>
    <td>RTM</td>
    <td>Radiative transfer model</td>
  </tr>
  <tr>
    <td>S-band</td>
    <td>a nominal frequency range, from 2 to 4 GHz (7.5-15 cm in wavelength) within the microwave portion of the electromagnetic spectrum</td>
  </tr>
  <tr>
    <td>S/C</td>
    <td>Spacecraft</td>
  </tr>
  <tr>
    <td>SAM</td>
    <td>Sample Analysis at Mars suite (MSL science payload)</td>
  </tr>
  <tr>
    <td>SAM</td>
    <td>Sample Analysis at Mars</td>
  </tr>
  <tr>
    <td>SEP</td>
    <td>Solar Electric Propulsion</td>
  </tr>
  <tr>
    <td>SfM</td>
    <td>Structure-from-Motion processing of descent images (DAVINCI, Sect. 5.3.4)</td>
  </tr>
  <tr>
    <td>SHARAD</td>
    <td>Mars SHAAllow RADar sounder (Mars Reconnaissance Orbiter Payload)</td>
  </tr>
  <tr>
    <td>SNR</td>
    <td>Signal-to-noise ratio</td>
  </tr>
  <tr>
    <td>SP1</td>
    <td>Science Phase 1 (VERITAS mission)</td>
  </tr>
  <tr>
    <td>SP2</td>
    <td>Science Phase 2 (VERITAS mission)</td>
  </tr>
  <tr>
    <td>SPICAV</td>
    <td>Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (Venus Express Science Payload)</td>
  </tr>
  <tr>
    <td>SRS</td>
    <td>Subsurface Radar Sounder (EnVision science payload)</td>
  </tr>
  <tr>
    <td>SRTM</td>
    <td>Shuttle Radar Topography Mission</td>
  </tr>
  <tr>
    <td>SSAS or SS-AS</td>
    <td>Subsolar to antisolar (wind circulation)</td>
  </tr>
  <tr>
    <td>SSPA</td>
    <td>Solid State Power Amplifier</td>
  </tr>
  <tr>
    <td>SVET</td>
    <td>Fourier Infrared Thermal Spectrometer (Venera-D Science Payload)</td>
  </tr>
  <tr>
    <td>SWIR</td>
    <td>Short Wavelength Infrared radiation, 1.4 – 3 µm in wavelength, within the infrared portion of the electromagnetic spectrum</td>
  </tr>
  <tr>
    <td>SZA</td>
    <td>Solar zenith angle, the angular distance between the vertical direction and the direction of the Sun from a specific location</td>
  </tr>
  <tr>
    <td>TanDEM-X</td>
    <td>TerraSAR-X satellite add-on for Digital Elevation Measurement</td>
  </tr>
  <tr>
    <td>Tb</td>
    <td>1 Tb = 1 terabit = 10<sup>12</sup> bits; 1 terabyte = 1 TB = 8 Tb</td>
  </tr>
  <tr>
    <td>TB</td>
    <td>1 TB = 1 terabyte = 10<sup>12</sup> bytes</td>
  </tr>
  <tr>
    <td>TDI</td>
    <td>Time delay and Integration</td>
  </tr>
  <tr>
    <td>TESS</td>
    <td>Transiting Exoplanets Survey Satellite</td>
  </tr>
</table>
<table>
  <tr>
    <td>Thermal IR (or LWIR)</td>
    <td>Long Wavelength Infrared radiation, 8 – 15 µm in wavelength, within the infrared portion of the electromagnetic spectrum</td>
  </tr>
  <tr>
    <td>TMT</td>
    <td>30-m Thirty-Meter Telescope, TMT International Observatory</td>
  </tr>
  <tr>
    <td>TRAPPIST</td>
    <td>TRAnsiting Planets and Planetesimals Small Telescope, a ground-based observatory on two sites: TRAPPIST-S in La Silla Observatory, Chile; TRAPPIST-N in Oukaïmeden Observatory, Morocco.</td>
  </tr>
  <tr>
    <td>TRAPPIST-1</td>
    <td>A cold dwarf star in Aquarius constellation, 40.55 ± 0.04 light-years away from Earth, with a planetary system of seven known exoplanets TRAPPIST-1b/h</td>
  </tr>
  <tr>
    <td>TT&C</td>
    <td>Telemetry, Tracking and Command</td>
  </tr>
  <tr>
    <td>TW</td>
    <td>1 TW = 1 terawatt = 10<sup>12</sup> watts</td>
  </tr>
  <tr>
    <td>TWTA</td>
    <td>Travel Waveguide Tube Amplifier</td>
  </tr>
  <tr>
    <td>USO</td>
    <td>Ultra-Stable Oscillator</td>
  </tr>
  <tr>
    <td>UVN-MSI</td>
    <td>Ultraviolet-Visible-Near Infrared Multi-Spectral Imager (VOICE Science Payload)</td>
  </tr>
  <tr>
    <td>VARTISS</td>
    <td>(Shukrayaan-1 Science Payload)</td>
  </tr>
  <tr>
    <td>VASI</td>
    <td>Venus Atmospheric Structure Investigation (DAVINCI Science Payload)</td>
  </tr>
  <tr>
    <td>VCMC</td>
    <td>Visible Camera for Cloud Monitoring (Shukrayaan-1 Science Payload)</td>
  </tr>
  <tr>
    <td>VEDA</td>
    <td>Electron Density (Shukrayaan-1 Science Payload)</td>
  </tr>
  <tr>
    <td>VEM</td>
    <td>Venus Emissivity Mapper (VERITAS science payload)</td>
  </tr>
  <tr>
    <td>VenDI</td>
    <td>Venus Descent Imager (DAVINCI Science Payload)</td>
  </tr>
  <tr>
    <td>VENIS</td>
    <td>IR grating spectrometer and imager, 2-5 µm (Venera-D Science Payload)</td>
  </tr>
  <tr>
    <td>VenSpec</td>
    <td>Venus Spectroscopy suite (EnVision science payload)</td>
  </tr>
  <tr>
    <td>VenSpec-H</td>
    <td>Venus Spectroscopy High Resolution (EnVision science payload)</td>
  </tr>
  <tr>
    <td>VenSpec-M</td>
    <td>Venus Spectroscopy Mapper (EnVision science payload)</td>
  </tr>
  <tr>
    <td>VenSpec-U</td>
    <td>Venus Spectroscopy Ultraviolet (EnVision science payload)</td>
  </tr>
  <tr>
    <td>VERITAS</td>
    <td>Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy</td>
  </tr>
  <tr>
    <td>VEx</td>
    <td>ESA Venus Express orbiter mission 2007-2014</td>
  </tr>
  <tr>
    <td>VfOx</td>
    <td>Venus Oxygen Fugacity Experiment (DAVINCI Student Collaboration Experiment)</td>
  </tr>
  <tr>
    <td>VIKA</td>
    <td>near-IR spectrometer suite 1.05 – 1.65 µm (Venera-D Science Payload)</td>
  </tr>
  <tr>
    <td>VIRAL</td>
    <td>Venus Infrared Atmospheric gas Linker, a high-resolution echelle spectrograph 2.3–4.4 µm (Shukrayaan-1 Science Payload)</td>
  </tr>
  <tr>
    <td>VIRTIS</td>
    <td>Visible and Infrared Thermal Imaging Spectrometer (Venus Express science payload)</td>
  </tr>
  <tr>
    <td>VIRTIS-H</td>
    <td>The high spectral resolution channel of the Venus Express VIRTIS IR spectrometer, aboard Venus Express</td>
  </tr>
  <tr>
    <td>VIS</td>
    <td>Visible spectral range (0.38 – 0.75 µm, or 380 – 750 nm)</td>
  </tr>
  <tr>
    <td>VISAR</td>
    <td>Venus Interferometric Synthetic Aperture Radar (VERITAS science payload)</td>
  </tr>
  <tr>
    <td>VISOR</td>
    <td>Venus Imaging System for Orbital Reconnaissance (DAVINCI Science Payload)</td>
  </tr>
  <tr>
    <td>VISWAS</td>
    <td>(Shukrayaan-1 Science Payload)</td>
  </tr>
</table>
<table><tr><td>VMC</td><td>Venus Monitoring Camera (Venus Express Science Payload)</td></tr><tr><td>VMS</td><td>Venus Mass Spectrometer (DAVINCI science payload)</td></tr><tr><td>VODEX</td><td>Venus Orbiter Dust Experiment (Shukrayaan-1 Science Payload)</td></tr><tr><td>VOI</td><td>Venus Orbit Insertion</td></tr><tr><td>VOLNA</td><td>(Venera-D Science Payload)</td></tr><tr><td>VSAR</td><td>(Shukrayaan-1 Science Payload)</td></tr><tr><td>VSEAM</td><td>(Shukrayaan-1 Science Payload)</td></tr><tr><td>VTC</td><td>Venus Thermal Camera (Shukrayaan-1 Science Payload)</td></tr><tr><td>VTLS</td><td>Venus Tunable Laser Spectrometer (DAVINCI Science Payload)</td></tr><tr><td>VZ</td><td>Venus Zone, 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</td></tr><tr><td>X-band</td><td>a nominal frequency range, from 8 to 12 GHz (2.5-3.8 cm in wavelength) within the microwave portion of the electromagnetic spectrum</td></tr><tr><td>XRF</td><td>X-ray fluorescence</td></tr></table>

**Acknowledgements** T.W. and G.A. acknowledge France's Centre National d'Études Spatiales (CNES) for funding support of Venus related studies. J.B.G. (as well as S.G. G.A. N.J, and E. K) acknowledges NASA's Discovery program for support of Venus related studies and the DAVINCI mission. Work by S.E.S., S.H., and D.N was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Members of the VERITAS team are grateful for funding by NASA's Discovery Program and JPL's proposal support. C.G. acknowledges the support of Rice University and the CLEVER planets group (itself supported by NASA and part of NExSS) and ET-HOME Excellence of Science programme. A.S. acknowledges support from NASA's Habitable Worlds Program (No. 80NSSC20K0226). Section 7 was prepared with the assistance of Oleg Sedykh (Venera-D). Finally, the authors thank the International Space Institute (ISSI) in Bern, Switzerland, for supporting the "Venus: Evolution through Time" workshop and the subsequent book, of which this paper forms a chapter.

**Funding** Open access funding provided by University of Oslo (incl Oslo University Hospital).

## Declarations

**Competing Interests** The authors declare that they have no conflict of interest.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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