surrounding shell (Pappalardo and Barr 2004). For more details on how the Europa Clipper mission will investigate the geological processes within the shell, we refer to Daubar et al. (2024, this collection).

Processes that transport material through the ice shell's interior are likely to leave fingerprints in the form of locally variable concentrations of salts, clathrates or melts (e.g., Fagents 2003; Schmidt et al. 2011; Buffo et al. 2020; Chivers et al. 2021), though surface non-ice materials will be modified by radiolysis and space weathering (Sect. 6.6). Spectra obtained by the MISE investigation will inform the composition and relative ages of features at <1 km resolution (e.g., Geissler et al. 1998; Hansen and McCord 2004; Dalton et al. 2013; Blaney et al. 2024; 2025) (see Fig. 3). Color images obtained by the EIS investigation will constrain abundances of non-ice species at meter scales (Trumbo et al. 2019a,b; Hibbitts et al. 2019; Turtle et al. 2024, this collection). If oceanic water is chemically evolved through ice thickening, more chlorides are expected in younger endogenic features. By determining stratigraphic relationships derived from EIS images of geologic formations it may be possible to constrain the compositional evolution of the ice-ocean system. However, the link between salt assemblage at the surface and the initial salt composition in the crystallizing solutions is not trivial (e.g., Zolotov and Shock 2001) as it depends on transport processes and cooling rate (e.g., Vu et al. 2016). Any systematic relationship between salt assemblage and stratigraphy will be valuable information but should not be considered as a direct constraint on ocean composition.

Thermal emissions from current and/or recent geologic activity potentially measured by the E-THEMIS instrument in combination with MISE and EIS measurements could constrain the composition of the freshest surface material. Radar reflections obtained with REASON within the ice shell could inform about compositional stratigraphy (e.g., refrozen melts, disconformities) and about compositional variations such as a layer of salts or the presence of melt (e.g., brine pockets) (Blankenship et al. 2009). Data on specific geologic features observed by EIS, MISE, Europa-UVS, MASPEX, SUDA, and E-THEMIS would constrain formation mechanisms of these features and surface-ice-ocean exchanges. If detected with REASON, salt-water eutectic interfaces could inform about geological processes in the icy shell such as convection. Attenuation of the radar signal could constrain both temperature and the concentration of impurities (Moore 2000; Pettinelli et al. 2015) as well as porosities and voids (Heggy et al. 2017).

## 6.6 Surface Processes

Much of the compositional information about the geological processes on Europa will be inferred from observations of the surface and atmosphere, which are physically and chemically affected by heavy irradiation, impacts, mass wasting, and thermal processing. These types of processes, though they may be constrained to the uppermost layer of Europa's surface (Sect. 2.4), have strong effects on the surface composition and complicate interpretations of the remote sensing data. It is therefore important for Europa Clipper investigations to constrain the extent of these processes and to acknowledge that measurements of Europa's surface and atmospheric compositions must be interpreted within the context of the following processes.

Some of these processes, and the depths to which they change the surface material, are shown in Fig. 18. For comparison, the maximum observable penetration depth as a function of wavelength is also shown, indicating how different investigations on Europa Clipper will be sensitive to, and constrain, different surface processes. Here, the penetration depth describes the decay of the intensity or power of the electromagnetic waves inside a material, which decays to 1/e of its surface value. For the visible range, it means that about 63% of
<figure><img src="image_19.png" /><figcaption>**Fig. 18** The maximum penetration depth into bulk H₂O ice of remote sensing observations as a function of wavelength. This curve was created based on the imaginary optical constants from Warren and Brandt (2008) at 1/e attenuation, assuming the ice is a perfect glass, not granular, with no internal scattering or surface roughness. The bandpasses of the Europa Clipper investigations are indicated by the transparent vertical bars and labeled. The vertical bars that begin at the top of the figure indicate the depths at which different surface processes can affect the surface material and composition. The radiation processing depths are approximated to the maximum stopping ranges of S⁺, O⁺, H⁺ (NIST PSTAR online program), e⁻ (NIST ESTAR online program), and Bremsstrahlung (NIST X-ray attenuation factors). Micrometeoroid gardening data are from Costello et al. (2021)</figcaption></figure>

light is absorbed or scattered. We note that the calculation shows the maximum depth because it assumes the ice is a perfect glass with no internal scattering or surface roughness. Therefore, properties like grain size, which affect penetration depth as is shown in (Fig. 7), are not considered here.

### 6.6.1 Radiation-Induced Processes

Energetic charged particles trapped in Jupiter's magnetosphere continually irradiate Europa's surface, altering its physical state and chemical composition through radiolysis while also ejecting surface material into the exosphere. The flux of energetic particles onto Europa will be constrained using data acquired by the RadMon and other opportunistic radiation data from science instruments (Meitzler et al. 2023, this collection). The RadMon is an engineering instrument that will also provide a coarse energetic particle spectrum, which can be used to assess Europa's radiation environment and surface weathering by charged particles greater than ~1 MeV. Charged particles at lower energy (less than ~6 keV) will be measured by the PIMS instrument, while Europa-UVS is sensitive to penetrating electrons in the ~10 MeV range. While the investigations will not directly measure the full energy spectrum of these particles, the data will 'bracket' the energetic bombardment that the Europa Clipper spacecraft will experience. Observations by the MISE, EIS, and Europa-UVS instruments will synergistically map Europa's surface albedo, maturity, and ice crystalline fraction, which are affected by the impinging plasma (Sect. 2).
Much of the ice on Europa appears crystalline in the interior of grains while the uppermost layer appears amorphous due to continuous exposure to radiation that destroys its crystal structure (e.g., Leto and Baratta 2003; Hansen and McCord 2004; Ligier et al. 2016). This is likely the result of competing radiation-driven amorphization and temperature-driven crystallization processes. Potential detections of areas with crystalline ice on the grains' very top surface in MISE data could be indicative of a recent deposit of water ice from the subsurface and/or plumes, which can potentially be confirmed with other MISE and/or Europa-UVS observations, and/or an anomalously warm surface, the existence of which may be confirmed with E-THEMIS data. In addition, REASON reflectometry data could reveal fresh (low density) icy plume deposits (Blankenship et al. 2024, this collection). At surface temperatures higher than ~130 K, any amorphous ice will irreversibly transition to crystalline ice (likely cubic or possibly hexagonal) within hours (Mastrapa et al. 2008), while at temperatures below 110 K this conversion could take years or longer. It should be noted that there are still inconsistencies between the spectral observations of Europa's surface and models incorporating radiation-induced amorphization measurements in the laboratory (Berdis et al. 2020). This indicates the need for continued laboratory irradiation studies at the appropriate temperatures and with the appropriate array of candidate surface materials and mixtures on the surface of Europa.

Radiation processes can also contribute to thermal sintering of near-surface ice and/or hydrate. For a porous Europa surface composed primarily of water ice, thermal sintering rates and surface consolidation may operate over timescales ranging from decades to hundreds of millions of years (Clark et al. 1983; Molaro et al. 2019; Choukroun et al. 2020). On Saturn's moon Mimas, the near-surface water ice appears to have been partially sintered by induction heating by electrons from the Saturnian co-rotating magnetic field (e.g., Schaible et al. 2017). Near the surface of Europa, it is possible that ice and even the hydrated materials could be sintered by Jupiter's much stronger magnetic field and/or by 'hot spots' above diapirs bringing warm ice close to Europa's surface. Derivation of thermal inertia from E-THEMIS data could provide insight into this process, and REASON reflectometry data will provide information on porosity of the near-surface layer (Sect. 2.1.1) and inform the investigation of sintering processes across the surface.

Continuous bombardment of the ice and non-ice surface materials could also result in surface-bound radiolytic products in their electronic and vibrational excited states. As a result, luminescence from these excited atoms, molecules, and ions is expected across a wide portion of the electromagnetic spectrum, which could constrain compositional information of the surface (Nulsen et al. 2020; Gudipati et al. 2021). Spectral color centers, which are optically active crystal defects in halide salts such as NaCl, can form through irradiation by high energy electrons and ions but can subsequently relax through thermally induced defect center migration (e.g., Poston et al. 2017). NaCl has been tentatively identified on Europa's surface through the presence of a shallow absorption feature near 460 nm and 230 nm (Trumbo et al. 2019a, 2019b, 2022; Brown et al. 2022; Denman et al. 2022; Sect. 2). The shallowness of the band, yet absence of a 720-nm feature, could be indicative of thermal relaxation, or healing of the crystal defects responsible for the color centers. EIS observations that capture these bands could therefore constrain both surface composition and radiation effects on surface compounds.

Energetic ions release radiolytic species from the surface into the exosphere through sputtering. Investigations of atmospheric gases by MASPEX and Europa-UVS will constrain the abundance and distribution of radiolytic volatiles (e.g., H₂O₂, H₂, O₂, CO₂; see Sect. 4) and identify salts, acid hydrates, and trace organic compounds that result from radiolytic processing and sputtering, if they are present. Organic matter in the atmosphere can be studied
to reveal chemical modifications that indicate a history of irradiation (e.g., Court et al. 2006). Abundance and vertical structure of the exospheric species and their spatiotemporal variations (especially on the night side hemisphere), coupled to the plasma environment, may constrain source processes and rates, including sputtering. These investigations are critical to decoding the compositional relationship between Europa's exosphere and surface and how their compositions are altered via radiolytic processing. Interpretation of these measurements made by the Europa Clipper instrument suite will depend on continued laboratory experiments and computational modeling efforts that provide a foundational understanding of the role of radiation-induced processes on the chemical and physical changes of candidate materials on Europa's surface.

### 6.6.2 Impact Gardening, Thermal Processing, and Mass Wasting

In addition to bombardment by charged particles, bombardment by micrometeoroids and larger impactors serves to mix the material at Europa's surface. This process, referred to as “impact gardening,” buries irradiated surface material and exposes fresher subsurface material to depths ranging from centimeters to meters (Chyba and Phillips 2001; Cooper et al. 2001; Phillips and Chyba 2001; Costello et al. 2020, 2021). Impact gardening affects the composition, grain size, crystallinity, and porosity in the upper surface layers, all of which will be assessed by the Europa Clipper investigations (Sect. 2.1). SUDA measurements of the abundance, trajectory and composition of exogenic grain particles in the Jovian system (e.g., grains from Io's volcanoes) will constrain physical and chemical effects of those grains on the garnering. Evaluations of impact gardening rates, sputtering rates, and diffusion rates through the ice, in conjunction with geological resurfacing rates, are needed to estimate delivery rates of radiolytic compounds (e.g., O₂, H₂O₂, H₂SO₄) to the deeper subsurface and in the ocean (Chyba and Phillips 2002; Hand et al. 2007; Greenberg 2010). Compositional data from MISE and Europa-UVS in combination with high-resolution ELS images will be used to constrain gardening of surface materials.

Europa's surface of ice and hydrated non-ice material is susceptible to thermal processing. At the maximum surface temperature observed at Europa of ~132 K (Spencer et al. 1999), water ice will sublime at a rate of greater than 1 km/Ga (Vasavada et al. 1999) with the molecules potentially traveling global distances to contribute to the ice at the poles or more local cold traps. This rate is likely a lower limit for Europa because irradiation and micrometeoroid bombardment should enhance the sublimation. Sublimation of water ice from Europa's surface results in the concentration and eventually mass movement of the residual non-ice material, or lag. Lag deposits have been identified at the bases of fresh linea and in older regions, where they accumulate over time (e.g., Greeley et al. 2004) and protect the underlying terrain from weathering processes. Volatile segregation and migration should produce distinct deposits. The refractory lags created by volatile sublimation might provide strong spectral signatures of the non-ice components of features of interest. Non-volatile lag deposits can be observed through remote spectroscopy conducted by the MISE and Europa-UVS instruments, imaged by the EIS cameras, and sampled above the surface after sputtering by both the SUDA and MASPEX instruments. The exposure of hydrated salts of oceanic origin at the surface could lead to their dehydration and formation of lag deposits with a hydration profile that increases with depth (Zolotov and Shock 2001). This prediction will be tested with high spatial and spectral resolution data from MISE observations and complemented by EIS imaging. The condensation of sublimed and sputtered water molecules in polar regions creates surface frost deposits. The smallest ice patch due to cold trapping of sputtered or sublimed ice may be tens of centimeters wide (e.g., Spencer 1987). Sufficiently
cold polar frost deposits may even trap more volatile species than H₂O (e.g., CO₂, light hydrocarbons), and could be targeted for observations by MISE and Europa-UVS to identify some of these frosts (Sect. 2).

Mass wasting on Europa will likely fall into two categories: classic down slope movement of loose material, and volatile (e.g., H₂O, CO₂) segregation and migration, whereby the latter could initiate the former. Geologically recent large down slope movements may exhibit fresh exposures of compositionally distinct materials (Sect. 2.3.2). However, such materials may be difficult to observe because the scarp or cliff from which they are exposed may also block that material from view of the spacecraft due to the observational geometry.

## 6.7 Photochemical and Radiation Processing in the Atmosphere/Exosphere

The science investigations of the Europa Clipper mission will provide important insight into the composition of Europa's exosphere (Sect. 4). Processes in this region are dominated by interactions with photons, electrons, and ions, which are currently not well characterized, particularly with respect to their relative abundances. This complex and dynamic region encompasses the transition between Europa and Jupiter's magnetosphere. Compositional measurements of Europa's atmosphere and local space environment will provide key insight into local interaction processes, particle escape, and magnetospheric interaction, which in turn facilitates the ability to distinguish between endogenic and exogenic material. Close flybys over large endogenic geological structures (e.g., chaotic terrains) will inform the contribution of endogenic materials in data obtained with MASPEX and UVS for atmospheric gases. These compositional observations will help resolve relative particle interaction rates, the significance of each process, and ultimately how the exosphere is populated.

Multiple exospheric particle interaction processes increase the probability of escape and thus interaction with the magnetospheric environment. In particular, the interaction with solar UV photons and ionospheric and magnetospheric electrons ionizes and/or dissociates Europa's endogenic particles (Smyth and Marconi 2006). Previously observed neutral particles (H, O; Hansen et al. 2005) and minor species, such as Na (Brown and Hall 1996) and K (Brown 2001), tend to have larger energies and attain higher altitudes, thus increasing the possibility of escape. Detection of additional particle species would identify currently unknown interactions and source processes. Additionally, neutral particles can interact with surrounding ions through charge-exchange processes, which neutralize the ion and charge the neutral particle (Schreier et al. 1993). The neutralized ions can thereby have a higher velocity than neutral source particles. MASPEX data on neutral species will inform about particle interactions and will enable the differentiation of particles originating from Europa from those with external sources. Europa-UVS observations of auroral variations and of stellar occultations and Jupiter transits that vary in time, longitude, latitude, and altitude are essential to interpret Europa's dynamic exosphere and space environment. Compositional data obtained using the PIMS instrument could provide an understanding of the ambient plasma environment, which constrains the charge-exchange and ionization processes, while the REASON measurements will constrain the ionospheric electron density in the vertical column between the spacecraft and Europa's surface. Additionally, grain composition data obtained by the SUDA investigation could possibly identify minor species more difficult to identify from data collected by the other investigations.

The photochemical and radiation processes described in the previous sections increase the number of atoms and molecules escaping from Europa's tenuous atmosphere. Compositional data collected by MASPEX, Europa-UVS, and PIMS will be used together with spatial, directional, and energy information to quantify escaping particles, while the SUDA investigation will capture ice/dust grains that achieve escape velocity.
Escaping particles together with magnetic fields will constrain the interaction of Europa with the Jovian magnetosphere. Such interactions affect the local exospheric, atmospheric, surface, and subsurface environments, and they must be understood to make inferences on the subsurface ocean and, ultimately, Europa's habitability. Measurements by MASPEX, Europa-UVS, PIMS, and SUDA may also help to distinguish the flux of exogenic material and particles entering the atmosphere from other sources, including from Io's volcanoes.

## 6.8 Volatile Transport Within the Atmosphere and Cold Traps

As with Mercury (e.g., Ingersoll et al. 1992; Paige et al. 2013), the Earth's Moon (e.g., Watson et al. 1961; Hurley et al. 2012), and Ceres (e.g., Platz et al. 2016), the small obliquity of Europa leads to the possibility of long-term cold traps near its north and south poles in permanently shadowed regions (PSRs). Volatiles from localized endogenic (e.g., irradiated, low-latitude regolith, outgassing, and possible plumes) and exogenic (e.g., comet and asteroid impacts, dust accretion) sources can migrate to PSRs through ballistic transport, a process referred to as thermal hopping. A recent model (Ashkenazy 2019) suggests polar surface temperatures on Europa of 46 K compared with equatorial temperatures between ~86 K and 130 K. At temperatures this low, CO, $CO_2$, $SO_2$, $NH_3$, HCN, $CH_2O$, $CH_3OH$, Kr, Xe, and some low order hydrocarbons could migrate across the surface and would be long-lived at Europa's pole (e.g., Zhang and Paige 2009). Cassini's Ion and Neutral Mass Spectrometer data provided evidence of seasonal trapping of $CO_2$ at Rhea's poles (Teolis and Waite 2016) at similar temperatures that were verified by the Cassini Visible and Infrared Mapping Spectrometer (VIMS) investigation (Howett et al. 2010) and modeled by Miles et al. (2020). Migration and trapping have been modeled for the case of Europa's plumes (Teolis et al. 2017).

Both the MASPEX and Europa-UVS investigations will be used to track volatiles in Europa's atmosphere, which may also be detected on the surface with the remote sensing investigations (Sect. 2). The MASPEX investigation has the capability to detect noble gases *in situ*, including their isotopes. In addition, the Europa-UVS investigation is capable of detecting He through its EUV resonance line at 58.4 nm, as has been demonstrated at the Moon using LRO-LAMP (e.g., Grava et al. 2021). Transport of these gases will be inferred from either time-dependent behavior (i.e., if a sudden change in abundance is observed) or by spatial abundance gradients, along with modeling using E-THEMIS surface temperature measurements. The MISE and Europa-UVS spectral investigations will be used to determine the presence of surface volatiles by their spectral signatures in the cold trap or PSR regions. In addition, it is likely that the sublimation and condensation volatiles could affect the microstructure of the surface, e.g., the porosity, in a detectable way, as has been observed with the lunar PSRs (Gladstone et al. 2012; Sect. 2).

Vapor pressure differences spanning many orders of magnitude are to be expected across Europa, owing to surface temperatures ranging from ~46 K at the poles, to between ~86 and ~132 K at the equator for the nightside and dayside, respectively (Ashkenazy 2019; Spencer et al. 1999). Massive icy planetary bodies like Europa have sufficient gravity to retain thermally accommodated molecules in the exosphere, and most volatiles heavier than $H_2$ have scale heights no more than a few tens of kilometers, impacting and interacting with the surface many times before escaping into space (Shematovich et al. 2005). Europa's exosphere may contain a number of as yet undetected sputtered and possibly plume-supplied species, such as $CO_2$ and $NH_3$ (Teolis et al. 2017), which are semi-volatile over Europa's surface temperature range. These minor species, and to an extent, even $H_2O$, may sublimate on Europa's dayside surface and refreeze on the nightside (Teolis et al. 2017). This same
physics determines the distribution of the lunar argon exosphere (Grava et al. 2015) and the CO₂ exospheres of the icy moons Dione and Rhea (Teolis and Waite 2016), which also have gas densities far greater over the dayside hemisphere because of dayside sublimation process. We may therefore anticipate that some semi-volatile species in Europa's exosphere would likewise exhibit a highly nonuniform distribution centered on the dayside, or a diurnally 'pulsating' exosphere.

Temporal changes of Europa's entire exosphere may be driven by localized gas sources, such as localized outgassing or possibly high-speed plume sources, or 'Macula' type terrains from which a unique suite of volatiles are sputtered. Any time-variable plumes (Paganini et al. 2020; Roth et al. 2016; Sparks et al. 2016, 2017) would add another source for variations in global exospheric dynamics.

Measurements by the Europa-UVS and MASPEX investigations will be used to verify the density of H₂O and other species relative to the amount of radiolytic O₂. In a sputtered exosphere, the amounts of H₂O and O₂ are expected to remain fixed relative to one another as they are both ejected in relatively consistent proportions from the surface ice. Based on modeling and analysis of laboratory data on ice, Teolis et al. (2017) suggest that an H₂O/O₂ ratio greater than ~15 would indicate a source of H₂O other than sputtering, which might suggest a nearby plume. If such a plume is detected, measurements from early flybys could be used as inputs into exospheric models to predict the evolution of the atmospheric species and establish if observations later in the tour are consistent with (1) a changing or constant exosphere, and (2) the presence or absence of a localized gas source. With the species column densities measured along the spacecraft track to provide rough constraints on exospheric composition, exospheric models will be iterated to fine-tune both the composition of material supplying the exosphere and the surface residence time of each species, which may depend on surface composition and regolith structure. Observations over the course of the mission enable the assessment of the atmosphere as a function of Europa's local time, and whether localized sources produce diurnal variability in the exosphere. Measurements of the spatial distribution and temporal responses of many different species with drastically different volatilities will help considerably to constrain exospheric models.

# 7 Conclusions and Broader Relevance

The Europa Clipper mission will assess the habitability of the enigmatic, icy moon of Jupiter through detailed studies of Europa's composition, geology, and interior. The unique and complementary suite of *in situ* and remote sensing investigations facilitate a powerful and robust set of observations to achieve the mission's primary goal.

The compositional investigations focus on identifying the composition and sources of key non-ice constituents on Europa's surface and in its atmosphere and local space environment. High spatial-resolution maps covering the majority of Europa's surface will be constructed from the Europa Clipper's remote-sensing suite and complemented by *in situ* measurements. The distribution of surface materials and correlation with local geological landforms will provide key insights into sources and ongoing processes on Europa's surface and interior, including radiation effects, space weathering, geological processes, and thermo-chemical evolution. Through the characterization of the composition and sources of volatiles, particulates, and plasma in Europa's atmosphere and local space environment, the Europa Clipper mission will determine the structure of Europa's atmosphere, constrain the relationship between the subsurface, surface, atmosphere, and local environment, and provide information about photochemical and radiation processing of materials on Europa's
surface and in its atmosphere. The possible presence of plumes, which may be directly ob-
served or indirectly detected by their effects on the atmosphere or fallout on the surface, will
enable a unique compositional study of material recently expelled from within Europa, pos-
sibly sourced from its underlying ocean. These compositional measurements of the moon’s
environment, atmosphere, surface, subsurface, and activity enable key analyses for assessing
the habitability of Europa.

**7.1 Connection with Habitability**

Compositional measurements by the Europa Clipper investigations will be used to assess the habitability of Europa's interior (Vance et al. 2023; Pappalardo et al. 2024, both this collection). Data from the Europa-UVS, MASPEX, and SUDA instruments, with contributions from PIMS and REASON, on non-ice species in the atmosphere and local environment, together with spectral, geological, and thermal measurements of surface materials from MISE, EIS, Europa-UVS, and E-THEMIS, will be used to distinguish endogenic compounds, yielding implications about habitability at depth. The composition of salts in endogenic surface features and possible plume deposits as observed by the MISE, Europa-UVS, SUDA, and REASON investigations will be used to assess inorganic speciation and the pH of oceanic water, composition, and salinity of brine pockets in the icy shell, and evaluate mineralogy of suboceanic rocks and/or sediments.

The ratios between sulfate/chloride/carbonate and Ca/Mg/Na/K in salts will provide in-
sight about the oxidation state, geologic history of oceanic water, and habitability of Eu-
ropa through time. Relative abundances of oxidized and reduced species in plume and fresh
surface materials (e.g., sulfate/sulfide, sulfate/organic matter, SO₂/H₂S, CO₂/CH₄, N₂/NH₃,
H₂O/H₂) will be used to assess redox conditions, redox disequilibria, and chemical sources
of energy that could support metabolism in the shell, in oceanic water, and at the ocean-rock
interface. MASPEX, Europa-UVS, and SUDA data on organic compounds, which may be
found in potential plume emissions, would constrain the organic composition in the ocean
and the ice shell, together with possible origins and transformation pathways of organic mat-
ter. REASON measurements that are sensitive to the electromagnetic properties and struc-
ture of the ice shell will highlight the presence of any brine pockets and major impurities that
could affect the moon's habitability. The abundances of radiolytic compounds observed in
surface and local atmospheric materials by MISE, EIS, Europa-UVS, SUDA, and MASPEX
and their association with geological features or hotspots observed by E-THEMIS will help
assess whether the delivery of strong oxidants and acids to habitable niches in the ice shell,
and even to oceanic water upon a major ice melting event, could have been a major process
at Europa.

**7.2 Supportive Laboratory, Telescopic, and Mission Science**

Previous missions and telescopic observations of Europa not only provide the foundational
knowledge for the planned investigations at the satellite, but also contribute independent
results at different wavelengths and capture changes over time that can bolster findings from
the data returned by the Europa Clipper mission. The observations made during close flybys
by NASA's Juno mission, with its own unique suite of instrumentation, add new, high quality
data of Europa and the Jupiter system, including the intensity and variability of the radiation
environment anticipated for the Europa Clipper mission (e.g., Kollman et al. 2022).

The JUICE mission, led by the European Space Agency, launched in April 2023 and
will arrive at Jupiter in 2031. Its anticipated simultaneous presence with Europa Clipper
in orbit around Jupiter presents unparalleled opportunities for concurrent observations of the system from two separate vantage points. The JUICE and Europa Clipper spacecraft carry complementary instrumentation, and the ultraviolet spectrographs on these missions are nearly identical (Retherford et al. 2024, this collection). The data that JUICE will collect during multiple flybys of Europa and Callisto, and following a year-long orbit of Ganymede (Grasset et al. 2013), should provide new, independent insights into the primordial material and evolution of all of the Galilean satellites. Further, the inherent differences between the icy satellites will help us to better understand and characterize the uniqueness of Europa.

Exploration and monitoring from Earth-based facilities and space telescopes (e.g., HST, JWST) continue to produce exciting possibilities for the interpretation of Europa's composition and habitability, which may pose new questions for the Europa Clipper mission to explore and provide the necessary, complementary data to address those puzzles. The detailed information gained about Europa's composition, interior, and geology through Europa Clipper data will provide critical constraints for any future missions to Europa, especially a possible lander (Grima et al. 2014b; Hand et al. 2022; Phillips et al. 2023).

New and ongoing laboratory studies will continue to support the interpretation of the Europa Clipper to enhance its scientific return well into the future. Current efforts to identify species and constrain grain properties under Europa-relevant temperatures will be important for deciphering the observed non-ice material on Europa (e.g., Dalton et al. 2007, 2012b, 2012a; Clark et al. 2013) and potentially the material spewed from within subsurface water in the form of plumes. Cryogenic fallouts or other geologically recent emplacement of surface material may appear to have different spectral properties in the ultraviolet to the infrared, offering a new approach for potentially detecting and dating recent geological activity with Europa-UVS, EIS, and MISE, and should continue to be studied in the lab (e.g., Raut and Baragiola 2013). Laboratory studies that simulate the processing of surface material by space weathering and Jupiter's radiation environment have important implications for interpretation of compositional measurements of the moon's surface and atmosphere (e.g., Hibbitts et al. 2019; Brown et al. 2022). Existing and emerging sputtering measurements and models for condensed ices (Famá et al. 2008; Raut and Baragiola 2013; Teolis et al. 2017; Meier and Loeffler 2020) and for non-ice constituents (e.g., hydrated salts and acids) will improve constraints on the abundance of exospheric species (Cipriani et al. 2009). These studies can be fine-tuned after direct measurements are made by the Europa Clipper instruments, and will be used to quantify the effects of the competing processes, such as crystallization vs. radiation-induced amorphization, unfolding on Europa's surface.

## 7.3 Implications for Composition and Composition-Related Processes on Other Moons, Planetary Bodies and Beyond

The data to be returned by Europa Clipper will likely shed light on composition and processes on other solar system and exoplanetary bodies. Compositional constraints on Europa's formation (Sect. 6.2) have implications for the origin of the other Galilean satellites. Insights into the composition and geological histories of ocean worlds may be applicable to water-bearing objects in the main asteroid belt, such as Ceres and Pallas, D and P type asteroids, Jovian Trojans, Triton, and other ocean worlds throughout our Solar System and beyond. A detection of elevated carbon contents and ammoniated species together with some isotopic enrichments (e.g., ¹⁸O, ¹⁵N) through Europa Clipper investigations would indicate migration of outer Solar System materials to the Jovian system and place constraints on the large-scale dynamical evolution of the early solar system.
The composition of endogenic salts, mainly the sulfate, chloride, and carbonate content found at Europa, will provide insights into the composition and oxidation state of deep-water oceans on Ganymede and Callisto. Inferred composition of Ionian grains, ions and molecules near Europa and in surface materials will reveal new insights about Io's volcanism and its contribution to surface and atmospheric materials on Ganymede and Callisto. Likewise, data on other exogenic constituents, such as inorganic/organic species in chondritic and cometary dust, will have implications for exogenic accumulation on Galilean satellites. The Europa Clipper data on speciation of radiolytic products in surface and atmospheric materials will be indicative of radiolysis on other icy bodies in the current and early solar system, and in the universe. If plumes are found at Europa, their sampling will serve as direct insight into their source in the ice shell, ocean, or volatile rich (e.g., liquid) bodies within the crust. Comparisons with Enceladus's plumes will further constrain the differences between the Jovian and Saturnian systems. The rich, synergistic compositional dataset promised by the Europa Clipper mission will not only illuminate this icy world's formation, evolution, and habitability; its in-depth investigation will provide context for the numerous ocean worlds that exist within our Solar System and beyond.

**Acknowledgements** This work was supported by NASA through the Europa Clipper Project. FP acknowledges the support by a European Research Council (ERC) Consolidator Grant 724908-Habitat OASIS.

## Declarations

**Competing Interests** This work was financially supported by NASA through the Europa Clipper project. Partial support was provided to FP by a Europa Research Council (ERC) Consolidator Grant 724908-Habitat OASIS.

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## Authors and Affiliations

T.M. Becker<fnref n="1" />,<fnref n="2" /> ID • M.Y. Zolotov<fnref n="3" /> ID • M.S. Gudipati<fnref n="4" /> ID • J.M. Soderblom<fnref n="5" /> ID •

M.A. McGrath⁶ ID • B.L. Henderson<fnref n="4" /> ID • M.M. Hedman⁷ ID • M. Choukroun<fnref n="4" /> ID •

R.N. Clark⁸ ID • C. Chivers⁹ ID • N.S. Wolfenbarger¹⁰ ID • C.R. Glein<fnref n="1" /> ID •

J.C. Castillo-Rogez<fnref n="4" /> ID • O. Mousis¹¹ ID • K.M. Scanlan¹⁰ ID • S. Diniega<fnref n="4" /> ID •

F.P. Seelos¹² ID • W. Goode¹³ ID • F. Postberg¹⁴ ID • C. Grima¹⁰ ID • H.-W. Hsu¹³ ID •

L. Roth¹⁵ ID • S.K. Trumbo¹⁶ ID • K.E. Miller<fnref n="1" /> ID • K. Chan¹⁰ ID • C. Paranicas¹² ID •

S.M. Brooks<fnref n="4" /> ID • K.M. Soderlund¹⁰ ID • W.B. McKinnon¹⁷ ID • C.A. Hibbitts¹² ID •

H.T. Smith¹² ID • P.M. Molyneux<fnref n="1" /> ID • G.R. Gladstone<fnref n="1" />,<fnref n="2" /> ID • M.L. Cable<fnref n="4" /> ID •

Z.E. Ulibarri¹³ ID • B.D. Teolis<fnref n="1" /> ID • M. Horanyi¹³ ID • X. Jia¹⁸ ID • E.J. Leonard<fnref n="4" /> ID •

K.P. Hand<fnref n="4" /> ID • S.D. Vance<fnref n="4" /> ID • S.M. Howell<fnref n="4" /> ID • L.C. Quick¹⁹ ID • I. Mishra¹⁶ ID •

A.M. Rymer¹² ID • C. Briois²⁰ ID • D.L. Blaney<fnref n="4" /> ID • U. Raut<fnref n="1" />,<fnref n="2" /> ID • J.H. Waite<fnref n="1" /> ID •

K.D. Retherford<fnref n="1" />,<fnref n="2" /> ID • E. Shock<fnref n="3" /> ID • P. Withers²¹ ID • J.H. Westlake¹² ID • I. Jun<fnref n="4" /> ID •

K.E. Mandt¹⁹ ID • B.J. Buratti<fnref n="4" /> ID • H. Korth¹² ID • R.T. Pappalardo<fnref n="4" /> ID •

the Europa Clipper Composition Working Group

<figure><img src="image_20.png" /></figure>

T.M. Becker

tracy.becker@swri.org

<footnote n="1">¹ Southwest Research Institute, San Antonio, TX, USA</footnote>

<footnote n="2">² University of Texas at San Antonio, San Antonio, TX, USA</footnote>

<footnote n="3">³ Arizona State University, Tempe, AZ, USA</footnote>

<footnote n="4">⁴ Jet Propulsion Laboratory, Pasadena, CA, USA</footnote>

<footnote n="5">⁵ Massachusetts Institute of Technology, Cambridge, MA, USA</footnote>
6 NASA Marshall Space Flight Center, Huntsville, AL, USA

7 University of Idaho, Moscow, ID, USA

8 Planetary Science Institute, Tucson, AZ, USA

9 Georgia Institute of Technology, Atlanta, GA, USA

10 University of Texas, Austin, TX, USA

11 Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France

12 Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

13 University of Colorado, Boulder, CO, USA

14 Freie Universität, Berlin, Germany

15 KTH Royal Institute of Technology, Stockholm, Sweden

16 Cornell University, Ithaca, NY, USA

17 Washington University, St. Louis, MO, USA

18 University of Michigan, Ann Arbor, MI, USA

19 NASA Goddard Space Flight Center, Greenbelt, MD, USA

20 Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), Orléans, France

21 Boston University, Boston, MA, USA