The icy ocean moons Enceladus and Europa offer potentially habitable environments below their icy crusts. Ice grains ejected from cryovolcanic plumes [1,2] and micrometeorite bombardment can be sampled by impact ionization mass spectrometers, as performed in the past by the Cosmic Dust Analyzer (CDA; [3]) onboard Cassini in the Saturnian system. Successor instruments to the CDA include the SUrface Dust Analyzer (SUDA; [4]) onboard NASA’s Europa Clipper mission and the HiFi instrument for a future Enceladus mission [5]. The strongly enhanced capabilities of SUDA and contemporary instruments, relative to CDA, allow the identification of molecular biosignatures. Among possible molecular biosignatures, amino acids are essential building blocks of proteins and play a crucial role in the formation of water-based life as we know it, thus their identification on extraterrestrial water worlds is key to the search for life beyond Earth.Laboratory analogue experiments using laser-induced liquid beam ion desorption (LILBID [6]) have demonstrated that impact ionization mass spectrometers can detect amino acids [7] down to the ppm or ppb level, if they are entrapped in emitted ice grains, and can distinguish between abundance patterns of abiotic and biotic formation processes [8]. However, at any given molecular mass of an amino acid, several isomers (identical molecular formula but distinct arrangements of atoms in space) exist, which are indistinguishable by their molecular peaks in recorded mass spectra. Until now, it was unclear whether isomeric amino acids can be discriminated from each other, e.g., by fragmentation patterns in impact ionization mass spectra.Here, using LILBID mass spectrometry, we conducted a cation mode analysis of eight isomeric amino acids with an identical molecular mass of 131.173 u and formula C6H13NO2 [9]. The recorded mass spectra were investigated for spectral features that enable differentiation of the different isomeric amino acids, with the aid of quantum chemistry calculations.We show that the amino acid isomers (including diastereoisomers) can be uniquely identified due to their distinct mass spectral features and fragmentation patterns. Several observed fragments and their intensities can be explained through intramolecular hydrogen bonding and other structural effects originating from the parent molecules. Importantly, α-amino acids can be clearly differentiated from non-α-amino acids, because they have lower proton affinities than non-α-amino acids, which result in lower ionization efficiencies for α-amino acids. Additionally, we further complement the LILBID database [10], which already contains a large variety of analogue mass spectra of both organic and inorganic compounds, for upcoming missions to icy ocean moons.The ability to discriminate amino acid isomers in a robust and reliable manner highlights a novel ability of impact ionization mass spectrometers that has significant implications for the search of biosignatures in the solar system, in particular for SUDA on Europa Clipper and other future instruments onboard missions exploring ocean worlds.[1] F. Spahn et al., Science, 311, 1416-1418 (2006)[2] L. Roth et al., Science, 343, 171-174 (2014)[3] R. Srama et al., Space Sci. Rev., 114, 465-518 (2004)[4] S. Kempf et al., Space Sci. Rev. 221, 10 (2025)[5] O. Mousis et al., The Planetary Science Journal, 3(12), 268 (2022)[6] F. Klenner et al., Rapid Commun. Mass Spectrom., 33, 1751-1760 (2019)[7] F. Klenner et al., Astrobiology, 20, 179-189 (2020)[8] F. Klenner et al., Astrobiology, 20, 1168-1184 (2020)[9] J. Bönigk, et al. Astrobiology 15311074261443835 (2025)[10] F. Klenner et al., Earth Space Sci., 9, e2022EA002313 (2022)

Erupting from a subsurface ocean, the plume of Enceladus contains ice grains composed of varying amounts of salts and organics. How the organics in the plume's jets partition during freezing relative to the salts and ice grains is not fully understood. While prior works have focused on amino acids and chondritic organics in analog frozen brines for Enceladus and Europa, more complex molecules such as sugars have not been studied. Here we evaluate the hydrophilic sugars trehalose and hyaluronic acid using micro-Raman imaging. We conducted flash-freezing and slow-freezing experiments of these sugars in an analog Enceladus brine made of NaCl, Na2CO3, NH4Cl, KCl, and Na2SiO3. Raman maps revealed that in both freezing scenarios, trehalose and hyaluronic acid have similar spatial distributions to those of the salt minerals natron (Na2CO3•10H2O) and hydrohalite (NaCl•2H2O). This demonstrates that the solute-solute interactions between the sugars and salts dominate so that the salts concentrate the sugars, despite the hydrogen bonding capabilities of both sugars. Due to the preferential spatial association of organics with salts, future missions should prioritize Type III, salt-rich grains, from the Enceladus plume as prime candidates for biosignatures.

Saturn's moon Titan exhibits remarkable parallels to the Earth in many geophysical and geological processes not found elsewhere in the solar system at the present day. These include a nitrogen atmosphere with a condensible gas - methane - replacing the Earth's water, leading to an active meteorology with rainfall and surface manifestations including rivers, lakes and seas, and the dissolution of karstic terrain. Other phenomena such as craters, dunes, and tectonic features are found elsewhere - e.g. on Mars and Venus - but their continuing alteration by pluvial, fluvial and lacustrine processes can be studied only on Earth and Titan. Meanwhile Titan also hosts an interior liquid water ocean with similarities to the Earth as well as to ocean worlds such as Europa and Enceladus. Our focus in this review paper is twofold: to describe the geophysical and geological parallels between Earth and Titan, and to evaluate the yet-underexploited possibilities for field analog research to gain new knowledge about these processes. To date, Titan's much colder temperature and different atmospheric and crustal materials have led to a skepticism that useful analogs can be found on Earth. Our conclusion, however, is that a much larger range of useful analog field work is possible and this work will substantially enhance our knowledge of both worlds. Such investigation will supplement the existing sparse data for Titan returned by space missions, will greatly enhance our understanding of such datasets, and will help to provide science impetus and goals for future missions.

In this paper, we report a fully automated, end-to-end (sample-in/data-out) capillary electrophoresis system. The system's dimensions conform to the cylindrical shape and power/data requirements of the science payload compartment of the Exobiology Extant Life Surveyor (EELS), a snake-like robot capable of autonomously navigating challenging terrain on Earth or other worlds. The capillary electrophoresis system is equipped with three contactless conductivity detectors: two dedicated to analyte detection and one for characterizing bulk sample flow. The system enables simultaneous detection of cations and anions, including K+, Na+, Ca2+, Mg2+, Cl-, and SO4 2-, at submicromolar concentrations. For the first time, we deployed and demonstrated autonomous capillary electrophoresis operation on a glacier with the system tested in three environments: on-ice, partially submerged in an active stream, and fully submerged in a glacial pond.

As an organic-rich world with a subsurface ocean, Titan is an object of great astrobiological interest. However, geological signs of surface-subsurface exchange are limited, and Titan’s thick icy crust may significantly impede delivery of organic-rich surface materials to the subsurface ocean. Heating of accreted complex organic material in Titan’s interior may provide an additional source of organic molecules to serve as building blocks for life and/or chemical energy sources for ocean habitability. Here, we investigate the composition of Titan’s subsurface ocean with a focus on organic materials derived from the rocky interior. We develop organic-rich compositional models, and consider what effects heating of these organics may have on the total noble gas signature in Titan’s atmosphere. We compare this signature to existing constraints to provide a consistent framework for the abundance of organic material derived from the interior. Present day constraints generally allow contributions of both noble gas ices and Phase Q noble gases from heating of abundant organic material, but the Xe signature may be inconsistent with ices. We combine this compositional framework with a thermal profile for Titan’s interior that includes the thermophysical properties of organic material, and perform thermodynamic calculations for extraction of volatiles and formation of the hydrosphere. Our results suggest that formation of Titan’s massive hydrosphere requires a large initial ice mass, including a minimum ice/dust mass ratio of 1.4 for anhydrous cometary dust. Finally, we consider the composition and abundance of organic material produced by such processes and them to dissolved organic carbon in terrestrial oceans. We find that Titan’s accreted complex organic material may be a significant contributor to organic material in the subsurface ocean, producing organic abundances that are comparable to terrestrial oceans.

Enceladus and Europa are compelling targets for astrobiology investigations due to their potentially habitable subsurface oceans connected to the icy surface by geological processes. Both moons emit ice grains either ejected via micrometeoroid surface impacts or erupted from their interiors through plume activity. These grains can be sampled by spacecraft flybys, and their composition can be analyzed by impact ionization mass spectrometers, such as the SUrface Dust Analyzer (SUDA) onboard Europa Clipper, or similar instruments proposed for future Enceladus missions. These instruments can identify potential biomolecules, such as amino acids, down to nanomolar concentrations, as demonstrated through previous laboratory experiments. However, the identical masses of isomeric compounds could hinder the mass spectrometric identification and assignment of molecular biosignatures. Here, we investigate the general capability of impact ionization mass spectrometry to distinguish between isomeric compounds, validated with a test case of eight amino acid isomers with an identical molecular mass of 131.173 u and formula C6H13NO2, using quantum chemical calculations. We show that the amino acid isomers (including diastereoisomers) can be uniquely identified due to their distinct mass spectral features and fragmentation patterns, explained through intramolecular hydrogen bonding and other structural specificities of the individual isomers. Importantly, α-amino acids can be clearly differentiated from non-α-amino acids owing to several major mass spectral features. We show that SUDA-type instruments have sufficient capabilities to differentiate certain isomers and to identify biosignatures from ocean worlds with high confidence.

Surface observations of Saturn's moon Titan revealed features characterized as dissected, elevated plateaus with high valley density known as labyrinth terrains. Of this terrain class, a subtype referred to as radial labyrinth is described as dome-shaped uplifts with radial channel patterns. Uplift of these radial labyrinths has previously been explained as cryomagmatic intrusions at the brittle-ductile transition zone. Here we propose an alternative hypothesis that crustal heterogeneities in Titan's upper clathrate crust introduce density differentials due to ethane-methane substitution, as ethane-rich liquids percolate into methane clathrate, inducing solid state flow and generating domal topography. This mechanism is analogous to salt tectonics on Earth and has similarly been evoked for dome formation on the dwarf planet Ceres. We show that the elevation and width of the observed radial labyrinths are consistent with domal uplift driven by a hydraulic head within the uppermost portion of Titan's crust, given a plausible set of elastic parameters for clathrate hydrates. Additionally, the insulating effect of clathrate, combined with partial mixing with water-ice, allows for sufficiently low viscosity for geologic flow on a relevant timescale: uplift of the domes could have occurred within the last billion years.

Field studies at terrestrial analogue sites represent an important contribution to the science of ocean worlds. The value of the science and technology investigations conducted at field analogue sites depends on the relevance of the analogue environment to the target ocean world. We accept that there are no perfect analogues for many of the unique environments represented by ocean worlds but suggest that a one-to-one matching of environmental characteristics and conditions is not crucial to the success or impact of the work. Instead, we must determine which processes and parameters are required to map directly to the target ocean world environment with high fidelity to address the science question. In this review paper, we discuss the outcomes of a workshop aimed at developing a new framework for evaluating the suitability of analogue field locations for ocean worlds research. Here we present a two-step approach to (a) identify the most crucial processes and parameters associated with a given science question and (b) assess the fidelity of these processes and parameters at a proposed field site to those expected for the target ocean world. We demonstrate this approach in a test case evaluating three types of ocean world analogue environments with respect to a science question. The consensus document presented here equips veteran and new investigators with valuable tools to better assess and justify their analogue site selections.

Titan, with its organic-rich atmosphere and global subsurface ocean, has long been considered a prime candidate to potentially host life beyond Earth. The detection of species frozen within ocean fluids when extruded on the surface via cryovolcanism, crust-breaching impacts, or other means, presents an intriguing possibility for the characterization of the ocean composition and its habitability. On Titan, in situ spectra of organic- and salt-bearing ices could serve as evidence of life in the ocean and reveal the presence of salts or other nutrients. To this end, we present infrared (IR) reflectance spectra of organic- and salt-bearing ices under Titan surface conditions. For a subset of these species, a dilution series of spectra is presented to begin to constrain the limits of detection for Titan. Ultimately, this work shows that IR reflectance spectroscopy has little hope of detecting such species frozen in water ice from orbit, or even from a landed spacecraft, due to the low expected concentrations of these target species. The effect of abundant IR absorbers in the atmosphere presents an additional complicating factor. Such detections would require more sensitive in situ analytical techniques such as fluorescence or mass spectrometry, along with the sampling systems that such methods would require.

Impact craters are rare on the icy ocean world Europa: only 24 ≧ 10 km in diameter are currently observed. Craters ≳20 km in diameter, such as Pwyll and Manannán, display unusual morphologies, perhaps because they excavate down close to the subsurface ocean. Previous work suggests that Manannán may have impacted into liquid, slushy ice or thinner ice than at Pwyll. Here we show that impact craters, via their intrinsic morphologic and compositional properties, as well as impact‐excavation of otherwise unobservable materials, provide windows into subsurface structure and composition, making them natural laboratories for investigating Europa's habitability. We find that both Pwyll and Manannán excavated dark material from ∼3.1 km deep, which could be intriguing non‐ice materials such as H 2 SO 4 or hydrated minerals. We calculated that Pwyll impacted into a solid ice shell of ≧10.7 km thickness. We hypothesize that Manannán may have impacted into less viscous material, and detail how this can be tested with future Europa Clipper data. We also hypothesize that a reservoir of sub‐surface impact‐induced material sourced the lobate material within Pwyll. This hypothesis can also be tested with Europa Clipper data and, if confirmed, would provide targets to observe salts derived from potentially some of the most processed brines on Europa.

Propionitrile (also known as ethyl cyanide, CH3CH2CN) and acetylene (C2H2) are two organic molecules that have been detected in Titan's atmosphere. Over time, they may interact with each other as they are transported to Titan's surface. We sought to determine if any reactions or associations such as co-crystal formation might occur between the two molecules. Using micro-Raman spectroscopy, we characterized band shifts, new bands, and morphological changes, which are characteristic of co-crystal formation. We found that the propionitrile:acetylene co-crystal forms within minutes at 90 K and is stable from 90 to 160 K. A cryogenic powder X-ray diffraction study confirms co-crystal formation at 90 K and indexes to a monoclinic unit cell, P21/a. A thermal expansion study between 90 and 140 K indicates that the co-crystal exhibits anisotropic thermal expansion, with a limited change in the b axis over the temperature range. This information gives insight into the preferred form of propionitrile:acetylene and the nature of these molecular interactions under Titan-relevant conditions. We discuss broader implications of the propionitrile:acetylene co-crystal's participation in forming Titan's geologic features such as the karstic, labyrinth terrain. Additionally, co-crystals that include acetylene as a coformer may provide a source of energy for acetylenotrophs to harness, should putative life exist on Titan's surface or in the subsurface. The Dragonfly mission to Titan will explore the nature and distribution of Titan's organics at the surface; thus, characterizing these organics in the laboratory before surface operations will inform the likely phases Dragonfly may encounter and support data analysis and interpretation of this exciting mission.

We explore a hypothesis in which the detection of classes of lipid-like molecules with similar abundance-averaged lengths would constitute a biosignature for other worlds. This is based on the functional requirements of membrane molecules: they must have enough hydrophobic length to not diffuse away from the membrane, be capped by one or two hydrophilic polar groups, and also maintain a semipermeable membrane. Our hypothesis is that once membrane thickness is set in a biological system, it is very difficult to modify it, due to the necessity to redesign all the other associated molecules; the membrane thickness will be constant across all molecular classes that constitute membranes resulting from a common ancestor. In such a scenario, similar thickness values would thus constitute a biosignature and cross-correlate between different molecular classes. We tested this hypothesis by developing a simple method to use modeled lengths of lipid-like molecules to estimate the thicknesses of membranes formed by these molecules. We examined abundance patterns of four different classes of membrane molecules used by terrestrial life: fatty acids, glycerol dialkyl glycerol tetraether lipids, carotenoids, and ladderanes from microbial isolates and environmental samples, as well as abiotic samples of fatty acids. We found that the modeled cell membrane thicknesses from each of these molecular classes were similar and gave results consistent with the observed values. From these results, we propose that our approach provides a framework to identify potential membrane component molecules as an agnostic biosignature. The power of our approach is that our method enables multiple molecular classes to be compared and provides increasing confidence of a biological detection.

Aqueous mixtures of inorganic and organic solutes display complex chemical behavior upon freezing. These systems are analogous to the ocean waters of icy moons in the outer solar system and play an important role in impacting the habitability and astrobiological potential of microenvironments contained in the ice shells. In this study, putative Enceladus ice cores are synthesized in the presence of a range of organic compounds typically found in carbonaceous chondrite meteorites (glycerol, glyceric acid, and 2,3-dihydroxybenzoic acid), and their spatial distribution is investigated using Raman imaging. The results demonstrate an intricate interplay among the water, ice, organic, and salt phases in dictating the partitioning of these components within the ice. Specifically, glycerol and glyceric acid are found to preferentially associate with hydrohalite, the primary salt hydrate that forms upon freezing (carbonates are also observed in these samples). However, the 2,3-dihydroxybenzoic acid (DHBA) system is found to exhibit a drastically different outcome, where the DHBA molecules agglomerate into organic-rich pockets instead of being colocated with the salt hydrates. Based on the molecular interactions previously demonstrated in the crystal structure of DHBA, we assign this behavior to the hydrophobicity of its aromatic ring and the presence of intramolecular hydrogen bonding, coupled with the slow-freezing condition that typically expels impurities from the ice and allows the DHBA molecules to come together. The findings provide insights into the chemical composition of brine channels, which hold important implications for the search for organics in ocean world ice shells. Particularly, while salt-rich zones may present enticing targets to look for simple organics, larger and more complex species with hydrophobic aromatic rings may be occluded elsewhere inside the ice, potentially requiring a more dedicated sampling/detection strategy.

Energy and nutrient sources for life could be delivered to Titan’s subsurface water ocean from both its surface above and its core below. Organic matter forming de novo in Titan’s atmosphere and depositing on the surface may hydrolyze upon descent into the ocean with impact-generated melt pools sinking through the ice, adding to a primordial inventory released by the core during differentiation and/or across geologic time. This raises the possibility that abiotic organic carbon could fuel heterotrophic carbon assimilation into biomass in Titan’s ocean if it is inhabited. Glycine fermentation is one possible metabolism of interest, because mechanisms exist to transport glycine to Titan’s ocean and anaerobic fermentations do not rely on additional strong oxidants which may not be present on Titan. Using bioenergetic modeling, we show that while conditions favorable to glycine fermentation may exist, they are highly dependent on temperature. Additionally, the ability of that metabolism to fuel a global biosphere is limited by the slow delivery of glycine by impact melt pools (<10 nmolal yr ^−1 optimistically, with a >1 mmolal primordial component). A total population of 10 ^14 –10 ^17 cells (a few kilograms of carbon) can be sustained, amounting to less than 1 cell kg ^–1 water when diluted through the entire ocean. Constraining notionally detectable biospheres on Titan will therefore require (i) considering localized environments that may concentrate cells, (ii) better characterizing other candidate metabolisms (e.g., degradation of acetylene or polyaromatic hydrocarbons) for quantitative bioenergetic modeling, and (iii) resolving new mechanisms to deliver organics and oxidants for life.

We analyzed spectral cubes of Callisto's leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25-micron absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 microns over both hemispheres, confirming the global presence of CO2 gas in Callisto's tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto's trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto's atmosphere. We detected a 4.38-micron absorption band that likely results from solid-state 13CO2. A prominent 4.57-micron absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto's leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially anti-associated. The distribution of the 4.57-micron band is more consistent with a native origin and/or accumulation of dust from Jupiter's irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide (OCS), and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface-atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA's Europa Clipper and ESA's JUICE spacecraft.

Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a habitable environment. In this work, we determined the amount of organic material that can be delivered from Titan's surface to its ocean through impact cratering. We assumed that Titan's craters produce impact melt deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 1011 mol/Gyr for HCN hydrolysis and from 0 to 1014 mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass productivity of ∼103 kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than recent estimates of the hypothetical biomass production supported by Enceladus's subsurface ocean. Unless biologically available compounds can be sourced from Titan's interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere.

We use hyperspectral-imaging observations from the Visual and Infrared Mapping Spectrometer (VIMS) to identify and explain processes of Titan's equator through the qualitative spatial relationships between geographic features (i.e. geospatial-topology). Our geographic features are defined by their spectra and geomorphology. We use tens of millions of VIMS pixels between 30 degrees S and 30 degrees N with incidence and emission angles <75 degrees, and a pixel spatial scale of 200 km or less. Our dataset is several orders of magnitude larger than previous studies. This is possible through our use of novel techniques to reduce scattering and improve inter-flyby comparisons. We validate the dataset produced by these techniques by reproducing the results of previous studies. We use vector quantization, dimension reduction, and the Monte Carlo method to identify 14-16 spectrally and spatially distinct units within our dataset, a priori maps or images. These spectral units occur in distinct sequences, indicating that there is a discrete number of spectral pathways to describe the transitions across the surface. Using the same methodology used to identify the spectral units, we determine the spectral transition between units can be explained by five spectroclines. We define a spectrocline as the geographic expression of change in spectra between two spectrally distinct units; it is the spectral equivalent of an ecocline. We compare the spectra of the five spectroclines to the USGS spectral library to identify candidates for the change in compositions across the equator. We find evidence among the spectra of the spectroclines for changes in abundance of water-ice, acetylene, benzene, and alkane species. With the help of the geomorphological units identified in Cassini RADAR images, we discuss the significance of the spectral units and the spectroclines based on their geospatial-topology including their distribution, frequency, size, patterns, and sequences. From the correlations we identify between the spectra and geomorphology, we propose mechanisms for the formation and evolution of Titan's equatorial surface features.

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories. References: [1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764. Acknowledgements: This work was conducted at the California Institute of Technology (Caltech) under contract with NASA. Y.M. and A.S. (partly) was supported by the Czech Science Foundation (grant no. 20-27624Y). ©2021 California Institute of Technology. Government sponsorship acknowledged.