Infrared Mapping Spectrometer Research Articles

The history and processes of Titan's equator from the geospatial-topology of spectrally distinct units

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°S and 30°N with incidence and emission angles <75°, 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.

Water‐Ice Dominated Spectra of Saturn's Rings and Small Moons From JWST

AbstractJWST measured the infrared spectra of Saturn's rings and several of its small moons (Epimetheus, Pandora, Telesto, and Pallene) as part of Guaranteed Time Observation program 1247. The NIRSpec instrument obtained near‐infrared spectra of the small moons between 0.6 and 5.3 microns, which are all dominated by water‐ice absorption bands. The shapes of the water‐ice bands for these moons suggests that their surfaces contain variable mixes of crystalline and amorphous ice or variable amounts of contaminants and/or sub‐micron ice grains. The near‐infrared spectrum of Saturn's A ring has exceptionally high signal‐to‐noise between 2.7 and 5 microns and is dominated by features due to highly crystalline water ice. The ring spectrum also confirms that the rings possess a 2%–3% deep absorption at 4.13 microns due to deuterated water ice previously seen by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. This spectrum also constrains the fundamental absorption bands of carbon dioxide and carbon monoxide and may contain evidence for a weak aliphatic hydrocarbon band. Meanwhile, the MIRI instrument obtained mid‐infrared spectra of the rings between 4.9 and 27.9 microns, where the observed signal is a combination of reflected sunlight and thermal emission. This region shows a strong reflectance peak centered around 9.3 microns that can be attributed to crystalline water ice. Since both the near and mid‐infrared spectra are dominated by highly crystalline water ice, they should provide a useful baseline for interpreting the spectra of other objects in the outer solar system with more complex compositions.

Near Infrared Spectral Radiance at Multiple Wavelengths From Io's Volcanoes 1: The Low Spatial Resolution Night‐Time Galileo NIMS Data Set

AbstractWe have calculated surface‐leaving spectral radiances for 285 detections of volcanic thermal emission from Io observations made by the Galileo Near Infrared Mapping Spectrometer (NIMS) in the 0.7‐ – 5.2‐μm wavelength range, the first such analysis of the entire low‐spatial‐resolution night‐time data set. Most of these detections are of 53 individual hot‐spots. We establish that 4.8‐μm spectral radiance is a good proxy for total thermal emission from a hot spot. Many detections reported here have not been previously modeled. This set of hot‐spot detections is derived from NIMS night‐time observations obtained between June 1996 (orbit G1) and October 2001 (Orbit I32), where the hot‐spot is sub‐pixel. Modeling of the spectra yields total thermal emission from each volcano in each observation, as well as the 2:5‐μm spectral radiance ratio and radiant flux density. Surface‐leaving spectral radiance is derived at 2.0, 3.5, 3.8, 4.8, and 5.0 μm allowing direct comparison with data obtained by other spacecraft and ground‐based telescope data. The most energetic volcanic eruptions are identified. We find an unusual number of sources in a C30 observation suggesting that a vigorous volcanic activity was taking place simultaneously in multiple locations, possibly tidally controlled.

First Optical Constants of Laboratory-generated Organic Refractory Materials (Tholins) Produced in the NASA Ames COSmIC Facility from the Visible to the Near Infrared (0.4–1.6 μm): Application to Titan’s Aerosols

We have measured the complex refractive indices, from 0.4 to 1.6 μm, of five laboratory-generated organic refractory materials (tholins) produced at low temperature (150 K) using plasma chemistry in the stream of a supersonic expansion in NASA Ames’ COsmic SImulation Chamber (COSmIC) facility. Three samples were produced from N2:CH4 gas precursors (with different voltages inducing different degrees of ionization in the plasma), one sample was produced from N2:CH4:C2H2, and one sample was produced from Ar:CH4 in order to produce a purely carbonaceous sample. The optical constants, n and k, of the samples were determined using spectral reflectance measurements. We observe that both n and k appear to be correlated with the nitrogen content in the solid sample, with samples containing more nitrogen having higher n and k. Comparisons to previous laboratory studies and Titan aerosol optical constants derived from observations show that the COSmIC tholins with a higher nitrogen content (higher n and k) are closer analogs of Titan aerosols. We also present a new analysis of Cassini Visible Infrared Mapping Spectrometer observations of Titan’s atmosphere in the visible to near infrared using the COSmIC tholin optical constants in a radiative transfer model. The COSmIC tholin sample produced from N2:CH4 with the lowest energy level has a spectral behavior that appears well suited to reproduce the observed Titan aerosol properties. This study has therefore demonstrated that this COSmIC tholin sample has valuable and promising optical properties for the analysis of Cassini’s Titan atmospheric observations.

Titan’s North–South Haze Asymmetry Ratio and Boundary at Visible Wavelengths over the Cassini Mission

We document the evolution of the north–south asymmetry (NSA) of Titan’s haze albedo during the Cassini mission between 2004 and 2017. We analyze coadded cube images taken at 96 distinct wavelengths between 0.35 and 1.05 μm by the Cassini Visual and Infrared Mapping Spectrometer (VIMS-V) instrument from 14 Titan flybys. Over half of a Titan year, we observe a near-complete transition in the NSA boundary latitude across the geographic equator from the southern to the northern hemisphere, including a 3 yr fading of the boundary for several years after the equinox. The fading transition of the NSA matches previous observations of a reversal of the NSA in Hubble Space Telescope images of Titan before the winter solstice between 1997 and 2000. A comparison of NSA images taken at similar times but different phase angles shows the NSA boundary is detectable, albeit with less contrast, at moderately high phase angles (∼90°). Analysis of the NSA boundary in T61 and T67 VIMS images further supports a small tilt between the superrotating atmosphere and the solid body of Titan, as suggested in a previous analysis of 0.890 μm images from the Cassini Imaging Science Subsystem.

New Insights into Variations in Enceladus Plume Particle Launch Velocities from Cassini-VIMS Spectral Data

Enceladus’s plume consists mainly of a mixture of water vapor and solid ice particles that may originate from a subsurface ocean. The physical processes underlying Enceladus’s plume particle dynamics are still being debated, and quantifying the particles’ size distribution and launch velocities can help constrain these processes. Cassini’s Visual and Infrared Mapping Spectrometer observed the Enceladus plume over a wavelength range of 0.9–5.0 μm for a significant fraction of Enceladus’s orbital period on three dates in the summer of 2017. We find that the relative brightness of the plume on these different dates varies with wavelength, implying that the particle size distribution in the plume changes over time. These observations also enable us to study how the particles’ launch velocities vary with time and observed wavelength. We find that the typical launch velocity of particles remains between 140 and 148 m s−1 at wavelengths between 1.2 and 3.7 μm. This may not be consistent with prior models where particles are only accelerated by interactions with the vent walls and gas and could imply that mutual particle collisions close to the vent are more important than previously recognized.

Updated Radiative Transfer Model for Titan in the Near-infrared Wavelength Range: Validation against Huygens Atmospheric and Surface Measurements and Application to the Cassini/VIMS Observations of the Dragonfly Landing Area

We present an analysis of Titan data acquired by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) at the landing site of the Dragonfly mission, using a new version of our radiative transfer model for Titan, with significant updates for the spectroscopic parameters of atmospheric gases and photochemical aerosols. Our updated radiative transfer model is validated against the in situ spectroscopic measurements of the Huygens probe during its descent and once landed. We confirm that aerosols with a fractal dimension of 2.3–2.4 provide the best fit to the observations. We apply our radiative transfer model to four VIMS data cubes over the Selk crater region including the Dragonfly landing and exploration areas, further validating our model by producing consistent aerosol population and surface albedo maps. These infrared albedo maps, further corrected from the photometry, enable us to study the Selk crater region in terms of surface composition, landscape formation, and evolution. Our results suggest that the Selk crater is in an intermediate state of degradation and that the mountainous terrains of the area (including the crater rim and ejecta) are likely to be dominated by fine grains of tholin-like sediment. This organic sediment would be transported to the lowlands (crater floor and surrounding plains), possibly with water ice particles, by rivers, and further deposited and processed to form the sand particles that feed the neighboring dune fields. These results provide information for the operational and scientific preparation of the Dragonfly mission, paving the way for future exploration of Titan’s surface composition and geology.

Geomorphological Map of the Soi Crater Region on Titan

AbstractWe mapped the Soi crater region at 1:800,000 scale and produced a geomorphological map using methodology presented by Malaska, Lopes, Williams, et al. (2016), https://doi.org/10.1016/j.icarus.2016.02.021 and Schoenfeld et al. (2021), https://doi.org/10.1016/j.icarus.2021.114516. This region spans longitude 110° to 180°W and latitude 0° to 60°N and is representative of the transition between the equatorial, mid‐latitude, and high‐latitude northern regions of Titan. We used Cassini Synthetic Aperture Radar (SAR) as our primary mapping data set. For areas where SAR was not available, we used lower resolution data from the Imaging Science Subsystem, the Visible and Infrared Mapping Spectrometer, radiometry, and high‐altitude SAR for complete mapping coverage of the region. We identified 22 geomorphological units, 3 of which have been discussed in existing literature but have not yet been incorporated into our mapping investigations. These include sharp‐edged depressions (bse), ramparts (brh), and bright gradational plains (pgh). All six major terrain classes are represented in this region: Craters, Labyrinth, Hummocky/mountainous, Plains, Dunes, and Basin and Lakes. We find that plains dominate the surface of the Soi crater region, comprising ∼73% of the mapped area, followed by dunes (∼14%), mountains/hummocky terrains (∼12%), basin and lakes (∼0.7%), labyrinth terrains (∼0.5%), and crater terrains (∼0.4%). We also observe empty lakes as far south as 40°N. The Soi crater region largely has the same collection and proportion of geomorphological units to other mapped regions on Titan. These results further support the hypothesis that surface processes are, broadly speaking, the same across Titan's middle and equatorial latitudes, with the exception of Xanadu.

Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations

Solar system observations that serve as analogs for exoplanet remote sensing data can provide important opportunities to validate ideas and models related to exoplanet environments. Critically, and unlike true exoplanet observations, solar system analog data benefit from available high-quality ground- or orbiter-derived “truth” constraints that enable strong validations of exoplanet data interpretation tools. In this work, we first present a versatile atmospheric retrieval suite, capable of application to reflected light, thermal emission, and transmission observations spanning a broad range of wavelengths and thermochemical conditions. The tool—dubbed rfast—is designed, in part, to enable exoplanet mission concept feasibility studies. Following model validation, the retrieval tool is applied to a range of solar system analog observations for exoplanet environments. Retrieval studies using Earth reflected light observations from NASA’s EPOXI mission provide a key proof of concept for exo-Earth direct imaging concept missions under development. Inverse modeling applied to an infrared spectrum of Earth from the Mars Global Surveyor Thermal Emission Spectrometer achieves good constraints on atmospheric gases, including many biosignature gases. Finally, retrieval analysis applied to a transit spectrum of Titan derived from the Cassini Visual and Infrared Mapping Spectrometer provides a proof of concept for interpreting more feature-rich transiting exoplanet observations from NASA’s James Webb Space Telescope. In the future, solar system analog observations for exoplanets could be used to verify exoplanet models and parameterizations, and future exoplanet analog observations of any solar system worlds from planetary science missions should be encouraged.

Dark ray craters on Ganymede: Impactor or endogenous origin

Through the spectral analysis of Galileo Near Infrared Mapping Spectrometer (NIMS) observations we now have a better understanding of the origin of dark ray craters (DRCs) on Ganymede. The analysis of NIMS observations show that DRCs are not a single compositional group, and at least some DRCs are the result of the excavation of endogenous material. DRCs overlying bright terrain may be entirely due to the excavation of the darker Ganymede hydrate material; the dark ejecta associated with the Tammuz impact is an example with its rays composed entirely of Ganymede hydrate. In contrast, three DRCs on the trailing hemisphere imaged by NIMS (Kittu, Amon, and Mir craters) show that a third component is present, with the ejecta being a mixture of water-ice, the hydrated non-ice material endogenous to Ganymede, and a less-hydrated, red, non-ice component. The spectrum of this second non-ice component is consistent with hydrated carbonaceous material, implying either a primitive impactor origin or excavation of primordial Ganymede non-ice material. This carbonaceous component is required at an abundance of about 40 to 50% in the ejecta of the three DRCs, assuming discrete mixing. It is absent in icy terrain but often present in regiones unassociated with DRCs, ranging in abundance in that terrain from 0 to again, about 50%. Its abundance in older, more primordial terrain suggests the carbonaceous material is endogenous to Ganymede. Thus, excavation of subsurface material appears capable of accounting for the diversity of DRCs. Additional spectral imaging observations of these and other DRCs are needed to verify this conclusion.

Analysis of four solar occultations by Titan’s atmosphere with the infrared channel of the VIMS instrument: Haze, CH4, CH3D, and CO vertical profiles

Titan, the largest satellite of Saturn, has a dense atmosphere mainly composed of nitrogen, methane at a percent level, and minor species. It is also covered by a thick and global photochemical organic haze. In the last two decades, the observations made by the Cassini orbiter and the Huygens probe have greatly improved our knowledge of Titan's system. The surface, haze, clouds, and chemical species can be studied and characterised with several instruments simultaneously. On the other hand, some compounds of its climatic cycle remain poorly known. This is clearly the case of the methane cycle, which is, however, a critical component of Titan's climate and of its evolution. We reanalysed four solar occultations by Titan's atmosphere observed with the infrared part of the Visual Infrared Mapping Spectrometer (VIMS) instrument. These observations were already analysed, but here we used significantly improved methane spectroscopic data. We retrieved the haze properties (not treated previously) and the mixing ratios of methane, deuterated methane, and CO in the stratosphere and in the low mesosphere. The methane mixing ratio in the stratosphere is much lower (about 1.1%) than expected from Huygens measurements (about 1.4 to 1.5%). This is consistent with previous results obtained with other instruments. However, features in the methane vertical profiles clearly demonstrate that there are interactions between the methane distribution and the atmosphere circulation. We also retrieved the haze extinction profiles and the haze spectral behaviour. We find that aerosols are aggregates with a fractal dimension of Df ≃ 2.3 ± 0.1, rather than Df ≃ 2 as previously thought. Our analysis also reveals noticeable changes in their size distribution and their morphology with altitude and time. These changes are also clearly connected to the atmosphere circulation and concerns the whole stratosphere and the transition between the main and the detached haze layers. We finally display the vertical profiles of CH3D and CO for the four observations. Although the latter retrievals have large error bars due to noisy data, we could derive values in agreement with other works.

Observations and Modeling of the Opposition Surges of the Icy Moons of Saturn Based on Cassini Visual Infrared Mapping Spectrometer Data

Observations of the opposition surges on the main moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus) during Cassini's prime and extended missions are reduced and analyzed. The main data set comes from the Visual Infrared Mapping Spectrometer (VIMS) with wavelength coverage in the visible and near-infrared out to 3.60 μm, covering 99% of the solar spectrum. Imaging Science Subsystem images and Ultraviolet Imaging Spectrometer data augment the VIMS data set. Hapke models are fit to Dione and Rhea, and Irvine's simpler shadowing model is fit to the sparser data sets of Enceladus, Dione, and Rhea. The high porosities (∼95% void space in the optically active portion of the regolith) and forward-scattering properties of the surfaces are similar to other icy moons and to Pluto. A change in the character of their opposition surge at 3.60 μm can be attributed largely to the noninteraction of long-wavelength photons with small particles on these moons’ surfaces. The opposition surge on the low- and high-albedo regions of Dione are similar. However, the low-albedo terrain of Iapetus exhibits a less robust surge than the high-albedo regions, which we attribute to differences in surface texture. The low-albedo hemisphere of Iapetus seems to be akin to the small number of bodies in the solar system that lack an appreciable opposition surge, possibly due to the accumulation of low-albedo dust. With observations over the range of the solar spectrum, we compute new values for the bolometric Bond albedos of these moons.

Packed media radiative-transfer modeling with Gaussian particles: Application to spectra of icy regolith of Saturnian satellites

The radiative transfer theory with packed media correction (RTT-PM) is a balanced, physically rigorous, and practical light scattering model that is suitable for modeling reflectance and similar spectra of densely packed particulate media. The static structure factor correction to the classical radiative transfer solution and actualization of fundamental particles of media with spheres or aggregates are key properties of this model that ensure both physical rigor and practical efficiency. We improved upon the assumptions in the RTT-PM method by incorporating irregularly shaped Gaussian particles into its scheme. This incorporation of Gaussian particles is a notable advancement for applications of the RTT-PM method to planetary surfaces that often are layers of irregularly shaped particles. With the Gaussian particle RTT-PM method, we modeled spectra of Saturnian moons Dione, Rhea, and Tethys observed with the Cassini Visual and Infrared Mapping Spectrometer (VIMS), assuming pure water ice composition. For Rhea and Tethys, the Gaussian particle RTT-PM technique modeled VIMS spectra better than models with spherical or aggregate particles, strengthening prior suggestion that particles on Rhea and Tethys are solid, non-spherical particles. Dione's spectra were best modeled not with Gaussian particles but rather with an aggregate of 128 monomers. This makes Dione's icy regolith different from that on Rhea and Tethys. Prior studies have indicated that the cause for the difference might arise from the presence of surface macrostructures or absorbing materials on Dione, but according to our modeling results, increased multiple scattering from small fluffy aggregate particles is an alternative explanation.

Titan Stratospheric Haze Bands Observed in Cassini VIMS as Tracers of Meridional Circulation

We analyzed Cassini data to derive the nature and evolution of circumglobal annuli observed in the stratosphere of Titan, Saturn's largest moon. The annuli were observed between 2004 and 2017 in data acquired by the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft. We observed a north polar annulus, an equatorial annulus, and several secondary annuli. Pre-Cassini telescopic observations by the Hubble Space Telescope and Keck reported an atmospheric feature consistent with the presence of a south polar annulus between 1999 and 2001, although this feature was not observed by Cassini. Relative to the atmosphere near the annuli, they appear dark at 300–500 nm and bright in methane absorption channels such as the ones at 900 and 1150 nm. The stratosphere seems to rotate around the north pole. Alternatively, it seems to rotate about a point offset from solid-body rotation axis by a few degrees; this point in turn rotates around the solid-body rotation axis.

Kronoseismology. VI. Reading the Recent History of Saturn’s Gravity Field in Its Rings

Abstract Saturn’s C ring contains multiple structures that appear to be density waves driven by time-variable anomalies in the planet’s gravitational field. Semiempirical extensions of density wave theory enable the observed wave properties to be translated into information about how the pattern speeds and amplitudes of these gravitational anomalies have changed over time. Combining these theoretical tools with wavelet-based analyses of data obtained by the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft reveals a suite of structures in Saturn’s gravity field with azimuthal wavenumber 3, rotation rates between 804° day−1 and 842° day−1, and local gravitational potential amplitudes between 30 and 150 cm2 s−2. Some of these anomalies are transient, appearing and disappearing over the course of a few Earth years, while others persist for decades. Most of these persistent patterns appear to have roughly constant pattern speeds, but there is at least one structure in the planet’s gravitational field whose rotation rate steadily increased between 1970 and 2010. This gravitational field structure appears to induce two different asymmetries in the planet’s gravity field, one with azimuthal wavenumber 3 that rotates at roughly 810° day−1 and another with azimuthal wavenumber 1 rotating three times faster. The atmospheric processes responsible for generating the latter pattern may involve solar tides.

Saturn’s icy satellites investigated by Cassini - VIMS. V. Spectrophotometry

Albedo, spectral slopes, and water ice band depths maps for the five midsized saturnian satellites Mimas, Enceladus, Tethys, Dione, and Rhea have been derived from Cassini-Visual and Infrared Mapping Spectrometer (VIMS) data. The maps are systematically built from photometric corrected data by applying the Kaasalainen-Shkuratov model (Kaasalainen et al., 2001; Shkuratov et al., 2011). In this work a quadratic function is used to fit phase curves built by filtering observations taken with incidence angle i≤70∘, emission angle e≤70∘, phase angle 10∘≤g≤120∘, and Cassini-satellite distance D≤100.000 km. This procedure is systematically repeated for a subset of 65 VIMS visible and near-infrared wavelengths for each satellite. The average photometric parameters are used to compare satellites’ properties and to study their variability with illumination conditions changes. We derive equigonal albedo, extrapolated at g = 0∘, not including the opposition effect, equal to 0.63 ± 0.02 for Mimas, 0.89 ± 0.03 for Enceladus, 0.74 ± 0.03 for Tethys, 0.65 ± 0.03 for Dione, 0.60 ± 0.05 for Rhea at 0.55μm. The knowledge of photometric spectral response allows to correct individual VIMS spectra used to build maps through geolocation. Maps are rendered at a fixed resolution corresponding to a 0.5∘×0.5∘ bin on a longitude by latitude grid resulting in spatial resolutions of 1.7 km/bin for Mimas, 2.2 km/bin for Enceladus; 4.7 km/bin for Tethys; 4.5 km/bin for Dione; 6.7 km/bin for Rhea. These spectral maps allow establishing relationships with morphological features and with endogenic and exogenic processes capable to alter satellites’ surface properties through several mechanisms. The hemispheric dichotomies in albedo and spectral indicators between leading and trailing hemispheres are common properties of all midsized satellites: the accumulation of fine E ring grains is responsible for the higher albedo measured across the leading hemispheres of Tethys, Dione, Rhea, and the trailing side of Mimas. Conversely, the dark and red-colored material visible across the trailing hemispheres of Tethys, Dione, and Rhea is associated with the implantation of cold plasma particles. The thermal anomaly lenses located on the leading equatorial regions of Mimas and Tethys have been resolved and mapped. VIMS data evidence that the distribution of water ice band depths on Mimas lens is biased by the presence of the large Herschel impact crater pointing to a different regolith size distribution with respect to Tethys. Maps show local changes of albedo and spectral indicators in correspondence of recent impact craters (Inktomi on Rhea, Creusa on Dione) and on Dione’s wispy terrains are caused by the exposure of pristine water ice. Enceladus’ tiger stripes, the active sources of plumes in the southern polar region, despite being partially resolved on VIMS maps allow measuring an exceedingly high band depths in comparison with the rest of the satellite’s surface. Moreover, Enceladus’ smooth terrains located on the leading hemisphere around (lon,lat)=(90∘,30∘) show peculiar properties (low infrared albedo, positive 0.35-0.55μm slope and maximum band depth) possibly associated with the presence of a buried diapir in this area. The variability of average spectral albedos and indicators induced by phase angle is investigated and exploited to establish a comparison among satellites’ properties as a function of orbital distance from Saturn.

A Comprehensive Revisit of Select Galileo/NIMS Observations of Europa

Abstract The Galileo Near Infrared Mapping Spectrometer (NIMS) collected spectra of Europa in the 0.7–5.2 μm wavelength region, which have been critical to improving our understanding of the surface composition of this moon. However, most of the work done to get constraints on abundances of species like water ice, hydrated sulfuric acid, hydrated salts, and oxides has used proxy methods, such as absorption strength of spectral features or fitting a linear mixture of laboratory-generated spectra. Such techniques neglect the effect of parameters degenerate with the abundances, such as the average grain size of particles, or the porosity of the regolith. In this work we revisit three Galileo NIMS spectra, collected from observations of the trailing hemisphere of Europa, and use a Bayesian inference framework, with the Hapke reflectance model, to reassess Europa’s surface composition. Our framework has several quantitative improvements relative to prior analyses: (1) simultaneous inclusion of amorphous and crystalline water ice, sulfuric-acid-octahydrate (SAO), CO2, and SO2; (2) physical parameters like regolith porosity and radiation-induced band-center shift; and (3) tools to quantify confidence in the presence of each species included in the model, constrain their parameters, and explore solution degeneracies. We find that SAO strongly dominates the composition in the spectra considered in this study, while both forms of water ice are detected at varying confidence levels. We find no evidence of either CO2 or SO2 in any of the spectra; we further show through a theoretical analysis that it is highly unlikely that these species are detectable in any 1–2.5 μm Galileo NIMS data.

The surface of (4) Vesta in visible light as seen by Dawn/VIR

Aims.We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology.Methods.We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite meteorites data as a reference, we derived the lithology of Vesta using the variations of the 930 and 506 nm (spin-forbidden) band centers observed in the VIR dataset.Results.Our spectral parameters highlight a significant spectral diversity at the surface of Vesta. This diversity is mainly evidenced by impact craters and illustrates the heterogeneous subsurface and upper crust of Vesta. Impact craters also participate directly in this spectral diversity by bringing dark exogenous material to an almost entire hemisphere. Our derived lithology agrees with previous results obtained using a combination of infrared and visible data. We therefore demonstrate that it is possible to obtain crucial mineralogical information from visible wavelengths alone. In addition to the 506 nm band, we identified the 550 nm spin-forbidden one. As reported by a laboratory study for synthetic pyroxenes, we also do not observe any shift of the band center of this feature across the surface of Vesta, and thus across different mineralogies, preventing use of the 550 nm spin-forbidden band for the lithology derivation. Finally, the largest previously identified olivine rich-spot shows a peculiar behavior in two color composites but not in the other spectral parameters.

Laboratory Investigations Coupled to VIR/Dawn Observations to Quantify the Large Concentrations of Organic Matter on Ceres

Organic matter directly observed at the surface of an inner planetary body is quite infrequent due to the usual low abundance of such matter and the limitation of the infrared technique. Fortuitously, the Dawn mission has revealed, thanks to the Visible and InfraRed mapping spectrometer (VIR), large areas rich in organic matter at the surface of Ceres, near Ernutet crater. The origin of the organic matter and its abundance in association with minerals, as indicated by the low altitude VIR data, remains unclear, but multiple lines of evidence support an endogenous origin. Here, we report an experimental investigation to determine the abundance of the aliphatic carbon signature observed on Ceres. We produced relevant analogues containing ammoniated-phyllosilicates, carbonates, aliphatic carbons (coals), and magnetite or amorphous carbon as darkening agents, and measured their reflectance by infrared spectroscopy. Measurements of these organic-rich analogues were directly compared to the VIR spectra taken from different locations around Ernutet crater. We found that the absolute reflectance of our analogues is at least two orders of magnitude higher than Ceres, but the depths of absorption bands match nicely the ones of the organic-rich Ceres spectra. The choices of the different components are discussed in comparison with VIR data. Relative abundances of the components are extrapolated from the spectra and mixture composition, considering that the differences in reflectance level is mainly due to optical effects. Absorption bands of Ceres’ organic-rich spectra are best reproduced by around 20 wt.% of carbon (a third being aliphatic carbons), in association with around 20 wt.% of carbonates, 15 wt.% of ammoniated-phyllosilicate, 20 wt.% of Mg-phyllosilicates, and 25 wt.% of darkening agent. Results also highlight the pertinence to use laboratory analogues in addition to models for planetary surface characterization. Such large quantities of organic materials near Ernutet crater, in addition to the amorphous carbon suspected on a global scale, requires a concentration mechanism whose nature is still unknown but that could potentially be relevant to other large volatile-rich bodies.

Geomorphological map of the South Belet Region of Titan

We mapped in detail Titan's South Belet region which spans from longitude 60°E to 120°E and from latitude 60°S to 0°, encompassing both equatorial and southern mid-latitude regions. We used Cassini RADAR in its Synthetic Aperture Radar (SAR) mode data as our basemap, which covers 31.8% of the region, supplemented with data from the RADAR's radiometry mode, the Imagining Science Subsystem (ISS), the Visual and Infrared Mapping Spectrometer (VIMS), and topographic data. This mapping work is a continuation of the detailed global mapping effort introduced in Malaska et al. (2016a) and continued in Lopes et al. (2020). We followed the mapping procedure described in Malaska et al. (2016a) for the Afekan Crater region and identified four major terrain classes in South Belet: craters, hummocky/mountainous, plains, and dunes. Each terrain class was subdivided into terrain units by characteristic morphology, including border shape, texture, general appearance, and radar backscatter. There are two terrain units that were not included in previous studies but were identified in our mapping of South Belet: “bright alluvial plains” and “pitted hummocky”. Similar to the Afekan Crater region, we find that plains dominate the surface make-up of South Belet, comprising ~47% of the mapped area. Unlike Afekan, the areal extent of the dunes closely rivals the dominance of plains, making up 43% of the mapped area. The next most widespread unit by area in the region following the dunes are the mountains/hummocky terrains (10%), and finally, crater terrains (0.01%). The introduction of two new units, “bright alluvial plains” and “pitted hummocky”, are necessary to capture the full range of morphologies seen in South Belet and expands our understanding of processes typical of Titan's equatorial and mid-latitude regions. For example, the presence of alluvial fans indicates a period in Titan's past where discharges and slopes were such that sediment could be mobilized and deposited. Similarly, the pits associated with the “pitted hummocky” may represent an important erosional feature, with implications for the removal of volatiles from Titan's crust. However, analysis of our geomorphological mapping results suggests the geology of South Belet is consistent with the narrative of organics dominating the equatorial and mid-latitudes. This is similar to the conclusion we arrived at through our mapping and analysis of the Afekan region. Lastly, the applicability of the terrain units from our mapping of the Afekan region, which bears a similar latitude but in the northern hemisphere, to our mapping of South Belet suggests latitudinal symmetry in Titan's surface processes and their evolution.