Stereo Surface Imaging System Research Articles

A Global Dataset of Potential Chloride Deposits on Mars as Identified by TGO CaSSIS

Chloride deposits are markers for early Mars’ aqueous past, with important implications for our understanding of the martian climate and habitability. The Colour and Stereo Surface Imaging System (CaSSIS) onboard ESA’s Trace Gas Orbiter provides high-resolution color-infrared images, enabling a planet-wide search for (small) potentially chloride-bearing deposits. Here, we use a neural network to map potentially chloride-bearing deposits in CaSSIS images over a significant fraction of the planet. We identify 965 chloride deposit candidates with diameters ranging from <300 to >3000 m, including previously unknown deposits, 136 (~14%) of which are located in the highlands north of the equator, up to ~36°N. Northern chloride candidates tend to be smaller than in the south and are predominantly located in small-scale topographic depressions in low-albedo Noachian and Hesperian highland terranes. Our new dataset augments existing chloride deposit maps, informs current and future imaging campaigns, and enables future modelling work towards a better understanding of the distribution of near-surface water in Mars’ distant past.

Evidence for transient morning water frost deposits on the Tharsis volcanoes of Mars

The present-day water cycle on Mars has implications for habitability and future human exploration. Water ice clouds and water vapour have been detected above the Tharsis volcanic province, suggesting the active exchange of water between regolith and atmosphere. Here we report observational evidence for extensive transient morning frost deposits on the calderas of the Tharsis volcanoes (Olympus, Arsia and Ascraeus Montes, and Ceraunius Tholus) using high-resolution colour images from the Colour and Stereo Surface Imaging System on board the European Space Agency’s Trace Gas Orbiter. The transient bluish deposits appear on the caldera floor and rim in the morning during the colder Martian seasons but are not present by afternoon. The presence of water frost is supported by spectral observations, as well as independent imagery from the European Space Agency’s Mars Express orbiter. Climate model simulations further suggest that early-morning surface temperatures at the high altitudes of the volcano calderas are sufficiently low to support the daily condensation of water—but not CO2—frost. Given the unlikely seasonal nature of volcanic outgassing, we suggest the observed frost is atmospheric in origin, implying the role of microclimate in local frost formation and a contribution to the broader Mars water cycle.

Novel quantitative methods to enable multispectral identification of high-purity water ice exposures on Mars using High Resolution Imaging Science Experiment (HiRISE) images

Reliable detection and characterization of water ice on the Martian surface is pivotal to not only understand its present and past climate, but to also provide valuable information on in-situ resource availability and distribution for future human exploration missions. Ice-rich features are currently identified with visible/near-IR (VNIR), thermal IR and radar data. However, their coarse spatial scale sometimes limits confident characterization of small (i.e., meter-scale) icy exposures resulting from recent activity like new impacts. Water ice bearing materials possess weaker spectral characteristics at wavelengths shorter than ∼1030 nm that may be resolved by VNIR imaging instruments like the High Resolution Imaging Science Experiment (HiRISE) and the Colour and Stereo Surface Imaging System (CaSSIS). Our study assesses the spectral capability of HiRISE colour observations to help distinguish high purity water ice exposures from ice-poor materials. We report detailed methodologies for reliable colour characterization of icy surface using unfiltered HiRISE images. We present the first quantitative approach to uniquely characterize high-purity ice-rich materials through spectral shape and spectral parameterization methods at high spatial resolution (∼50 cm/pixel). We also present three spectral parameters to aid detection of pure water ice features, while also providing statistical constraints to enable a quantitative interpretation scheme. Our methods are observed to work well in characterizing and separating ice-rich features uniquely from ice-poor and ferrous materials. However, we do observe that these methods have a lower grain size detection limit of ∼250–300 μm, and may not be able to uniquely separate frosts from ground ice exposures. We also apply these methods to better constrain the composition of bright materials exposed by recent impacts identified in previous surveys, where substantial evidence for ice-bearing materials was previously unavailable. Overall, our work proposes HiRISE colour-based methods as a novel approach for high-resolution multispectral characterization of ice-rich features on the Martian surface, which is of particular value since the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has ceased operations.

Insights into the interaction between defrosting seasonal ices and gully activity from CaSSIS and HiRISE observations in Sisyphi Cavi, Mars

Martian gullies are surface features, typically composed of an alcove, a channel and a depositional apron. They have been extensively studied since their first observations in the 2000s and were initially attributed to the action of liquid water. Later studies highlighted that their activity is spatially and temporally correlated with the seasonal presence of surface CO2 ice, suggesting a link between seasonal frost and gully activity. However, the exact mechanisms leading to gully formation are still under debate. Establishing whether or not gullies are formed by liquid water has important implications for Mars recent climate and habitability. Here, we study the evolution of seasonal frost and its connection to gully activity in Sisyphi Cavi, located in the southern circumpolar region (68°S - 74°S; 345°E − 5°E). The high latitude of this site allows for detailed temporal monitoring, owing to the high frequency imaging by spacecraft in polar orbits, largely unavailable for other martian gullies, hence this site has the potential to reveal new insights on gullies in general. In particular, we used repeat images from MRO HiRISE (Mars Reconnaissance Orbiter, High Resolution Imaging Science Experiment) and TGO CaSSIS (Trace Gas Orbiter, Colour and Stereo Surface Imaging System). Our findings show that the general timing and characteristics of the defrosting patterns in Sisyphi Cavi follow seasonal patterns that are consistent from year to year. The timing of the start of defrosting and its temporal evolution are controlled by the overall orientation of the host hillslope, with equator-facing slopes defrosting before pole-facing ones and the steepest pole-facing slopes defrosting last. Gully alcoves defrost before channels and aprons located on the same hillslope independent of the orientation of the hillslope likely because the aprons have a lower thermal inertia than the alcoves that counteracts the orientation effect, which on pole-facing slopes would mean the alcoves should defrost last. We observe the presence of seasonal and ephemeral dark spots and flows which are interpreted to be a result of sediment deposition on top of the seasonal ice deposits, triggered by basal sublimation. Our observations suggest that the appearance of dark spots and dark flows can be spatially correlated with surface roughness, including presence of boulders, contrasts in material types, irregular lobes/levees and braiding caused by gully activity, and therefore could be used to help detect recently active gullies on Mars in areas with seasonal frost without the need of repeat imaging. Finally, we propose that presently observed gully activity in Sisyphi Cavi is driven by defrosted material flowing on to a frosted apron. We infer that the presence of a frosted apron could be common precursor for this type gully activity. We note that this activity only involves the mobilisation of loose materials and we have not observed any erosion of the wall rock present in Sisyphi Cavi.

Preface for special issue on the Colour and Stereo Surface Imaging System (CaSSIS) at Mars

Preface for special issue on the Colour and Stereo Surface Imaging System (CaSSIS) at Mars

Seasonal low albedo streaks at the edge of the Martian south polar layered deposits

Low Albedo Streaks (LAS) are narrow dark streaks observed at the edges of the South Polar Layered Deposits (SPLD) on Mars. We investigate the seasonal morphology, surface topography of LAS-rich areas using monochrome and multiband images from both the High Resolution Imaging Science Experiment (HiRISE) and the Color and Stereo Surface Imaging System (CaSSIS). We complement the image analysis with spectral data from the Compact Reconnassiance Imaging Spectrometer (CRISM). In addition, we analyze the thermal environment of LAS-rich areas with data from a Martian global climatological model. Our analysis suggests that streaks (i) originate from similar scarps and outcrops on the edge of layered structures at the end of local winter during different Martian years, (ii) lengthening in spring forming sinuous branches on terrain with rippled ridges, and (iii) eventually fade leaving distinguishable bright deposits within the streaked area. The surface topography analysis indicates that streaks advance on slopes between 4° and 34°, while the dust flows emanated from mass-wasting scars propagate on the slope steeper than 20°. Our spectral data analysis suggests that pyroxene- or basaltic-bearing materials are deposited on the surface mainly composed of dust by LAS. Thermal analysis indicates that LAS are active when the surface temperature rises from 145.5 K to 162 K, fading when the temperature increases suddenly and significantly. Finally, we evaluate several possible mechanisms to explain the formation of LAS. LAS might be formed by liquids, and such liquids might be brines. However, the presence of salts with lower eutectic temperatures seems to be necessary for the existence of brine on the Martian surface, and should be investigated at the target area in the future.

Dendritic ridges in Antoniadi basin, Mars: Fluvial or volcanic landforms?

Antoniadi basin displays dark-toned dendritic ridges previously interpreted as inverted fluvial channels. Detailed observations of these dark-toned ridges as well as the geological units in the central region of Antoniadi basin are provided emphasizing images from the Colour and Stereo Surface Imaging System (CaSSIS), the High Resolution Stereo Camera (HRSC) and the High Resolution Imaging Science Experiment (HiRISE) instruments. Results show that the dark-toned ridges are part of the most recent geological unit as they overlie, and thus postdate all plains of the central Antoniadi basin, which is Early Amazonian based on its crater size-frequency distribution. Our observations of the dark-toned ridges are not consistent with inverted fluvial channels: they do not widen in the expected downstream direction, they display a rubbly texture and lack layering at high resolution, and have lobes with local levees in place of channel heads. In addition, the branched ridges are more mafic in composition and display a relatively higher thermal inertia than their surroundings. This suite of characteristics is better explained by volcanic flows developed as distributary channels rather than fluvial tributary channels. The occurrence of dikes in the east and west of the studied region supports that these flows were formed by lava, perhaps a'a like flows as suggested by the rubbly texture, but with an unusually high degree of digitation. Alternatively, such a geometry could be explained by the emplacement of the lava along pre-existing fluvial valleys, but neither the underlying topography, nor two nearby older craters, exhibit signs of fluvial erosion.

Change detection and monitoring of active Martian surface phenomena with the Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO)

Mars is host to a variety of active surface processes that relate to changes in seasonal ice/frost, slope activity, wind and processes potentially relating to liquid water. Regular monitoring and change detection of these phenomena is crucial to not only provide us insights into present day Martian surface conditions, but also its past geologic and climatic scenarios. Visual comparisons of high-resolution remote sensing images of the surface from the Mars Reconnaissance Orbiter (MRO) have allowed detailed tracking and monitoring of these changes over time. The CaSSIS instrument onboard the ExoMars Trace Gas orbiter also provides a unique pathway to track areally-large surface changes in colour, that permits us to better constrain origin and evolution of various surface features. Since TGO operates in a non-sun-synchronous orbit, previously adopted qualitative methods like visual image comparisons for MRO cannot be adopted for CaSSIS-based change detection. Consequently, this study lists and describes in detail, the techniques that need to be adopted to conduct such change detection campaigns with CaSSIS, for the variety of active processes currently identified on Mars. This work also proposes and describes two additional semi-quantitative techniques for CaSSIS-based change detection, that may be used in concert with existing visual comparison methods to enable reliable change identification and tracking. It is observed that these supplementary methods work well in characterizing a variety of surface changes related to ice/frost and slope processes, and can provide reasonable spectral constraints to better understand their origin. These methods are not effective in providing quantitative constraints for purely physical changes. Instead, for long-period changes like aeolian bedform movement, digital image correlation techniques on ortho-rectified images, are recommended; while for short period changes like real-time dust devil activity, measurements based on CaSSIS stereo pairs of the region can be used to provide quantitative estimates of change.

Hydrothermal Alteration of Ultramafic Rocks in Ladon Basin, Mars—Insights From CaSSIS, HiRISE, CRISM, and CTX

AbstractThe evolution of the Ladon basin has been marked by intense geological activity and the discharge of huge volumes of water from the Martian highlands to the lowlands in the late Noachian and Hesperian. We explore the potential of the ExoMars Trace Gas Orbiter/Color and Stereo Surface Imaging System color image data set for geological interpretation and show that it is particularly effective for geologic mapping in combination with other data sets such as HiRISE, Context, and Compact Reconnaissance Imaging Spectrometer for Mars. The study area displays dark lobate flows of upper Hesperian to early Amazonian age, which were likely extruded from a regional extensional fault network. Spectral analysis suggests that these flows and the underlying rocks are ultramafic. Two distinct altered levels are observed below the lobate flows. The upper, yellow‐orange level shows hundreds of structurally controlled narrow ridges reminiscent of ridges of listwanite, a suite of silicified, fracture‐controlled silica‐carbonate rocks derived from an ultramafic source and from serpentine. In addition to serpentinite, the detected mineral assemblages may include chlorite, carbonates, and talc. Kaolin minerals are detected in the lower, white level, which could have formed by groundwater alteration of plagioclase in the volcanic pile. Volcanism, tectonics, hydrothermal activity, and kaolinization are interpreted to be coeval, with hydrothermal activity and kaolinization controlled by the interactions between the aquifer and the hot, ultramafic lobate flows. Following our interpretations, East Ladon may host the first listwanite ridges described on Mars, involving a hydrothermal system rooted in a Hesperian aquifer and affecting ultramafic rocks from a magmatic source yet to be identified.

Seasonal southern circum-polar spots and araneiforms observed with the colour and stereo surface imaging system (CaSSIS)

The southern polar area of Mars is home to various seasonal activity commonly explained by the Kieffer model. During southern spring, the ice covering the polar area sublimates and leaves distinct features (spiders, spots, fans) observable from orbit. The Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO), provides high-resolution multi-filter images of the Martian surface offering high sensitivity to colour contrasts. Its stereo capability is pivotal for momentary processes and offers a unique perspective for studying surface sublimation processes and their relation to atmospheric features. For the first time, we identify clouds well correlated with surface features (araneiforms and spots at southern circum-polar latitudes) hence motivating a new campaign to refine these observations over time periods where CO 2 sublimation processes occur. We focus here on the structure of spot deposits and their evolution through time. We identify and describe seven structures: dark spot, bright-haloed spot, ringed spot, inverted spot, dark-haloed spot, banded spot, and bright spot. By morphological and spectral analyses, we hypothesize a new chronology of events that characterize the origin, formation and evolution of these features. • We analysed dune spots and araneiform terrain during southern spring on Mars. • Various structures of spots are observed with the Colour and Stereo Surface Imaging System through time and provide grounds for a classification of spots. • We hypothesize a new chronology of events that characterize the origin, formation and evolution of these features. • Diffuse atmospheric features (clouds and haze) have been observed correlating with surface features (spot, araneiform terrains).

Assessing slope uncertainties of martian Digital Elevation Models from numerical propagation of errors on synthetic geological surfaces

Digital Elevation Models (DEM) are widely used in planetary sciences, including for the specific case of Mars. DEMs allow us to extract topography parameters necessary in geomorphological studies. However, DEMs are not free from vertical errors, which yields uncertainties in calculations of parameters such as local slopes. In addition, slope maps computed from DEMs often display slope patterns which are not spatially correlated with the original images. We suspect such slope patterns to originate from DEM vertical errors. To investigate this question, we propose a fully numerical method to provide a quantitative analysis of slope errors based on DEM error propagation using synthetic models. We find that the addition of vertical errors following a normal distribution (random noise) leads to the occurrence of slope patterns comparable to those in observed data. Results are similar for the two models of spatially correlated errors. We also provide estimations of slope errors for four martian cameras: HiRISE (High Resolution Imaging Science Experiment), CaSSIS (Colour and Stereo Surface Imaging System), HRSC (High Resolution Stereo Camera) and MOC (Martian Orbiter Camera). These estimations aim to be used as first order uncertainty constraints on local slopes for geomorphological studies.

Geometric processing of TGO CaSSIS observations

The Colour and Stereo Surface Imaging System (CaSSIS) is the scientific imager onboard the European Space Agency/Roscosmos ExoMars Trace Gas Orbiter. CaSSIS has one of the most advanced capabilities for colour imaging of the Martian surface on any spacecraft currently orbiting Mars. Such capability can only be utilized, however, by accurate geometric processing of CaSSIS datasets. Here we describe the dataset processing carried out by the CaSSIS geometry pipeline. This pipeline takes a number of single CaSSIS exposures and applies co-registration and map projection algorithms to produce full CaSSIS image mosaics in single and multiple wavelength bands. This process is described in detail along with the software packages that are implemented. The need for accurate co-registration between the different wavelength channels is shown. A description of the output from the pipeline is also given, with example images being shown to illustrate the result of using different CaSSIS wavelength channel combinations for image reconstruction.

Patterns of martian glacial deformation: Implications for glacio-geology, internal structure, and regional climate

Viscous Flow Features (VFF) are widespread in the martian mid-latitudes and indicative of near-surface ice deposits. Their distribution and morphology hint at the regional history of ice deposition and ablation, as well as changes in surface temperature. Here we interrogate the deformation history of a type of VFF, a Lobate Debris Apron (LDA), located in the eastern Hellas region, from its surface morphology, discussing the implications it poses for its internal structure and regional climate variability. Our observations integrate data from the Colour and Stereo Surface Imaging System (CaSSIS), the SHAllow RADar (SHARAD), the Context Camera (CTX), the High Resolution Imaging Science Experiment (HiRISE), and the Mars Orbiter Laser Altimeter (MOLA). Morphological observations, spectral analysis of characteristic wavelengths, and ice deformation stability analysis place constraints on the dynamics and deformation history of the deposit. We discuss contrasting hypotheses for the origin of the different surface structures, including the possibility of gelifluction in addition to glacial creep. Our results provide a guide to interpret glacial deformation patterns in martian VFFs in the light of internal structure, regional climate history, and underlying topography.

Pre-landslide topographic reconstruction in Baetis Chaos, mars using a CaSSIS Digital Elevation Model

Planview detailed morphological analysis of martian landslides is usually performed using orbital imagery such as from the ConTeXt camera (CTX) at 6 ​m/pix, the Colour and Stereo Surface Imaging System (CaSSIS) at 4.5 ​m/pix or the High-Resolution Imaging Science Experiment (HiRISE) at 0.25–0.5 ​m/pix. However, topographic information is key to fully understand a landslide's formation mechanism and its mobility, by estimating the material volumes mobilised and the spatial distribution of erosion and deposition. Digital Elevation Models (DEM) are required to carry out these analyses; nevertheless, there is currently a gap in landslide-volume studies between those using Mars Orbiter Laser Altimeter (MOLA) dataset at ∼450 ​m/pix or HRSC at 50–200 m/pix and those using HiRISE data at 1–2 m/pix, which is only partially filled by CTX elevation data at ∼20 ​m/pix. The CaSSIS camera on board the ESA/Trace Gas Obiter (TGO) can be used to produce DEMs, but so far, such data have not yet been used to conduct a landslide volume analysis. Here, we use three reconstruction methods (semi-automatic, morphology-based and tilted) on a CaSSIS DEM to estimate the initial topography and hence the volume and the distribution of erosion and deposition of a 6 ​km long landslide in Baetis Chaos. Despite the complex topography of the surrounding area due to the presence of an ejecta deposit beneath the landslide, we were able to estimate the landslide's volume and mass distribution. Using a tilted plane as part of estimating the initial topography produced the best results. We evaluated the success by considering the quantifiable balance between erosion and deposition (given the uncertainties) and more subjectively by considering whether the volume distribution matched with what was expected based on the morphology in images alone. Therefore, we recommend the use of this method for individual landslide studies in complex topography where detailed knowledge of the deposit-thickness distribution is required. The semi-automatic reconstruction method produces satisfactory volume estimates and would be better suited to studies where hundreds of landslides are present. We found that CaSSIS data can be used to successfully conduct such analyses, providing additional DEM coverage to study martian medium-scale landslides or other landforms of similar scale (5–15 ​km) with the notable benefit that it provides single-pass stereo image acquisition. • Digital Elevation Model are needed to estimates landslide's material volumes and distribution in order to fully understand their mobility. • CaSSIS (Colour and Stereo Surface Imaging System) DEM have not yet been used to conduct such landslide analysis. • Three reconstruction on a CaSSIS DEM have been used to estimate the volume and the material distribution of a landslide in Baetis Chaos on Mars. • CaSSIS data can be used to conduct analysis and provide additional coverage to study medium-scale landslides or other landforms (scale 5–15 km).

CaSSIS-based stereo products for Mars after three years in orbit

The Colour and Stereo Surface Imaging System (CaSSIS) on board the ExoMars Trace Gas Orbiter (TGO), operating in push-frame mode, provides multiband images at four different wavelengths thanks to a Filter Strip Assembly with a panchromatic filter and three broadband filters within the visible and near infrared range. The camera acquires stereo pairs fundamental for the initialization of the photogrammetric process to perform three-dimensional reconstruction of the Martian surface at the best resolution of 4.6 m per pixel for regions up to ∼400 km2 in one imaging sequence.

Generation and Optimization of Spectral Cluster Maps to Enable Data Fusion of CaSSIS and CRISM Datasets

Four-band color imaging of the Martian surface using the Color and Stereo Surface Imaging System (CaSSIS) onboard the European Space Agency’s ExoMars Trace Gas Orbiter exhibits a high color diversity in specific regions. Not only is the correlation of color diversity maps with local morphological properties desirable, but mineralogical interpretation of the observations is also of great interest. The relatively high spatial resolution of CaSSIS data mitigates its low spectral resolution. In this paper, we combine the broad-band imaging of the surface of Mars, acquired by CaSSIS with hyperspectral data from the Compact Reconnaissance Imaging Spectrometer (CRISM) onboard NASA’s Mars Reconnaissance Orbiter to achieve a fusion of both datasets. We achieve this using dimensionality reduction and data clustering of the high dimensional datasets from CRISM. In the presented research, CRISM data from the Coprates Chasma region of Mars are tested with different machine learning methods and compared for robustness. With the help of a suitable metric, the best method is selected and, in a further step, an optimal cluster number is determined. To validate the methods, the so-called “summary products” derived from the hyperspectral data are used to correlate each cluster with its mineralogical properties. We restrict the analysis to the visible range in order to match the generated clusters to the CaSSIS band information in the range of 436–1100 nm. In the machine learning community, the so-called UMAP method for dimensionality reduction has recently gained attention because of its speed compared to the already established t-SNE. The results of this analysis also show that this method in combination with the simple K-Means outperforms comparable methods in its efficiency and speed. The cluster size obtained is between three and six clusters. Correlating the spectral cluster maps with the given summary products from CRISM shows that four bands, and especially the NIR bands and VIS albedo, are sufficient to discriminate most of these clusters. This demonstrates that features in the four-band CaSSIS images can provide robust mineralogical information, despite the limited spectral information using semi-automatic processing.

A CaSSIS and HiRISE map of the Clay-bearing Unit at the ExoMars 2022 landing site in Oxia Planum

In preparation for the operations of the ExoMars Rosalind Franklin rover, characterising its landing site in Oxia Planum is essential. Of particular interest is the extensive Clay-bearing Unit present at the site, a key target in the search for biosignatures. In this paper we provide a map based on variations in colour and spectral information within this unit, covering the 1σ landing envelope of the rover along with a 1 ​km buffer to account for minor shifts of the landing envelope ahead of launch (referred to going forward as the 1σ+ landing envelope). We used imagery from the Colour and Stereo Surface Imaging System (CaSSIS) and High Resolution Imaging Science Experiment (HiRISE) instruments, along with CaSSIS Band Ratio Composites with enhanced colour sensitivity to the presence of ferric (Fe3+) and ferrous (Fe2+) iron bearing materials. Our map is of a far higher resolution (map-scale 1:2000) than those previously available and, in contrast to previously available maps of this unit, differentiates between an Orange Subunit and a Blue Subunit which make up the Clay-bearing Unit. This mapping covered the ∼91% of the 1σ+ landing envelope where there was CaSSIS coverage and split the Clay-bearing Unit into three categories: one for each of the clay subunits, and another for exposures of the Clay-bearing Unit where either both subunits were too intermixed to reliably separate, or where it was difficult to determine which of the two were present.The results from our mapping shows that at least ∼35% of the 1σ+ envelope is covered by exposures of the Clay-bearing Unit: ∼18% by the Orange Subunit, ∼9% the Blue Subunit, and ∼12% were classified as Indeterminate. The spread of these two subunits varied substantially over the 1σ+ landing envelope, with the south-east half of the landing envelope dominated by the Orange Subunit (∼70% exposures in this area belonging to the Orange Subunit, ∼10% to the Blue Subunit and ∼20% to the Indeterminate category), while the north-west has more sporadic exposures of the Clay-bearing Unit (∼22% Orange, ∼37% Blue and ∼41% Indeterminate). The colour distinction between the two subunits is thought to be due to constituent mineralogical differences rather than differences in dust coverage of the two subunits. The scale of the fracturing present in the two subunits has also been assessed in this study, via qualitative observations of the fracture length and quantitative mapping out of fracture networks. While there were differences in the scale of fracturing between the two subunits, these were not as great as had previously been identified.

Subpixel-Scale Topography Retrieval of Mars Using Single-Image DTM Estimation and Super-Resolution Restoration

We propose using coupled deep learning based super-resolution restoration (SRR) and single-image digital terrain model (DTM) estimation (SDE) methods to produce subpixel-scale topography from single-view ESA Trace Gas Orbiter Colour and Stereo Surface Imaging System (CaSSIS) and NASA Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE) images. We present qualitative and quantitative assessments of the resultant 2 m/pixel CaSSIS SRR DTM mosaic over the ESA and Roscosmos Rosalind Franklin ExoMars rover’s (RFEXM22) planned landing site at Oxia Planum. Quantitative evaluation shows SRR improves the effective resolution of the resultant CaSSIS DTM by a factor of 4 or more, while achieving a fairly good height accuracy measured by root mean squared error (1.876 m) and structural similarity (0.607), compared to the ultra-high-resolution HiRISE SRR DTMs at 12.5 cm/pixel. We make available, along with this paper, the resultant CaSSIS SRR image and SRR DTM mosaics, as well as HiRISE full-strip SRR images and SRR DTMs, to support landing site characterisation and future rover engineering for the RFEXM22.

Absolute calibration of the Colour and Stereo Surface Imaging System (CaSSIS)

The Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter (TGO) has observed several standard stars and Jupiter with the aim of deriving the absolute calibration factors that convert observed signal into reflectance (”I over F”). The targets were observed through all four colour filters and hence a relative calibration between the filters was possible. In addition, observations of Phobos acquired during the TGO capture orbits in 2016 were analysed in support of the relative colour calibration. The results show that CaSSIS was around 13% less sensitive than predicted (normalised to the RED filter) - a value that is larger than the formal uncertainty and therefore significant. The relative colour calibration shows CaSSIS to be 4% more sensitive in the BLU than modelled which is close to the relative uncertainty. The text describes the analysis approach and methods for the various targets and newly recommended conversion factors are provided in table form.

The geography of Oxia Planum

ABSTRACT We present the geography of Oxia Planum, the landing site for the ExoMars 2022 mission. This map provides the planetary science community with a framework to understand this, until recently, unexplored area. The map comprises (1) a mosaic of the panchromatic Context Camera (CTX) Digital Elevation Models (DEM) and Ortho Rectified Images (ORI) controlled to the High Resolution Stereo Camera (HRSC) multiorbit Digital Elevation Models (DEM) and (2) a mosaic of Colour and Stereo Surface Imaging System (CaSSIS) synthetic colour data products, registered to the CTX ORI mosaic. We define a grid of exploration quadrangles (quads) and an informal group of geographic regions to describe Oxia Planum. These regions bridge the scale gap between features observed on large areas (∼100s km2) and the local geography (10s km2) relevant to the Rosalind Franklin rover’s operations in Oxia Planum.