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Phyllosilicate minerals have been detected at Oxia Planum, the ExoMars landing site, which indicate this area has been subject to aqueous activity during the Noachian [1,2,3]. The aim of the ExoMars rover mission is to search for signs of past or present life at Oxia Planum and to characterise the geochemical environment [1]. Evidence of past life may include physical biosignatures such as fossilised cells or stromatolites, or chemical biosignatures, such as biominerals and organic molecules produced by biological activity (biomarkers) [1,4,5]. The rover will be equipped with a suite of instruments, such as Multispectral Panoramic cameras (PanCam), Close-up imager (CLUPI), Raman Laser Spectrometer (RLS), and Mars Organic Molecule Analyser (MOMA), the latter of which includes gas chromatography-mass spectrometry (GC-MS) and laser desorption/ionisation-mass spectrometry (LDI-MS) capable of detecting biomarkers [6,7,8,9]. Mineralogy plays a vital role in the preservation of organic molecules [10]. Phyllosilicate minerals have been shown to adsorb organic compounds on and between their layered structures [11,12], protecting them from degradation by radiation and oxidation [13]. Given the identification of phyllosilicate mineralogy at Oxia Planum, detecting biomarkers there is plausible. However, alteration of biosignatures may have occurred at Oxia Planum through a range of geological processes, such as impacts. Geomorphological data of Oxia Planum indicates that extensive impact cratering has occurred, which would have altered the surface mineralogy, and may have modified any biomarkers present in the sediments [14]. However, organic molecules detected in impact craters on Earth have been shown to have survived the impact process [15] and impact experiments have shown impact energy, angle of ejection and mineralogy will influence the decomposition rate of organic molecules [16]. In addition, glasses formed by impact processing have been shown to trap organic components and aid their preservation [17, 18]. However, the elevated temperatures and pressures experienced during impact events can cause the modification of organic molecules, such as bond breaking and formation, thermal oxidation and racemization [19,20]. Depending on the reaction dynamics of these changes, these modifications may differ according to particular mineral hosts [20]. In this study, we perform laboratory impact experiments using the all-axis light gas gun (LGG), at the Open University, to understand the effects of impacts on the local mineralogy at Oxia Planum. These experiments use a mineralogical simulant for Oxia Planum, which is composed of a mixture of unaltered basaltic minerals, a phyllosilicate component, iron oxides and an amorphous component, representing the possible mineral assemblage at Oxia Planum[21].The LGG will expose the simulant to the high pressures and temperatures associated with planetary impacts at a range of velocities relevant to impacts at Oxia Planum. Samples taken of the simulant will be analysed using Near-IR, Raman and XRD analysis to assess the changes to its mineralogy. Here, we will present the preliminary results from these experiments, and will outline their application to understanding the survival of biomarkers at the Oxia Planum landing site. [1] Vago et al., Astrobiology Vol 17 No. 6 and 7, 2017 [2] Carter et al., LPSC abstract Vol. 47 No. 2064 [3] Quantin-Nataf et al., Astrobiology Vol. 21 No. 3, 2021 [4] Parnell et al., Astrobiology Vol. 7 No. 4, 2007 [5] Simoneit et al., Origins of Life and Evolution of the Biosphere Vol.28 p. 475–483, 1998 [6]Coates et al., Astrobiology, Vol. 17 No. 6-7, 2017[7]Rull et al., Astrobiology, Vol. 17 No 6-7[8] Goesmann et al., Astrobiology, Vol. 17 No 6-7 [9]Josset et al., Astrobiology, Vol. 17 No 6-7[10] Wiseman et al., Geoderma Vol. 134 p. 109–118, 2006 [11]Kleber et al., European Journal of Soil Science Vol. 56 No. 717–725, 2005 [12] Pearson et al., Meteoritics & Planetary Science Vol. 37, p.1829-1833 (2002) [13] dos Santos et al., Icarus Vol. 277 p. 342–353, 2016[14] Roberts et al., Journal of Maps, Vol. 17 No. 2, 2021[15]Burchell et al., Astrobiology Vol. 14 No. 6, 2014[16] Bowden et al., Astrobiology Vol. 8 No. 1, 2009[17]Sapers et al., Earth and Planetary Science Letters Vol. 430 p.95–104, 2015 [18]Edwards et al., LPSC Vol. 40 No. 2524 [19] Bowden et al., Journal of Analytical and Applied Pyrolysis Vol. 82, p.312–314 (2008) [20] Furukawa et al., Orig Life Evol Biosph, Vol. 48 p.131–139. 2018[21] Dugdale et al., LPSC (2020)

Introduction Antoniadi basin is a 330 km diameter Noachian basin localized in the eastern Arabia Terra that contains a network of dendritic ridges. Branched, dendritic ridges, such as these can form by a variety of processes including the inversion of fluvial deposits, thus potentially highlighting aqueous processes of interest for understanding Mars’ climate evolution. Here, we test this hypothesis by analyzing in details data from Colour and Stereo Surface Imaging System (CaSSIS), High Resolution Imaging Science Experiment (HiRISE) and High Resolution Stereo Camera (HRSC). Age of landforms Antoniadi’s interior plains are filled by deposits interpreted to be Hesperian or younger and volcanic in origin, possibly coeval with the volcanic episodes of the nearby Syrtis Major Planum [1]. The dendritic ridges lie on these volcanic plains to the south of a 28 km-diameter well-preserved crater (Fig. 1). Although this crater predates the branched ridges it is devoid of any fluvial erosion and appears mostly similar to many fresh, Amazonian age craters. While some erosion has affected the ridges, there is no indication of exhumation of these ridges from putative layers above the current plains surface. The crater size frequency distribution of the volcanic plains gives a model age of 2.5±0.5 Gy, using diameters of 200 m to 2 km. Thus, the branched ridges are Early Amazonian or younger. Figure. 1: Four Cassis images projected on CTX mosaic. The fresh crater to the north has been covered by fresh lava flows which themselves have been covered by the dendritic features. Morphometry and topography Individual ridges are easily recognizable with their dark tone in visible images compared to surrounding plains (Fig. 2), which display a slightly lighter tone. The ridges are organized as a dendritic network reaching a Strahler order of 4, i.e. the degree of hierarchy from the primary branches counting the number of junctions. Assuming tributary flows, this organization indicates a northward flow direction. However, the local slope is of 0.2° toward South, and thus contrary to the apparent network organization (assuming tributary flows). The branched ridges are also present on some of the lowest areas of these plains, but there is no evidence of terminal fans nor of erosional features affecting the plains in their more elevated areas. The texture of ridges at HiRISE scale (25cm/pixel) is rubbly with the occurrence of blocks up to ~1 m in size and a complete lack of layering in all HiRISE images where they are present unlike sedimentary strata usually observed within inverted channels on Mars. A HiRISE elevation model shows that branched ridges are between 1-5 m high, although erosion can explain some variations in height among the ridges. Ridges are up to 10 km long and 10-200 m wide without any obvious trends in width, i.e. ridges are not wider with increasing Strahler order. The order 1 branches are also peculiar in plan-view shape. They frequently display lobate shapes and are up to 100 m wide, which is wider than the ridge measured after the junction with several order 1 branches. North of the main pattern, a 500 m wide sinuous ridge present in the middle of the plain displays the same internal texture (rubbly) suggesting that it formed by the same process (white arrow at the top of Fig. 1). Figure 2: Close-up on the dendritic network with CaSSIS images. Conclusion Previous assessments of these landforms favored an origin as inverted channels [2]. Yet, our observations show many inconsistencies with this interpretation: (i) The rubbly texture lacks any layering at meter scale, a typical feature of inverted channels observed elsewhere on Mars [e.g., 3, 4]. (ii) The order 1 branches display a lobate shape and a larger width than higher order branches unlike river flows as observed on Earth. (iii) There is no increase in width from degree 1 branches of the network towards the north as would be expected for channels with increasing discharge rates downstream. (iv) The slope towards the south is contrary to the inferred flow direction to the north assuming a tributary network. Wrinkle ridges may be evidence for post-depositional changes in topography, but these uplifts appear localized (Fig. 1). Thus, the detailed analysis of these branched ridges shows characteristics difficult to reconcile with inverted fluvial channels. Subglacial drainages are known to locally flow against topography, but rarely display a dendritic pattern [5]. Assuming that deposition occurred along the current slope from north to south, the organization of the network requires a control by distributary channels rather than tributary channels. Distributary fluvial channels are possible for fluvial flows, but generally limited to braiding regimes or deltaic deposits, of which no further evidence is observed here. The lobate digitate shapes of the degree 1 branches are actually more in line with deposits of viscous flows, thus as terminal branches (Fig. 3). Such an interpretation is consistent with lava flows or mudflows that formed along the current topography. This conclusion would explain the presence of these landforms in an Early Amazonian plain lacking any evidence of fluvial activity, including the fresh crater that is stratigraphically older than the landforms studied. Acknowledgments: French authors are supported by the CNES. The authors wish to thank the spacecraft and instrument engineering teams. CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA’s PRODEX. The instrument hardware development was also supported by the Italian Space Agency (ASI) (agreement no. I/018/12/0), INAF/Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw. Support from SGF (Budapest), the Univ. of Arizona (Lunar and Planet. Lab.) and NASA are gratefully acknowledged. References : [1] Tanaka K. L. et al. (2014) USGS map 3292. [2] Zaki et al. (2019) EPSC-DPS2019-244-1. [3] Burr et al. (2010) JGR-Planets, 115, E07011. [4] Mangold et al. (2008) JGR-Planets, 113, E08009. [5] Menzies, J (2002), Modern and Past Glacial Environments, Oxford.

The geologic origin of the ancient, phyllosilicate-rich bedrock at Oxia Planum, Mars, the landing site of ESA’s ExoMars rover, is unknown. The phyllosilicates record ancient aqueous processes, but the processes that formed the host bedrock remain elusive. Here, we use high-resolution orbital image and topographic datasets (HiRISE, CTX, CaSSIS) to investigate and characterize fluvial sinuous ridges (FSRs), found across the Oxia Planum region. The FSRs form segments up to 70 km long, with sub-horizontal layering commonly exposed in ridge margins. Some FSRs comprise multi-story ridge systems; many are embedded within and are being exhumed from the phyllosilicate-rich bedrock. We interpret the FSRs as deposits of ancient, episodically active, alluvial river systems (channel-belt and overbank deposits) at Oxia Planum. Thus, the phyllosilicate-rich bedrock was formed at least partly by ancient alluvial rivers, active across the wider region. Our alluvial hypothesis does not exclude other sources of sediment playing a role in building the terrain as well. Future exploration by ExoMars can verify this interpretation and provides an opportunity to investigate some of the oldest river deposits in the Solar System.

Introduction: Inverted channels are sinuous ridges comprising fluvial facies, which have gained a positive relief due to differential erosion [e.g., 1, 2]. Many have been found preserved on Mars in terrains dated between the Mid-Noachian to Early Hesperian in the region of Arabia Terra [1, 3, 4]. The distribution of these features is well known [e.g., 2, 3] but timing, geological settings, their relative timescales of liquid water stability and the nature of the early climate under which fluvial deposition was possible are still under debate. We present results from studies conducted on three morphologically different inverted channel systems in the region of Arabia Terra: Arago Dorsa (within the Arago crater), Cantabras Dorsum and Piscinas Serpentes (in Meridiani Planum). The analysis includes interpretations of the formation settings of the three systems and estimation of the volume of preserved alluvial materials and the consequent required period of fluvial activity for such deposition. Methodology: Digital morphostratigraphic maps (scale 1:10,000) were constructed using ArcGIS software using 25 cm/pixel HiRISE images [5]. A CTX mosaic [6] was used to study the general context of the study area, and to assist the analysis of stratigraphy. HiRISE DEMs [7] were used to calculate thicknesses of units interpreted as fluvial facies and to characterise and interpret the stratigraphic relationships of the different geological units. When HiRISE DEMS were not available, the HRSC-MOLA DEM [8] was used. The ridges’ morphometric parameters (length, thickness, width) were used to estimate both the preserved volume of alluvial material and the duration of fluvial activity of the systems, assuming sedimentation rates of 0.01-0.7 m/Kyr from terrestrial comparisons [4, 9]. Inverted channel types and settings: The Arago Dorsa ridges are the result of channel belt aggradation [10], as shown by the presence of a well-organized multi-level structure made of candidate channel fills and overbank deposits. We interpret the overall structure as a candidate relict aggrading fluvial fan preserved within a continental highland basin in a distal portion from the source where inverted channel belts are usually formed [11]. Cantabras Dorsum represents a complex package of fluvial sediments, and presents two distinct ridges. The smaller, a sinuous flat-topped ridge with a rectangular cross-section [following 2], is developed on top of a wider one (Fig. 1). We interpret the structure as alluvial deposits deposited by an avulsed channel on the floor of a wider valley which progressively reduced the cross-sectional area, terminating in a smaller channel now expressed by the upper inverted ridge. Piscinas Serpentes presents a network of fine-scale valleys that transition downstream (north of the studied site) into pitted channels and finally ridges (Fig. 2). The ridges of Piscinas Serpentes, are more likely the result of channel fill in a ribbon-like channel. Figure 1: HiRISE-DEM oblique view of Cantabras Dorsum. Vertical exaggeration = 5 times. a- Main ridge (9.13°N, -5.47°E). b- Lateral oblique view of the upper ridge (9.13°N, -5.48°E). Figure 2: Detail of Piscinas Serpentes system (ESP_048489_1845; 4.51°N, -1.17°E), where the passage from positive inverted relief in the northern part of the image to negative relief in the south is visible. Estimation of volume and timing for fluvial deposition: The estimated vertical thickness of the ridges and the estimated duration of fluvial activity (Table 1) are in line with those measured for terrestrial analogues [12]. Aggrading systems like Arago Dorsa and Cantabras Dorsum should have been stable for between 104-106 years. Table 1: Morphometric data of the studied systems and estimation of volume and formation timescale of fluvial activity. System Vertical Thickness (m) Exposed Length (m) Width (m) Volume of alluvial material preserved (km3) Terrestrial analogues sedimentation rates (m/kyr) Formation time scale (Myr) Approximate fluvial activity (Yr) Arago Dorsa 50 25,000 18,000 22.5 0.01–0.7 0.07-5 105-106 Cantabras Dorsum 35 100,000 2000 7 0.01–0.7 0.05-3.5 104-106 Piscinas Serpentes 10 10,000 90 0.009 0.01–0.7 0.01-1 104-106 Conclusions and implications for martian climate: Our study supports long-duration fluvial and aqueous inputs to allow significant thicknesses of fluvial aggradations and/or avulsions to occur in a variety of settings across Arabia Terra (a crater/basin and valleys). It also shows a distributed source of water in Arabia Terra. Geological settings where inverted relief developed, their morphometry and the estimated formation time are similar to terrestrial analogues [12], suggesting that similar environmental conditions are likely to have characterised the two planets at the time of fluvial activity. The obtained results are better explained with a stable hydrological cycle which would have been required to allow migration and aggradation in such a large region, probably sourced from precipitation both in the area and upstream. References: [1] Davis et al., (2019), JGR Planets, 10.1029/2019JE005976. [2] Mirino et al.,(2020), LPSC 2020, LPI Contribution No. 2326, 2020, id.1492. [3] Davis et al., (2016), Geology, G38247.1. [4] Balme et al.,(2020), JGR Planets, https://doi.org/10.1029/2019JE006244. [5] McEwen et al., (2007), JGR, 112:E05S02. [6] Dickson et al., (2018), LPSC 2018, LPI Contrib. No. 2083. [7] Davis (2019), https://doi.org/10.6084/m9.figshare.c.4330838.v1. [8] Fergason et al., (2018). U.S. Geological Survey, http://bit.ly/HRSC_MOLA_Blend_v0. [9] Colombera et al., (2015), Geology, 10.1130/G36385.1. [10] Mirino et al., (2021), EPSC 2021, https://doi.org/10.5194/epsc2021-311, 2021. [11] Ventra & Clarke (2018), Geological Society of London, 10.1144/SP440.16. [12] Zaki et al., (2021), Earth-Science Reviews, https://doi.org/10.1016/j.earscirev.2021.103561.

There is conflicting evidence for an ancient ocean which occupied the northern hemispheric basin on Mars. Along different regions of the dichotomy boundary, sediment fans have been interpreted as either forming into a large water body or a series of smaller paleolake basins. Here, we investigate fluvial systems in the Memnonia Sucli region of Mars, set along the dichotomy, which comprise erosional valley networks, paleolake basins, inverted channel systems, and sediment fans. We focus our analysis on the evolution of the upslope catchment and characterizing the ancient environment of a large, downslope basin, bound by the topographic dichotomy and the Medusae Fossae Formation. The catchment fluvial systems comprise highly degraded valley networks and show a complex history of incision and filling, influenced by paleolake basin overflow, impact crater damming, aggradation, and possibly a downstream water body. The morphology of the sediment fans is consistent with either fluvial fans or deltas and they form at discrete elevations, rather than a common elevation plane. Our analysis is consistent with the sediment fans forming into a series of paleolake basins set along the dichotomy, rather than into a large inner sea or ocean-sized water body. The fluvial systems were likely active between the mid Noachian and early Hesperian periods. Our results demonstrate the complex, multi-phase evolution of fluvial systems on ancient Mars and highlight the importance of regional and local studies when characterising ancient regions of the dichotomy.

We present a map of Oxia Planum, Mars, the landing site for the ExoMars Rover. This shows surface texture and aeolian bedform distribution, classified using a deep learning (DL) system. A hierarchical classification scheme was developed, categorising the surface textures observed at the site. This was then used to train a DL network, the ‘Novelty or Anomaly Hunter – HiRISE’ (NOAH-H). The DL applied the classification scheme across a wider area than could have been mapped manually. The result showed strong agreement with human-mapped areas reserved for validation. The resulting product is presented in two ways, representing the two principle levels of the classification scheme. ‘Descriptive classes’ are purely textural in nature, making them compatible with a machine learning approach. These are then combined into ‘interpretive groups’, broader thematic classes, which provide an interpretation of the landscape. This step allows for a more intuitive analysis of the results by human users.

Oxia Planum, the landing site for the ExoMars rover mission, is a shallow basin on the southern margin of Chryse Planitia that hosts remnants of fan-shaped sedimentary deposits associated with the ancient channel system Coogoon Vallis. This indicates runoff from a catchment in Arabia Terra has transported sediment into the landing site. To explore this fluvial system we created a model catchment for Oxia Planum and, using 6 m/pixel ConTeXt camera (CTX) orbital remote sensing image data, we digitised the fluvial and lacustrine landforms in Western Arabia Terra in and around this catchment. We find: (1) The catchment has a minimum area of ∼2.1×105 km2 and has been episodically deformed by tectonic activity; (2) There were at least two phases of fluvial activity. The first created a mature landscape associated with Coogoon Vallis, which may have deposited alluvial or deltaic deposits in the Oxia Basin. After a substantial hiatus, a second phase of activity incised U-section channels into the pre-existing landscape and channel systems; and (3) Evidence for numerous possible paleolake deposits within the catchment. These are not well connected to the fluvial system and were probably sustained by groundwater activity contemporaneous with both phases of fluvial activity. This groundwater might have modified the mineralogy of Oxia Planum. Oxia Planum probably experienced an alluvial or distal deltaic/lacustrine depositional environment during the mid Noachian, which was later overprinted by a younger phase of fluvial activity.

There is conflicting evidence for an ancient ocean which occupied the northern hemispheric basin on Mars. Along different regions of the dichotomy boundary, sediment fans have been interpreted as e...

Introduction: Inverted channel belts have been recognized on Mars [1,2]. A channel belt represents an extensive fluvial package of sediments which usually formed from lateral migration and aggradation of a river over time [3]. An inverted channel belt is created because the sediment package developed an inversion of relief as some sections were infilled by, or formed from, a more resistant material than the neighboring terrains. The surrounding terrains, less resistant to the erosion were removed, leaving the more resistant sediments upstanding [3] and often organized into characteristic sinuous ridges. This process implies that the fluvial system that deposited the material associated with the channel belt structure must have been active and stable for a long time (about 105-107 years [1]), and then a major change occurred, moving from a period of deposition to a period of erosion. The discovery of these features on Mars could have a major role for the dry-cold versus warm-wet climate debate [4]. Here, we present results from a mapping study in support of an inverted channel belt system developed within the degraded and partially buried Arago crater,in south-east Arabia Terra. Methodology: HRSC-MOLA DEM [5] and CTX mosaic [6,7] data were ingested into ArcGIS software to study the general context of the study area, and to assist analysis of stratigraphy. A digital morphostratigraphic map (scale 1:10,000) was constructed using the available HiRISE [8] image and represents our interpretation of the area. Units are divided into three main groups:(i) The ‘Ridge formation‘ (units with strong correlation to the main ridges and likely represent fluvially-deposited material), (ii) ‘Basal units, (terrains stratigraphically below the inverted systems, or unrelated to them), (iii) ‘Miscellaneous units’ (heterogeneous materials, erosional remnants). The focus of the mapping was to determine the relationships between the sinuous ridge segments and the proximal surrounding terrains. Observations: Arago crater is in Middle Noachian [9] terrains, and is bordered by valley networks [10] at north and south, suggesting that the area was influenced by fluvial processes. Water probably filled Arago crater and the surrounding topographic lows, evolving into temporary paleo-lakes or basins. The studied system is found at the south-west of Arago crater and here we refer to it informally as Arago Dorsum. Arago Dorsum is a continuous, branched, inverted network. Each branch of the system presents a variation in cross section style, expressing a different erosional stage[11]. The branches present smooth or cratered upper surfaces, and the longest segments are connected to smaller, secondary ridges. The Arago Dorsum ridges are upstanding and set within a low-reliefelevated material, demonstrating that they are resistant to erosion. The ridges are both laterally and vertically separated, organised in a multilevel structure. Results: In the Ridge Formation sequence (Fig. 1), the main mapped ridges units (Urf 1-2, Srf 1-2, Lrf 1-2-3) are set at different stratigraphical levels and distributed within both Marginal units (Mrf 1-2) and the lower Transitional units (Trf 1-2-3), which correspond to ~65 m of vertical deposits (Fig. 2). Mapping allowed the recognition of other fine-scale features including: (i) more resistant thin layers visible at the flanks of the main ridges, or distributed in the Marginal units, (ii) the presence of smaller and secondary sinuous ridges which connect and disconnect at different stratigraphical levels, (iii) the presence of polygonal networks, possibly connected with secondary groundwater infilling, (iv) boulders and mass wasting falling from the ridges. Figure 1: Gemorphic map of the “Arago Dorsum” system. The track A-A1 is presented in Fig. 2. Figure 2: Morphostratigraphic profiles(vertical exaggeration x 20) presenting the main mapped units (indicated with labels on the top) and the interpretation (bottom). The cross-section A-A1 track is indicated in Fig. 1. Interpretation: Based on the recognised fine-scale features, ridge morphologies, distribution and relationship of the mapped units, we interpret Arago Dorsum to represent the expression of an aggrading fluvial system, now preserved as an inverted channel belt. Specifically, the alternation of more resistant (ridges and layers) and the less resistant (Marginal-Transitional) units is in line with a flood plain sequence with preserved overbank deposits and discrete or stacked channel bodies. The three main ridges identified in the mapping area represent three periods of deposition. As other ridges have been recognised at CTX observation scale, the sequences reported here include only part of the Arago Dorsum stratigraphy, which may be even more complex, and probably even more long lived. Conclusions: We interpret the Arago ridge system to be fluvial in origin, preserved in positive relief and now modified by differential erosion. Our observations and mapping reveal the inverted channel candidate to be the sedimentary deposits of an extensive aggradational fluvial channel-belt system, probably active in the Mid-Noachian. Valley networks inflowing Arago crater possibly supplied the channel-belt. There is a succession with at least 65 m vertical thickness of fluvial sediments here. Aram Dorsum, in western Arabia Terra (over 2000 km distance and over 1500 m elevation difference from Arago) presents very similar characteristics, including (i) age, (ii) morphology, (iii) vertical thicknesses of sediments, and (iv) the presence of polygonal networks developed in the units associated with the ridge development. These similarities suggest that similar thicknesses, volumes, and secondary processes (burial, erosion, groundwater alteration) of mid-Noachian fluvial sediments may be extensive and common in Arabia Terra. The presence of two (probably more) inverted channel belts, dated to the Mid-Noachian provide supports a past climate with widespread precipitation across Arabia Terra during this period. References:[1] Balme et al.,(2020), JGR Planets,https://doi.org/10.1029/2019JE006244. [2] Liu et al.,(2020), Earth and Planetary Science Letters, https://doi.org/10.1016/j.epsl.2021.116854. [3] Hayden et al.,(2019), Icarus, https://doi.org/10.1016/j.icarus.2019.04.019.[4] Wordsworth et al.,(2016), Annual Review of Earth and Planetary Sciences, 10.1146/annurev-earth-060115-01235.[5] Fergason et al.,(2018), Astrogeology PDS Annex, U.S. Geological Survey. http://bit.ly/HRSC_MOLA_Blend_v0. [6] Dickson et al.,(2018), LPSC, 2480. [7] Malin et al., (2007), JGR Planets10.1029/2006JE002808. [8] McEwen et al.,(2007), JGR Planets, https://doi.org/10.1029/2005JE002605. [9] Tanaka et al.,(2014), GeologicMap of Mars: USGS Scientific Investigations SeriesMap 3292, scale 1:20,000,000, pamphlet 43 p. [10] Hynek et al.,(2010), Journal of Geophysical Research, https://doi.org/10.1029/2009JE003548. [11] Mirino et al.,(2020), LPSC, 1500. [10] Mirino et al.,(2020), LPSC 1492.

Introduction: Numerous inverted fluvial channels occur on the surface of Mars [e.g. 1, 2, 3], and are found mainly in Late Noachian-Early Hesperian terrains [e.g. 6], and often within Arabia Terra [e.g. 1, 3]. A well-studied example is Aram Dorsum, a sinuous branched ridge interpreted as an inverted channel belt [7]. Sub-units within Aram Dorsum present a strong association with different types of decameter-scale, polygonally-patterned terrains:(i) Narrow-Fracture polygons, (ii) Wide-Fracture polygons, (iii) Narrow-Ridge polygons, (iv) Vein-like ridges, and (v) “meter-scale” polygons [7]. The mapping of the Aram Dorsum system shows that these types of polygons present both a distinct spatial distribution and a consistent stratigraphic pattern[7]. Developing a better understanding of the possible causes (local or regional) and formation processes of these polygonal terrains may offer an opportunity for a deeper understanding of the processes involved in inverted channel formation and development. We performed a regional survey within Arabia Terra to identify similar polygonal terrains and morphologies associated with other inverted systems. The main focus was on determining whether these polygon types are present within other terrains associated with inverted channels on Mars, and determine if the spatial/stratigraphic distribution observed in Aram Dorsum is consistent across the region. Method: A database of inverted channels systems in Arabia Terra [8]was compared with HiRISE (25 cm/pixel, [9]) and CTX (6 m/pixel, [10]) image coverage using ArcGIS software. 205 HiRISE images were used for the identification of the polygonised terrains (observation-scale 1:2000). The observations have been made for all the inverted channels which were covered by at least one HiRISE image (Fig.1a). The result of the survey was a new database of polygonal terrain types associated with inverted channels, to which a simplified Aram-like morphological classification division was also applied (Fig.1b). Figure 1: a-Studied HiRISE images which cover part of inverted channels(blue lines [8]). White colours indicate presence of polygonal-grounds. Black colours indicate absence of polygonal-grounds. b-Distribution of Aram Dorsum-like polygonal terrains. Results: From the regional survey, Aram Dorsum-like polygons have been observed in association with many of inverted channels covered by HiRISE images found in Arabia Terra (Fig.2). 100 HiRISE images (out of 205 observed) have shown at least one polygonal type. The polygons have been found mainly associated with the main ridge itself or with the surrounding material adjacent to the inverted feature. Some materials close to the inverted channels may be characterised by only one type of polygonal morphology, and in this case fracture polygons or narrow ridge polygons are the most common types. Some examples of inverted channel deposits contain several types of polygonal features and it is difficult to define when a given type of polygon ends and another type begins. From the regional survey it was difficult to define a unique stratigraphic relationship between the polygonal terrains, and several inverted channels in Arabia Terra present unique morphologies and stratigraphy. However, a trend has been observed: fracture polygon types with clear morphology are usually found on top of the inverted channel, within erosional windows where the ‘capping unit’ has been removed, or on the closest associated materials. Narrow ridges or Vein-like polygons are usually developed within surrounding materials or sometimes in the lower parts of the main ridge (where exposed).Meter-scale polygons, or densely polygonal terrains, were found in proximity of craters linked to the inverted channels, or they have been observed between other polygonal fractures and between the narrow ridges polygons. Interpretation: The Polygonal terrains have been observed in proximity to inverted channel features, on top of the ridge, along the flanks and within ridge-related materials (sometimes interpreted to be overbank deposits [7]) which could be distributed within the inverted channels in erosional windows. The identification of the same types of Aram Dorsum-like polygonal features in so many areas in Arabia Terra suggest that a common phenomenon could be found to explain their formation and development within these fluvial deposits. The persistence of the spatial distribution and a regional stratigraphical trend suggest a connection with the physical proprieties (e.g. grain size, induration, thickness) of the inverted channel deposited materials (fluvial origin) and their response to either (i) a regional stress, if the polygons formed underground as joints, or (ii) a surface process such as desiccation, thermal contraction or possibly a combination of the two. HiRISE images, selected in other areas with no inverted channels have been also checked to evaluate if such a fine-scale polygonal features could be distributed also somewhere else not in association with the inverted channel bodies. Based on preliminary observations on 10 HiRISE samples (randomly chosen) we have not been able to identify these small-scale polygonal terrains. However, since dust and impact materials were present, the survey will be expanded in future studies. Figure 2: Example of Aram Dorsum-like polygonal terrains found in Arabia Terra region. a- Narrow ridge polygons, b- Wide fracture polygons, c- Meter-scale polygons (small domes) and Narrow fractures polygons. Conclusions: Many other Aram Dorsum-like polygonal terrains have been observed and mapped in and around inverted channels in Arabia Terra, suggesting that inverted channels are likely to be observed associated with this type of polygonised terrains. A general spatial-stratigraphical trend has been observed, but deeper studies on selected candidates are necessary to confirm this. We will therefore perform more detailed morphological mapping choosing the a variety of inverted channel candidates in the area to further explore this possibility. More HiRISE images not associated to inverted channel bodies are also going to be controlled. References: [1]Davis et al.,(2016), Geology, G38247.1. [2]Williams et al.,(2013), Icarus 225:308- 324. [3]Chuang & Williams,(2018), Journal of Maps, 14:2, 652-660. [4]Williams et al.,(2013), Icarus 225:308-324. [5]Williams et al.,(2007), Utah Geological Association, pp. 221-235. Salt Lake City, Publication, 36. [6]Tanaka et al.,(2014), Geologic Map of Mars: USGS Scientific Investigations Series Map 3292, scale 1:20,000,000, pamphlet 43 p. [7]Balme et al.,(2020), JGR Planets, https://doi.org/10.1029/2019JE006244. [8]Mirino et al.,(2020), LPSC 1492. [9]McEwen et al.,(2007), JGR Planets,https://doi.org/10.1029/2005JE002605. [10]Malin et al.,(2007), JGR Planets, 10.1029/2006JE002808.

MURFI was financially supported in part by the UK Space Agency, and by the following grants: Balme: UK SA grants ST/L00643X/1 and ST/R001413/1; Bridges: UK SA grant ST/R00143X/1; Butcher and Wright: STFC studentship grant ST/N50421X/1; Coates and Griffiths: UK SA grant ST/R002223/1; Cousins: Royal Society of Edinburgh Research Fellowship and UK SA grant ST/P001297/1; Davis: UK STFC grant ST/K502388/1; Grindrod: UK SA and STFC grants ST/J005215/1, ST/L006456/1, ST/N000528/1; Gunn: UK SA grants ST/P001408/1, ST/P001394/1, ST/N006410/1; Muller: UK SA grant ST/P002145/1; Preston: UK SA grant ST/P001254/1; Stabbins: UK SA studentship grant ST/N002377/1. We thank the UK Harwell Campus Satellite Applications Catapult for providing access to the control room that became the MOC, and the STFC Rutherford Appleton Laboratory for other ROC support. The 2016 Mars Utah Rover Field Investigation (MURFI) was a Mars rover field trial run by the UK Space Agency in association with the Canadian Space Agency's 2015/2016 Mars Sample Return Analogue Deployment mission. MURFI had over 50 participants from 15 different institutions around the UK and abroad. The objectives of MURFI were to develop experience and leadership within the UK in running future rover field trials; to prepare the UK planetary community for involvement in the European Space Agency/Roscosmos ExoMars 2020 rover mission; and to assess how ExoMars operations may differ from previous rover missions. Hence, the wider MURFI trial included a ten-day (or ten-‘sol’) ExoMars rover-like simulation. This comprised an operations team and control centre in the UK, and a rover platform in Utah, equipped with instruments to emulate the ExoMars rovers remote sensing and analytical suite. The operations team operated in ‘blind mode’, where the only available data came from the rover instruments, and daily tactical planning was performed under strict time constraints to simulate real communications windows. The designated science goal of the MURFI ExoMars rover-like simulation was to locate in-situ bedrock, at a site suitable for sub-surface core-sampling, in order to detect signs of ancient life. Prior to “landing”, the only information available to the operations team was Mars-equivalent satellite remote sensing data, which were used for both geologic and hazard (e.g., slopes, loose soil) characterisation of the area. During each sol of the mission, the operations team sent driving instructions and imaging/analysis targeting commands, which were then enacted by the field team and rover-controllers in Utah. During the ten-sol mission, the rover drove over 100 m and obtained hundreds of images and supporting observations, allowing the operations team to build up geologic hypotheses for the local area and select possible drilling locations. On sol 9, the team obtained a subsurface core sample that was then analyzed by the Raman spectrometer. Following the conclusion of the ExoMars-like component of MURFI, the operations and field team came together to evaluate the successes and failures of the mission, and discuss lessons learnt for ExoMars rover and future field trials. Key outcomes relevant to ExoMars rover included a key recognition of the importance of field trials for (i) understanding how to operate the ExoMars rover instruments as a suite, (ii) building an operations planning team that can work well together under strict time-limited pressure, (iii) developing new processes and workflows relevant to the ExoMars rover, (iv) understanding the limits and benefits of satellite mapping and (v) practicing efficient geological interpretation of outcrops and landscapes from rover-based data, by comparing the outcomes of the simulated mission with post-trial, in-situ field observations. In addition, MURFI was perceived by all who participated as a vital learning experience, especially for early and mid-career members of the team, and also demonstrated the UK capability of implementing a large rover field trial. The lessons learnt from MURFI are therefore relevant both to ExoMars rover, and to future rover field trials. Postprint

Mars Polar Science is an integrated, compelling system that serves as a nearby analogue to numerous other planets, supports human exploration, and habitability. Mars possesses the closest and most easily accessible layered ice deposits outside of Earth, and accessing those layers to read the climate record would be a triumph for planetary science.

Oxia Planum (OP), located at the transition between the ancient terrain of Arabia Terra and the low lying basin of Chryse Planitia, will be the landing site for the ESA-Roscosmos ExoMars Programme’s 2022 mission [1]. The descent module and landing platform, Kazachock, will transport the Rosalind Franklin Rover to search for signs of past and present life on Mars, and investigate the geochemical environment in the shallow subsurface over a 211-sol nominal mission.OP forms a shallow basin, open to the north, characterized by clay-bearing bedrock, and episodic geological activity spans from the ~mid-Noachian to ~early Amazonian in age [2,3,4]. Building a thorough understanding of Oxia Planum prior to operations will provide testable hypotheses that facilitate interpretation of results, and hence provide an effective approach to address the mission’s science objectives. To this end, we have run a detailed group mapping campaign at HiRISE-scale using the Multi-Mission Geographic Information System (MMGIS) [5], co-registered HRSC [6], CaSSIS and HiRISE mosaics [7], and 116 1km2 quads covering the 1-sigma landing ellipse envelope. Complementary CTX-scale mapping covers the wider area around the landing site and is described elsewhere [8].Throughout 2020, 84 mapping volunteers associated with the mission’s Rover Science Operations Working Group followed a pre-formulated programme of training, familiarisation and mapping. With the mapping phase complete, a small sub-team are focused on map reconciliation phase, comprising data cleaning and science decision making. The process will culminate in map finalisation and submission for publication, and use in activities to plan rover science activities.This campaign yields important advances for overall science readiness of the ExoMars 2022 mission:Team experience working, communicating and learning together, valuable for operations. Building team knowledge of the landing site, and the main scientific interpretations. Curated datasets and software available for team use in ongoing planning. High-resolution map data representing our geologic understanding of Oxia Planum. This is an input to ongoing RSOWG work to construct the mission strategic plan, which provides science traceability from mission objectives to rover activities.Acknowledgments: We thank Fred Calef and Tariq Soliman at JPL for their support regarding MMGIS.References: [1] Vago, J. L. et al., (2017) Astrobiology 17 (6–7), 471–510. [2] Carter, J. et al., (2013) J. Geophys. Res. 118 (4), 831–858. [3] Quantin-Nataf, C. et al., (2021) Astrobiol. 21 (3), doi:10.1089/ast.2019.2191. [4] Fawdon P. et al., (2019) LPSC50 #2132. [5] Calef, F. J. et al., (2019) in 4th Planet. Data Work., Vol. 2151. [6] Gwinner, K. et al., (2016) Planet. Space Sci. 126, 93–138. [7] Volat, M. et al., (2020), EPSC, #564. [8] Hauber, E. et al. (2021), LPSC52.

Antoniadi basin is a 330 km diameter Noachian basin localized in the East of Arabia Terra that contains a network of ridges with a tree-like organization. Branched ridges, such as these can form by a variety of processes including the inversion of fluvial deposits, thus potentially highlighting aqueous processes of interest for understanding Mars’ climate evolution. Here, we test this hypothesis by analyzing in details data from Colour and Stereo Surface Imaging System (CaSSIS), High Resolution Imaging Science Experiment (HiRISE) and High Resolution Stereo Camera (HRSC).Branched ridges are up to 10 km long and from 10 to 200 m wide without obvious organization in width. The branched ridges texture is rubbly with the occurrence of blocks up to ~1 m in size and a complete lack of layering. A HiRISE elevation model shows the local slope is of 0.2° toward South, and thus contrary to the apparent network organization (assuming tributary flows). There is no indication of exhumation of these ridges from layers below the current plains surface. Our observations are not consistent with the interpretation of digitate landforms such as inverted channels: (i) The rubbly texture lacking any layering at meter scale is distinct from inverted channels as observed elsewhere on Mars. (ii) Heads of presumed inverted channels display a lobate shape unlike river springs. (iii) There is no increase in width from small branches toward North as expected for channels with increasing discharge rates downstream. (iv) The slope toward South is contrary to the inferred flow direction to the North. The detailed analysis of these branched ridges shows many characteristics difficult to reconcile with inverted channels formed by fluvial channels flowing northward. Subglacial drainages are known to locally flow against topography, but they are rarely dendritic. Assuming that deposition occurred along the current slope, thus from North to South, the organization of the network requires a control by distributary channels rather than tributary ones. Distributary channels are possible for fluvial flows, but generally limited to braiding regimes or deltaic deposits, of which no further evidence is observed here. The lobate digitate shapes of the degree 1 branches are actually more in line with deposits of viscous flows, thus as terminal branches. Such an interpretation is consistent with lava or mudflows that formed along the current topography. The next step in this study will be to determine more precisely the rheology of these unusual flows.Acknowledgments: French authors are supported by the CNES. The authors wish to thank the spacecraft and instrument engineering teams. CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA’s PRODEX. The instrument hardware development was also supported by the Italian Space Agency (ASI) (agreement no. I/018/12/0), INAF/Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw. Support from SGF (Budapest), the Univ. of Arizona (Lunar and Planet. Lab.) and NASA are gratefully acknowledged.

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.

International audience; The lack of adequate stereo coverage and where available, lengthy processing time, various artefacts, and unsatisfactory quality and complexity of automating the selection of the best set of processing parameters, have long been big barriers for large-area planetary 3D mapping. In this paper, we propose a deep learning-based solution, called MADNet (Multi-scale generative Adversarial u-net with Dense convolutional and up-projection blocks), that avoids or resolves all of the above issues. We demonstrate the wide applicability of this technique with the ExoMars Trace Gas Orbiter Colour and Stereo Surface Imaging System (CaSSIS) 4.6 m/pixel images on Mars. Only a single input image and a coarse global 3D reference are required, without knowing any camera models or imaging parameters, to produce high-quality and high-resolution full-strip Digital Terrain Models (DTMs) in a few seconds. In this paper, we discuss technical details of the MADNet system and provide detailed comparisons and assessments of the results. The resultant MADNet 8 m/pixel CaSSIS DTMs are qualitatively very similar to the 1 m/pixel HiRISE DTMs. The resultant MADNet CaSSIS DTMs display excellent agreement with nested Mars Reconnaissance Orbiter Context Camera (CTX), Mars Express’s High-Resolution Stereo Camera (HRSC), and Mars Orbiter Laser Altimeter (MOLA) DTMs at large-scale, and meanwhile, show fairly good correlation with the High-Resolution Imaging Science Experiment (HiRISE) DTMs for fine-scale details. In addition, we show how MADNet outperforms traditional photogrammetric methods, both on speed and quality, for other datasets like HRSC, CTX, and HiRISE, without any parameter tuning or re-training of the model. We demonstrate the results for Oxia Planum (the landing site of the European Space Agency’s Rosalind Franklin ExoMars rover 2023) and a couple of sites of high scientific interest.

A beneficial outcome of ExoMars Rosalind Franklin Rover (ERFR) 1,2 landing site selection process has been the spinout science from detailed studies of parts of Mars that had not previously been examined in detail. Here, we present the geological description of Aram Dorsum3 (Fig. 1), a well-preserved, flat‐topped, branching, ~85 km long and ~ 1 km wide ridge system in western Arabia Terra that was a ‘top 3’ candidate site during ERFR site selection. Fig 1. CTX Mosaic Showing Aram Dorsum (sinuous ridge running top right to lower left). We use morphostratigraphic mapping of the Aram Dorsum ridge and surrounding area, and detailed morphological observations, to propose a consistent working hypothesis for the geological history of the region. Our observations and mapping reveal Aram Dorsum to be the sedimentary deposits of an extensive aggradational fluvial channel belt system, now preserved in positive relief by differential erosion. The existing ridge was once a large river channel belt set in extensive flood plains, many of which are still preserved. Aram Dorsum is part of a wider set of similar inverted channels found across Arabia Terra4,5, and thus was probably part of a regional fluvial system, demonstrating movement of water and sediment across large distances. Furthermore, several smaller palaeochannel belts feed into the Aram Dorsum ridge from within the local regions, and their setting and network pattern suggest a distributed and local source of water. Aram Dorsum therefore appears to record both regionally and locally distributed sources of water. Combining mapping with HiRISE6 and CTX7 Digital Elevation Model data reveals that the Aram Dorsum alluvial succession is up to 60 m thick, suggesting a formation time of 105 to 107 years by analogy to Earth8. Correlating our observations with previous regional‐scale mapping9 shows that Aram Dorsum formed in the mid‐Noachian, a result supported by impact crater size frequency distribution measurements. The Aram Dorsum formation comprises a succession of what are, by analogy with terrestrial fluvial systems, probably coarse‐grained fluvial channel belt sandstones and finer‐grained overbank deposits. The vertical thickness of alluvial succession equates to several cubic kilometres of fluvial sediments in this study region alone. That other inverted channels elsewhere in Arabia Terra4,5,10 are similar in morphology and scale suggest that similar thicknesses and volumes of mid‐Noachian to late‐Noachian fluvial sediments may be extensive and common in the wider region. Aram Dorsum was an extensive long-lived fluvial system with distributed sources. This suggests that local and regional precipitation (either as rain or as seasonal or repeated snow melt) was the source of water. That Aram Dorsum is one of several similar systems suggests that precipitation was widespread across western Arabia Terra during this period. In contrast, Aram Dorsum's low elevation and distance from the majority of the Valley Networks, argues against the source of water being melting of a distant, high‐altitude, ice sheet, or ice cap11,12. Similarly, the aggradational fluvial depositional setting and the scale of the system do not suggest deposition from multiple short‐lived fluvial flows, as might have occurred due to impact cratering or catastrophic volcanic outgassing temporarily altering the climate12–15. We conclude that Aram Dorsum is one of the oldest fluvial systems described on Mars and indicates climatic conditions that sustained surface river flows on early Mars. References cited Vago, J. L. et al. ESA ExoMars program: The next step in exploring Mars. Sol Syst Res 49, 518–528 (2015). Vago, J. L. et al. Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover. Astrobiology 17, 471–510 (2017). Balme, M. R. et al. Aram Dorsum: An Extensive Mid-Noachian Age Fluvial Depositional System in Arabia Terra, Mars. Journal of Geophysical Research: Planets 125, e2019JE006244 (2020). Davis, J. M., Balme, M. R., Grindrod, P. M., Williams, R. M. E. & Gupta, S. Extensive Noachian fluvial systems in Arabia Terra: Implications for early Martian climate. Geology G38247.1 (2016) doi:10.1130/G38247.1. Davis, J. M. et al. A Diverse Array of Fluvial Depositional Systems in Arabia Terra: Evidence for mid-Noachian to Early Hesperian Rivers on Mars. Journal of Geophysical Research: Planets 124, 1913–1934 (2019). McEwen, A. S. et al. Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE). J. Geophys. Res. 112, doi:10.1029/2005JE002605 (2007). Malin, M. C. et al. Context Camera investigation on board the Mars Reconnaissance Orbiter. J. Geophy. Res. 112, doi:10.1029/2006JE002808 (2007). Colombera, L., Mountney, N. P. & McCaffrey, W. D. A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: Implications for sequence stratigraphy. Geology 43, 283–286 (2015). Hynek, B. M. & Di Achille, G. Geological map of the Meridiani Region. (2017). Newsom, H. E. et al. Inverted channel deposits on the floor of Miyamoto crater, Mars. Icarus 205, 64–72 (2010). Fastook, J. L. & Head, J. W. Glaciation in the Late Noachian Icy Highlands: Ice accumulation, distribution, flow rates, basal melting, and top-down melting rates and patterns. Planetary and Space Science 106, 82–98 (2015). Palumbo, A. M., Head, J. W. & Wordsworth, R. D. Late Noachian Icy Highlands climate model: Exploring the possibility of transient melting and fluvial/lacustrine activity through peak annual and seasonal temperatures. Icarus 300, 261–286 (2018). Halevy, I. & Head, J. W. Episodic warming of early Mars by punctuated volcanism. Nature Geoscience 7, ngeo2293 (2014). Segura, T. L., Toon, O. B., Colaprete, A. & Zahnle, K. Environmental Effects of Large Impacts on Mars. Science 298, 1977–1980 (2002). Toon, O. B., Segura, T. & Zahnle, K. The formation of martian river valleys by impacts. Annual Review of Earth and Planetary Sciences 38, 303–322 (2010).

Introduction In 2023 the Rosalind Franklin rover will land on Mars at Oxia Planum, as part of the European Space Agency’s ExoMars programme. Transverse Aeolian Ridges (TARs), periodic bedrock ridges (PBRs), dust devils, dust devil tracks, and/or windstreaks are common within the rover’s landing ellipse. These aeolian features can be used to characterize the paleo and contemporary aeolian environment of the landing region before the start of rover operations. Here we present new measurements of the orientation, morphologies, and directionality of TARs, PBRs, windstreaks, and active dust devils, in order to better understand the wind regime at the landing site, on a continuum of spatial and temporal scales. Methods We used machine learning (Novelty or Anomaly Hunter – HiRISE [NOAH-H])[1] and manual mapping-derived TAR and PBR distributions, as well as feature orientation and crest-line lengths to elucidate regional wind patterns in a 640 km2 area of Oxia Planum (Fig.1). The area was gridded into 160 four km2 quadrants, with every second quadrant analyzed. TAR crestlines were manually digitized as continuous and discontinuous ripples, based on NOAH-H ontological classes (Barrett et al., in prep). Continuous ripples are found at a variety of scales (metres to tens of metres in across-bedform length) and consist of patches of parallel ripples which merge into one another. Discontinuous ripples are features in which the aeolian bedforms are separated by areas of non-aeolian material, including those that are isolated on the surface and those which form part of a sparse patch overlying any non-aeolian material. PBRs were identified based on NOAH-H bedrock classes, as well as their morphology and texture, similar to PBRs found elsewhere on Mars[2] and Earth[3]. We used multiple change detection techniques on HiRISE, HRSC, and CaSSIS imagery to determine migration rates of TARs and to track active dust devils and analyze wind streaks [4-8]. Results and Discussion We digitized the crestlines of 7989 continuous ripples and 2764 discontinuous ripples. TAR along-crestline lengths range from 1.1 m to 556 m. Meter-scale TARs are common across the study area but decameter-scale TARs are uncommon, and are primarily found in topographic lows. Irrespective of the scale of the bedform, we found crestline azimuths to be consistent across the study area. Based upon terrestrial megaripple research using the difference in albedo to determine the stoss and lee slopes of ripples in the Argentinian Puna[9], coupled with the crestline azimuths determined from digitizing the landforms, we posit that the winds responsible for the expression of TARs in Oxia Planum blew from the NW-NNW towards the SE-SSE (Fig.2). Crestlines for 457 PBRs were digitized, and found to be longer than TAR crestline-lengths: PBRs have ridge-crests that range in length from ~4 m to just over 600 m. Azimuths of PBRs were found to be substantially different than TARs, requiring winds to have blown from the N-NNE or the S-SSE, accounting for 180° ambiguity. However, analysis of coregistered and orthorectified repeat HiRISE imagery over 11 Earth years showed no appreciable movement in TAR fields in the study area. Two active dust devils were identified in the study area using HRSC images. One dust devil moved from the NW to the SE, while the second travelled from the SW to the NE. From a pair of CTX images taken 50 Earth days apart, 6 active dust devils and 649 dust devil tracks and/or windstreaks were identified. The features are predominantly oriented in a WNW-ESE direction with a secondary NEN-SWS noted for a few of the tracks (Fig.2). The computed azimuths for TARs, PBRs, dust devils and dust devil tracks and/or windstreaks are markedly different (Fig.2), suggesting that different formative winds would have been necessary to result in the directionality of bedforms, landforms, and dust devils we currently see in Oxia Planum. These results pose important questions about the wind regime in this area. When were the winds responsible for TARs and PBRs active? And if contemporary winds are not responsible for TAR and PBR orientation, when did the wind regime shift? To begin answering these questions, our work will focus on comparing the results from this morphological study with modelled climate data in order to help determine when and why the wind regime changed. Looking forwards to the surface mission, the surface platform for the Rosalind Franklin rover will carry a meteorological package that will provide in situ wind measurements. We can compare those data to our morphologically- and modelled-derived results. The rover itself will be able to analyze the granulometry of TARs and the bedrock strata of PBRs using imaging data, further assisting us to characterize the winds from a geomorphological standpoint. These datasets will provide further understanding of the aeolian regime of the Oxia Planum area and provide valuable insight into the climatic history of Mars. References [1] Barrett et al. (In Prep). NOAH-H, a deep-learning, terrain analysis system: results for the ExoMars Rover candidate landing sites. [2] Montgomery et al. (2012). Journal of Geophysical Research: Planets, 117(E3). [3] Hugenholtz et al. (2015). Aeolian Research, 18, 135-144. [4] Balme et al. (2008). Geomorphology, 101(4), 703-720. [5] Greeley et al. (2004). Geophysical research letters, 31(24). [6] Leprince et al. (2007). IEEE Transactions on Geoscience and Remote Sensing, 45(6), 1529-1558. [7] Reiss et al. (2011). Icarus, 215(1), 358-369. [8] Grindrod et al (2018). Journal of Geophysical Research: Planets, 123(7), 1881-1900. [9] Favaro et al. (2020). Icarus, 113765.

Introduction: Oxia Planum (OP) will be the landing site of ‘Rosalind Franklin’ (RF), the ExoMars 2020 rover (Figure 1). With the primary goal of searching for signs of past and present life on Mars, the ExoMars rover will investigate the geochemical environment in the shallow subsurface over a nominal mission of 218 martian days (sols) [1]. Two science criteria for landing site selection were ancient terrains, representing the time at which Mars was at a peak of habitability, and a recent surface exposure age, minimizing cosmic ray bombardment and associated damage to organic biomarkers - key targets for Rosalind Franklin’s analytical instruments [1]. Oxia Planum contains ancient clay-bearing terrains and has undergone significant burial and erosion, with some terrains having been recently exposed, so it is an ideal site. The landing site contains impact craters, the distribution, and degradation of which can tell us where and when burial and erosion processes have occurred. Here we: (1) Present an impact crater degradation classification scheme and database (Figure 2) [2]; (2) Report the likelihood of degradation morphologies occurring together; (3) Use the database to explore the timing of resurfacing events in OP, and; (4) Consider the implications for the geological evolution of the OP site and the wider Chryse/Arabia margin. Figure 1: The location of Oxia Planum study area (blue square), the Exo-Mars landing site (yellow ellipses), impact craters (red). Data: We developed a crater degradation classification scheme based on morphologies observable in 5–6 m/pixel CTX [3] and digitized on Murray Lab global CTX mosaic tiles; E028N16 and E024N16 [4]. We also used THEMIS [4] data overlain by the MOLA topography data to identify subtle relief patterns indicating buried “ghost craters”. Methodology: All impact craters >500 m in diameter in the OP region (Figure 1) were recorded and classified. Classification is based on the deviation from a pristine morphology by post-impact burial and degradation processes in four aspects of their morphology: (1) Stratigraphic relationship, (2) cross-sectional shape, (3) crater rim morphology, and (4) ejecta morphology. From the database, we performed a quantitative analysis to assess the probability of different degradation morphologies occurring together using the open-source Anaconda Software Python/R data science packages. We used to use the Impact Crater Size Frequency Distributions (CSFD) of the different populations of craters, based on their ‘stratigraphic relationship’ criterion, to explore the timing of regional burial and erosion process events. Figure 2: The distribution of impact craters in the Oxia Planum study area and the symbolization illustrating the 4 classes of degradation morphology. Results: The classification shows that there is a generally consistent relationship between different morphological aspects of impact crater morphology as craters degrade, forming a general trend. The distribution of different degradation morphologies (Figure 2) highlights where different burial and erosion processes have occurred history of Oxia Planum. To understand when these events happened we evaluated the CSFD for populations of craters with different degradation states (Figure 3). Craters that have not been infilled tell us the time since there has been substantial sedimentation and craters that retain sharp rims and do not have eroded ejecta tell use the time since there has been substantial regional erosion (Figure 4a). By considering buried craters, and those with removed rims embayed by younger terrain, we determine a minimum age of the basement (Figure 4b). Figure 3: Cross-sections illustrating the time sequence of different classes within the ‘stratigraphy’ classification of impact craters [2]. Discussion & Conclusions: The model ages and distribution of impact craters with different states of burial and erosion suggest that different processes have occurred in OP at different times. Close inspection of the geological setting of these crater populations shows that: (i) The ~4.0 Ga regional basement contains light-toned, layered, clay-bearing terrains. The light-toned surfaces host the most heavily degraded craters, and in the areas where craters have been buried, this is likely to be the underlying material in which the now buried craters formed. Furthermore, these impact craters are systematically found at lower elevations. Given the context of the study area, at the termination of a substantial ancient channel system (Coogoon Valles) into a very large basin (Chryse Planitia), we hypothesize that fluvial, lacustrine or possibly thalassic processes eroded and infilled these craters. (ii) Although the depositional processes had stopped by ~3.5 Ga there was still widespread erosion, degradation, modifying the rims, and removing ejecta from craters. The bowl-shaped (not infilled) floors of those craters which still retain degraded ejecta suggest that all fluvial, or other heterogeneous depositional, activity in the region had ceased by this time. (iii) All impact craters with sharp rims and preserved ejecta are younger than ~3.0 Ga. This suggests that since this time there have been substantially lower rates of erosion compared to the preceding ~0.5 Ga. This is consistent with global trends and implies that the removal of any regional overlying strata, such as might be represented by remnant rounded, buttes [6,7 ], occurred before this time. Figure 4: The CSFD for impact craters which (a) overly the clay-bearing terrains and (b) underlay, or are found within, these clay-bearing units Acknowledgment: The Open University’s ‘Space Strategic Research Area’ for funding A. Roberts’ References: [1] Vago, J. L. et., al. 2017 Astrobiology 17, 471–510. [2] Roberts et al., 2020 in prep, [3] Malin et al., (2006) [4] Dickson et al., 2018; (http://murray-lab.caltech.edu/CTX/index.html [5] Christensen et al., (2004) Space Science Reviews, 110,1 [6] Quantin-Nataf et al., 2020 in press at Astrobiology. [7] McNeil et al., 2020; This conference

Introduction Oxia Planum is a clay-rich plain located on the boundary between Arabia Terra and Chryse Planitia, and will be the landing site of the ExoMars 2022 rover, ‘Rosalind Franklin’. The region is situated between the ancient, rugged terrain of the southern highlands, and the young, smooth plains of the northern lowlands; in effect, the area acts as part of a transitional zone between the two hemispheres of the planet. Distributed across the Oxia and Circum-Chryse regions are thousands of kilometre-scale isolated landforms that resemble terrestrial mesas, buttes, hills, and plateaus (Figure 1). As a widespread geomorphological feature in this highland-lowland transitional area, they are important in the geological history of the region, but their origins are not well understood and are not yet documented. This study investigates the morphologies and distributions of the mounds in order to improve our understanding of their mode of formation and place in the regional stratigraphy. Figure 1: Study area showing the location of mounds (white dots) in Chryse Planitia. Data and Methods 14,346 mounds were identified and investigated within a study area of ~500,000 km2 using an HRSC (High Resolution Stereo Camera; Jaumann et al., 2007) and orthorectified CTX (Context Camera; Malin et al., 2007) base map that was incorporated into an ArcMap GIS (Geographical Information Systems) environment. The mounds were identified by manual inspection and were distinguished from subjacent plains by their characteristic high albedo and low thermal inertia. Mound margins and prominences (such as peaks and plateaus) were identified by their visible morphology and were digitised by hand. A circular buffer of 100 m radius was generated around mound apexes, and a 200 m buffer generated around the lower mound margin in order to accurately capture the relief of the surrounding topography. Mound height was determined by extracting the maximum elevation of the mound tops and the mean elevations of the surrounding plains from the MOLA PEDR (Mars Orbiter Laser Altimeter Point Experimental Data Records) (Zuber, 1992) and the HRSC Digital Terrain Model (DTM) raster data sets. Results Mounds are classified into four categories: (1) Hills have a single prominent peak or smooth crest (Figure 2a). (2) Mesas have a single plateau at its maximum elevation (Figure 2b). (3) Clustered mounds are instances where multiple hill peaks or mesa plateaus are juxtaposed, but the gap between them does not reach the elevation of the surrounding plains (Figure 2c). (4) Compound mounds are when hills or mesas appear to be vertically stacked on top of one another, forming distinct tiers (Figure 2d). MOLA and HRSC-derived measurements reveal that the heights of the mounds increase with areal extent, up to a maximum of ~500 m, after which the height does not increase further. This maximum height limit is reached when mounds have an area of ~10km2 in area (Figure 3). Figure 2: Examples of mounds (CTX): A: Hills, B: Mesa, C: Clustered Mounds, D: Compound Mounds. Figure 3: Graph showing the relationship between height and area for 2366 mounds of different classes. Discussion Mounds with clear, regolith-free margins show compelling evidence for being mantled and embayed by the dark plains material mapped as the HNCC1 and HCC2 units by Tanaka et al. (2005). They therefore predate the Late-Noachian to Early-Hesperian emplacement of these units. In the Oxia Planum region, the mounds directly overlie the Early Noachian (Quantin-Nataf et al., 2020) clay-bearing units. This suggests that the mounds were deposited—and partially eroded—between the Early and Late Noachian. The morphology and morphometry of a given mound are an expression of its erosional condition (Figure 3). Hills, with their rounded appearances, have the lowest average heights and have clearly been eroded more than the flat-topped mesas, which include some of the tallest mound examples. Compound mounds are intermediate in both morphology and height, suggesting that they represent a transitional erosional state between hills and mesas, and that the population represents the remnants of an eroded layer. The population of mounds extends several hundred kilometres into the northern lowlands, implying that the layer the mounds formed from (if this is their origin) once covered a substantial amount of the northern lowlands, proximal to the dichotomy boundary. Many of the mounds in the study area occur in curvilinear or ring-like arrangements, which are suggested to demarcate the location of underlying buried craters (Rodríguez et al., 2007). The preferential occurrence of mounds around the edges of buried impact craters implies that these areas were commonly more resistant to the erosion that created the mounds’ present-day relief. We suggest that these crater margin-delineating mounds are the result of post-impact hydrothermal/groundwater activity that exploited pre-existing impact-generated fractures. This process would have reinforced the material directly above the fractures through the precipitation of minerals that increased the mechanical competence of the superimposing ‘mound’ layer. Subsequent differential erosion then removed the non-indurated material, leaving arcuate patterns of mounds above crater-edges. Many mounds do not occur in arcuate patterns. We attribute these to localised fractures in the underlying bedrock that acted as individual conduits for fluid flow, or to craters that have had most of their mounds buried or removed through erosion. Conclusions Through studying the morphology, morphometry and distribution of mounds of Chryse Planitia we show that: (1) We have identified 14,346 mounds in the south Circum-Chryse region. The mounds can be divided into 4 classes, hills (isolated, rounded top), mesas (isolated flat topped), clustered mounds (non-isolated hills or mesas) and tiered mounds (stacked mesas and hills). (2) The height–area relationship of the mounds plateaus at a maximum of ~500 m. Combined with the observation of the degree of erosion of different mound morphologies, this result suggests that the mounds formed from at least one layer that was regionally extensive in the past. (3) The local distribution of mound-associated buried impact structures supports hydrothermal activity as part of the preferential preservation process.

Introduction: Oxia Planum (OP) is the selected landing site for the ESA-Roscosmos ExoMars Programme’s 2022 mission (Figure 1). The descent module and landing platform, Kazachock, will transport the Rosalind Franklin Rover to OP. With the primary goal of searching for signs of past and present life on Mars, Rosalind Franklin will investigate the geochemical environment in the shallow subsurface [1]. Figure 1: A) Locations of OP on (A) Mars and (B) in Arabia Terra. Phyllosilicate detections are in red [2]. Estimated/study landing ellipses are in yellow. -3000 m contour is shown in blue. Fluvial channels and sediment fan remnants are evident. C) Designation of mapping ‘Areas’. Oxia Planum: OP lies at the transition between the ancient terrain of Arabia Terra and Chryse Planitia (Figure 1). OP forms a shallow basin, open to the north, characterized by clay-bearing bedrock, and contains units from ~mid-Noachian to ~early-Amazonian in age [3, 4]. There have been at least two distinct phases of aqueous activity within the landing site area. A first phase during the mid-Noachian (estimated age is 4.0 Ga) resulted in ~100 m of layered clay-rich deposits. These comprise Mg/Fe smectites overlain by more Al-rich materials. After a substantial hiatus the second phase of aqueous activity included a fluviodeltaic system. This fluvial activity post-dates the clay-rich layered unit, and is associated with the younger set of channels in Coogoon Vallis, which feeds into the Oxia basin. There is abundant evidence for intense erosion across OP. There are isolated buttes, perhaps remnants of a once-extensive layer that superposed the clay-bearing unit, and a dark, mafic-rich, resistant unit of Amazonian age ( High resolution mapping campaign: Gaining a thorough understanding of the OP landing site prior to operations will provide testable hypotheses that facilitate interpretation of results, and hence provide an effective approach to address the mission’s science objectives. In pursuit of this, the ‘Macro’ sub-group, part of the Rover Science Operations Working Group (RSOWG), has been tasked to perform a detailed group mapping exercise of the OP landing site. The work prioritizes the area of highest estimated landing probability, but incorporates interpretations from elsewhere in the wider landing ellipse envelope and beyond. Complementary CTX scale mapping covers the wider area around the landing site and is described elsewhere [5]. The objectives of the campaign include to: (1) Provide a detailed geospatial dataset comprising mapped photo-geologic units, potential science targets, and surface features at OP. Via a follow-on reconciliation phase, and incorporating additional information from orbital or analogue data, a morpho-stratigraphic map of the landing site will be produced. (2) Familiarize scientists from across the different instrument teams and disciplines represented on RF with the geology and geography of the landing site in anticipation of rover operations. (3) Using the detailed scientific understanding enabled by the map, and elaborating on work completed during the landing site selection process, develop testable hypotheses that will (i) link mission-level science objectives to specific activities in the rover mission science operations strategy, and (ii) provide a basis for fine detail ‘rapid reconnaissance’ mapping (e.g. [6]) done after landing, when the precise location of the rover is established. Schedule and Organisation: The campaign coordination/leadership team begun planning in Q4 2019 and team mapping exercise began in Q3 2020 with reconciliation underway in Q4 2020 (Figure 2). We use a grid of 1×1 km ‘quads’ to cover the estimated 3-sigma landing ellipse envelope (Figure 3). Of these, ~150 quads cover the 1-sigma estimated landing ellipse envelope, which we divide into 3 ‘Areas’ informally referred to as Pannonia, Dalmatia and Aquitania. 88 individuals associated with RSOWG who volunteered to map, are assigned quads amongst these Areas. Mapping is intended to be completed by August 2020. Figure 2: Mapping campaign timeline and phases. Data and Tools: The Multi-Mission Geographic Information System (MMGIS) [7] provides capabilities for group mapping and centralised data management for this project. An enhancement to MMGIS with additional functionality for geospatial analysis and mapping was developed for the NASA Mars 2020 Rover mission. We have deployed an instance of this tool at ESA ESTEC to facilitate group mapping. Volunteers map on a HiRISE orthorectified image and DTM mosaic. These data are co-registered with a CTX DTM and orthomosaic, which in turn are georeferenced to the HRSC MC11W mosaic [8]. In the mapping phase, many users concurrently map geologic contacts and surface features in their assigned quads. During the reconciliation phase, a smaller team is responsible for reconciling the maps produced by volunteers over all quads. The map data output are planned to be published and made accessible to the community. Results: At the time of abstract submission the mapping phase is ongoing. Having successfully completed the initial training mappers are undertaking an ‘exploration phase’ in which they discover the contents of their quads to refine an overall geological unit definition to be used during the mapping phase. We present the preliminary mapping results and a status of the project, and provide an outlook towards publication of the map as a community resource, and for use in continued development of the ExoMars 2022 Mission science operations strategy. Figure 3: Left – Distribution of quads and areas in MMGIS. Right – example terrain and mapping in MMGIS: HiRISE ESP_039299_1985. References Vago, J. L. et al., (2017) Astrobiology 17 (6–7), 471–510. Carter, J. et al., (2013) J. Geophys. Res. 118 (4), 831–858. Quantin-Nataf, C. et al., (2020) Submitted to Astrobiology. Quantin-Nataf, C. et al., (2019) 9th Mars, Abs.# 6317. Hauber, E. et al., EGU Gen. Assem. (2020), Abs.# EGU2020-7773. Balme, M. R. et al., (2019) Planet. Space Sci. 165, 31–56. Calef, F. J. et al., (2019) in 4th Planet. Data Work., Vol. 2151. Gwinner, K. et al., (2016) Planet. Space Sci. 126, 93–138.

The European Space Agency’s ExoMars Rosalind Franklin rover is scheduled to launch in 2022, with the goal of searching for signs of past and present life on Mars by investigating the geochemical environment in the shallow subsurface and characterising the surface environment. The landing site, Oxia Planum, is a clay bearing plain approximately 200 km wide at the margin of Chryse Planitia. In this presentation we show results of CRISM hyperspectral analysis within the ~2.1 × 10^5 km^2 Oxia Planum model catchment area, which contributes to the geological context for the landing site.

A major debate in Mars science is the nature of the early Mars climate, and the availability of precipitation and runoff. Observations of relict erosional valley networks have been proposed as evidence for extensive surface run‐off around the Noachian‐Hesperian boundary. However, these valley networks only provide a time‐integrated record of landscape evolution and thus the timing, relative timescales and intensity of aqueous activity required to erode the valleys remain unknown. Here, we investigate an ancient fluvial sedimentary system in western Arabia Terra, now preserved in positive relief. This ridge, ‘Aram Dorsum’, is flat‐topped, branching, ~ 85 km long, and particularly well‐preserved. We show that Aram Dorsum was an aggradational alluvial system and that the existing ridge was once a large river channel‐belt set in extensive flood plains, many of which are still preserved. Smaller, palaeochannel‐belts feed the main system; their setting and network pattern suggest a distributed source of water. The alluvial succession is up to 60 m thick, suggesting a formation time of 105 to 107 years by analogy to Earth. Our observations are consistent with Aram Dorsum having formed by long‐lived flows of water, sourced both locally, and regionally as part of a wider alluvial system in Arabia Terra. This suggests frequent or seasonal precipitation as the source of water. Correlating our observations with previous regional‐scale mapping shows that Aram Dorsum formed in the mid‐Noachian, making it one of the oldest fluvial systems described on Mars and indicating climatic conditions that sustained surface river flows on early Mars.

There is abundant evidence for aqueous processes on Noachian terrains across Mars; however, key questions remain about whether these processes continued into the Hesperian as the martian climate became less temperate. One region with an extensive Hesperian sedimentary record is Valles Marineris. We use high‐resolution image and topographic data sets to investigate the fluvial systems in the southwest Melas basin, Valles Marineris, Mars. Fluvial landforms in the basin exist across a wide area, and some are preserved as inverted channels. The stratigraphy of the basin is complex: Fluvial landforms are preserved as planview geomorphic features and are also interbedded with layered deposits in the basin. The fluvial morphologies are consistent with formation by precipitation‐driven runoff. Fluvial processes in the basin were episodic, suggesting multiple wet and dry periods. During dry periods, mantling material accumulated, and significant volumes of sediment were eroded, inverting fluvial channels. During wet periods, inverted channels and mantling material infilling valleys were incised by further fluvial erosion. These trends for episodic fluvial processes are similarly reflected in the central depression of the southwest Melas basin, previously described as a paleolake. Ultimately, fluvial processes in the basin gradually shut down, becoming geographically restricted, and then ceased entirely. We show that branching valley networks are also present on the plateaus above Melas and Ius Chasma, which converge on the heads of tributary canyons. These suggest that precipitation‐driven runoff processes also extended onto the plateaus of Valles Marineris.

Nisha Ramkissoon, Mark Burchell, Peter Fawdon and Louisa Preston report from an RAS Specialist Discussion Meeting on finding and identifying evidence for life on Mars

The presence of snow and ice at midlatitudes of Mars cannot be explained by current climatic conditions, as surface ice is unstable. However, a large variety of debris‐covered glaciers have been observed at both midlatitudes. Here, we report the presence of local, small‐scale, and debris‐covered stagnant ice deposits on the floor of a valley system in Terra Cimmeria. These deposits, termed valley fill deposits (VFD), have a distribution that is restricted to the host valley floor and to the extent of the ejecta blanket associated with Tarq impact crater. The VFD are characterized by convex‐upward morphology, various crevasses, sublimation pits, an average area of a few square kilometers, and occasional ejecta streaks on their surface. Our model age estimation points to two possible time frames for the Tarq impact event; thus, we suggest two formation scenarios for VFD: (I) distribution of ice due to impact into shallow ice during the Middle Amazonian and (II) post‐impact deposition of VFD due to precipitation. In both scenarios, ice preservation is most likely due to a lag of dust and debris deposited in the valley's topographic lows. Scenario I is more consistent with our geomorphological observation of the VFD being overlain by ejecta streaks. Our results highlight the importance of local geological events and conditions in the distribution and preservation of buried ice deposits on Mars and suggest that more small‐scale and debris‐covered ice deposits may exist in the midlatitudes than previously thought. These deposits are of high importance for future human exploration missions to Mars.

Branching to sinuous ridges systems, hundreds of kilometers in length and comprising layered strata, are present across much of Arabia Terra, Mars. These ridges are interpreted as depositional fluvial channels, now preserved as inverted topography. Here we use high‐resolution image and topographic data sets to investigate the morphology of these depositional systems and show key examples of their relationships to associated fluvial landforms. The inverted channel systems likely comprise indurated conglomerate, sandstone, and mudstone bodies, which form a multistory channel stratigraphy. The channel systems intersect local basins and indurated sedimentary mounds, which we interpret as paleolake deposits. Some inverted channels are located within erosional valley networks, which have regional and local catchments. Inverted channels are typically found in downslope sections of valley networks, sometimes at the margins of basins, and numerous different transition morphologies are observed. These relationships indicate a complex history of erosion and deposition, possibly controlled by changes in water or sediment flux, or base‐level variation. Other inverted channel systems have no clear preserved catchment, likely lost due to regional resurfacing of upland areas. Sediment may have been transported through Arabia Terra toward the dichotomy and stored in local and regional‐scale basins. Regional stratigraphic relations suggest these systems were active between the mid‐Noachian and early Hesperian. The morphology of these systems is supportive of an early Mars climate, which was characterized by prolonged precipitation and runoff., Key Points Fluvial depositional systems, 50–100 m in stratigraphic thickness, are common across Arabia Terra, MarsThese deposits represent former rivers (at least 100–200 km long), floodplains, and lakes and are found filling fluvial valleys and basinsThese systems are mid‐Noachian to early Hesperian in age and represent significant periods of geologic time

We investigated the origin of the fan-shaped deposit at the end of Hypanis Valles that has previously been proposed as an ExoMars, Mars 2020, and human mission candidate landing site, and found evidence that the landform is an ancient delta. Previous work suggests that the deposit originated from a time of fluvial activity both distinct from and prior to catastrophic outflow, and crater counting placed the deposit’s age at ≥ 3.6 Ga. We found over 30 thin sedimentary strata in the proposed delta wall, and from our slope analysis conclude that the fluvial sequence is consistent with a lowering/retreating shoreline. We measured nearly horizontal bedding dip angles ranging from 0° to 2° over long stretches of cliff and bench exposures seen in HiRISE images and HiRISE stereo DTMs. From THEMIS night IR images we determined that the fan-shaped deposit has a low thermal inertia (150-240 Jm-2 K-1 s-1/2) and the surrounding darker-toned units correspond to thermal inertia values as high as 270-390 Jm-2 K-1 s-1/2. We interpret these findings to indicate that the fan-shaped deposit consists mostly of silt-sized and possibly finer grains, and that the extremely low grade and large lateral extent of these beds implies that the depositional environment was calm and relatively long-lived. We interpret the geomorphology and composition as incompatible with an alluvial fan or mudflow hypothesis. From our stratigraphic mapping we interpret the order of events which shaped the region. After the Chryse impact, sediment filled the basin, a confined lake or sea formed allowing a large delta to be deposited near its shoreline, the water level receded to the north, darker sedimentary/volcanic units covered the region and capped the light-toned deposit as hydro-volcanic eruptions shaped the interior of Lederberg crater, freeze/thaw cycles and desiccation induced local fracturing, and finally wrinkle ridges associated with rounded cones warped the landscape following trends in degraded crater rims and existing tectonic features. The ancient deltaic deposit we observe today was largely untouched by subsequent catastrophic outflows, and its surface has been only moderately reshaped by over 3 billion years of aeolian erosion.

Recent aeolian bedforms comprising loose sand are common on the martian surface and provide a mobility hazard to Mars rovers. The ExoMars rover will launch in 2020 to one of two candidate sites: Mawrth Vallis or Oxia Planum. Both sites contain numerous aeolian bedforms with simple ripple-like morphologies. The larger examples are ‘Transverse Aeolian Ridges’ (TARs), which stereo imaging analyses have shown to be a few metres high and up to a few tens of metres across. Where they occur, TARs therefore present a serious, but recognized and avoidable, rover mobility hazard. There also exists a population of smaller bedforms of similar morphology, but it is unknown whether these bedforms will be traversable by the ExoMars rover. We informally refer to these bedforms as “mini-TARs”, as they are about an order of magnitude smaller than most TARs observed to date. They are more abundant than TARs in the Oxia Planum site, and can be pervasive in areas. The aim of this paper is to estimate the heights of these features, which are too small to measured using High Resolution Imaging Science Experiment (HiRISE) Digital Elevation Models (DEMs), from orbital data alone. Thereby, we aim to increase our knowledge of the hazards in the proposed ExoMars landing sites. We propose a methodology to infer the height of these mini-TARs based on comparisons with similar features observed by previous Mars rovers. We use rover-based stereo imaging from the NASA Mars Exploration Rover (MER) Opportunity and PRo3D software, a 3D visualisation and analysis tool, to measure the size and height of mini-TARs in the Meridiani Planum region of Mars. These are good analogues for the smaller bedforms at the ExoMars rover candidate landing sites. We show that bedform height scales linearly with length (as measured across the bedform, perpendicular to the crest ridge) with a ratio of about 1:15. We also measured the lengths of many of the smaller aeolian bedforms in the ExoMars rover Oxia Planum candidate landing site, and find that they are similar to those of the Meridiani Planum mini-TARs. Assuming that the Oxia Planum bedforms have the same length/height ratio as the MER Opportunity mini-TARs, we combine these data to provide a probabilistic method of inferring the heights of bedforms at the Oxia Planum site. These data can then be used to explore the likely traversability of this site. For example, our method suggests that most of the bedforms studied in Oxia Planum have ridge crests higher than 15 cm, but lower than 25 cm. Hence, if the tallest bedforms the ExoMars rover will be able to safely cross are only 15 cm high, then the Oxia Planum sites studied here contain mostly impassable bedforms. However, if the rover can safely traverse 25 cm high bedforms, then most bedforms here will be smaller than this threshold. As an additional outcome, our results show that the mini-TARs have length/height ratios similar to TARs in general. Hence, these bedforms could probably be classified simply as “small TARs”, rather than forming a discrete population or sub-type of aeolian bedforms.

Nili Patera is a 50 km diameter caldera at the center of the Syrtis Major Planum volcanic province. The caldera is unique among Martian volcanic terrains in hosting: (i) evidence of both effusive and explosive volcanism, (ii) hydrothermal silica, and (iii) compositional diversity from olivine-rich basalts to silica-enriched units. We have produced a new geological map using three mosaicked 18 m/pixel Context Camera digital elevation models, supplemented by Compact Remote Imaging Spectrometer for Mars Hyperspectral data. The map contextualizes these discoveries, formulating a stratigraphy in which Nili Patera formed by trapdoor collapse into a volcanotectonic depression. The distinctive bright floor of Nili Patera formed either as part of a felsic pluton, exposed during caldera formation, or as remnants of welded ignimbrite(s) associated with caldera formation—both scenarios deriving from melting in the Noachian highland basement. After caldera collapse, there were five magmatic episodes: (1) a basaltic unit in the caldera's north, (2) a silica-enriched unit and the associated Nili Tholus cone, (3) an intrusive event, forming a ~300 m high elliptical dome; (4) an extrusive basaltic unit, emplaced from small cones in the east; and (5) an extreme olivine-bearing unit, formed on the western caldera ring fault. The mapping, together with evidence for hydrated materials, implies magmatic interaction with subsurface volatiles. This, in an area of elevated geothermal gradient, presents a possible habitable environment (sampled by the hydrothermal deposits). Additionally, similarities to other highland volcanoes imply similar mechanisms and thus astrobiological potential within those edifices.

BPSC 2020 abstract