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Chaos terrains are geologically young and extensively disrupted surface features of Europa, thought to be an expression of the subsurface ocean interacting with the surface. The most prominent examples of this terrain on Europa are Conamara Chaos, and Thera and Thrace Maculae, all prime targets for the upcoming JUICE and Europa Clipper missions to assess the astrobiological potential of Europa. Of the three features, Thrace Macula is currently the least studied and understood. It intersects both Agenor Linea to the north and Libya Linea to the south, two important regional-scale bands whose interaction with Thrace is yet to be fully unraveled, especially in terms of their relative ages of emplacement and activity. Through detailed structural mapping using Galileo Solid State Imager data and terrain analysis on Digital Terrain Models, we here develop a novel hypothesis on the mechanisms that have been involved in the study area. We find that Thrace Macula is bordered along most sides by preexisting strike-slip faults that have constrained its emplacement and areal distribution. We determine a sequence of events in the area involving the formation of Agenor Linea, followed by that of Libya Linea first and Thrace Macula later, and ultimately by strike-slip tectonic activity driven by Libya Linea and displacing a portion of Thrace Macula. Therefore, Thrace’s subsurface material, uprising along faults postdating its formation, likely represents the freshest possible that could be sampled by future spacecraft in this region, a major consideration for the upcoming Europa Clipper mission.

Faults and fractures may emplace fresh material onto Europa's surface, originating from shallow reservoirs within the ice shell or directly from the subsurface ocean. Ménec Fossae is a region of particular interest as it displays the interaction of several geological features, including bands, double ridges, chaotic terrains, and fossae, within a relatively small area. These features might affect the emplacement of buried material and subsequent exposure of fresh volatiles, prime targets for the upcoming JUICE and Europa Clipper missions in order to assess Europa's astrobiological potential. Previous studies have already revealed that a deep central trough is present at Ménec Fossae, flanked by several subparallel minor troughs and by a few asymmetrical scarps with lobate planforms. The presence of such features has motivated this study, given its potential to provide clear indications on the tectonic regime involved. Through detailed geomorphological-structural mapping using Galileo Solid State Imager data and terrain analysis on Digital Terrain Models, we could develop a novel hypothesis on the formation mechanisms that might have been involved in the study area. We propose that Ménec Fossae has been shaped by transtensional (strike-slip with an extensional component) tectonic activity, as indicated by the orientation and relationship of the tectonic features present. Likely, such transtensional tectonism occurred above or associated with shallow subsurface water, consistent with the overall morphology and topography of the study area and the presence of chaotic terrains and double ridges. These results strengthen the case for widely distributed shallow water reservoirs within Europa's ice shell.

Events which deposit fresh material onto Europa’s surface may be irregular and catastrophic, such as large-scale impacts, or localized, and potentially extant, processes in which faults, fractures, or brine transport bring subsurface liquid onto the surface. This liquid may originate from shallow reservoirs within the ice shell or directly from the subsurface ocean [1]. Emplacement may be a slow, extrusive process, or quicker, potentially with the formation of cryovolcanic plumes (e.g. [2,3,4]). Plume deposits or any other surface material can also be transported to high altitudes by another mechanism: when hypervelocity interplanetary micrometeoroids, or larger objects, impact the surface of an atmosphereless planetary body like Europa, these can generate impact ejecta with high enough velocities to reach altitudes of hundreds of kilometres [5]. Most of these ejecta particles are gravitationally bound, moving on ballistic trajectories lasting up to hundreds of seconds and producing an almost isotropic dust exosphere around Europa [6,7]. Subsurface oceans, in the Jovian and in other planetary systems, can therefore be characterized using the particles they emit, via either indirect or direct routes: 1) Detection and analysis of ejecta particles lofted by micrometeoroid impacts from those parts of the surface that recently interacted with subsurface water (e.g. certain landforms, such as chaos terrains on Europa, or plume deposits); 2) Direct sampling of plume particles in space (where plumes are present, as perhaps possible on Europa). The SUrface Dust Analyzer (SUDA), onboard the Europa Clipper spacecraft, is designed to measure the composition and trajectories of such impact ejecta particles and/or plume material. SUDA is an impact ionisation time-of-flight mass spectrometer, which uses the ions generated by hypervelocity impacts of dust grains onto the instrument target to generate mass spectra of the impinging particles. Depending on the altitude, SUDA will be able to detect up to tens of ejected surface particles per second during each flyby, each likely to contain a wide variety of organic and/or inorganic compounds, and via trajectory reconstruction, map them to their origins on the surface [8]. At global scales, landform evolution on atmosphereless bodies is primarily driven by impact gardening and tectonics. The tectonic regime that dominates within Europa’s ice shell is likely to be extensional, a hypothesis supported by numerous lines of evidence, such as the widespread presence of dilational bands that represent >40% of the total surface area [9]. Several types of geological features generated within such a tectonic regime, including chaos terrains and fossae, might affect the emplacement of buried material and subsequent exposure of fresh volatiles on Europa’s surface. Chaos Terrains are, on Europa, geologically very young and extensively disrupted surface features, interpreted as reflecting recent interaction with subsurface material [10]. Leading-hemisphere chaos regions have recently been shown to be compositionally distinct from their surroundings, probably indicating contributions from endogenous sodium chloride sourced from the subsurface ocean [11,12]. Fossae are long, narrow depressions (troughs). The term is used for topographic features that occur on extraterrestrial planetary surfaces, whose exact origin is uncertain, although they are thought to be the result of extensional tectonic processes [13]. Here we investigate two neighboring features on Europa, Ménec Fossae and Thrace Macula (a chaos terrain), for which we produced high-resolution photoclinometrically-derived [14] digital terrain models (DTMs, Fig. 1) and geomorphological-structural maps (Fig. 2). Our preliminary results suggest that this area of Europa has undergone transtensional (strike-slip paired with extension) tectonic activity, as indicated by the orientation and relationship of the faults and fossae. Such tectonic style has possibly created a pathway facilitating the ascent of subsurface material, especially volatiles, that play an important role in the formation of chaos terrain. These results will help ascertain the most likely regions on Europa in which to find fresh material, representative of the subsurface ocean, and be used as input data for dust ejecta trajectory models that will ultimately assist the mass spectrometer Surface Dust Analyzer (SUDA), onboard the upcoming Europa Clipper mission, in compositionally mapping Europa’s surface [15]. Figure 1. Regional DTMs, Ménec Fossae (top) and Thrace Macula (bottom) Figure 2. Geomorphological map of Ménec Fossae 1. Head, J.W., & Pappalardo, R.T., (1999). J. Geophys. Res. 104, 27143–27155. 2. Roth, L., et al., (2014). Science 343, 171. 3. Sparks, W.B., et al., (2017). The Astrophysical Journal Letters, 839(2), L18. 4. Jia, X., et al., (2018). Nat. Astron. 2, 459. 5. Koschny, D., & Grün, E., (2001). Icarus, 154(2), 402-411. 6. Krivov, A. V., et al., (2003a). Planetary and Space Science 51, 251-269. 7. Postberg, F., et al., (2011). Planetary and Space Science, 59(14), 1815–1825. 8. Goode, W., et al., (2021). Planetary and Space Science, 208, 105343 9. Kattenhorn, S.A. & Hurford, T.A., (2009). In: Europa, 199-236. 10. Schmidt, B. E., et al., (2011). Nature 502, v. 479 11. Trumbo, S. K., et al., (2019). SciA, 5, aaw7123 12. Trumbo, S. K., et al., (2022), PSJ, 3, 27 13. Schenk, P., et al., (2020). Geophys. Res. Let., 47, e2020GL088364 14. Lesage et al., (2021). Icarus, Volume 361, 114373. 15. Kempf, S., et al., (2019). AGU Fall Meeting 2019, abstract #P53D-3500

Introduction While the single image coverage of the currently available Context Camera (CTX, [1]) images converges to the full coverage of the surface of Mars, the individual images show offsets of tens to hundreds of pixels from their real ground position. Current efforts for ortho-rectification and lateral control use the MOLA data as a source for global reference and geometric correction (e.g., [2]). Towards the equator this approach is unprecise due to the large discrepancy of the spatial resolutions of the two datasets (6 m/px vs.463 m/px). Automatic point matching of image pixels to DTM pixels are not reliable, therefore usually the CTX pixels are matched to imagery datasets which are themselves controlled to MOLA, such as the THEMS IR dataset [3]. This of course is sensitive to error propagation. The HRSC team expects to finalise the creation of global mosaics consisting of bundle-block-adjusted DTMs and their corresponding image mosaics [4] by the end of the year 2023. These DTMs provide an internal photogrammetric precision better than the 50 m used as the grid size. The deviations from the MOLA profile heights are also less than this single pixel resolution value [5]. Their corresponding image mosaics provide precise pixel-by-pixel alignment to the respective DTM in 12.5 m/px resolution [6]. Here we show a first approach using these HRSC products as the global reference dataset for Mars, by creating a CTX quadrangle mosaic with absolute geodetic control to HRSC (and therefore also to MOLA). Methods Generally, first the HRSC DTM is used for the ortho-rectification of the CTX images instead of MOLA. After this geometric correction, the image mosaic is used as a reference for automatic point matching and co-registration. We then apply the well-established brightness correction in a similar way we are using it for HRSC [6]. Ultimately, we combine the multitude of images from a whole quadrangle together to a seamless mosaic and export it as a single image file. The general workflow of our processing pipeline is shown in Fig. 1. Figure 1: Flow chart of the proposed CTX processing workflow from single images to a complete mosaic. Owed to the large amount of single CTX images per HRSC tile, we use a database management system (DBMS) for storage and retrieval of the CTX data catalog. In the spatially-enabled DBMS we perform geometric queries by intersection with the HMC30 boundary with the CTX footprints. The map-projection and co-registration of the data is slow - processing time can be significantly increased by high-performance computing (HPC). We define the main tasks as array jobs: The ISIS [7] processing pipeline performs image ortho-rectification using the HMC30 DTM. The resulting nominally ortho-rectified CTX images show a significant offset to HRSC based on inaccurate spacecraft attitude control (see Fig. 2 left). For correction, we introduce a subsequent co-registration based on a phase correlation approach [8]. The consecutive brightness correction uses the same HRSC image mosaic as a radiometric reference. A final step for seamline creation and image export is performed outside of the cluster using Desktop GIS software. Figure 2: Image offsets between CTX image D02_027799_1580 (6.25 m/px) and HRSC HMC30 20W (12.5 m/px); left: CTX with nominal pointing after ISIS standard processing, offsets in red; middle: CTX co-registered to HRSC by global x/y shift, offsets in blue; right: CTX co-registered by warping to dense network of tie points, no visible offsets. Results In a first qualitative assessment of dividual single CTX images, we examined the pixel offsets between the nominal images and their co-registered counterparts. First we tried to translate the images only in x/y direction as a block, but there were significant remaining offsets, increasing in the along-track direction of the spacecraft (see Fig. 2 middle). We achieved better results by transforming the CTX image to the HRSC reference image by the use of a dense network of tie points (see Fig. 2 right). In the example given in Figure 2, the offsets measure about 170 m which corresponds to 27 CTX pixels, 13 HRSC image pixels and 3 HRSC DTM pixels. Based on the processing pipeline described above, we created a complete CTX mosaic of the HMC-20W quadrangle, which will be available on our mapserver (https://maps.planet.fu-berlin.de). The ISIS processing steps were successfully applied to all of the 2062 images intersecting the DTM boundary, and 1692 of the images were successfully co-registered to the HMC30 image mosaic. The processing of the complete CTX dataset consisting of 2062 single images with a resolution of 6.25 m/px took around 3 days on the cluster, but this time is highly dependent on the cluster utilization. Discussion The results of the co-registration with a success rate of 82% are better than we initially expected. The remaining visible image gaps might be filled either by adapted parameters for co-registration or by the addition of manual tie-points. The initial offsets between the nominally-processed CTX images and the HRSC DTM (3 pixels in our example) might be large enough to expect impacting geometric artifacts during the orthorectification process, which will not be removed by the co-registration. Therefore it is planned to include an additional iterative processing step by feeding the tie-points acquired from the co-registration back into the ISIS pipeline and perform a bundle-adjustment between the CTX image and HRSC as ground and height reference. Acknowledgements This work is supported by the German Space Agency (DLR Bonn), grant 50OO2204, on behalf of the German Federal Ministry for Economic Affairs and Climate Action. We thank the HPC Service of FU for computing time [9]. References [1] M. C. Malin et al., JGR Planets (E5 2007). [2] J. L. Dickson et al., LPSC 49, #2480. [3] S. J. Robbins et al., LPSC 52, #2066. [4] K. Gwinner et al., EPSC/DPS 13, 2019, #2006. [5] K. Gwinner et al., PSS 126 (2016). [6] G. G. Michael et al., PSS 121 (2016). [7] T. Sucharski et al. (2020), DOI: 10.5281/zenodo.3962369. [8] Q. Chen et al., IEEE TPAMI 16 (1994), pp. 1156–116. [9] L. Bennett et al. (2020), DOI: 10.17169/refubium-26754.

Direct link to the interactive map: https://maps.planet.fu-berlin.de/jezero Introduction: We welcome you to explore the landing site of the Mars 2020 Perseverance rover in an interactive hiking map loaded with orbital imagery, terrain data and virtual 3D panoramic views of Jezero crater and its surrounding area! The map allows fast zooming and panning for the exploration of the available data in several magnitudes of scale levels from far away until centimeter detail. It also seamlessly integrates several waypoints with pre-rendered virtual 360° panoramas and several real panoramas each stitched together from a multitude of single images of the MastcamZ instrument. System Design: The general architecture follows the principles used for the HRSC Mapserver [1], see Figure 1. In contrast to the Mapserver’s intended use as a science-based data-portal and dissemination platform, the Jezero map was also planned as an public outreach application. But it serves as the first prototype of a next-generation infrastructure for disseminating upcoming data products from Freie Universität. Major parts of the existing set-up could be re-used but had to be updated. The layer menu had to be completely re-designed as it was implemented statically and hard-wired into the main code, which made it impossible to maintain and update its module dependencies. The most fundamental changes to the HRSC Mapserver are: Integrated A-Frame panoramic views, enabled switching between map views and 3D/VR/AR scenes Sidebar element for improved user experience on mobile devices and small screens, by Tobias Bieniek (https://github.com/Turbo87/sidebar-v2) LayerSwitcher element integrated into the sidebar, by Matt Walker (https://github.com/walkermatt/ol-layerswitcher) Transition from OpenLayers 3 to OpenLayers 5 involving a major migration from the underlying Google™ Closure compiler infrastructure to ES-based modules Usage of the Parcel packager to bundle the application for production IAU2015-based Mars spatial reference system as the base reference Figure 1: Flow chart of the web-GIS application’s general system design outlining front-end (client) and back-end (server) components. Data: The base layer of the customized map consists of a merged data-set from three different instruments. The Mars Express High-Resolution Stereo Camera (HRSC, [2]) Mars-Chart 30 data [3] are used as reference data-sets providing the geodetically controlled topography base and the seamless base imagery covering the complete landing site. 33 single images of the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX, [4]), empirically corrected for their inherent ”smile” effect and bundle-adjusted with absolute geodetic control to the HMC-13E tile [5]. The single CTX images have then been radiometrically normalized using the HMC13 tile as brightness reference with the well-established methods from [6], re-implemented and adapted to CTX. To provide the orbital image data from the landing site in the highest available detail, we included image and topography data from the MRO High Resolution Imaging Science Experiment (HiRISE, [7]). We use the ortho-rectified image mosaic provided by the Terrain Relative Navigation (TRN) team of the JetPropulsion Laboratory (JPL) [8], which provided sufficient alignment to the underlying CTX and HRSC reference data. All three data-sets have been colourised by the HRSC color channels using pan-sharpening techniques and logarithmically stretched to Byte format. Outlook: The new web-GIS client presented here will also serve as the prototype for the re-design of the HRSC Mapserver for data dissemination and download. Its unique features for single image visualization and download will be re-implemented and integrated into the new design. It will serve more image data products for time-series analysis and vizualitation, mainly the complete co-registered set of CTX images. Figure 2: Jezero front-end showing the inflow-channel Neretva vallesJezero delta as a map frame. The droplet icons are clickable and switch the interface from the web-based view to the 3D-virtual panorama view as seen in Figure 3. Figure 3: Jezero front-end showing the 3D-virtual panorama view of the ”Pliva Vallis” outflow channel. The underlying A-Frame library allows 360 degree-orientation in 3D or VR. Acknowledgements: This work is supported by theGerman Space Agency (DLR Bonn), grant 50OO2204, on behalf of the German Federal Ministry for Economic Affairs and Climate Action. References: [1] S.H.G. Walter et al., ESS 7 (2018). [2] R. Jaumann et al., PSS 55 (2007). [3] K. Gwinner et al., PSS 126 (2016). [4] M. C. Malin et al., JGR Planets (2007). [5] A. Neesemann et al., LPSC 52, 2021, #2509. [6] G. Michael et al., PSS 121 (2016). [7] A. S. McEwen et al., JGR Planets (2007). [8] R. L. Fergason et al., LPSC 51, 2020, #2020.

Hydrothermal processes in impact environments on water-rich bodies such as Mars and Earth are relevant to the origins of life. Dawn mapping of dwarf planet (1) Ceres has identified similar deposits within Occator crater. Here we show using Dawn high-resolution stereo imaging and topography that Ceres’ unique composition has resulted in widespread mantling by solidified water- and salt-rich mud-like impact melts with scattered endogenic pits, troughs, and bright mounds indicative of outgassing of volatiles and periglacial-style activity during solidification. These features are distinct from and less extensive than on Mars, indicating that Occator melts may be less gas-rich or volatiles partially inhibited from reaching the surface. Bright salts at Vinalia Faculae form thin surficial precipitates sourced from hydrothermal brine effusion at many individual sites, coalescing in several larger centers, but their ages are statistically indistinguishable from floor materials, allowing for but not requiring migration of brines from deep crustal source(s)., Dawn mission’s second extended phase provided high resolution observations of Occator crater of the dwarf planet Ceres. Here, the authors show stereo imaging and topographic maps of this crater revealing the influence of crustal composition on impact related melt and hydrothermal processes, and compare features to those on Mars, Earth and the Moon.

Before acquiring highest-resolution data of Ceres, questions remained about the emplacement mechanism and source of Occator crater’s bright faculae. Here we report that brine effusion emplaced the faculae in a brine-limited, impact-induced hydrothermal system. Impact-derived fracturing enabled brines to reach the surface. The central faculae, Cerealia and Pasola Facula, postdate the central pit, and were primarily sourced from an impact-induced melt chamber, with some contribution from a deeper, pre-existing brine reservoir. Vinalia Faculae, in the crater floor, were sourced from the laterally extensive deep reservoir only. Vinalia Faculae are comparatively thinner and display greater ballistic emplacement than the central faculae because the deep reservoir brines took a longer path to the surface and contained more gas than the shallower impact-induced melt chamber brines. Impact-derived fractures providing conduits, and mixing of impact-induced melt with deeper endogenic brines, could also allow oceanic material to reach the surfaces of other large icy bodies., The second extended phase of the Dawn mission provided high resolution observations of Occator crater of the dwarf planet Ceres. Here, the authors show that the central faculae were sourced in an impact-induced melt chamber, with a contribution from the deep brine reservoir, while the Vinalia Faculae were sourced by the deep brine reservoir alone.

Wind-formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment [1-4]. Here we show that the landing site experienced multiple climatic changes recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs) [5, 6]. Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Superposition relationships of the PBRs with a dark-toned geological unit [4] indicate that such a change in the main wind condition likely occurred during the Amazonian. Active bedform migration from nearby craters (McLaughlin and Oyama) show winds coming from the North, matching the orientation of the wind streaks visible in the putative landing ellipse. Our results provide constrains on the wind regime in Oxia Planum and offer indications on present and past winds that will be crucial for understanding the landing site geology.For full details, see [1].[1] Silvestro, S. et al. 2021. Periodic Bedrock Ridges at the ExoMars 2022 landing site: Evidence for a Changing Wind Regime. GRL, 48, 4.[2] Favaro, E. et al. 2021. The Aeolian Environment of the Landing Site for the ExoMars Rosalind Franklin Rover in Oxia Planum, Mars. JGR, 126, 4.[3] Balme, M. et al. 2017. Surface-based 3D measurements of small aeolian bedforms on Mars and implications for estimating ExoMars rover traversability hazards. PSS, 153, 39-53.[4] Quantin, C. et al. Oxia Planum: The Landing Site for the ExoMars ‘‘Rosalind Franklin’’ Rover Mission: Geological Context and Prelanding Interpretation. Astrobiology, 21, 3.[5] Montgomery, D. R. et al. 2012. Periodic bedrock ridges on Mars. JGR, 117, E03005.[6] Hugenholtz, C. H. et al. 2015. Formation of periodic bedrock ridges on Earth. Aeolian Research, 18, 135–144.

Detailed mapping of topography is crucial for the understanding of processes shaping the surfaces of planetary bodies. In particular, stereoscopic imagery makes a major contribution to topographic mapping and especially supports the geologic characterization of planetary surfaces. Image data provide the basis for extensive studies of the surface structure and morphology on local, regional and global scales using photogeologic information from images, the topographic information from stereo-derived digital terrain models and co-registered spectral terrain information from color images. The objective of the Psyche topography and geomorphology investigation is to derive the detailed shape of (16) Psyche to generate orthorectified image mosaics, which are needed to study the asteroids’ landforms, interior structure, and the processes that have modified the surface over geologic time. In this paper we describe our approaches for producing shape models, and our plans for acquiring requested image data to quantify the expected accuracy of the results. Multi-angle images obtained by Psyche’s camera will be used to create topographic models with about 15 m/pixel horizontal resolution and better than 10 m height accuracy on a global scale. This is slightly better as global imaging obtained during the Dawn mission, however, both missions yield resolutions of a few m/pixel locally. Two different techniques, stereophotogrammetry and stereophotoclinometry, are used to model the shape; these models will be merged with the gravity fields obtained by the Psyche spacecraft to produce geodetically controlled topographic models. The resulting digital topography models, together with the gravity data, will reveal the tectonic, volcanic, impact, and gradational history of Psyche, and enable co-registration of data sets to determine Psyche’s geologic history.

Occator crater is one of the most recognizable features on Ceres because of its interior bright regions, which are called the Cerealia Facula and Vinalia Faculae. Here we use high-resolution images from Dawn (∼35 m/pixel) to create a detailed geologic map that focuses on the interior of Occator crater and its ejecta. Occator's asymmetric ejecta indicates that the Occator-forming impactor originated from the northwest at an angle of ∼30–45°, perhaps closer to ∼30° Some of Occator's geologic units are analogous to the units of other complex craters in the region: the ejecta, crater terrace material, hummocky crater floor material and talus material. The geologic units that make Occator unique are the bright Occator pit/fracture material (this is the unit that corresponds to the Cerealia Facula and Vinalia Faculae) and the extensive, well-preserved lobate materials. We propose that the lobate materials are a slurry of impact-melted and non-impact-melted target material, which flowed around the crater interior before solidifying to form deposits geomorphologically consistent with impact melts elsewhere in the Solar System. We sub-divide the lobate materials on the basis of their surface textures. It is likely that knobby or smooth lobate materials form if the lobate material entrains or does not entrain blocks, respectively. Post-impact inflation is suggested to form the hummocky lobate material. The Vinalia Faculae formed within the hummocky lobate material. We find that the knobby lobate material and the outer edge of the Cerealia Facula formed prior to the central pit. The central dome formed after the formation of the central pit, while the majority of the Cerealia Facula (besides the outer edge) could have formed, continued to form and/or have been modified after the formation of the central pit. The Cerealia Facula may have initially been emplaced in a similar process to the Vinalia Faculae, and the surface of the Cerealia Facula appear to have somewhat darkened over time. The insights into Occator crater and its faculae derived from our geologic mapping will be synthesized together with inputs from all of the studies in this special issue in Scully et al. (2018a ).

The distinctive bright regions within Occator crater are one of the most remarkable discoveries of the Dawn mission's exploration of Ceres. The central region is named Cerealia Facula and the additional regions in the eastern crater floor are named Vinalia Faculae. Here we summarize and synthesize the results of this special issue, which aimed to identify the driving forces behind the formation of Occator and the faculae, and thus lead us to a new understanding of the processes and conditions that occurred in Ceres’ past, and potentially in its present. The investigations presented here used Dawn data, theoretical modeling and laboratory experiments to deduce the sequence of events that led to the formation of Occator, Cerealia Facula and Vinalia Faculae, which are broken into stages 1–3. Stage 1: Occator's ejecta blanket, terraces and hummocky crater floor material formed during and shortly after crater formation. These features are located in many Cerean complex craters. However, Occator also contains the lobate materials and faculae, which are unique to Occator. We interpret the lobate materials as a slurry of water, soluble salts and boulders of unmelted silicates/salts, which flowed around the crater interior before solidifying. At least portions of the lobate materials were solidified prior to the formation of the central pit, which is suggested to form via the ‘melted uplift model’ and provides insights into central pit formation across the Solar System. We propose the outer edge of Cerealia Facula formed shortly after the Occator-forming impact, via impact-induced hydrothermal brine deposition or via salt-rich water fountaining (perhaps sourced in a pre-existing reservoir). Stage 2: the majority of Cerealia Facula is located within the central pit and is interpreted to have formed later, at least ∼18 Myr after the Occator-forming impact, via multiple depositional events. The Cerealia-Facula-forming brines may have flowed out of fractures in the walls of the central pit and/or been driven to the surface by freezing of a subsurface reservoir and/or deposited via salt-rich water fountains. Further investigations are required to identify whether the formation of the majority of Cerealia Facula is driven by processes triggered by an impact, i.e. an exogenic event, or by a combination of impact-driven and endogenic processes. Cryomagmatic intrusions are suggested to uplift the crater floor, resulting in concentric floor fractures and an asymmetric dome. Injection of a similar material is proposed to inflate part of the lobate materials, giving them a hummocky texture. The resulting stresses formed fractures in the hummocky lobate material, which allowed the Vinalia-Faculae-forming brines to travel to the surface, where they ballistically erupted. The central dome within the central pit was one of the last features to form, by laccolithic intrusion, or by volume expansion from freezing of volatiles, or by extrusion of brines. Stage 3: mixing with Ceres’ average materials and/or space weathering darken the faculae over time. Cerealia Facula and Vinalia Faculae are the brightest and freshest of the bright regions identified on Ceres’ surface. Bright regions darken over time until their eventual erasure. Thus, it is likely that faculae formation has occurred throughout Ceres’ history, but that Occator's faculae are visible today because they are geologically young. Through synthesis of the studies presented in this special issue, we find that entirely exogenic driving forces, triggered by the impact, or a combination of endogenic and impact-derived forces could explain the formation of Occator and its faculae. Whether activity is impact-triggered and/or endogenic in nature is a key question for all investigations of Ceres, and future studies may favor one possibility over the other. The investigations presented in our special issue indicate Ceres is an active world where brines have been mobile in the geologically recent past. As such, Ceres is an intriguing world that we have only begun to explore.

We characterized two sets of extensional faults that comprise the Nar Sulcus region of Ceres by applying a cantilever model for fault related flexure and derived flexural rigidity values for Nar Sulcus between 2.0 · 10E15 and 1.8 · 10E16 N·m. This range of flexural rigidity makes Nar Sulcus mechanically akin to extensional structures on Ganymede, Europa, and Enceladus. We combine these observations with an inferred strength profile for the upper mechanical layer of Ceres and estimate its thickness to be 2.9–9.5 km. Surface heat fluxes at Nar Sulcus during its formation were likely ≥10 mW/m2 for estimated strain rates of 10E−17–10E−14 sE−1, which is at least one order of magnitude larger than the current estimated global average. For geologically plausible heat fluxes between 10 and 100 mW/m2, we estimate an upper bound of ~30 vol.% mechanically silicate‐like phases in the near surface at Nar Sulcus, neglecting the effects of porosity.

Impact craters of different geological ages, sizes and morphologies are not only the most obvious surface features on Ceres’ surface. The investigation of their spectral properties in combination with Ceres’ geology and topography reveals not only lateral compositional variations in Ceres’ surface material but also possible stratigraphic differences within Ceres’ crust. Spectral properties of impact craters with different ages do show distinct trends implying variations with increasing exposure duration of the impact material onto Ceres’ surface. Local concentrations of H2O ice and carbonates are associated with the youngest, either recently emplaced or excavated, surface deposits. On the contrary, regionally higher amounts of ammoniated phyllosilicates originate from deeper regions of Ceres’ crust and strengthen the theory of ammonia being a primordial constituent of Ceres. The blue spectral slope, clearly associated with relatively weak absorptions of OH-bearing and/or ammoniated phyllosilicates, is limited to fresh impact material. Either, the blue spectral slope diminishes slowly with increasing geologic age due to space weathering processes, or shortly as a result of gravitation-induced slumping, forming a fine and loosely consolidated regolith.

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The ESA/ROSCOSMOS ExoMars 2022 will land in Oxia Planum an area that shows outcrops of clay-rich Noachian-aged phyllosilicates overlaid by an Early Amazonian volcanic dark resistant unit (Adru) [1]. Using HiRISE images, we identified NE-SW (53.9 ± 13.2°) oriented TARs overlying an enigmatic ~EW (95.4 ± 10°) oriented ridge pattern that we interpreted as periodic bedrock ridges (PBRs) [2]. Ridges (~ 38 m spaced) display Y-junctions, show cross-cutting fractures and share the same blocky texture of the bedrock they are associated with. Ridge crestlines are locally found in continuity outside and inside heavily eroded impact craters around the dark upstanding material (Adru) exposed in the center of many craters. These stratigraphic relationships suggest that the ridges (PBRs) formed after the event(s) that eroded the crater rims and thus after deposition of the Adru (2.6 Ga). Ridges are even visible in association with impact crater ejecta and are superimposed by 10-25 m craters and boulders, so they pre-date these impact events. When associated with crater ejecta, ridges locally show two different crests. Both crests are truncated by craters suggesting they were emplaced before the impacts. We interpret this double crest arrangement as megaripples detaching from PBRs. The ejecta deposited over the megaripple-PBRs favored the preservation of the megaripple crests from a subsequent episode/s of erosion that led to the complete exposure of the PBRs on the plain. Because the preserved megaripples are locally visible on the southern edges of the PBRs, the wind that formed the megaripple-PBR system should have blown from N-NNE because the megaripples are located at the downwind side of PBRs [3]. To better understand the relative age of the ridges, we mapped their occurrence on 316 craters in the study area that we qualitatively classified as relatively degraded/old and pristine/young. Results show that ridges are only found in degraded/old craters but are never found inside pristine/young craters. Thus, the ridge forming process was only active in-between the formation of degraded/old and pristine/young craters. A major change in the wind regime occurred during or after the event that exposed the PBRs: N-NNE winds that shaped the PBRs changed into dominant SE winds that led to the deposition of the TARs above the PBR/megaripples. This work unveils a complex history of aeolian erosion and deposition in Oxia Planum during the Amazonian. By visiting PBRs for the first time, the ExoMars 2022 mission will provide further constraints on PBR formation and paleo-winds, shedding light on a past Amazonian environment.This work is a summary of a manuscript that is currently in press on Geophysical Research Letters: Silvestro et al. 2021, Periodic Bedrock Ridges at the ExoMars 2022 Landing Site: Evidence for a Changing Wind Regime. DOI: 10.1029/2020GL091651.[1] Quantin-Nataf C. et al. (2021). Astrobiology, 21, N.3.[2] Silvestro S. et al. (2020). 6th Int. Planet. Dunes Work. 12-15 May, 2020. LPI No. 2188, id.3009.[3] Hugenholtz C. H. et al. (2015). Aeolian Res. 18, 135–144.

Wind‐formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment. Studies of aeolian bedforms at the landing site were focused on assessing the risk for rover trafficability, however their potential in recording climatic fluctuations has not been explored. Here we show that the landing site experienced multiple climatic changes in the Amazonian, which are recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs). Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Our results provide constrains on PBR formation mechanisms and offer indications on paleo winds that will be crucial for understanding the landing site geology., Key Points We present the first evidence for a periodic bedrock ridge (PBRs) pattern from the ExoMars 2022 landing siteFormative paleowind directions are extrapolated from PBRs and transverse aeolian ridgesEvidence for an Amazonian change in the wind regime are provided