AbstractThe Mars Reconnaissance Orbiter and its Context Camera (CTX) have acquired more than 100,000 separate panchromatic images that capture nearly the entire surface of Mars at ∼5–6 m/pixel. This paper describes a data processing workflow used to generate the first contiguous global mosaic of CTX data, which represents a large improvement in spatial resolution over existing 100 m/pixel contiguous global mosaics. We describe the overarching strategy for the mosaic's construction, which was to maximize the scientific utility of a continuous mosaic that is 5.7 trillion pixels in size. The pipeline used for data processing prioritized traceability and reproducibility of the final mosaic, such that the provenance of all pixels is reported, equipping scientists with information to differentiate mosaic artifacts from surface landforms and to incorporate critical image metadata into their analyses. The CTX data set synthesized into a global CTX mosaic facilitates ready analysis and provides a new capability in transitioning global studies of Mars from high‐resolution investigations of individual images to systematic studies of the entire Martian surface at outcrop‐resolving quality without regard to image boundaries.

Gullies on Mars resemble water-carved channels on Earth, but they are mostly at elevations where liquid water is not expected under current climate conditions. It has been suggested that sublimation of carbon dioxide ice alone could have formed Martian gullies. We used a general circulation model to show that the highest-elevation Martian gullies coincide with the boundary of terrain that experienced pressures above the triple point of water when Mars' rotational axis tilt reached 35°. Those conditions have occurred repeatedly over the past several million years, most recently ~630,000 years ago. Surface water ice, if present at these locations, could have melted when temperatures rose >273 kelvin. We propose a dual gully formation scenario that is driven by melting of water ice followed by carbon dioxide ice sublimation.

TRANSITION AT GALE CRATER. W. Rapin, G. Dromart, D. Rubin, L. Le Deit, S. Le Mouélic, O. Gasnault, G. Caravaca, N. Mangold, V. Fox, J.L. Dickson, B.L. Ehlmann, K. Herkenhoff, L.A. Edgar, R.B. Anderson, P. Pinet, S. Maurice, R.C. Wiens. IRAP, université de Toulouse, France, william.rapin@irap.omp.eu; Univ. Lyon, LGLTPE, France; University of California, Santa Cruz; LPG, CNRS-Univ. Nantes, France ; Caltech-JPL, Pasadena CA; Astrogeology Science Center, USGS, Flagstaff AZ; LANL.

The Curiosity rover is exploring Hesperian-aged stratigraphy in Gale crater, Mars, where a transition from clay-bearing units to a layered sulfate-bearing unit has been interpreted to represent a major environmental transition of unknown character. We present the first description of key facies in the sulfate-bearing unit, recently observed in the distance by the rover, and propose a model for changes in depositional environments. Our results indicate a transition from lacustrine mudstones into thick aeolian deposits, topped by a major deflation surface, above which strata show architectures likely diagnostic of a subaqueous environment. This model offers a reference example of a depositional sequence for layered sulfate-bearing strata, which have been identified from orbit in other locations globally. It differs from the idea of a monotonic Hesperian climate change into long-term aridity on Mars and instead implies a period characterized by multiple transitions between sustained drier and wetter climates.

Introduction: Martian gully channels [1], which typically incise indurated fine-grain material like the latitude-dependent mantle [2] and dunes [3], frequently emanate from amphitheater-shaped chutes that are eroded into bedrock at the crest of steep slopes [1], most commonly crater walls in the mid-latitudes of Mars. Though gully channels are morphologically similar to water-carved landforms on Earth, recent work has tied these features to seasonal CO2 activity [4]. If gully channels can be incised by processes that do not involve H2O [5], does the same hold true for bedrock chutes, which are made of considerably stronger material? This project aims to explore the role of dry rockfall in the erosion of the bedrock alcoves of Martian gullies. Bedrock chutes may also preserve a longer record of gully activity than their associated channels, which have been shown to be ephemeral features over long timescales [2]. To understand the capacity for bedrock erosion on steep slopes in a completely dry environment, we follow upon the work of Bart [6] and Kumar et al. [7], who have shown that features that are broadly visually similar to Martian gullies are observed on fresh crater slopes on the Moon [7]. The long duration success of both the Mars Reconnaissance Orbiter and the Lunar Reconnaissance Orbiter allow us to make observations and measurements using data not previously available. We performed a global survey of bedrock chute morphology on Mars using Digital Elevation Models (DEMs) we generated using Ames Stereo Pipeline [8] from Context Camera (CTX) [9] imagery. High resolution DEMs generated from High Resolution Imaging Science Experiment (HiRISE) [10], and Lunar Reconnaissance Orbiter Narrow Angle Camera (LRO NAC) [11] imagery were used for Mars and the Moon respectively, to compare the morphology of bedrock chutes on Mars to those on the Moon in closer resolution, and validate the CTX measurements. Lunar Survey: Target locations for Lunar erosion were identified using a slope map generated from Kaguya Terrain Camera [12] data to filter locations with slopes between 30-40°. All potential LRO NAC stereo pairs that overlap an area of sufficiently steep slope were visually inspected for erosional landforms. Chute measurements were taken in locations with sufficient DEM coverage. Mars Survey: After conducting a global survey of 4 – 9 km diameter craters with potential CTX stereo pairs, CTX DEMs were generated and used to systematically measure bedrock chute relief on North and South facing slopes. Martian slopes were categorized based on their orientation and the presence of regolithincised channels beneath the bedrock chutes. Comparison of Chute Morphologies: The morphologies of Lunar erosional landforms (Fig 1a) were analyzed using qualitative observations and quantitative measurements taken from the DEMs. These were compared with measurements of bedrock chutes in a range of Martian environments that do and do not host gullies (Figs. 1-2): equatorial (Fig 1b), midlatitude pole-facing slopes without channels (Fig 1c), and midlatitude polefacing slopes with channels (Fig 1d).

Sedimentary basins are the archives of ancient environmental conditions on planetary surfaces, and on Mars they may contain the best record of surface water and habitable conditions. While erosional valley networks have been mapped, the global distribution of fluvial sedimentary deposits on Mars has been unknown. Here we generated an eight-trillion-pixel global map of Mars using data from the NASA Context Camera (CTX), aboard the Mars Reconnaissance Orbiter spacecraft, to perform the first systematic global survey of fluvial ridges—exhumed ancient deposits that have the planform shape of river channels or channel belts, but stand in positive relief due to preferential erosion of neighboring terrain. We used large fluvial ridges (>70 m width) as a conservative proxy for the occurrence of depositional rivers or river-influenced depositional areas. Results showed that fluvial ridges are as much as 100 km long, common across the southern highlands, occur primarily in networks within intercrater plains, and are not confined to impact basins. Ridges were dominantly found in Noachian through Late Hesperian units, consistent with cessation of valley network activity, and occurred downstream from valley networks, indicating regional source-to-sink transport systems. These depositional areas mark a globally distributed class of sedimentary deposits that contain a rich archive of Mars history, including fluvial activity on early Mars.

Table of all fluvial ridge systems mapped in this study.

Introduction: Martian gully channels [1], which typically incise indurated fine-grain material like the latitude-dependent mantle and dunes [2], frequently emanate from amphitheater-shaped alcoves that are eroded into bedrock at the crest of steep slopes [1], most commonly crater walls in the mid-latitudes of Mars. Though gully channels are morphologically similar to watercarved landforms on Earth, recent work has tied these features to seasonal CO2 activity [3]. If gully channels can be incised by processes that do not involve H2O, does the same hold true for alcoves, which are made of considerably stronger material? This project aims to explore the role of dry rockfall in the erosion of the bedrock alcoves of Martian gullies. Bedrock alcoves may also preserve a longer record of gully activity than their associated channels, which have been shown to be ephemeral features over long timescales. To understand the capacity for bedrock erosion on steep slopes in a completely dry environment, we follow upon the work of Bart [4] and Kumar et al. [5], who have shown that features that are broadly visually similar to Martian gullies are observed on fresh crater slopes on the Moon [5]. The long duration success of both the Mars Reconnaissance Orbiter and the Lunar Reconnaissance Orbiter allow us to make observations and measurements using data not previously available. We generated high resolution Digital Elevation Models (DEMs) from High Resolution Imaging Science Experiment (HiRISE) [6] and the Lunar Reconnaissance Orbiter Narrow Angle Camera (LRO NAC) [7] imagery for Mars and the Moon respectively, to determine if Mars-like bedrock gully alcoves can form on an airless body (without volatile activity). Lunar Survey: Target locations for Lunar erosion were identified using a slope map generated from Kaguya Terrain Camera [8] data to filter locations with slopes between 30-40°. All potential LRO NAC stereo pairs that overlap an area of sufficiently steep slope were visually inspected for erosional landforms (Fig. 1). Alcove Comparison: DEMs of target locations on Mars and the Moon were produced using NASA’s Ames Stereo Pipeline [9]. The morphologies of Lunar erosional landforms were analyzed using qualitative observations and quantitative measurements taken from the DEMs. These were compared with measurements of characteristic alcoves in a range of Martian environments that do and do not host gullies (Figs. 2-3): high altitude (Fig. 3D), equatorial (steep slopes without gullies) (Fig. 3C), mid-latitude pole facing (steep slopes with gullies) (Fig. 3A), and mid-latitude equator facing (Fig. 3B). We used data extracted from the DEMs of each location to measure the depths of alcoves in a “suite”, which we defined as 4 or more alcoves of comparable size and shape forming sequentially on a slope. Depths of alcoves along equipotential contours within the crater rim were averaged for each location, and sorted by category (Fig. 2). Preliminary results show that alcove depths fall along a spectrum, with shallow Lunar alcoves (Fig 3E) rarely exceeding 10m in depth on one end, and Martian mid-latitude pole-facing alcoves (Fig 3A) at the other. Alcove suites on Martian slopes that are not known to

WITH THE LUNAR TRAILBLAZER MISSION. R. L. Klima1 (Rachel.Klima@jhuapl.edu), B. L. Ehlmann2,3, D. L. Blaney3, N. E. Bowles4 S. Calcutt4, J. Dickson2, K. L. Donaldson Hanna4,5, C. S. Edwards6, R. Evans4, R. Green3, W. Frazier3, R. Greenberger2, M. A. House7, C. Howe8, J. Miura2, C. Pieters9, M. Sampson10, R. Schindhelm10, E. Scheller2, C. Seybold3, D. R. Thompson3, J. Troeltzsch10, T. J. Warren1, K. Shirley1, and J. Weinberg10. 1Johns Hopkins Applied Physics Laboratory, Laurel, MD, 2California Institute of Technology, Pasadena, CA, US, 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, US, 4Department of Physics, University of Oxford, UK, US, 5Department of Physics, University of Central Florida, Orlando, FL, US, 6Northern Arizona University, Flagstaff, AZ, US, 7Pasadena City College, Pasadena, CA, US, 8STFC RAL Space, Didcot, UK, 9Brown University, Providence, RI, US, 10Ball Aerospace & Technologies Corporation, Boulder, CO, US.

OF MASSIVE IMAGING DATA SETS. J. L. Dickson1, B. L. Ehlmann1,2, L. Kerber2, C. I. Fassett3, T. M. Hare4, D. P. Quinn5, L. Plesea6, D. Noss7, 1California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, 91125 (jdickson@caltech.edu), 2Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, CA 91109. 3NASA Marshall Space Flight Center, Huntsville, AL 35805. 4U.S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ, USA. 5University of Wisconsin, Madison, Department of Geoscience, Madison, WI, USA. 6ESRI, Redlands, CA. 7Mars Space Flight Facility, 201 E Orange Mall, Arizona State University, Tempe, AZ 85287 USA.

TRAILBLAZER MISSION. D. R. Thompson1, R. O. Green1, B.L. Ehlmann1,2, R. Klima4, C. Pieters3, D. Blaney1, W. Williamson1, P. Mouroulis1, N. Bowles5, S. Calcutt5, M. Cannella6, J. Dickson2, K. Donaldson-Hanna7, C. Edwards8, R. Evans5, W. Frazier1, M.A. House8, C. Howe9, B. Marotta5, J. Miura1, M. Sampson5, E. Scire1, R. Schindhelm5, C. Seybold1, K. Shirley5, J. Troelzsch6, T. Warren5, J. Weinberg6. 1Jet Propulsion Laboratory, California Institute of Technology, 2California Institute of Technology, 3Brown University, 4Johns Hopkins Applied Physics Laboratory, 5Department of Physics, University of Oxford, UK, 6Ball Aerospace & Technologies Corporation, Boulder, CO, USA, 7Univ. of Central Florida, Orlando, FL, 8Northern Arizona Univ., Flagstaff, AZ

Introduction: The McMurdo Dry Valleys (MDV) of Antarctica have been considered a valuable martian analog since the Viking era of Mars exploration [1]. Over the past two decades, our understanding of martian environmental and geologic evolution has significantly improved thanks to the plethora of orbital and landed missions. New observations have raised many new enigmatic questions about how cold and dry geological systems evolve, which has revitalized the MDV as an important terrestrial analog for Mars, from its earliest recorded geologic history [2] to the present [3]. Global martian climate models struggle to produce consistently warm and wet conditions at the martian surface early in its history, even with the aid of additional greenhouse gases to offset the distance between Mars and the faint young Sun [4]. Together with studies suggesting that snowmelt can satisfy the known distribution of valley networks in parts of the southern highlands [e.g., 5], these lines of evidence suggest that early Mars may have been dominated by relatively cold and dry conditions as opposed to more clement conditions. The hyper-arid and hypo-thermal conditions that dominate the MDV today are some of the most comparable terrestrial conditions to those on modern (cold and dry) Mars [3,6,7] (Fig. 1). Shallow buried ice, glacial processes, and anhydrous oxidative weathering processes are pervasive across both landscapes. More recent work, however, has helped to appreciate the role of locally optimized conditions in driving many surface processes that generate habitable conditions, even in the extremely arid, cold, and radiative environment of the MDV [2]. For example, localized salt concentrations are able to facilitate deliquescence [8], melting of snowpacks and glaciers are facilitated by topographically controlled insolation [9] and, at least in the MDV, microbial ecosystems are able to suspend biological activity indefinitely until conditions are optimal [10]. These cold and dry conditions operate at one end of a hydrological continuum [11] in the MDV--the other end of which is the “cold and wet” endmember--where glacial melt, snow-fed streams, and ice-covered lakes persist, despite only ~2 months of melting [3,12]. The hydrological and habitability gradients operating in the MDV make them the ideal environment for exploring Mars-like surface processes that have shaped both planets under both ancient and modern climate conditions [2,3]. In this work, we highlight the recent developments in comparative planetology between the martian surface and the MDV. This work has helped to decipher many enigmatic climatological and geological features observed on Mars. We also highlight where additional work in the MDV is necessary to address outstanding questions in martian science. Cold and Icy Early Mars?: The apparent disagreement between observed fluvial and lacustrine landforms and the inability for global climate models to produce mean annual temperatures greater than 0o C suggest that the martian surface was possibly never clement from a terrestrial perspective. Instead of a longlived and continuously active hydrological system on early Mars, is it possible that hydrological activity was more episodic through punctuated climatic excursions on an otherwise cold and icy early Mars? The fluvial and lacustrine systems of the MDV are one possible analog where localized climatic optima drive local hydrological systems that can cease once conditions are unfavorable [2,12]. The MDV provide compelling evidence for comparable landforms found in ancient martian landscapes. It is therefore possible that hydrological