Lucus Planum, extending for a radius of approximately 500km around 181 degrees E, 5 degrees S, is part of the Medusae Fossae Formation (MFF), a set of several discontinuous deposits of fine-grained, friable material straddling across the Martian highland-lowland boundary. The MFF has been variously hypothesized to consist of pyroclastic flows, pyroclastic airfall, paleopolar deposits, or atmospherically deposited icy dust driven by climate cycles. Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), a low-frequency subsurface-sounding radar carried by European Space Agency's Mars Express, acquired 238 radar swaths across Lucus Planum, providing sufficient coverage for the study of its internal structure and dielectric properties. Subsurface reflections were found only in three areas, marked by a distinctive surface morphology, while the central part of Lucus Planum appears to be made of radar-attenuating material preventing the detection of basal echoes. The bulk dielectric properties of these areas were estimated and compared with those of volcanic rocks and ice-dust mixtures. Previous interpretations that east Lucus Planum and the deposits on the northwestern flanks of Apollinaris Patera consist of high-porosity pyroclastic material are strongly supported by the new results. The northwestern part of Lucus Planum is likely to be much less porous, although interpretations about the nature of the subsurface materials are not conclusive. The exact origin of the deposits cannot be constrained by radar data alone, but our results for east Lucus Planum are consistent with an overall pyroclastic origin, likely linked to Tharsis Hesperian and Amazonian activity. Plain Language Summary Lobe-shaped thick deposits, collectively known as the Medusae Fossae Formation, are found west of the Olympus Mons volcano on Mars. Visual observations of these smooth and relatively unremarkably looking materials have not definitively determined how they formed with hypotheses ranging from volcanic ash to remnants of a materials deposited at a previous location of the north pole, to accumulation of atmospheric dust. In this study we used the ground penetrating radar on board Mars Express to see through these deposits to derive information about Lucus Planum, the central lobe of the Medusae Fossae Formation. Through our analysis of the way the radar waves were reflected by subsurface layering, we concluded that the materials forming Lucus Planum are spatially variable: the east and west portions of the deposits are highly porous and probably composed of ashes and rocks from nearby volcanoes. In the north-west the deposits are much denser, but we could not unequivocally define their nature. Finally, our instrument could not detect signals from the central part of Lucus Planum, which suggests yet a different component in the deposits. This diversity points to a dynamic geological history in this unique region of Mars.

Mechanically interacting fault systems on Earth are often associated with groundwater flow (e.g. Curewitz and Karson, 1997) by facilitating water storage and flow through fracture conduits before, during, and after seismic events (e.g. Sibson, 1975). Similar associations between interacting fault segments and fluid flow are present on Mars (Davatzes and Gulick, 2007a). The Cerberus Fossae compose a system of elongate topographic lows, a portion of which coincides with the source region of the outflow channel Athabasca Valles. The Cerberus Fossae and source area were mapped using Thermal Emission Imaging System (THEMIS) daytime IR mosaics and Context camera (CTX) images to establish spatial relations of structural features. Mars Orbiter Laser Altimeter (MOLA) elevation data were plotted to construct the depth profiles of the fossae to test the hypothesis that the Cerberus Fossae are normal fault-bounded graben. High Resolution Imaging Science Experiment (HiRISE) images were mapped for fractures within the fault damage zones with the degree of fracture plotted as a function of distance along strike. This plot established the spatial relations between fractures, mechanically interacting fossae segments, and Athabasca Valles. The depth profiles of the Cerberus Fossae are consistent with the displacement distribution of terrestrial normal faults with a surface expression consistent with fault propagation from depth and mechanical interaction among segments. Similarly, regions of interpreted mechanical interaction indicated by slip distribution and segment overlap correspond to increased fracture intensity and density. On Earth, such regions of mechanical interaction tend to have high fracture intensity (e.g. Davatzes et al., 2005), are associated with hydrothermal fluid flow (Curewitz and Karson, 1997), and have evidence of extensive long-term fluid flow as evidenced by diagenetic alterations (Eichhubl et al., 2004). Higher fracture intensities and densities near the head of Athabasca Valles as a proxy for increased permeability provide a potential mechanism and a necessary condition for the localized fluid flux necessary to supply the outflow channel. Thus, I conclude the Cerberus Fossae are mechanically interacting normal fault-bounded graben with highly permeable damage zones that would act to quickly dewater an aquifer resulting in the carving of Athabasca Valles. / Geology

Yardangs are streamlined ridges carved by the action of wind into consolidated yet erodible substrates. The direction of sediment transport is indicated by their elongate shapes and steep sides which redirects most of the sediment transport layer onto adjacent low-lying surfaces, resulting in lower erosion rates on the elevated areas. Despite this simple premise, the rates of erosion and transport as well as the material properties necessary to form yardangs have until now been largely unknown. This study aims to determine how material properties affect yardang dimensions in order to use yardang morphometry to derive the mechanical properties of a surface that has yet to been explored in situ. As a terrestrial analog for planetary yardangs, we use ignimbrite samples and a Digital Terrain Model (DTM) derived from drone imagery of the Campo Piedra Pomez, Argentina. For comparison, we examine a martian analog using a DTM generated by the HiRISE instrument team for a section of the Medusae Fossae Formation northeast of Aeolis Dorsa. We use the DTMs along with thin sections, porosity, density, and strength measurements of yardang materials to understand the conditions contributing to yardang morphology. This method reveals microscopic evidence of nuanced differences in terrestrial ignimbrite depositional processes that create strong, lightweight, yardang-forming ignimbrite like that suggested to occur in the Medusae Fossae Formation. On average, the CPP ignimbrite samples have 49.51 ± 0.43 percent porosity, density of 1.26 ± 0.13 g/cm^3, and uniaxial compressive strength of 4.88 ± 2.86 MPa. Using the topographic structure of yardangs in the DTMs, we automatically extract yardang polygons and characterize their length, width, height, and spacing in four directions. We ratio these measurements and find that yardang width over minimum crosswind spacing has a geometric mean near one for 4102 terrestrial yardangs (~0.7) as well as for 1269 martian yardangs (~1.3). The ratio of ~1 for closely spaced yardangs is probably caused by increased windspeeds enhancing erosion in the gaps between yardangs until the gaps achieve the same cross-sectional area as the yardangs. Finally, we use regressions of the material properties and morphometry data to suggest that if formation conditions are the same as in the Campo Piedra Pomez, the Medusae Fossae Formation surface would have 52.04 + 1.41 / - 1.37 percent porosity, density of 1.19 ± 0.02 g/cm^3 and strength of 0.64 + 0.84 / -0.36 MPa. These results indicate that topography, porosity, strength, and density attenuate the formation processes that ultimately determine the morphometric properties of yardangs. This establishes a framework on which to make progress towards a quantitative understanding of yardang morphology and evolution. The ArcGIS toolboxes and Python scripts used to obtain our results are available at https://github.com/dmcdoug/yardangtools.

Mars est un objet privilégié pour comprendre l'évolution d'une planète. Des témoins géologiques de son activité interne et des échanges surface-atmosphère sont préservés sur 4 Ga. Cette thèse étudie ces échanges au travers de la circulation des fluides dans la croûte. Le climat froid et sec de l'Amazonien (< 3 Ga) implique la condensation, sublimation et diffusion des espèces volatiles dans le régolithe. Ce paradigme est modifié par la découverte de l'importance de la convection d'air dans les sols poreux (aérothermalisme). Ce processus a été mis en évidence par l'imagerie thermique (Mars Odyssey/THEMIS) et la morphologie (e.g., Mars Express/HRSC) pour Cerberus Fossae et le volcan Arsia Mons. La période Hespérienne est marquée par la libération massive d'eau aboutissant à la formation des terrains chaotiques et des chenaux de débâcle. Nous proposons que ces objets résultent de la convection d'argiles. Cette hypothèse originale est corroborée par les détections de phyllosilicates (données CRISM et OMEGA).