[1] The degradation of impact craters provides a powerful tool to analyze surface processes in the Martian past. Previous studies concluded that large impact craters (20-200 km in diameter) were strongly degraded by fluvial erosion during early Martian history. Our goal is to study the progression of crater degradation through time with a particular emphasis on the craters with alluvial fans and on the relative chronology of these craters. The geometric properties of 283 craters of >20 km in diameter were analyzed in two highlands of Mars, north of Hellas Planitia, and south of Margaritifer Terra, both known to contain craters with alluvial fans. Three classes were defined from morphology: strongly degraded craters with fluvial landforms and without ejecta (type I), gently degraded craters with fluvial landforms and preserved ejecta (type II), and fresh craters with ejecta and no fluvial landforms (type III). Our main result is that the type II craters that present alluvial fans have characteristics closer to fresh craters (type III) than degraded craters (type I). The distinctive degradation characteristics of these classes allowed us to determine a temporal distribution: Type I craters were formed and degraded between $4 Gyr and $3.7 Gyr and type II craters with alluvial fans were formed between Early Hesperian and Early Amazonian ($3.7 to $3.3 Gyr). This chronology is corroborated by crosscutting relationships of individual type II craters, which postdate Late Noachian valley networks. The sharp transition at $3.7 Gyr suggests a quick change in climatic conditions that could correspond to the cessation of the dynamo.

No abstract

The identification of widespread pyroclastic vents and deposits on Mercury has important implications for the planet's bulk volatile content and thermal evolution. However, the significance of pyroclastic volcanism for Mercury depends on the mechanisms by which the eruptions occurred. Using images acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse-edged spectral anomaly, a configuration previously used to identify sites of explosive volcanism. We find that these sites cluster at the margins of impact basins and along regional tectonic structural trends. Locally, pits and deposits are usually associated with zones of weakness within impact craters and/or with the surface expressions of individual thrust faults. Additionally, we use images and stereo-derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circumpit relief within a wider, thinner deposit. These eruptions were therefore likely driven by a relatively high concentration of volatiles, consistent with volatile concentration in a shallow magma chamber prior to eruption. The colocation of sites of explosive volcanism with near-surface faults and crater-related fractures is likely a result of such structures acting as conduits for volatile and/or magma release from shallow reservoirs, with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases. Our findings suggest that widespread, long-lived explosive volcanism on Mercury has been facilitated by the interplay between impact cratering, tectonic structures, and magmatic fractionation.

The Hokusai (H05) quadrangle is in Mercury's northern mid-latitudes (0-90°E, 22.5-65°N) and covers almost 5 million km 2 , or 6.5%, of the planet's surface. We have used data from the MESSENGER spacecraft to make the first geological map of H05. Linework was digitized at 1:400,000-scale for final presentation at 1:3,000,000-scale, mainly using a ∼166 m/pixel monochrome basemap. Three major photogeologic units of regional extent were mapped: intercrater, intermediate, and smooth plains. Materials of craters ≥ 20 km in diameter were classified according to their degradation state. Two classification schemes were employed in parallel, one with three classes and the other with five classes, for compatibility with existing MESSENGER-era quadrangle maps and the first global geologic map. This map will provide science context and targets for the ESA-JAXA BepiColombo mission to Mercury.

On Mars, locally warm surface temperatures (~293 K) occur, leading to the possibility of (transient) liquid water on the surface. However, water exposed to the martian atmosphere will boil, and the sediment transport capacity of such unstable water is not well understood. Here, we present laboratory studies of a newly recognized transport mechanism: “levitation” of saturated sediment bodies on a cushion of vapor released by boiling. Sediment transport where this mechanism is active is about nine times greater than without this effect, reducing the amount of water required to transport comparable sediment volumes by nearly an order of magnitude. Our calculations show that the effect of levitation could persist up to ~48 times longer under reduced martian gravity. Sediment levitation must therefore be considered when evaluating the formation of recent and present-day martian mass wasting features, as much less water may be required to form such features than previously thought.

International audienceLiquid water may exist on the Martian surface today, albeit transiently and in a metastable state under the low atmospheric surface pressure. However, the identification of liquid water on Mars from observed morphological changes is hampered by our limited understanding of how metastable liquids interact with sediments. Here, we present lab experiments in which a block of ice melts and seeps into underlying sediment, and the resulting downslope fluid propagation and sediment transport are tracked. In experiments at Martian surface pressure, we find that pure water boils as it percolates into the sediment, inducing grain saltation and leading to wholesale slope destabilization: a hybrid flow mechanism involving both wet and dry processes. For metastable brines, which are more stable under Martian conditions than pure water, saltation intensity and geomorphological impact are reduced; however, we observed channel formation in some briny flow experiments that may be analogous to morphologies observed on Mars. In contrast, under terrestrial-like experimental conditions, there is little morphological impact of seeping water or brine, which are both stable. We propose that the hybrid flow mechanism operating in our experiments under Martian surface pressure could explain observed Martian surface changes that were originally interpreted as the products of either dry or wet processes

No abstract

This paper explores the origins and evolution of ice-rich interior mounds found within craters of the north polar region of Mars. We present a systematic study of impact craters above 65°N, and identify 18 craters that have interior mounds. At least eleven of these mounds are composed of water ice and geometric similarities suggest that dune-covered mounds may also have a water ice core. The mounds are found in the deeper craters in the north polar area and we suggest that these form a specific microclimate favorable for mound initiation and growth. It is likely that at least seven of the mounds have evolved as individual outliers, rather than conterminous with the main polar cap. Our observations suggest that the mounds are built up by atmospheric deposition, similar to that of the north polar layered deposits. Using a combination of remote sensing techniques enabling topographic, spectral, radar and image data analyses, we have documented the morphology, composition and stratigraphy of selected mounds. We advance and test four hypotheses for formation of these mounds: artesian outpouring from a deep aquifer, hydrothermal activation of ground ice, remnants of a more extensive polar cap, and atmospheric deposition on ice caps in meteorologically isolated locations. We propose that during periods when the perihelion was located in northern summer (most recently 10-25 ka before present) the microclimate in these craters retarded the sublimation of CO 2 and water ice in northern spring, thus creating a cold trap for volatiles released as the seasonal cap retreated. This created a thick enough deposit of water ice to withstand sublimation over the summer and initiate a positive feedback leading to mound-building. Mounds without complete dune-cover may be in dynamic equilibrium with the ambient climate and show evidence of both present-day and past periods of erosion and aggradation. We conclude that the water ice mounds formed in deep impact craters in Mars" north polar region may contain sensitive records of past polar climate that may enhance our understanding of the CO 2-H 2 O system in the polar regions.