N. Mangold

Perennial water ice identified in the south polar cap of Mars.

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2u2009km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

Geomorphic analysis of lobate debris aprons on Mars at Mars Orbiter Camera scale: Evidence for ice sublimation initiated by fractures

[1]xa0Lobate debris aprons, known to be geomorphic landform indicators of the presence of ground ice, are of special interest for future missions devoted to the research of water on Mars. Lobate debris aprons in fretted terrains of Deuteronilus and Protonilus Mensae (35°–50°N) show typical convex shapes interpreted to be the result of viscous deformation. At the scale of Mars Orbiter Camera (MOC) high-resolution images the surface of these debris aprons shows complex patterns with small pits and buttes. These patterns can be explained by the mantling of dust, the accumulation of interstitial ice, and the subsequent removal of ice by sublimation. The sublimation of the ground ice is especially initiated and accelerated by subsurface heterogeneities like fractures. Theoretical quantification of sublimation rates therefore minimizes sublimation, which is not a homogeneous process, at least over the landforms studied. Crater counts show that the sublimation occurred in the last tens of millions of years up to the recent past. In the point of view of future searching of subsurface ice, only surface layers are submitted to sublimation favoring the conservation of ground ice in deeper layers since the formation of the landform. The geophysical survey of lobate debris aprons would give interesting insights into the subsurface distribution of ice and its seasonal variations, especially in order to measure current sublimation of ground ice.

Mineralogy of the Nili Fossae region with OMEGA/Mars Express data: 1. Ancient impact melt in the Isidis Basin and implications for the transition from the Noachian to Hesperian

[1]xa0The Nili Fossae region located on the northwestern quadrant of the Isidis Basin, Mars, displays superb exposures of bedrock outcrops that reveal mineralogy and composition of the crust. Previous work has shown that this region exhibits the largest exposures of olivine-dominated rock units on Mars. Visible-infrared imaging spectrometer data acquired by OMEGA were calibrated to surface reflectance and analyzed to determine surface mineralogy. The dominant minerals identified are iron-bearing mafic silicates (low- and high-calcium pyroxene, olivine) and water-bearing phyllosilicate (iron-rich smectite clay). The strength and position of mineral absorption features were used to produce mineral indicator maps for the dominant species, and the maps were integrated with high-resolution imaging from the THEMIS, MOC, and HRSC instruments, and MOLA topography. We show that olivine and phyllosilicate occur in spatially distinct outcrops; the olivine-bearing rock unit is a meters- to tens of meters-thick cap unit resting on phyllosilicate-bearing bedrock, and the phyllosilicate units predate the Isidis basin-forming event. On the basis of superposition, crosscutting, and geomorphic relationships, we interpret the emplacement of the olivine-bearing units as having been contemporaneous with the Isidis impact event. By analogy with the Orientale basin on the Moon, we propose that the olivine-bearing unit represents the surface exposure of the impact melt from the Isidis impact event. These results demonstrate that large regions of crust had been altered in the presence of water prior to the date of the Isidis basin-forming event in the Late Noachian (≈3.96 Ga).

Fluvial and lacustrine activity on layered deposits in Melas Chasma, Valles Marineris, Mars

[1] Valley networks on Mars are the most obvious features attesting that different geologic processes and possibly climatic conditions existed in the past. THEMIS images reveal valley networks within Melas Chasma, in Valles Marineris, a Hesperian-age canyon system. The valley networks in Melas Chasma are dense and highly organized, and the heads of the valleys are scattered at different elevations. All these features suggest that the networks were fed by precipitation. The morphological details reveal inner channels on some valley floors, attesting that water flowed within these valleys. On the DEM, the valleys flow into a completely enclosed depression. The edge of this feature follows a MOLA contour line, and the depression shows many sedimentary morphologies suggesting lacustrine environment. These landforms are located on remnants of layered deposits possibly composed of sulfate layers suggesting that fluvial activity could have contributed to the erosion of the layered terrains in Valles Marineris. Collectively, the features in Melas Chasma are a maximum of Hesperian in age. These results suggest that warm, wet environmental conditions on Mars persisted through the Hesperian and were present during the formation of Valles Marineris. The evidence for a paleolake in Melas Chasma attests to adequate environmental conditions for life development through the Hesperian period.

Debris flows over sand dunes on Mars: Evidence for liquid water

[1]xa0This study focuses on the formation and physical properties of the gullies observed over large Martian dunes, especially those of the Russell crater (54°S, 347°W). Geomorphic features like sinuosities and connections of the channels show that gullies over dunes involve flows with a significant proportion of liquid. The occurrence of levees implies that these flows are debris flows with a yield strength characteristics of Bingham plastic materials. We apply terrestrial methods to estimate viscosity and velocity of these flows from levee size and sinuosities. We obtain average velocities in the range of 1 to 7 m s−1 and apparent viscosities of 2.8 to 46,000 Pa s, with an average at 740 Pa s, compared with the 0.001 Pa s of pure water. These viscosities and velocities are in the range of terrestrial debris flows with a proportion of 10 to 40% of H2O. These properties are typical of water-holding debris flows but not of pure water surface runoff or CO2 driven flows. The debris flows over dunes are oriented on south-facing slopes like other recent gullies. Meltwater from ground ice formed during a recent period of high obliquity is the more likely explanation for the formation of such flows over dunes.

Spatial relationships between patterned ground and ground ice detected by the Neutron Spectrometer on Mars

[1]xa0Patterned grounds, like polygonal features, are the signature of climatic effects in periglacial regions on Earth. Identifying similar features on Mars is important for an understanding of the past Martian climate. In this study we mapped fresh patterned landforms from the systematic analysis of Mars Orbiter Camera high-resolution images. We show that most of them are distributed at latitudes poleward of ±55°, making a climatic control likely. This distribution correlates to the distribution of ground ice detected by the Neutron Spectrometer aboard Mars Odyssey. This correlation is likely the consequence of the Neutron Spectrometer detecting ice no deeper than about 1 m. Patterned ground formation requires ice in this range of depth because these features are triggered by the propagation of a thermal wave that is driven by seasonal or diurnal changes in insolation, which affect the temperature in the uppermost ground layers. Sublimation seems to play a role in the shaping of many of the small patterns observed at latitudes between 55° and 70°. No widespread polygonal features are correlated to the equatorial regions where hydrogen is detected by the Neutron Spectrometer.

Geomorphic study of fluvial landforms on the northern Valles Marineris plateau, Mars

[1]xa0Fluvial landforms are observed on the plateau near Echus Chasma and other locations on the Valles Marineris plateau using high resolution (10 to 50 m/pixel) Mars Express images and topography. Branching valleys have a 20- to 100-m deep V-shape profile typical of fluvial processes. Their incision occurred in a thin (<150 m) and weak, dark unit that overlies the plateau basement. The valleys are distributed close to watershed boundaries as expected for overland flows, and different from pure glacial or hydrothermal processes. The 2D geometry of valley networks show drainage densities reaching 1.3 km−1 that indicate a strong 2D extension stage that usually requires a minimum of thousands of years, as established from terrestrial examples. However, the 3D valley geometry shows a limited incision, and a lack of concavity, suggesting a limited development in time (millions of years of evolution are unlikely). Many outlets connect to the heads of canyons of Echus Chasma that might involve sapping processes. These canyons might have formed coevally with shallow valleys: their difference in geometry is a consequence of the difference in lithology which induced a difference in the erosion capacity, and an enhanced infiltration conducing to sapping. Valleys are submitted to modification by aeolian processes, sometimes leading to the formation of inverted channels as observed more broadly through the region. These landforms formed late in the Early Mars history and might be considered as examples of episodic fluvial events due to short-term climate changes and/or regional fluvial activity after the Noachian.