Nicolas Mangold

Orientation and distribution of recent gullies in the southern hemisphere of Mars: Observations from High Resolution Stereo Camera/Mars Express (HRSC/MEX) and Mars Orbiter Camera/Mars Global Surveyor (MOC/MGS) data

Geologically recent small gullies on Mars display morphologies consistent with erosion by water or by debris flows. Suggested formation models are divided into two main categories: (1) groundwater or (2) melting of near-surface ice/snow sourced from the atmosphere. We have measured location and orientation and recorded the local contexts of gullies to constrain the likely models of gully formation. More than 22,000 Mars Orbiter Camera Narrow Angle (MOC NA) and >120 Mars Express High Resolution Stereo Camera (HRSC) images in the southern hemisphere were searched for gullies. Discrete gullied slope sections with consistent orientation were recorded rather than individual gullies. Slope setting (impact crater, valley wall, etc.), location, and orientation were recorded for each slope section. More than 750 MOC images with gullies (>900 distinct gullied slope sections) and more than 40 HRSC images (>380 distinct gullied slope sections) were identified. From both MOC and HRSC, gullies were found to be most common between 30 and 50 degrees latitude and to have an overall pole facing preference. The preferred gully orientation for HRSC is southeast rather than south in MOC, owing to illumination effects that make gullies difficult to detect on south- to southwest-facing slopes in HRSC. In both MOC and HRSC surveys, higher-latitude gullies show less preference for pole facing than those at mid latitudes. Both data sets produced similar results, demonstrating that our data are reliable. We suggest that the observed latitudinal and orientation distributions of gullies show that insolation and atmospheric conditions play a key role in gully formation.

Topography of valley networks on Mars from Mars Express High Resolution Stereo Camera digital elevation models

[1]xa0Martian valley networks have been identified mainly in the Noachian heavily cratered uplands. The geometry of valley networks can be studied using Mars Orbiter Laser Altimeter (MOLA) altimetry, which is sufficient to map large valleys without a detailed 3-D shape of valley networks. Imaging from the Mars Express High Resolution Stereo Camera (HRSC) is used to generate digital elevation models (DEMs) with resolution ≤50 m and vertical accuracy <60 m. We studied valleys near Huygens crater and in the Aeolis region both in the Noachian bedrock and on the West Echus plateau in Hesperian bedrock. HRSC DEMs in these areas show that (1) drainage density is 3 times higher than is observed in MOLA data, (2) degree of ramification is 1 order more than with MOLA, (3) transverse valley profiles show a V shape more accurately and a minimum depth of ∼20 m, and (4) higher drainage density shows greater headward extension that is not correlated to greater valley depth. The deepest valleys (400 m) are found in the Huygens region, where the density in the DEM is 0.1 km−1, compared to shallow valleys (<100 m) of the Echus region, where the density is higher (∼0.3 km−1). These regional differences are due to spatially variable preservation and bedrock lithology. Longitudinal profiles suggest variations in duration of activity: profile concavity is only developed in some Noachian terrains. Valleys visible in HRSC images correspond to topographic features in DEMs showing the same geometry as terrestrial valleys thought to be formed by overland flows and seepage.

Local geoelectrical models of the Martian subsurface for shallow groundwater detection using sounding radars

Received 15 February 2002; revised 20 July 2002; accepted 28 August 2002; published 6 March 2003. [1] Low-frequency sounding radars should be able to probe the Martian subsurface layers down to varying depths, depending on the geoelectrical properties of the sounded sites. We present in this work four frequency-dependent geoelectrical models of the Martian subsurface in the 1–20 MHz frequency band, based on laboratory electromagnetic characterization of Martian soil analogues. Those models correspond to local Martian sites that we considered to be of particular interest in the search for water using mainly the Ground-Penetrating Radar (GPR) instrument of the Netlander mission. Results and discussion are also valid for both sounding experiments MARSIS and SHARAD. The four models of the Martian subsurface are designed to represent terrains where recent fluviallike features suggest the presence of near-subsurface ground ice and probably liquid water. We performed measurements on volcanic and sedimentary materials that may be present on these sites under the appropriate geophysical conditions that may exist in those terrains. We then simulated the backscattered radar echo arising from each site in the 2 MHz frequency band, using the Finite Difference Time Domain (FDTD) algorithm, in order to evaluate the instrument performances to probe the subsurface stratigraphy of each site. Our results confirm that the near-subsurface rich iron oxide mineralogy controls the instrument performances in terms of penetration depth and signal-to-noise ratio in the 2 MHz frequency band. We finally discuss the geophysical and geoelectrical sounding conditions that could lead to an ambiguous detection of shallow subsurface water on Mars for the Netlander GPR. INDEX TERMS: 3210 Mathematical Geophysics: Modeling; 1794 History of Geophysics: Instruments and techniques; 5144 Physical Properties of Rocks: Wave attenuation; 5109 Physical Properties of Rocks: Magnetic and electrical properties; KEYWORDS: Mars, hydrology, GPR, sounding, simulation, FDTD

The Geology of Mars: Debris flows in Greenland and on Mars

The observation of small gullies on Mars was one of the more unexpected discoveries of the Mars Observer Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft (Malin and Edgett, 2000). Gullies are the flutes and narrow troughs formed by the debris flows process and not the process itself. They mostly occur in a latitudinal band higher than 30 . The upper parts of the slopes (mostly south facing slopes in the southern hemisphere) exhibit alcoves, with generally broad and deep channels. They are characterized by their distinct V-shaped channels with well-defined levees. Individual channels exhibit low sinuosity and deep erosion down to the fans that bury the lower parts of the crater walls (Figure 10.1). These debris fans correspond to one or several lobes. The characteristics of these gullies suggest that they were formed by flowing water mixed with soil and rocks transported by these flows. They appeared to be surprisingly young, as if they had formed in the last few million years or even more recently. In their initial analysis, the MGS Camera investigators Mike Malin and Ken Edgett (2000) proposed a scenario involving ground water seepage from a sub-surface liquid water reservoir located a few hundred meters or less below the surface. However, the process capable of maintaining such a shallow aquifer at temperatures above the freezing point of water remains unclear. Several other hypotheses have been proposed taking into account the geothermal activity (Hartmann, 2001; Gaidos, 2001)