Recent gullies on Mars: MOC and HRSC survey of the southern hemisphere

Small gullies on Mars, first observed in Mars Global Surveyor Mars Orbiter Camera Narrow Angle (MOC NA) images, are geologically recent and display morphologies consistent with erosion by water or by debris flows with significant water content. Suggested formation models are divided into two main categories: discharge of water from deep/shallow aquifers or near-surface ice/snow melting driven by obliquity changes. We have measured location, orientation, and geologic contexts of gullies to assess which is the more realistic model for gully formation. More than 22,000 MOC NA and 120 Mars Express High Resolution Stereo Camera (HRSC) images in the southern hemisphere were searched for gullies. MOC NA images are extremely numerous, have high-resolution but individually low spatial coverage. HRSC images have lower resolution (although still sufficient to resolve gullies) and are fewer in number, but individually have extremely high spatial coverage. By using both datasets we avoid the drawbacks of each. Discrete gullied slope sections with consistent orientation were recorded rather than individual gullies. Geologic context, location, and orientation were recorded for each slope section. In the MOC survey, more than 750 images with gullies were identified, some with up to 10 distinct slope sections. When multiple observations of individual gullies were removed, more than 600 images with over 900 slope sections remained. In the HRSC survey, gullies were found in more than 40 images, some containing as many as 30 individual slope sections. More than 380 gullied slope sections were identified from HRSC. From MOC, Gullies are most common between ~30 and~50 degrees latitude and have a strong pole facing preference. From HRSC, gullies are also most common between ~30 and ~50 degrees latitude and again have a preference for pole facing orientations (although the preference is for SE rather than S). In both MOC and HRSC surveys, higher latitude gullies show less preference for pole facing than those at mid latitudes. The pole-facing orientation suggests that insolation is important in gully formation and that latitude controls both distribution and orientation. This appears to confirm the role of climate in gully formation.