Relation of Groundwater and Surface Water to Gradation on Mars

Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images show unambiguous evidence of landforms that have been buried and exhumed, ranging from mounds of layered rock occupying craters hundreds of kilometers in diameter to small inverted channels only a few meters across. Most of the repetitively layered materials are found within craters, but there are very few examples where conduits by which the materials entered the depressions are preserved. Equally perplexing, in many cases these materials have been subsequently eroded, yet there is no evi-dence of how the materials were exported from within the depressions. Planation appears to have been an important component of degradation: scarp retreat seems to have been more rapid than surface lowering, but there are no net-works of small valleys or channels across the surfaces subjacent to the escarpments. Enormous volumes of material were involved. It is unclear what processes operated to create these landscapes. Only a few pieces of evidence indicate sustained surface flow and standing surface fluid occurred on Mars. Poten-tially the best examples of these factors are found together in a fan-like feature in Eberswalde Crater northeast of Holden Crater, where cut-off meanders and multiple levels of channels are expressed by erosion in inverted relief. Interbedded with light-toned layered outcrops also seen elsewhere in Eberswalde, this feature is likely to be a delta formed on the margin of a crater-filling lake, although no other shore-related features are preserved or exhumed. Although it is tempting to speculate that most of the crater-filling deposits on Mars were formed in an analogous manner, there is little evidence to support this speculation. In a very few areas, mostly on the high plains adjacent to the Valles Marineris, small networks of ridges appear to be inverted channel systems. Their topographic position likely excludes a groundwater source, and they may be the first and best evidence for precipitation-derived surficial flow. The absence of landforms created by overland flow contributing to the valley networks, and the box-heads of tribu-taries, led to the early recognition that groundwater processes contributed to and often controlled the morphology of the valleys; which despite displaying strikingly arborescent patterns, would, on Earth, be generally termed illusory or deranged. Their longitudinal topographic profiles rarely show monotonic altitude decrease, often apparent branches do not integrate with trunk valleys, central channels are exceptionally rare, and other wall and floor land-forms do not point to formation by through-flowing fluid. Many martian valley systems have been buried and partly exhumed, suggesting that their patterns may be relict from pre-burial processes, yet the processes by which they have been exhumed are unclear.