Contents — I through J Multiple Generations of Martian Valley Networks: Reconciling Extensive Fluvial Erosion with Immature Drainage Systems

Introduction: Most studies of martian erosion have focused on crater statistics, aeolian landforms and deposits, and valley network morphology. Previous workers have noted that the martian crater population is deficient in craters less than ~30 km in diameter, presumably due to erosion [1], and that ancient martian craters were subjected to different erosional processes than craters on airless bodies [2]. The martian record of craters that are visible in imaging is also deficient between 150-300 km diameter relative to the Moon [1], although many highly degraded or buried craters in this size range are now visible in Mars Orbiter Laser Al-timeter (MOLA) topography [3] (Fig. 1). Extensive regions of the martian surface have been resurfaced by airfall deposits [4-7], although some areas retain 20-50 m deep valley networks and other small scale Noachian landforms [8]. Martian valley networks have a similar appearance to some terrestrial arid-zone counterparts. However, martian drainage densities are spatially variable and can be quite low [9], appearing highest on crater walls and other steeply sloping terrain [8]. MOLA topography and Viking Orbiter imaging also show numerous remnant highland massifs, with hundreds of meters of vertical relief and relatively steep sides (Fig. 1). These features and the degraded impact basins collectively suggest that the extant martian valley networks and impact craters represent only a small fraction of the total erosion and deposition that has affected the martian highlands. Here we describe evidence that martian highland erosion was accomplished by multiple generations of valley networks, which were disrupted and buried by cratering, airfall deposition, and basin infilling. This interpretation reconciles observations of extensively eroded terrain with the limited development of valley networks. Extensive fluvial erosion: Observations of valley networks in the Terra Cimmeria/Terra Sirenum constrain our understanding of erosional processes: 1) Erosional processes could be focused at specific locations, rather than representing ubiquitous de-lamination or burial. Crater rims are landforms with known original geometries, and are often widely breached on the side that is downslope relative to pre-crater topography [8] (Fig. 1). This suggests that crater degradation was accomplished by a through-flowing fluid rather than by a process local to the crater. 2) Slopes on the order of ~1° magnitude controlled the erosional processes, such that sediments were transported down these slopes from elevated areas and deposited on basin floors. It is unlikely that aeolian erosion would deeply mantle only basin floors without ramping onto this gently sloping …