Small, Young Fluvial Features in Icy Terrains on Mars

Background: The success of recent missions has led to important advances in our understanding of the martian fluvial record. There has been considerable expansion in the number and extent of mapped valley networks [1], as well as recognition of many valley-related sedimentary deposits [e.g., 2]. These new data also revealed that many valley systems feed basins with outlets, implying that numerous open-basin lakes existed (~215) at a wide range of spatial scales when valley networks were active on Mars [3]. In some cases, these basins were linked in lake chains up to thousands of km in length [2,3], illustrating the continuity and intensity of the surface hydrological system. Our recent study using buffered crater counting [4] suggests that the large, integrated highland valley networks on Mars were last active before the end of the Early Hesperian (N(5)>155)(the Late/Early Hesperian boundary is at N(5)=125; the Hesperian/Noachian boundary is at N(5)=200 [5]). If the end of the major phase of highland valley network activity was a single point in time, our data suggest this occurred at approximately the Noachian/Hesperian boundary (N(5)~187-214). However, these results do not imply that all valley formation entirely ceased on Mars at this time. Indeed, it has long been suggested that some activity continued at later periods in specific environments, such as on the flanks of some volcanoes [6] and around Valles Marineris [7]. Formation of valleys on the flanks of the volcanoes may have been aided by special local conditions (e.g., energy availablity provided by the volcanoes themselves) which helped enable melting of surface or subsurface ice in conditions otherwise inhospitable to surface melting [8, 9]. Gullies, which appear to be a distinct class of features formed exclusively on steep slopes in mid-to high-latitudes, are another example of very young fluvial systems on Mars; both stratigraphic constraints and crater counts imply activity within the last ~5 Myr [10-12]. Thus, some valley formation on Mars has occurred on Mars after the Early Hesperian, but at greatly reduced scale and intensity. This shift in scale likely reflects a major change in conditions from a more clement environment to the cold, hyperarid desert that exists today. As environmental conditions changed, local conditions became of paramount importance for valley formation. In this abstract we document a distinct class of small valleys which appear to be directly associated with ice at the surface or in the near-surface, and which most likely formed …