Late-Stage Fluvial Erosion in a Changing Climate on Early Mars

The decline of heavy bombardment in the solar system coincided with incision of many branching fluvial valleys in the martian highlands. However, these valley networks are underdeveloped relative to typical terrestrial networks, suggesting that valley incision was geologically brief or slow on Mars. Most previous studies have attributed the end of martian fluvial erosion to a monotonic decline of the atmosphere and climate around the Noachian/Hesperian transition. Identification of fluvial valleys on some younger surfaces, including Hesperian volcanoes, and the occurrence of morphologically pristine and degraded reaches in the same valley networks challenged the simplicity of this model. More recently, fluvial valleys and deposits have been recognized on a variety of Hesperian surfaces, including the plateau around Valles Marineris, certain impact craters, and the crustal dichotomy boundary scarp. The extent to which this late-stage erosion represents localized event floods or more widely distributed precipitation and runoff remains to be determined. To evaluate whether Hesperian resurfacing processes were concurrent with (and may have caused) late-stage fluvial erosion, we are identifying any geologically rare or long-lived events that occurred between significant resurfacing events and fluvial erosion of those surfaces. In a variety of locations, we have identified small primary craters that formed between local resurfacing and fluvial dissection of those surfaces, suggesting a gap in time between resurfacing and dissection. These small, otherwise fresh craters have rims or ejecta that were incised by late-stage flows. In other cases, thick stratified deposits accumulated on Hesperian surfaces, and those deposits were later dissected by running water. We also found that highland intercrater plains generally have Early to mid-Hesperian crater populations at diameters less than about 4 km. All smaller primary and secondary craters from the Noachian Period were eradicated. These observations suggest the following geomorphic history. 1) Crater degradation and intercrater resurfacing extended into the Early Hesperian Epoch, but perhaps at a declining rate relative to the Noachian Period. 2) Most of the relict valleys formed as crater degradation declined and intercrater geomorphic surfaces began to stabilize, late in the Noachian or early in the Hesperian Period. 3) Impact cratering, volcanism, tectonism, and wind continued to modify the martian surface during the Hesperian Period. Older valley networks experienced some wall retreat and infilling, forming the classic flat-floored morphology. 4) In one or more intervals during the Late Hesperian or Early Amazonian Epochs, many older valleys reactivated, and some new ones formed on Hesperian surfaces. Late-stage erosion was most effective on steep, high-relief slopes, including Late Noachian and Hesperian crater walls, as well as tectonic scarps. In ongoing work, identifying clear stratigraphic relationships between older valleys, interposed geologic features, and younger valleys is key to determining the number and relative ages of erosional events in the martian highlands.