Prolonged Fluvial Activity From Channel-Fan Systems on Mars

Abstract

Alluvial fans on Mars, which are primarily sourced from erosional alcoves incised into crater rims, record a period of increased surface runoff which ended >1 Ga. However, we lack quantitative constraints on the frequency and duration of river-forming processes and the climatic conditions that accompanied these long-term habitable episodes. Here we use bedrock erosion and sediment transport models to show that the cumulative time span of wet activity (i.e., nonzero erosion and deposition) was between 100 years to 1 Myr excluding dry years. We use Context Camera (CTX) digital elevation models to compile a data set of >200 channels upstream of depositional fans and determine key fluvial geometry metrics. Results from calculating Mars stream power parameters are compared to great escarpment channels and globally distributed bedrock rivers on Earth. Although Martian channel profile morphologies fall within the range of those on Earth, they are slightly less concave-up (concavity index, (Formula presented.)) and steeper for a given drainage area (reference steepness index, (Formula presented.), for reference drainage area, Ar = 1 ×107 m2). Timescales depend strongly on poorly constrained variables such as erodability and grain size. Channel morphologies, intermittencies, spatial distributions, and orientations collectively suggest an arid climate and a source from snowmelt on steep crater rims, possibly from obliquity-paced insolation variations or orographic accumulation. Derived timescales are consistent with erosion rates and intermittencies observed in arid environments on Earth and do not support short-lived or catastrophic triggers for the warm climate conditions (such as impacts or individual volcanic eruptions).