Fresh Shallow Valleys in Northern Arabia Terra: Evidence for a Late, Widespread Period of Aqueous Activity on Mars

Abstract:

Concentrations of fresh shallow valleys (FSVs) on Mars occur between ~30-45° in both hemispheres as well as near the equator (e.g., Gale crater and vicinity). FSVs in the northern hemisphere occur along the dichotomy boundary, with the highest concentration in northern Arabia Terra from 35-40°N between 0-20ºE. In this region, FSVs developed both on and away from ejecta of relatively fresh craters, making the direct association between impact processes and formation less likely. Crater statistics and cross-cutting relationships indicate the formation of FSVs terminated prior to about 1.4 Ga, suggesting they may be contemporaneous with alluvial fan and delta formation in the equatorial and mid-latitudes. Many FSV systems are 150+ km long, and in several cases appear to cross depressions that were likely filled with ice or water during FSV formation. Most FSV systems could have formed from a single episode of erosion but incision of the main channel in some locations may imply episodic formation. One long valley system in N. Arabia Terra with an incised channel (100 m wide, up to 10 m deep) yields formative discharges in the range of 100-200 m³/s assuming sand sized particles and a flow depth of 0.25 m, consistent with formation via snowmelt. The grain size is unknown, therefore discharges could range from 10s m³/s for active transport of fine sand to 1000 m³/s for gravel sized grains. Approximately 25 small craters (diameters ranging from ~2-10 km) have single channels extending outward from their rim, implying overflow of the crater and the possible presence of a deep lake. Widespread occurrence of FSVs, their similar morphology, and modest state of degradation is consistent with most forming during one or more global intervals of favorable climate, likely through snowmelt from surface or sub‐ice flows during the Hesperian.