Fresh Shallow Valleys in Northern Arabia Terra, Mars: Implications for the Post-Noachian Climate

English Abstract:

The working paradigm of the hydrologic evolution of Mars for many years envisioned a sudden decline in fluvial activity around the Noachian-Hesperian boundary, possibly related to the collapse of the early, dense atmosphere. A growing inventory of post-Noachian fluvial features, however, date to the late Hesperian to early Amazonian and may represent the last widespread episode(s) of aqueous activity that occurred during a climate characterized by a relatively thin atmosphere and thick global cryosphere. For example, concentrations of fresh shallow valleys (FSVs) are more widespread than previously reported, occurring in a latitude band from ~30-45° in both hemispheres. FSVs occur in the equatorial regions as well (e.g., possibly Gale crater and vicinity) but are often harder to distinguish from the older, Noachian valleys. 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. 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. 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. Craters with a single fluvial exit breach (nicknamed “pollywog craters”) imply the involvement of artesian groundwater flow. 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.