Asynchronous formation of Hesperian and Amazonian-aged deltas on Mars and implications for climate

Abstract

Fluvial and lacustrine landforms on Mars are thought to be old and haveformed more than ~3.8 Gyr ago, in the Noachian period. After a majorclimatic transition, surface liquid water became less abundant andfinally disappeared almost completely. Recent work has shown thatobservational evidence for Hesperian and Amazonian aqueous processes ismore common than previously recognized, but their nature is poorlyunderstood. Moreover, it is not clear how the paleoclimate of Mars canbe constrained by this activity. Here we report our investigation of apopulation of deltas around the ancient impact basin Chryse Planitia. Totest whether the results are globally applicable, we also studiedselected deltas with similar morphologies in the eastern hemisphere andfound that the results are consistent. We compared the morphology ofdeltas, feeder channels, and receiving lakes, dated deltas by cratercounting and searched for alteration minerals in hyperspectral images.The valleys and associated late-stage deltas were formed by short-livedaqueous processes, as suggested by their morphology and the general lackof associated aqueous alteration minerals. The likely source of waterwas neither widespread precipitation nor a regionally connectedgroundwater aquifer, but water mobilized locally from the cryosphere.Delta formation in our study areas occurred from the Early Hesperian tothe Late Amazonian and did not require sustained periods of globalclimatic conditions favoring widespread precipitation. Liquid surfacewater has been locally present on Mars even after the Noachian, althoughonly episodically, for transient intervals, and widely separated inspace.