Old or not so old: that is the question for deltas and fans in Xanthe Terra, Mars

Abstract

Most aqueous activity on Mars (except the outflow channels) is thought to be older than ~3.8 Ga [1]. After a major and global environmental change [e.g., 2] of yet unknown reasons [3], the planet approached its current hyperarid climate. Despite this general agreement, many open questions remain: How abrupt was the climatic change at the Late Noachian/ Early Hesperian boundary? Was the type of aqueous processes different before and after? How quickly disappeared the surface water? To address some of these questions, we revisited a part of Xanthe Terra where a number of fluvial valleys and channels and associated deltaic deposits can be observed [4]. Our main objective was to test the hypothesis that deltas (and, implicitly, lakes) on Mars formed mainly during the period of more intense fluvial activity more than ~3.8 Ga ago. We analyzed the morphology, determined absolute model ages and searched for the spectral signatures of alteration minerals that might have formed in response to fluvial and/or lacustrine processes. To test whether our results apply only regionally (to Xanthe Terra), or perhaps globally, we also investigated similar deltas in the eastern hemisphere (e.g., Aeolis region) which were proposed as indicators for an ancient ocean [5] and, therefore, expected to be old (> ~3.8 Ga).