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Insights on the origin and evolution of the Martian valley networks from erosion models : reconciling climate modeling and geomorphological observations. Grau Galofre, Anna


The surface of Mars is incised with hundreds of ancient valley networks, the physical record of flowing liquid water during the early stages of Mars evolution (3.5-3.8 Byr ago). Their remarkable similarity to terrestrial rivers has historically motivated their interpretation in terms of rainfall and surface runoff, indicating that Mars’ climate was significantly warmer than present day. Protracted surface liquid water stability is, however, hard to reconcile with results from state of the art Global Climate Models, which predict that under a fainter young Sun and a thicker primitive atmosphere, the Martian southern hemisphere would be largely under ice cover. Distinguishing whether early Mars harbored surface water or was covered by an extensive cryosphere is key to understanding the nature of any habitable environments. The goal of this dissertation is to reconcile the climate and geomorphological characterizations of early Mars by establishing quantitative constraints on the origin of the Martian valley networks. In Chapter 2, I develop a methodology to quantitatively characterize valley networks in terms of their predominant erosional mechanism, including fluvial, glacial, sapping, and subglacial regimes. Chapter 3 uses constraints from a detailed field characterization of subglacial channels to establish their reliable identification from remote sensing data. In Chapter 4, I present the main results: the identification of subglacial channels among the Martian valley networks. These results support climate model predictions and are consistent with morphological observations. Chapter 5 builds on Chapter 4 to further understand the dynamics of landscape evolution on early Mars. In particular, I demonstrate that only a small fraction of valley networks are in a steady-state, and that erosion rates were likely very low on early Mars. I conclude (1) that subglacial erosion is widespread on the Martian Highlands and best explains the puzzling characteristics of valley networks, (2) that fluvial erosion was short-lived and only concentrated in narrow topographic corridors, and (3) that glacial and sapping erosion were rare on early Mars. In marked contrast to the popular view that Mars was "warm and wet'', my results show that early Mars had a climate akin to Antarctica: extensive ice sheets with localized melting. Supplementary materials: http://hdl.handle.net/2429/67020

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