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SynoPSIs -- The 1997 Mars Pathfinder Spacecraft Landed on Spillover Flood Deposits from an Early Mars Inland Sea

Wednesday, September 6, 2017

In 1997 the first in-situ evidence of Martian fluvial landforms was observed by Mars Pathfinder at the lower reaches of the planet’s most extensive outflow channel system, thereby adding evidence to a then-controversial idea that peak activity in catastrophic floods generated a northern ocean ~3.4 Ga. Nevertheless, this hypothesis remains challenged by proposed alternatives that attribute the formation of the huge channels to erosion by glaciers, debris flows, and/or lava flows. We show that the outflow channel system’s lowest areas constitute an immense basin, which is located upstream from the landing site. The fact that the basin was not buried by inflowing lithics implies a discharge history dominated by catastrophic floods that formed a transient inland sea. While we observe no basin-bounding wave-cut terraces, the proposed sea’s areal extent is covered by a sedimentary deposit, which exhibits upper reaches bounded by a constant elevation. By analogy to eastern Aral Sea, we attribute the lack of terraces to rapid seafront retreat driven by high water depth loss rates throughout shallowly submerged low inclination coastal plains. Our results restrict the flows reaching the landing site to spillover floods and recharacterize its geologic setting as part of an early Mars inland sea spillway.

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