Basal melting of snow on early Mars: A possible origin of some valley networks
He writes that:
The perception of an early, warm Mars is, however, being increasingly questioned because of (1) failure to detect weathering products from orbit [Christensen, et al., 2001], (2) the vulnerability of an early atmosphere to losses by impact erosion [Melosh and Vickery, 1989], (3) the likely rapid scavenging of CO2 from the atmosphere by weathering under warm, wet conditions Pollack et al., 1987], and (4) climate modeling studies which show that it is difficult, if not impossible, to sufficiently warm Mars with a CO2-H2O greenhouse so that rainfall could occur [Haberle, 1998; Kasting, 1991]. The latter problem is particularly acute early in the planet’s history, when the Sun’s output was likely significantly less than it is today [Newman and Rood, 1987].
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