QUOTE (elakdawalla @ Apr 21 2013, 07:09 PM)
Mike, one question that I had about this paper was that it takes as fact the notion that most of the observed sedimentary rocks postdate the valley networks...
I am neither an expert nor a big believer in age dating by crater counting, especially on Mars where we have burial and exhumation and complex erosional processes to worry about. Certainly Malin and Edgett assign most of the sedimentary rocks to the Noachian; see the whole discussion of age relations starting on page 1929 of "Sedimentary Rocks of Early Mars" Science 290, December 2000. But I'm not convinced anyone really knows in detail.
I reread Haberle ("Early Mars Climate Models", JGR 103 E12, November 1998) and it has plenty of "escape clauses" for how early Mars could have been warm enough to have large amounts of liquid water, so I think the motivation for a more exotic mechanism is not absolute. That said, the last few paragraphs of Malin and Edgett they advance a somewhat "Kite-like" hypothesis (before Kite did, obviously):
QUOTE
The second scenario is substantially more
exotic and attempts to conceive a plausible but
uniquely martian explanation for what is ob-
served. In this model, modulation of atmospher-
ic pressure by astronomical perturbations, com-
bined with catastrophic modulation of sediment
sources, gives rise to conditions recorded by the
layered, massive, and thin mesa units.