Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13e..03i&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13E-03
Mathematics
Logic
[5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Studies of degraded impact craters and valley networks have shown that Mars experienced a severe climate change around the end of the Noachian Period, but the decline in landscape denudation appears to be complex. Prolonged, ubiquitous Noachian crater degradation included smoothing of the crater rims and ejecta, wall backwasting, and infilling. Late Noachian valley networks are also widespread but more limited in many aspects of their development, suggesting relatively short-lived activity or arid conditions by terrestrial standards. Younger fluvial features that appear to have more limited spatial distributions may reflect later clement environments on some parts of the planet. However, distinguishing post-Noachian fluvial erosion is challenging, because it requires slopes such as volcanoes, tectonic scarps, crater rims, or airfall deposits that can be convincingly dated to the Hesperian or later. Moreover, the slope or contributing surface must have been large enough to generate erosive quantities of runoff. Several locations described in the literature meet these conditions. Most large alluvial fans occur in Late Noachian to Hesperian craters within the 15-30° south band. In Margaritifer Terra, recent studies show that large alluvial deposits significantly post-date Late Noachian valley networks. A speculative possible explanation involves seasonal precipitation (snowmelt, rain, or both) that generated more runoff in this latitude band than elsewhere, sometime during the Late Hesperian to Early Amazonian Epochs. Gale crater crosscuts the Early Hesperian crustal dichotomy boundary scarp, but a valley network south of the crater appears to have reactivated sometime after the Gale impact and breached the crater rim. Late Noachian valley networks in Aeolis Mensae are hanging with respect to the boundary scarp but exhibit some later dissection and knickpoint propagation. Late fluvial activity in Valles Marineris and some Tharsis volcanoes has also been described, as has dissection of a deposit in the Electris region. Whether these features represent concurrent activity of global scale or a variety of local short-lived environments is not known. Estimates of the dominant discharge for fluvial channels are not consistent with intense meteorological floods, but perhaps with up to cm/day runoff production from watersheds. In Eberswalde crater, the best-constrained case, meandering inverted channels on the delta surface reflect a dominant discharge of hundreds of cubic meters per second and event runoff production up to 1 cm/day, but annual runoff production of <10 cm/yr was necessary to maintain the lake level. In nearby Holden crater, inverted channels on alluvial fans suggest transport of finer-grained sediment, which does not require intense runoff. These and other sites suggest that any late clement interval on Mars was not necessarily very wet relative to Earth.
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