Mathematics – Logic
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........34p&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF SOUTHERN CALIFORNIA, 1994.Source: Dissertation Abstracts International, Volume: 56-09, Section: B,
Mathematics
Logic
5
Lakes, Water
Scientific paper
Martian fluvial channels, particularly the large, catastrophic flood systems (outflow channels), are often described as forming in an ancient Martian climate similar to the modern one. Even the relatively low-discharge, long-duration, "dendritic" valley networks common in the planet's southern highlands, are often attributed to groundwater sapping in the absence of atmospheric precipitation, though a warmer surface environment is usually allowed. It must be remembered, however, that all terrestrial sapping channel systems used as analogs to the Martian valley networks are ultimately recharged through atmospheric precipitation, either as rain or snow. In addition, channels on Venus, although volcanic in origin, are nonetheless subterranean -source system and lack tributaries. Most Martian channels debouch into plains regions, such as the northern lowland plains and numerous highland basins, yet there is little or no morphologic evidence to suggest that channeling continued far into the plains. This is in spite of a continued basinward regional topographic gradient in many cases. The immediate fate of the water discharged from the channels was dependent on the prevailing paleoclimate at the time of its emplacement and the rate at which it was supplied to the surface environment. Most current models of the Martian paleoclimate suggest that mean annual temperatures were likely below freezing throughout most of geologic time, but the geomorphic evidence described herein suggests that coastal erosion on a scale comparable to that of well-known terrestrial paleolakes has occurred, at least in the planet's northern plains. These landforms can be traced to nearly complete closure of the northern plains at two distinct levels, with as many as seven levels identifiable in regions with very-high-resolution (~10-50m/pixel) Viking Orbiter coverage. They appear to record highstands of an ocean with temperatures above freezing at least for geologically brief periods of time, to enable wave erosion or surface ice motion. The latest highstand may have been as recent as Early Amazonian time, perhaps younger than 1.8Ga. The elevations and areal extent of these landforms provide independent estimates of the Martian water budget that can be compared to prevailing models of Martian volatile evolution. Estimated volumes of water and sediment discharged by the circum-Chryse outflow channels alone are sufficient to have produced large bodies of standing water within the northern plains, and are comparable to the basin volume contained within the younger, least extensive highstand that can be traced globally. The earlier, more extensive highstand delineates a basin with a much larger implied volume that probably requires the presence of a semi-permanent, possibly ice-covered ocean in the northern plains prior to the latest major channeling events. (Abstract shortened by UMI.).
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