Topographic Evidence for a Massive Ocean on Early Mars.
Abstract
The recent identification of putative shorelines in the Utopia Basin and in the plains north of Alba Patera by Head and others (1999) in MOLA topography has revealed that a number of broad topographic terraces may not be apparent in the image data, possibly due to subsequent draping by younger deposits. At about the same time as this study, one of our team (Franklin) realized that similar terraces are evident in the topography in the east Acidalia/west Deuteronilus Mensae region. Contacts in this area, which we interpreted as possible shorelines, tend to parallel topographic contours and often fall along what appear to be erosional escarpments, without "favoring" either the top or base of the escarpment. These escarpments appear subdued relative to the younger contacts, with the surface textures associated with the contacts overlying the subdued topography, similar to the expression in Utopia Basin. But the lack of well-defined geomorphic contacts in the Viking mosaics in the Utopia region required MOLA topography to enable their identification. We have begun examining selected areas elsewhere along the highlands boundary for additional evidence for similar erosional features that might be ancient coastal terraces. Beginning with the east Cydonia Mensae through northwest Arabia Terra region, we have made a preliminary identification of a number of regional terraces along the highlands boundary. Most notable are two "steps" in the topography at about -3900m and at about -4100m elevation. The first of these corresponds to the position of the "gradational boundary", or "Contact 1." Craters along this boundary have been eroded, not just buried, because hundreds of meters of rim material is "missing" on the plainward side of the craters. Topography above -3900m elevation is degraded, and below -3900m is filled in, resulting in a topographic "terrace" that can be traced laterally for hundreds of kilometers at this elevation. Our second example, from the crustal dichotomy south of Elysium Planitia, verifies an earlier assertion in Parker and others (1993) and Parker and Schenk (1995) that the mouths of many valley networks terminate at an elevation that may be hundreds of meters to kilometers up into the highlands even though regional slopes continue beyond them toward the plains. This suggests a common base level for the channels, inferred at the time to have debouched into an ancient ocean at that elevation. MOLA topography verifies that Al Qahira, Durius, and Ma'adim Valles all terminate at approximately the same elevation, around -1400 to -1500m elevation, though the regional slope continues northward and down for at least another kilometer into southern Elysium.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.P21C..10P
- Keywords:
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- 5455 Origin and evolution;
- 5464 Remote sensing;
- 6225 Mars