Mathematics – Logic
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p31b0439g&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P31B-0439
Mathematics
Logic
6225 Mars
Scientific paper
As of early September 2007, HiRISE has returned over 3,200 images of the surface of Mars, including over 400 images of gullied locations. The HiRISE images (with resolution as high as 25 cm/pixel) provide an opportunity to test the current MOC-derived understanding of gully provenance and various suggested formation mechanisms. The HiRISE images show that the gullies exhibit a great deal of morphological diversity. They range in size with widths as large as several tens of meters down to the HiRISE resolution. Lengths range from several tens of meters to several kilometers. Some form tributaries that coalesce into gully networks, while other gullies exhibit the canonical single source alcove, incised middle reach and terminal debris fan. Some gully sources blend in gradually with the surrounding uplands, while others start full-borne from blunt, theater heads. Gully systems displaying different morphologic patterns can be located physically adjacent to each other. The most morphologically complex gully systems exhibit point bars, cut banks, undercutting of walls and source regions, erosion into underlying surfaces, braided and anastomosing reaches, multiple terraces located along gully margins, and erosion and deposition of materials along the gully and overlapping adjacent systems. This complex suite of morphological features suggests formation by fluvial processes. Other gully-like systems, such as those located on dunes, lack key morphological indicators and are little more than sets of parallel troughs without apparent debris fans. Other simple forms have distinct source regions and debris fans, but lack incised middle reaches. These particular features are on steep slopes, such as the inner walls of several volcano calderas, as well as on some crater, valley and canyon walls and may be more akin to debris chutes where material is transported down steep slopes mostly by gravity alone. We conclude from these observations that there may be a continuum of processes involved in the formation of gullies and gully-like forms, ranging from fluvial erosion to mass movement processes involving dry flows and slides. HiRISE imaging also shows that gullies in a single locale may emerge at a variety of elevations and may display strikingly different morphologies. For example, miniature gully systems, some less than a kilometer long, are found along a crater wall in the Terra Sirenum region. These small gullies are adjacent to larger ones and exhibit typical gully morphologic characteristics, however they emerge much further downslope than their nearby counterparts. In another example, along a crater wall in the Terra Cimmeria region, adjacent gully systems emerge from source regions at a variety of elevations. Some gullies have characteristics typically associated with runoff-dominated fluvial processes while others have characteristics of terrestrial sapping-dominated fluvial systems. Other adjacent gullies have transitional morphologies. Another intriguing set of gullies is found in Hale Crater, where pristine gullies are located on two sides of the eroded crater rim, flowing in opposite directions. In one location, only a narrow ridge separates eastward and westward oriented gullies. Associations such as these may challenge any single gully formation mechanism.
Davatzes A.
Gulick Virginia Claire
HiRISE Team
Keszthelyi Laszlo
Kolb K.
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