Other
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusm.p32a..04s&link_type=abstract
American Geophysical Union, Spring Meeting 2007, abstract #P32A-04
Other
6225 Mars
Scientific paper
Victoria crater is an impact crater about 800 meters in diameter that lies roughly 5 km south of the landing site of the Opportunity rover. After a drive of more than 900 sols, Opportunity has arrived at Victoria crater, and is now engaged in exploration of this, the largest impact crater encountered by either rover. The shape of Victoria crater is unusual. The rim is characterized by a number of prominent alcoves that are U- shaped in plan view, separated by sharp promontories that project toward the crater interior. The alcoves are clearly sites of mass wasting, and this distinctive shape indicates that the crater has been enlarged significantly by mass wasting processes. Since arriving at the crater's rim, Opportunity has been traversing along the rim imaging the stratigraphy exposed on the promontories. The promontories are steep, and most of them expose several to many meters of intact bedrock on near-vertical faces. Because Victoria crater is so large, by imaging these faces we can for the first time study lateral sedimentary facies variations at Meridiani over horizontal length scales of hundreds of meters. On most of the promontories, the uppermost unit is crater ejecta consisting a jumbled breccia of blocks up to a meter or more in size. The ejecta blanket is entirely dominated by blocks of sulfate-rich sandstone; no other material is evident in significant quantities. The ejecta blanket overlies a zone in which sulfate bedrock is extensively fractured in place, with an abrupt contact that is clearly visible around most of the crater rim. We interpret this contact to represent the pre-impact surface. Beneath the fractured zone lies intact bedrock. Mini-TES observations reveal this bedrock to be sulfate-rich over all depths observed to date. Pancam observations of the bedrock show the promontories on the north side of Victoria crater to be dominated by eolian facies. No clasts are visible in Pancam images; evidently the grain size is too small everywhere to be revealed by Pancam, as is the case everywhere else at Meridiani. Bedding, however, is prominent. In several locations, most notably a promontory named Cape St. Mary, there is spectacular eolian cross stratification exposed on the cliff faces, with high-angle truncations indicative of former dune deposits. Elsewhere we see the subparallel bedding and low-angle truncations expected for eolian sand sheets. Generally, then, the stratigraphy at Victoria is consistent with what was observed at Endurance crater, although much better exposed over significantly larger vertical and horizontal scales. Small cobbles have been found on the plains near the crater that are markedly different in composition from the sulfate-rich sandstone that is otherwise so dominant at Victoria. The cobbles lie well within the crater's annulus, and so may represent a minor fraction of the crater ejecta that is more resistant to wind erosion than the very friable sulfate-rich material, and hence better preserved. Compositional data on the cobble Santa Catarina indicate an elemental composition similar to that of the rock Barberton, a pebble near Endurance crater that we concluded was meteoritic on the basis of its kamacite content. Santa Catarina does not contain detectable kamacite, but does contain a significant quantity of troilite. Santa Catarina is also probably meteoritic in origin.
ACS Science Team
Squyres Steve W.
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