Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p34a..05a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P34A-05
Other
5220 Hydrothermal Systems And Weathering On Other Planets, 5419 Hydrology And Fluvial Processes, 5464 Remote Sensing, 6225 Mars
Scientific paper
We report evidence for spring deposits in Vernal Crater, Arabia Terra. The Vernal structures are low mounds, each approximately 250 m by 500 m in extent, with terraced flanks, apical depressions, river-like channels, concentric fractures, and elliptical tonal anomalies. All of these features are common in terrestrial springs such as the Dalhousie complex in Australia. The structures occur in an apparent unit of interdune, water-laid sediment and are associated with evidence of subsurface fluid flow in sets of aligned outcrops. The Vernal springs may be part of a larger complex of spring deposits and lineations, suggesting that fluid flow in this region was relatively extensive. The structures in Vernal Crater are coated with a thin layer of dust, which prevents mineral identification from orbit. In an attempt to find evidence for additional spring mounds, we conducted a survey of nearly 2,000 locations for which CRISM spectral images are available. We used CRISM data to identify dust-free, hydrated areas and HiRISE, CTX, and MOC images to evaluate morphology. This study covered all longitudes and latitudes from 50N to 70S, except near Tharsis where data were analyzed south of 15S. No location exhibited morphological features that closely resembled those in Vernal Crater, suggesting that these putative spring mounds are not common in the highlands of Mars. Our search led us to concentrate on a dust-free area, centered at 41.8N, 332.5E in Acidalia Planitia where Farrand et al. (2005) identified features resembling spring mounds or mud volcanoes. Tanaka et al. (2005) mapped this region as part of the Early Amazonian Vastitas Borealis Unit, interpreted as reworked sediments from outflow channels and highland sources. We mapped over 20 high-albedo pitted domes in the area covered by one HiRISE frame, with dome diameters ranging from 350 m to 1 km. Nearby, similar domes have measured heights ranging from 36 to 65 m. The dome material is darker in THEMIS nighttime IR than the surrounding plains, indicating that the domes have relatively lower thermal inertia. The dome material is also very smooth, and appears smeared across the textured plains and in local depressions as if it were emplaced by low viscosity flows. CRISM spectral data (Leah Roach, Brown Univ.) were used to assess the mineralogy of selected features. Dome spectra exhibit a steep shoulder from 0.4 to 0.6 microns and are generally featureless in the near IR, overall resembling the spectra of bright Martian soil. The steep shoulder is due to the oxidization of iron to Fe3+. Dome spectra do not exhibit evidence for hydrated minerals or precipitates such as carbonates or silica. While a range of origins has been suggested for the Acidalia domes, we believe that they are most consistent with a mud volcano analog. A volcano or pseudocrater origin is unlikely, as no lava flows or volcanic features are observed in the vicinity. A spring mound origin is also unlikely, due to the absence of hydrated or spring precipitate mineral signatures in the CRISM spectral data as well as the absence of terracing, channels, and circumferential faults that typify the Vernal springs. In addition to the Vernal springs and the Acidalia mud volcanoes, several other recent studies have pointed to evidence of subsurface fluid flow. These include the large spring deposits proposed by Rossi et al. (2008), the resistant knobs in Candor Chasma (Chan, 2008), and the bleached zones along faults in Valles Marineris (Treiman, 2008). Together, these features are changing our understanding of the hydrologic history of Mars.
class="ab'>
Allen Christine
Baker Mark D.
Oehler Dorothy Z.
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