HR: 17:15h
AN: P32C-06    [PDF]
TI: Geologic Evidence for Late-Stage Equatorial Surface and Ground Ice on Mars
AU: * Chapman, M G
EM: mchapman@usgs.gov
AF: Mary G. Chapman, U.S. Geological Survey 2255 N. Gemini Drive, Flagstaff, AZ 86001
AB: New imagery data from the Mars Observer Camera suggest that the equatorial canyon of Valles Marineris contained surface and ground ice relatively late in Martian history. Some troughs (or chasmata) of Valles Marineris contain large mounds and mesas of interior layered deposits (ILDs) that formed in the Late Hesperian to Early Amazonian. Although the origin of the ILDs remains controversial, their characteristics suggest that the strongest hypotheses origin are lacustrine or volcanic processes; some workers have suggested a compromise origin, noting that many MOC observations of ILDs are similar to those of terrestrial sub-ice volcanoes that erupt in meltwater lakes. Lacustrine deposition and sub-ice volcanism require that chamata water or ice would have had to remain stable on the surface long enough to form either (1) extremely thick (1 km to $> 4$ km) deposits of fine-grained suspended lacustrine materials or (2) numerous sub-ice volcanic edifices with heights that compare to those of Hawaiian oceanic volcanoes. However, a dust cover on top of ice or an ice-covered lake could aid in preventing rapid sublimation. If the ILDs are sub-ice volcanoes than new MOLA topographic data can be used to (1) measure the heights of their subaerial caprock and (2) estimate corresponding volumes of ice. For example, the largest ILD mound in the 113,275 km3 void of Juventae Chasma resembles a capped sub-ice volcanic ridge. The mound is about 2 km high; with the highest point of the cap reaching an elevation of about +80 m. GIS measurement indicate that the maximum volume of ice below the elevation of +80 m is 56,423 km3, so roughly half of the Chasma could have been filled with ice. If the ILDs are lacustrine, then the heights of some other mounds that rival the surrounding plateau elevation would have required a volume of water almost equal to their enclosing chasma. Later in the Amazonian, after sublimation of any putative surface water or ice, MOC imagery attests to ground ice within some ILD mounds. Theatre-headed gullies cut into the flanks of mounds in Hebes, Juventae, Gangis, and Ophir Chasmata. At the MOC scale these gullies display no impact craters and could therefore be extremely young. Finally, one new image attests to possibly recent ground ice within floor material of Juventae Chasma. At the MOC scale, surfaces within this chasma have few impact craters, indicating a very young surface age. MOC image M0804669 shows some interesting geologic/geomorphic relations that occur within the chasma. For instance, on the south side of a chaotic knob, a talus deposit with a flat, possible pediment cap has been cut by late-stage erosion from wind, water, or ice. This relation indicates that relatively steady, talus-forming erosion was interrupted by a period of downcutting that incised the talus and caprock. On the north part of image M08-04669, an impact crater rim and its ejecta blanket are pitted with irregularly shaped depressions that appear similar to terrestrial thermokarst pits found in active glaciated and periglacial terrain due to the meltout of buried ice. The south end of this same image shows possible brittle fracture of channel-confined, dune-covered material. The possible thermokarst pits and brittle fractures may indicate melting of late-stage ground ice. Valles Marineris is a possible volcano-tectonic graben or collapse structure. Dark material within the chasma has been suggested to be very young volcanic material and MOC data appears to show several associated possible volcanic vents. Perhaps late-stage to recent volcanism drove water into the chasma, changed the local atmospheric circulation to create a unique microclimate, and issued forth a fine-grain, protective dust cover of dark ash.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1620 Climate dynamics (3309)
DE: 1800 HYDROLOGY
DE: 6225 Mars
DE: 8409 Atmospheric effects (0370)
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting