HR: 15:25h
AN: P32B-08    [PDF]
TI: South Polar Residual Cap Geomorphology and Inferred Environmental Changes
AU: * Byrne, S
EM: shane@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, MC 150-21, Pasadena, CA 91125 United States
AU: Ingersoll, A
EM: api@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, MC 150-21, Pasadena, CA 91125 United States
AU: Pathare, A
EM: avp@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, Caltech, MC 150-21, Pasadena, CA 91125 United States
AB: The CO$_2$ southern residual cap (SRC) both controls circulation patterns regionally and buffers the atmospheric pressure globally. In turn this CO$_2$ deposit is affected by changes in environmental conditions wrought by external forces such as dust storm activity. Mars Global Surveyor data of this area have revealed a rich variety of geomorphic features (1) of which there are several distinct classes. These different classes may be end members of the same basic process of insolation driven ablation. We are currently investigating two types of SRC features. Swiss-cheese features (SCF) are depressions characterized by flat floors and steep walls, which retreat 1-3 meters each Martian year (2). In some regions they have a definite symmetry axis along the north-south direction (3). After the seasonal frost disappears the residual ice exposed in the walls has a lower albedo (4). Previously (5) we modeled the evolution and growth of these depressions as a hole in a layer of CO$_2$ ice underlain by water ice, which best explains their morphologic and thermal properties. The observed thickness of the CO$_2$ slab can be as high as 8 meters but in general is much lower. Larger SCF?s commonly possess a raised central island of CO$_2$ surrounded by a moat that penetrates to the underlying water ice (3). The fast rate of wall retreat observed (2) combined with the small sizes of the SCF?s indicate that all SCF?s visible today were created geologically recently. Within a particular region the size distribution is quite narrow (3): no larger (older) or smaller (younger) features were seen indicating that some relatively abrupt change in environmental conditions initiated the growth of this particular population of features. Fingerprint terrain (1) are areas with evenly spaced parallel ridges, which are steeper on one side. These ridges may have small areas of water ice exposed in the intervening troughs. Their wavelength is on the order of 70-90m with the steep edges facing northeast although this varies from area to area. These features are smaller in scale than SCF?s and so may have information pertaining to more recent environmental events. We will present results from several avenues of research that we are pursuing: We are investigating the overall mass budget of the SRC. If the mass lost from expanding depressions is not condensed elsewhere on the cap then the SRC will disappear within a few Martian centuries. It seems unlikely to us that we are observing Mars at such a special time in its history. A large range of expansion rates is possible depending on the subsurface albedo profile (3,5). We will attempt to measure the subsurface albedo by examining images of exposed SCF walls. We are also improving our model to more accurately date features and by extension the environmental events that triggered their initiation. Previously we always initiated our modeled depressions from small pre-existing surface features. We are more closely investigating the genesis of SRC features and what environmental changes are required to cause them. We will continue to catalogue new population statistics for different regions in the SRC Each distinct feature population that we can identify may give us information on previous environmental events. Investigations into SRC features have the potential to describe changes in the Martian polar environment over timescales of millennia. It will provide a link from present conditions to longer-term variations in Martian climate, which are perhaps recorded in the layered deposits. [1] Thomas et al., Nature, 404. [2] Malin et al., Science, 294. [3] Byrne and Ingersoll, GRL, 30. [4] Malin and Edgett, JGR, 106. [5] Byrne and Ingersoll, Science, 299.
DE: 5407 Atmospheres--evolution
DE: 5462 Polar regions
DE: 5464 Remote sensing
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting