HR: 14:40h
AN: P32B-05 INVITED     [PDF]
TI: Constraints on the Distribution of Hydrogen in the Polar Regions of Mars and Implications for Ice Formation Processes
AU: * Boynton, W V
EM: wboynton@LPL.Arizona.EDU
AF: University of Arizona, LPL, Tucson, AZ 85721
AU: Prettyman, T H
AF: Los Alamos National Laboratory, LANL, Los Alamos, NM 87545
AU: Kim, K
AF: University of New Mexico, Inst. of Meteoritics, Albuqerque, NM 87131
AU: Taylor, G J
AF: University of Hawaii, Inst. of Geophysics and Planetology, Manoa, HI 96822
AU: Evans, L G
AF: Computer Sciences Corp., Science Programs, Lanham, MD 20706
AU: Feldman, W C
AF: Los Alamos National Laboratory, LANL, Los Alamos, NM 87545
AU: Mitrofanov, I G
AF: Space Research Institute, Moscow, Moscow, none Russian Federation
AU: Reedy, R C
AF: University of New Mexico, Inst. of Meteoritics, Albuqerque, NM 87131
AU: Squyres, S W
AF: Cornell University, Radiophysics & Space Research, Ithaca, NY 14853
AU: Starr, R
AF: The Catholic University of America, Physics, Washington, DC 20064
AU: Hamara, D K
AF: University of Arizona, LPL, Tucson, AZ 85721
AU: Janes, D M
AF: University of Arizona, LPL, Tucson, AZ 85721
AU: Kristopher, K
AF: University of Arizona, LPL, Tucson, AZ 85721
AU: Chamberlain, M
AF: University of Arizona, LPL, Tucson, AZ 85721
AB: Early results of the Mars Odyssey Gamma-Ray Spectrometer investigation suggested that the south polar region of Mars contained large amounts of ice buried just beneath the surface. Based on a preliminary normalization to the Viking-1 lander site, the GRS data indicated the ice content in the buried layer was 35% by weight, and it was buried under an ice-free layer that was about 40 g/cm$^2$ thick [1]. More recently we have re-normalized the GRS data to a better-known "ground truth." The gamma-ray flux was normalized to 100% H$_2$O in the northern residual cap, and the neutron fluxes were normalized to the thick seasonal CO$_2$ frost of the northern winter cap. With the new normalization the amount of ice in the lower layer is even greater than calculated before, but the two-layer model for the distribution of hydrogen is no longer consistent with the observations. The problem is that the thermal neutron flux implies a burial depth on the order of 30 to 60 g/cm$^2$, but the gamma-ray flux is too great, even if the ice-rich layer were pure ice, to be buried by more than 20 g/cm$^2$ of ice-free material. A lower limit on the subsurface ice content can be calculated by assuming there is no ice-free upper layer, i.e. that there is no overburden attenuating the gamma rays. In this case the minimum ice content is about 40% by weight. Thus the constraints provided by the H gamma-ray flux yield an ice layer that is between 40% and 100% ice and that is buried by a depth from 0 to 20 g/cm$^2$.. This lower limit of 40% ice by weight is equivalent to 70% by volume. This much ice is difficult to accommodate simply by filling pore space with ice formed from atmospheric vapor condensation. The high ice content suggests that the ice was emplaced by another means capable of depositing ice with a high ice/dust ratio. Such mechanisms include surface deposition of ice in the form of snow or frost where a wide range of dust/ice ratios would be permitted. Nevertheless, it is clear that the process of sublimation and re-condensation of ice in pore spaces is an active process on Mars. The evidence for this conclusion come from the close correlation between the predicted regions of ice stability [2] and the location of the ice-rich deposits as noted in [1]. We now consider a 3-layer model in which the first (upper) layer is ice-free, the second layer has ice deposited by vapor diffusion filling the pores, and the third layer has a high ice/dust ratio formed by snow or frost. Though we have not yet modeled three-layer soils to see if any of them is consistent with the gamma flux as well as the thermal and epithermal neutron fluxes, the high ice content and indications of active sublimation independently drive us to a three-layer model. We expect that the depth of the boundaries between the layers will change with changes in climate, but layer three, once desicated during a warm epoch, could not reform with the same high ice/dust ratio as the high pore space would collapse to some extent. Thus the depth to layer three can put a constraint on the maximum warming in any epoch since the last significant deposition of snow or frost. [1] Boynton et al. Science 297:81 (2002). [2] Mellon \& Jakosky, JGR 98:3345 (1993)
DE: 5410 Composition
DE: 5462 Polar regions
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting