Quantitative studies of the Martian south polar region using spacecraft images

[摘要] Spacecraft observations must be calibrated absolutely in order to investigate the photometric properties of the Martian surface and atmosphere. The accuracy of the Mariner 9 and Viking Orbiter television system calibration was evaluated by comparing the two data sets with each other and with Earth-based spectrophotometry of Mars and Phobos. The Viking imaging data are consistent with published estimates of the geometric albedo of Phobos, which is uncertain by about 20%. Mariner 9 data are calibrated to within about ±20% by comparing Phobos images with Viking data. Better photometric observations of Phobos are necessary to improve the calibration of the Viking Orbiter and Mariner 9 television systems. Similarly, inflight Phobos observations should be used to calibrate imaging systems on future Mars missions.Mariner 9 images were processed for comparison with nearly simultaneous infrared spectra of the south polar cap of Mars recorded in 1971-72. Combined analysis of these observations indicates that the southern residual cap was covered by carbon dioxide frost throughout the summer, in agreement with Viking Orbiter measurements made three Mars years later. Thermal modeling of the spectra shows that areas of intermediate albedo are cooled to the sublimation temperature of CO2, suggesting that frost is present but not visible. Topographic roughness may shade the CO2 from the sun and produce the variegated appearance of the residual cap.Five color/albedo units, including polar frost, have been recognized and mapped in the southern layered deposits on Mars. Atmospheric dust scattering was measured in shadows and modeled in order to remove the component of brightness due to the atmosphere and quantify the albedo and color of the surface. The layered deposits appear to be mantled by red dust, except where eolian stripping has exposed the underlying bedrock. Frost and bare ground are mixed below the resolution of the images in many areas adjacent to the polar cap, some of which appear to be younger than the surrounding layered terrain. Dark material has been deposited in topographic depressions in much of the south polar region, including the layered deposits. The available observational data suggest that the layered deposits are composed of bright dust, ice, and a small amount of dark material. If the dark material is sand, a periodic change in polar winds seems required in order to transport the sand poleward into the layered terrain. In any case, the observations are not consistent with the layered deposits being composed only of bright dust and ice.Maximum slopes of 10-20 degrees occur on an exposure of layered deposits within the south polar residual cap of Mars. A new photoclinometric technique is used to produce profiles of slope and albedo using high resolution Mariner 9 images. Stereophotogrammetry is also used to constrain the photoclinometric solutions, which resolve layer thicknesses of 100-300 meters. The results are limited by the ~200 meter resolution of the images, and thinner (unresolved) layers are likely. The ~25% maximum albedo variations are correlated with slope, indicating that frost is present on level areas. There is evidence for temporal changes in frost distribution in the 7 days (4° of L5) between the two images used in this study, demonstrating that future photoclinometric studies of the polar regions must be attempted carefully. The magnitude of the slopes derived here suggest that the layers are competent, perhaps due to the presence of a weathering rind.Weathering of the layered deposits by sublimation of water ice can account for the data presented here and previous observations of the north polar deposits. The non-volatile component of the layered deposits appears to consist mainly of bright red dust, with small amounts of dark dust or sand. Deposition of sand in the layered deposits is problematical, so inclusion of dark dust is preferred. The dark dust may be similar to the magnetic material found at the Viking Lander sites, and may therefore preferentially form ~ 100[mu] filamentary residue particles upon weathering. Once eroded from the layered deposits, these particles may then saltate to form the dark sand dunes found in both polar regions. Eventual destruction of the particles could allow recycling of the dark dust into the layered deposits via atmospheric suspension.

[发布机构] University:California Institute of Technology;Department:Geological and Planetary Sciences

[关键词] Planetary Science and Geolo0gy