Other
Scientific paper
Sep 1966
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1966saosr.221.....s&link_type=abstract
SAO Special Report #221 (1966)
Other
3
Scientific paper
This paper is an analysis of radar observations of Mars, particularly at 12.5-cm wavelength, performed at the Goldstone facility of the Jet Propulsion Laboratory, during the 1965 opposition. Then the entire range of Martian longitudes at North latitude 21.6° was observed with an effective topographical resolution in the quasi-specular component of some 6 planetocentric degrees. Both for the 1963 and for the 1965 oppositions a very striking correlation is noted between high reflectivity at radar frequencies, and dark areas at optical frequencies. The correlation applies to the central longitude of the Martian features, to the extent of the features, and to their relative reflectivity rankings at the two frequencies. Despite these correlations, there are occasional displacements between the reflectivity maxima and the centers of dark areas of the order of 10°. This is greater than the expected errors in Martian cartography and in the location of the radar subterrestrial point on Mars. For example, Syrtis Major appears as a radar reflectivity relative minimum, while the 10° longitude strips east and west of Syrtis Major appear as relative maxima. Such observations can be explained-while retaining the hypothesis that the Martian dark areas are intrinsically more reflective at microwave frequencies-if the dark areas have slopes of several degrees. On this basis alone the dark areas might be either elevations or depressions. Conditions on slopes can also be set for those cases in which strong radar reflection comes from a bright area adjacent to a dark area. In this case, a dark area must have an appropriate two-dimensional alignment (in the plane of the sky) with respect to the radar station. From the fact that only the near sides of elevations, but the far sides of depressions, are suitably oriented for specular backscattering, the comparative alignments of near and far sides make it unlikely that dark areas are depressions, a conclusion also supported by other radar evidence. The hypothesis that the dark areas are elevations is systematically examined in the 36 Doppler spectrograms of the 1965 opposition, and is found to be consistent with these data. The radar data suggest that the dark areas examined in the 1965 opposition were generally not truncated, but rather sloped gently downward from the center of the dark area scanned. From this fact elevations can be deduced from slopes. It is found that some regions that characteristicaly undergo marked secular changes have very shallow slopes (1-2°) and elevations ~6 km; classical canals have steeper slopes (> 4°) but elevations of the same order; and large dark areas such as Syrtis Major and Moeris Lacus may have elevations between 10 and 20 km. These elevations and slopes are inconsistent neither with optical searches for elevations at the limb, nor with the yield and tensile strengths of ordinary solids under the reduced gravitational acceleration of Mars. The general picture of Mars that emerges is similar to that expected for the Earth, were the oceans removed, the effects of water erosion eliminated, and the relief enhanced by the ratio of the gravitational accelerations on the two planets. The dark areas are similar to continental blocks and the bright areas to dry ocean basins. This conclusion bears adversely on those theories of continental origins that depend on the existence of a liquid planetary core.
Goldstein Raymond M.
Pollack James B.
Sagan Carl
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