A Study of Variable Features on Mars During the Viking Primary Mission

Very few surface changes were seen during the Viking primary mission in 1976, a result consistent with predictions of relatively low wind velocities during northern summer. No eolian activity was detected from orbit in the vicinity of either landing site. This lack of eolian activity was representative of Mars as a whole at this season with two exceptions: (1) Persistent eolian activity was observed on the flanks of the three large Tharsis volcanos (Arsia Mons, Pavonis Mons, and Ascraeus Mons) but most markedly on Arsia Mons. The observed changes are best explained in terms of the erosion of bright albedo material by downslope winds. (2) The formation of a number of dark crater-associated streaks by E to W winds was documented on the eastern edge of Hellas. Comparison of specific albedo boundaries in the 1972 Mariner 9 coverage and in the 1976 Viking coverage revealed that in many cases, subtle changes in outline and/or contrast have occurred during the past 4 years. In a few areas (Syria Planum, for example) the albedo patterns in 1976 are dramatically different from those in 1972. As in 1972, the most conspicuous wind markers on the planet in 1976 are light crater-associated streaks whose pattern still defines the wind flow expected during southern summer. While some new light streaks have formed since 1972 and a few old ones have disappeared, by and large, most light streaks appear essentially unchanged in outline and in direction since 1972. The Viking observations confirm that the mean lifetime of dark streaks is short in comparison to that of light streaks. Many dark streaks have changed conspicuously in both outline and direction since 1972. It appears that contemporary albedo variations on the surface of Mars are due solely to eolian effects or to the deposition or sublimation of volatiles in the polar regions. The combined Mariner 9 and Viking observations contain no convincing evidence to support the once traditional concept of a seasonal wave of darkening.

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