ABSTRACT: Surface Modification of Venus as Inferred from Magellan Observations of Plains and Tesserae
ARVIDSON, RAYMOND E., McDonnell Center for Space Sciences, Washington University, St. Louis, MO
Magellan data are consistent with a planet that produces landforms randomly in space and time by volcanism, tectonism, and impact events. Weathering, mass wasting, and aeolian activity operate on a continuing basis and are capable of modifying the upper several meters of plains and tens of meters for tesserae, averaged over the several hundred million year average age of the surface. Evidence for these assertions includes: (1) Stratigraphically young lava flow units in Sedna Planitia that have radar signatures similar to those found for both a'a and pahoehoe flows on Earth and older flow units with backscatter characteristics similar to terrestrial basalt flows that have been degraded by mass wasting and burial by aeolian deposits. (2) The observation that only a small fraction of the impact craters have
extensive ejecta deposits. The deposits are typically <1 m thick and are removed over tens of millions of years by weathering and erosion. (3) The observation that reflection coefficients for tesserae in Alpha Regio and Ovda Regio below ~6054 km radius are relatively low (<~0.08) and consistent with the presence of an unconsolidated debris layer. The lack of scree slopes implies that mass wasting is not currently a major surface process, perhaps because the tesserae analyzed are mature and relatively stable. In other regions with pristine-appearing fractures and cliffs, mass wasted deposits are evident. These results imply that mountainous areas evolve in morphology by weathering and erosion. (4) The observation that elevated (>6054 km radius) tessera and plains in Ovda Regio have enhanced reflection coefficients (>0.20) implies that elevation-dependent atmosphere-surface reactions (as opposed to enhanced erosion on tesserae slopes) control the presence of high dielectric materials. High reflectivity plains exist in western Ovda Regio between 6054 km and 6053.5 km, where they merge with normal-appearing plains. These bright plains are interpreted to be covered with debris shed from adjacent tessera and blown downhill.The existence of the high reflectivity surfaces below 6054 km implies that weathering reaction rates are lower than the rate of addition of high reflection coefficient materials by mass wasting and aeolian activity. This result implies that such reactions occur over geological timescales.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)