SURFACE MODIFICATION OF VENUS AS INFERRED FROM MAGELLAN OBSERVATIONS OF PLAINS

In Sedna Planitia, clear stratigraphic relations can be discerned among volcanic flow units Young flows exhibit SAR specific cross section values similar to fresh terrestrial basalt flows, whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows. Total degradation of approximately 1 m depth over approximately 0.6 b.y. is inferred for the Sedna area from radar signatures, impact crater abundances, and ejecta superposition relations with respect to volcanic flow units. Analyses of parabolic ejecta deposits associated with the crater Stuart imply that the material is typically centimeters in thickness. A relatively small fraction (approximately 10%) of Venusian impact craters exhibit prominent parabolic ejecta deposits. These craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years. The inferred dispersal rate (<10(-3)mu-m/yr) is too low to produce the degradation of flows at Sedna Planitia, and it is concluded that the dominant flow modification process is in situ weathering. In addition, elevation dependent weathering is inferred in western Ovda Regio, where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations. The inferred rate of generation of high reflection coefficient materials is no more than approximately 10(-2)mu-m/yr, based on the inability of aeolian activity to cover high-reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high-reflectivity surfaces at high elevations. Surface modification rates on Venus are orders of magnitude lower than on Earth. Venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on Mars but are comparable to the estimated mte of weathering and erosion of Martian bedrock. Low surface modification rates imply that it will be possible to determine regional-scale age variations on Venus based on the degree of preservation of volcanic landforms and microwave signatures.