MASS MOVEMENTS ON VENUS - PRELIMINARY-RESULTS FROM MAGELLAN CYCLE-1 OBSERVATIONS

Mass movements on Venus, seen in radar images acquired by the Magellan spacecraft during its first mapping cycle, are easily interpreted within the scheme commonly used to classify terrestrial landslides. Rock slumps, rock and/or block slides, rock avalanches, debris avalanches, and possibly debris flows are seen in areas of high relief and steep slope gradients and are most abundant in the tectonic troughs that crisscross much of the equatorial region of Venus. Many classes of regolith and sediment movements are not seen; such features might be too small to resolve in the 75 m per picture element radar images, or their absence may reflect the relatively thin cover of fine sediments inferred from emissivity measurements and other observations. Venusian landslides, like those found within the Valles Marineris on Mars, tend to come from escarpments typically higher than those on Earth. They appear to fall between the terrestrial and Martian height to length trends; they are also somewhat larger (using length as a surrogate for volume) than terrestrial subaerial landslides but smaller than their Martian counterparts. Good morphologic analogs can be found in terrestrial volcanic slides (both subaerial and submarine): oversteepening of volcanic edifices by intrusion and subsequent lateral collapse appears responsible for shaping a number of large, isolated volcanos on Venus. Faulting and seismically induced accelerations are probably responsible for the majority of nonvolcanic mass movements. The atmosphere may participate in promoting the movement of some of the landslide debris, but environmental factors (e.g., rainfall, temperature cycling) do not appear to play as dominant a role as they do on Earth. Based on the types and locations of landslides seen in the Magellan data, it is possible to scale the terrestrial occurrence rate to Venus: if Venus is as seismically and volcanically active as Earth, than of order one major landslide (i.e., discernable in Magellan images or approximately 5-10 km in runout distance) should occur per year, which careful reexamination of Magellan images acquired during later mapping cycles may be able to detect.