EMPLACEMENT OF LONG LAVA FLOWS ON PLANETARY SURFACES

Three long lava flows on Mars, Venus, and the Moon were examined in or der to evaluate their possible emplacement rate and condition. On the Moon, flows of the last (phase III) effusion within the Imbrium impact basin were examined using Apollo photography. The longest phase III f low can be followed for 250 km, terminating similar to 400 km from the probable source vent. This flow has a width of 10 to 25 km, thickness of 10 to 30 m, and a medial channel preserved in its proximal reach, and it was emplaced on a regional slope of similar to 0.3 degrees: In the Tharsis region of Mars, a well-defined set of lava flows extends n orth from the topographic saddle between Ascraeus and Pavonis Montes. Viking Orbiter images show one flow that can be traced for 480 km, wit h a width ranging from 5 to 50 km, thickness of 30 to 100 m, and a pro minent medial channel in its proximal reach, and was emplaced on a reg ional slope of similar to 0.5 degrees to similar to 0.1 degrees. The S trenia Fluctus area on Venus consists of an array of intermixed radar- bright and radar-dark lobate flows, one of which can be traced for 180 km, with a width of 5 to 20 km, and an unknown thickness (but inferre d to be similar to 30 m), and was emplaced on the lowland plains where the regional slope is only similar to 0.03 degrees. When viewed at th e full Magellan resolution, this flow contains several flow margins, i ndicating its compound nature. Effusion rates were calculated for the simple lunar and Martian flows using published empirical and theoretic al relationships, resulting in a broad range of 500 to 10(8) (Moon) an d 600 to 2 x 10(8) (Mars) m(3)/s, with most likely values of similar t o 5 x 10(4) to similar to 10(5) for both flows. The compound Venus flo w would have required 494 years for emplacement at the typical Kilauea tate of similar to 5 m(3)/s, but the thermal balance of planetary tub e systems could also be consistent with a rate at least an order of ma gnitude larger. The distinction between simple and compound flows is i mportant to any evaluation of flow emplacement based solely on remote sensing data.