火星のローブ状地形の成因 --土石流堆積物逆解析による予察的検討--

Abstract

Gullies and lobate deposits commonly occur on the flanks of craters and dune foresets on Mars. These topographic features are thought to have been formed by debris-flow processes. Debris-flow processes suggest the existence of liquid water on the surface of Mars, which is believed to have an extremely cold and dry environment. Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge down slopes in response to gravitational attraction. Thus, recent activities of debris flows imply the existence of liquid water phase materials on the Martian surface. However, the dry granular flow caused by a slope avalanche can also form gullies and lobate deposits, which resemble debris-flow deposits, so there are still uncertainties over the occurrence and origins of liquid water on the modern Martian surface. This study proposes a method for estimating debris-flow properties, in order to distinguish debris-flow processes on Mars. Preliminary results suggest that a lobate deposit on Mars can be formed by a debris flow that shows flow properties similar to water-saturated debris flows on the Earth. If rheological parameters of the flows can be determined from the surface morphology of lobate deposits, it would be helpful for distinguishing debris-flow processes from dry-granular flows. Although our method is in the course of development and it is still difficult to determine flow types from rheological parameters alone, future field surveys and experimental studies will provide criteria for identifying debris flows.