抄録
Spiral troughs are known to exist on the polar ice caps of Mars. The formative conditions of the troughs, which should be affected by Martian climate processes, have been sources of much debate. Recently, they have been interpreted to be cyclic steps formed by katabatic winds blowing over the ice. Cyclic steps are spatially periodic bedforms observed on relatively steep slopes, characterized by regular upstream-migrating steps delineated by hydraulic jumps. They are relatives of upstream-migrating antidunes. Boundary waves often form at the interface between ice and fluid flowing adjacent to it. Examples include ripples on the underside of a river ice cover and steps on the bed of a supraglacial meltwater channel. Waves on ice may also be formed by wind, such as megadunes observed on the Antarctic ice sheet. There have been, however, few experimental studies on the formation of either cyclic steps or upstream-migrating antidunes on ice. The first part of this paper introduces features of spiral troughs on Mars' North Polar ice cap and recent hypotheses interpreting spiral troughs as cyclic steps created by katabatic winds. The cyclic step framework can explain trough initiation, migration, and all of the major physical characteristics of spiral troughs. The latter part of this paper introduces an analogue experiment on the formation of cyclic steps on ice by flowing water, with the aim of understanding the process whereby spiral troughs are formed on Mars' ice caps. Trains of steps form when the Froude number is larger than a value around unity. The features of those steps allow them to be identified as ice-bed analogs of cyclic steps in alluvial and bedrock rivers. Moreover, the results of experiments fall into a region where a linear stability analysis predicts interfacial instability.
