STAR AND LINEAR DUNES ON MARS

A field containing 11 star and incipient star dunes occurs on Mars at 8.8 degrees S, 270.9 degrees W. Examples of linear dunes are found in a crater at 59.4 degrees S, 343 degrees W. While rare, dune varieties that form in bi- and multidirectional wind regimes are not absent from the surface of Mars. The occurrence of both of these dune fields offe rs new insight into the nature of martian wind conditions and sand sup ply. The linear dunes appear to have formed through modification of a formerly transverse aeolian deposit, suggesting a relatively recent ch ange in local wind direction. The 11 dunes in the star dune locality s how a progressive change from barchan to star form as each successive dune has traveled up into a valley, into a more complex wind regime. T he star dunes corroborate the model of N. Lancaster (1989, Progr. Phys ical Geogr. 13, 67-91; 1989, Sedimentology 36, 273-289) for the format ion of star dunes by projection of transverse dunes into a complex, to pographically influenced wind regime. The star dunes have dark streaks emanating from them, providing evidence that the dunes were active at or near the time the relevant image was obtained by the Viking 1 orbi ter in 1978. The star and linear dunes described here are located in d ifferent regions on the martian surface. Unlike most star and linear d unes on Earth, both martian examples are isolated occurrences; neither is part of a major sand sea. Previously published Mars general circul ation model results suggest that the region in which the linear dune f ield occurs should be a bimodal wind regime, while the region in which the star dunes occur should be unimodal. The star dunes are probably the result of localized complication of the wind regime owing to topog raphic confinement of the dunes. Local topographic influence on wind r egime is also evident in the linear dune field, as there are transvers e dunes in close proximity to the linear dunes, and their occurrence i s best explained by funneling of wind through a topographic gap in the upwind crater wall. (C) 1994 Academic Press, Inc.