DUNES AND MICRODUNES ON VENUS - WHY WERE SO FEW FOUND IN THE MAGELLANDATA

A search through cycle 1, 2, and 3 Magellan radar data covering 98% of the surface of Venus revealed very few dunes. Only two possible dune fields and several areas that may contain microdunes smaller than the resolution of the images (75 m) were identified. The Aglaonice dune fi eld was identified in the cycle 1 images by the specular returns chara cteristic of dune faces oriented perpendicular to the radar illuminati on. Cycle 1 and 2 data of the Fortuna-Meshkenet dune field indicate th at there has been no noticeable movement of the dunes over an 8-month period. The dunes, which are oriented both parallel and perpendicular to the radar illumination, appear to be dark features on a brighter su bstrate. Bright and dark patches that were visible in either cycle 1 o r 2 data, but not both, allowed identification of several regions in t he southern part of Venus that may contain microdunes. The microdunes are associated with several parabolic crater deposits in the region an d are probably similar to those formed in wind tunnel experiments unde r Venus-like conditions, Bragg scattering and/or subpixel reflections from the near-normal face on asymmetric microdunes may account for the se bright and dark patches. Look-angle effects and the lack of suffici ent sand-size particles seem to be the most likely reasons so few dune s were identified in Magellan data. Insufficient wind speeds, thinness of sand cover, and difficulty in identifying isolated dunes may also be contributors to the scarcity of dunes. (C) 1994 Academic Press, Inc .