Table of Contents

Volume 517 Number 7532 pp6-116

1 January 2015

About the cover

Dunes on Titan in a Cassini radar mapper image taken on 21 December 2008 (upper part of image) resemble those in Conception Bay in the Namib desert, seen here from 283 km altitude in NASA crew image STS107-E-5380 (lower part of image). NASA’s Cassini spacecraft mission — still out there sending data from the Saturnian system — has revealed extensive aeolian (wind-formed) dunes on the surface of Titan, Saturn’s largest satellite. Devon Burr et al. used a high-pressure wind tunnel to simulate the thick near-surface atmosphere on Titan and, with numerical modelling of the low gravity and low sediment density, derived the wind speeds necessary to move dune sand on Titan. These speeds are significantly higher than those predicted by present models of sediment entrainment by wind that are based on wind-tunnel experiments under conditions relevant for Earth and Mars. Experimental results and theoretical work can be reconciled if the extremely low ratio of particle to fluid density on Titan is taken into account, a correction that is not required for high density ratio environments such as jets on comets. Cover: NASA/JPL-Caltech/ASI/ Johnson Space Center

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