22-12-10 DLR researchers compile atlas of Saturn's moon Rhea,an icy alien world

Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt;DLR) have compiled an atlas of Saturn's second largest moon,Rhea,which has been published by NASA today.The atlas includes a number of high-resolution images and a three-dimensional view of fractures on the icy world acquired by the Cassini spacecraft,which has been orbiting Saturn for six and a half years.The data was obtained during two fly-bys in September 2009 and March 2010,from distances as close as 100 kilometres.The atlas comprises the most detailed images of Rhea to date and provides an insight into the geological development and surface composition of the moon.Recently,two instruments on Cassini discovered a thin oxygen-carbon dioxide atmosphere,or exosphere,on Rhea.Of particular interest are a number of long,usually linear structures over 100 kilometres long,that are winding in some places.The origin of these thin lines was a mystery for a long time."The high-resolution image data from Cassini was the first to reveal that the bright lines are actually tectonic in origin,meaning that they are fractures in the ice crust,created during sudden releases of crustal stress," explains Roland Wagner from the DLR Institute of Planetary Research (Institut für Planetenforschung;IPF).They may have arisen during periods of extensional and shearing tectonic activity."By correlating the high-resolution images with spectroscopic data,we found that the conspicuous brightness of these lines is caused by the exposure of almost pure water ice on the escarpments of these tectonic structures," explains the planetary geologist.The tectonic graben on Rhea are up to four kilometres deep."This means that there must have been an enormous amount of stress in the brittle crust of the Moon in the past," adds Wagner."We can only speculate as to the cause of the stress." Rhea has a diameter of 1528 kilometres and orbits Saturn at more than 500,000 kilometres from the planet's centre.With an average density of 1.23 grams per cubic centimetre (a little denser than water,but less than a quarter of Earth's mean density),it appears that Rhea is a largely homogeneous body consisting of two-thirds water ice and one-third rock.With these latest observations,it is evident that Rhea is a cousin of Dione,the adjacent Saturnian moon,given the similar nature of their surfaces.These two moons,are almost like twins,similar even under their icy crust in many ways.