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Cassini flyby at Rhea

June 11, 2013; Ralf Srama

The age of Saturn's main ring is still a mystery. An international team of scientists under leadership of the University Stuttgart did a significant step forward to solve this long lasting open question in planetary science.
The NASA spacecraft Cassini encountered Saturn's moon Rhea at a distance of 997 on March 9 at 18:17 (UTC) with a relative speed of more than 9 km per second. The goal of the Cosmic Dust Analyser onboard this mission was to monitor the submicron sized dust particles in the environment of Rhea. Dust clouds around icy moons were discovered by the Galileo spacecraft in the Jovian system 10 years ago. Interplanetary micrometeoroids bombard continuously the icy surfaces of atmosphereless bodies and sputter off ejecta particles. Some of these tiny fragments from the surface reach altitudes as high as 1000 km or beyond. By measuring these grains, scientists want to learn both: The dust particles carry the compositional information of the moon surface to outer space, and the abundance of the ejecta is coupled to the infalling interplanetary dust flux, respectively.
A determination of the interplanetary dust flux at Saturn is relevant for the contamination of Saturn's main ring. The degree of contamination by external dust particles provides therefore a method to answer the question about the age of Saturn's ring. The longer the period has been in which water ice particles were bombarded by "dirty" interplanetary micrometeoroids, the older is the ring.
The dust sensor CDA measured the dust flux during the Rhea flyby and the data were linked to Earth. Saturn's moon Rhea is embedded in an outer faint dusty ring, the E-ring. Due to its low dust density, this ring cannot be observed by optical instruments at this distance from Saturn. But the CDA sensor is 1000 times more sensitive than optical cameras which lead to a strong impact rate of E-ring particles during Rhea's flyby. Now, scientists have to unfold the dataset and to distinguish between normal E ring dust particles providing a background signal, and dust ejecta originating from Rheas surface.
The dust sensor was built in Germany by a combined effort of the DLR e.V. and the Max Planck Institute for Nuclear Physics in Heidelberg. The instrument investigation is funded by the DLR e.V. Since two years, the instrument is controlled by the institute of space systems at the University of Stuttgart (IRS). Major partners in this investigation are the University of Potsdam, the University Heidelberg, the University Oulu and the University of Colorado.