Saturn moon Rhea may have its own rings. Image: NASA
An international team of astronomers has found evidence of a ring system around the Saturn moon Rhea. It probably has the shape of a flat disk and consists of dust and rocks up to a meter in size. The group around Geraint Jones from the Max Planck Institute for Solar System Research in Katlenburg-Lindau concludes this from measurements of the spacecraft Cassini, which records the particle flow within the Saturn rings. In the area around Rhea, the amount of measurable charged particles dropped significantly, which is why astronomers assume that a dust disk around the moon captures the particles. Rhea would be the first moon with its own ring system. Rhea lies within the so-called magnetosphere of Saturn, that is within the area, which is influenced by the magnetic field of the planet. This field contains many electrons and ions, charged particles that are held captive by Saturn's magnetism. The particles travel on the magnetic field lines of the magnetosphere from the north to the south pole of the planet and back, astronomers suspect. If the ions encounter obstacles such as dust rings or moons, they are caught by them like a vacuum cleaner. It was exactly this phenomenon that the Cassini spacecraft recorded in 2005 with the help of an electron detector called MIMI / LEMMS and sent the data back to Earth. As expected, an almost complete cessation of electron flow could be seen in the area of ​​Rheas. What astounded the astronomers around Jones, however, was the fact that not only behind the moon, but also in the metropolitan area, the particle flow significantly decreased.

With no absorbing layer of gas around the moon, Jones and his team suspected that the explanation would be only a build-up of dust that prevents the electrons from orbiting. They could support this assumption by further data Cassinis, since the dust catcher of the probe (CDA) had measured a much higher dust density around the moon than usual for the Saturn rings usual, however only in certain ranges. "We expected dust there, " remarks co-author Sascha Kempf, "but not in the form of a disk or even rings." Model calculations by astronomers have also shown that the formation of a small dust disk within the large dust disk around Saturn is physically possible and this even relatively stable. What is missing as the last proof, is only a photo of the moon rings.

Geraint Jones (Max Planck Institute for Solar System Research, Katlenburg-Lindau) et al .: Science, Vol. 319, p. 1380 ddp / science.de? Livia Rasche

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