March 14, 2006
Saturn’s E ring, the largest ring around any planet in our solar system, is not only remarkable for its huge size. What is also astounding is that current optical measurements show it to be made of ice particles of nearly equal size - a radius of 0.3 to 2 micrometres. The icy moon Enceladus was, however, expected to feed the ring with significantly larger pieces. The ring’s mass distribution must thus somehow be tied up with the dynamics of the particles. Until now, however, scientists did not know how. Directly measuring ice particles near Enceladus promised to deliver a better understanding of the E Ring’s complex nature.
On July 14, 2005, the CASSINI space probe came within 175 kilometres of Enceladus, which is suspected of being the source of Saturn’s E Ring. This made it possible to measure dust distribution deep inside the area of Enceladus’ gravitational pull. The scientists could thus investigate how the moon refreshed the ring with dust particles.
Until now, it was assumed that new ice particles were created by interplanetary micrometeorites, or ring particles themselves, bombarding the moon’s surface. In this model, most of the fresh particles form a nearly isotopic dust cloud around the moon. In the other model, faster particles feed the ring. Indeed, the dust detector on the space probe GALILEO had already discovered dust clouds surrounding Jupiter’s Galilean moons.
Measurements from the High Rate Detector (HRD) of CASSINI’s "Cosmic Dust Analyser" (CDA), taken by scientists from the Max Planck Institute for Nuclear Physics in Heidelberg and the University of Chicago are however not consistent with this assumption. Observations showed that the maximum rate of impact was reached before the probe moved to its closest point to Enceladus. This can only be explained if there exists a strong anisotropic - that is, directionally dependent - dust distribution on the moon.
The dust detector was not the only instrument which came to show unexpected discoveries. Photographs of Enceladus’ southern polar region clearly showed geologically young structures. Infrared images also indicated a warmer area - a "hot spot" - in that southern region. This led scientists from the University of Potsdam to model dust distribution on the moon under the assumption that there is an additional, tightly confined, dust source on its surface. The models indicated a contained dust source in the southern polar region. This turned out to be consistent with the probe’s measurements.
This convincing match between models and HRD measurements (see image 2) led the Cassini camera team to look for, and find, volcanic activity. Enceladus is thus the second moon in our solar system where volcanic activity has been discovered