This montage shows four major icy moons of Saturn that Cassini visited while surveying the Saturnian system during 2005. Even though all of these bodies are made largely of ice, they exhibit remarkably different geological histories and varied surface features.
Craters from meteorite impacts are common features on all of these moons. But since the major moons of Saturn are thought to have all formed at approximately the same time, the different distribution of sizes, shapes and numbers of craters on each of their surfaces tell scientists a great deal about the differences in their geologic histories.
Rhea and Iapetus are thoroughly peppered by impacts, suggesting their surfaces have lain exposed to the shooting gallery of space for eons. Dione appears to have regions of terrain that are smoother, with fewer craters suggesting a slightly younger surface. Dione also has a large system of bright, braided fractures that suggest tectonic activity took place there some time after the moon first formed.
Enceladus, however, possesses a region of terrain near its south pole (shown here) so dramatically devoid of impact sites that scientists suspected it was geologically active in the recent past, and perhaps even today. The discovery this year of material jetting from the pole and creating a great plume of icy particles confirmed these suspicions. (See PIA07758 for images of the Enceladus plume.)
The processes that power the activity on Enceladus remain elusive, as do those which produced the pronounced equatorial bulge on Iapetus. This feature was imaged for the first time by Cassini during a flyby of Iapetus that began the New Year. The bulge on Iapetus reaches 1,471 kilometers (914 miles) above the surrounding terrain in places, making it one of the tallest features in the Solar System.
Like many scientific journeys, Cassini's historic survey of Saturn's moons has raised more questions it has answered; for example, why small Enceladus (504 kilometers, 313 miles across) is presently geologically active while much larger Rhea (1,528 kilometers, 949 miles across) is not. Fortunately, such puzzles are the most exciting sort for scientists interested in uncovering the secrets of Saturn's realm.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.