This montage shows four major icy moons of Saturn that the Cassini
spacecraft 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 been 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), that is 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 that produced the pronounced equatorial bulge on Iapetus. This
feature was imaged for the first time by Cassini during a flyby of Iapetus
that began New Year's Day. The bulge on Iapetus reaches 20 kilometers (12
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. For example, why small Enceladus (505
kilometers, or 314 miles across) is presently geologically active while
much larger Rhea (1,528 kilometers, or 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 mission for NASA's Science Mission Directorate, Washington, D.C. The
Cassini orbiter and its two onboard cameras were designed, developed and
assembled at JPL. The imaging operations center is based at the Space
Science Institute in Boulder, Colo.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at
http://ciclops.org.