CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Thermal Runaway Model

This series of maps of Saturn's moon Iapetus shows how a computer model of migrating ice can explain most features of Iapetus' global appearance. The model starts with Iapetus uniformly covered in ice with some dark material mixed in, as shown in the top map. Additional dark material is then slowly added to the leading hemisphere (centered at 90 degrees west longitude). 260 million years later, the leading side has darkened and thus warmed slightly, and ice has begun to evaporate near the equator where temperatures are highest. The ice evaporation leaves dark material behind and thus darkens the surface further.

After 1,200 million years, a large dark region that is completely ice-free has developed on the leading side. By 2,400 million years the dark region has grown to closely resemble the size and shape of the dark region on the global mosaic map of Cassini images of the real Iapetus, shown in the bottom panel.

The Cassini Equinox Mission is a joint United States and European endeavor. 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 was designed, developed and assembled at JPL. 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.

For more information about the Cassini Equinox Mission visit, and

Credit: Southwest Research Institute/NASA/JPL/Steve Albers, NOAA
Released: December 10, 2009
Image/Caption Information
  Thermal Runaway Model
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Alliance Member Comments
mipsandbips (Dec 22, 2009 at 6:15 PM):
thanks carolyn, given that you are correct, of course, 'thermal' would not necessarily mean a volcanic eruption. Interesting also that you mention, in how the dust ring about Phoebe being of keen interest as well, I imagine that particular interest may be for a number of reasons also!
carolyn (CICLOPS) (Dec 13, 2009 at 8:16 AM):
mipsandbips: But there's no evidence for a massive volcanic explosion. If that were the case, you'd see craters that were partially buried. And the pattern wouldn't necessarily look like what we see. Computer models have verified that general outline you would see from a dust-initiated thermal runaway effect, and they agree beautifully with the observed pattern. Finally, a dust ring has just been found associated with Phoebe, an outer Saturnian satellite. That dust is making its way into Iapetus' orbit. So all holds together beautifully.
mipsandbips (Dec 12, 2009 at 7:37 PM):
I'm not convinced that the thermal model of migrating ice is a valid explanation. It seems more plausible, based on the imagery, that there was a massive volcanic eruption which jettied massive amounts of ejecta material that covered the icy surface. If this were so, then it is doublful that the process of the evolution of dark material deposits occurring over millions of years is valid either.
carolyn (CICLOPS) (Dec 11, 2009 at 5:13 PM):
rochelimit: Very clever way of looking at it. Bravo!
rochelimit (Dec 11, 2009 at 4:16 AM):
Very interesting, to think about it, it almost like Iapetus has 3 icy poles, the North icy pole, the South icy pole, and the trailing side icy pole :)