CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Superimposed Craters

Superimposed Craters
PIA 12742

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  The two large craters on the right of this image are overprinted with smaller, more recent craters in this Cassini view of Saturn's moon Rhea.

Rhea, at 1528 kilometers, or 949 miles, across, is Saturn's second largest moon. This view is centered on terrain at 13 degrees south latitude, 132 degrees west longitude.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 17, 2010. The view was acquired at a distance of approximately 44,000 kilometers (27,000 miles) from Rhea. Image scale is 259 meters (849 feet) per pixel.

The Cassini Solstice 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 Solstice Mission visit, and

Credit: NASA/JPL/Space Science Institute
Released: November 29, 2010 (PIA 12742)
Image/Caption Information

Alliance Member Comments
Mercury_3488 (Nov 30, 2010 at 12:21 PM):
Rhea's average surface temperature is so low, approx minus 180 Celsius, that Rhea can hold onto an exosphere. It is not really an atmosphere per se as it is far too tenuous, approx one in five trillionth of the Earth's atmospheric pressure at sea level. However it is a concentration of Oxygen, I guess Atomic Oxygen rather than molecular Oxygen (O2) or Ozone (O3) & CO2. However the pressure of this exosphere is lower than a good laboratory vacuum.

I expect the oxygen is liberated from the ice by irradiation from Saturn's magnetosphere ( I suspect Rhea may also be emitting Hydrogen) & the CO2 may be from outgassing & /o or irradiation.

With Rhea it is not obvious as so far the surface appears to lack geological features other than a few faults. Unlike Dione & Enceladus, which certainly show 'recent' geological activity & in the case of Enceladus is certainly ongoing, Rhea appears to have had nothing happen other than get hit.

Images particularly from Voyager 1 & Cassini show craters upon craters & gravity data suggest an undifferentiated interior. Further work will have to be done to examine the data & see whether or not possible sources on the surface of the large icy moon can be found.

Also it has been suggested that the Uranus moons Titania & Oberon could have exospheres too, as they are both more massive than Rhea & Titania is slightly larger (Oberon is slightly smaller but is denser than Rhea). Unlike Rhea, both Titania & Oberon clearly show signs of having geological activity in the past from Voyager 2 imagery.

Andrew Brown.
raketenflugplatz (Nov 30, 2010 at 3:47 AM):
it's fantastic information about oxygen and carbon dioxide in Rhea's thin atmosphere! congratulations!