CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Bizarre Temperatures on Mimas

This figure illustrates the unexpected and bizarre pattern of daytime temperatures found on Saturn's small inner moon Mimas (396 kilometers, or 246 miles, in diameter). The data were obtained by the composite infrared spectrometer (CIRS) on NASA's Cassini spacecraft during the spacecraft's closest-ever look at Mimas on Feb. 13, 2010.

In the annotated version, the upper left image shows the expected distribution of temperatures. The white sun symbol shows the point where the sun is directly overhead, which is at midday close to the equator. Just as on Earth, the highest temperatures (shown in yellow) were expected to occur after midday, in the early afternoon.

The upper right image in the annotated version shows the completely different pattern that Cassini actually saw. Instead of the expected smoothly varying temperatures, this side of Mimas is divided into a warm part (on the left) and a cold part (on the right) with a sharp, v-shaped boundary between them. The warm part has typical temperatures near 92 Kelvin (minus 294 Fahrenheit), while typical temperatures on the cold part are about 77 Kelvin (minus 320 Fahrenheit). The cold part is probably colder because surface materials there have a greater thermal conductivity, so the sun's energy soaks into the subsurface instead of warming the surface itself. But why conductivity should vary so dramatically across the surface of Mimas is a mystery.

The lower two panels in the annotated version compare the temperature map to Mimas' appearance in ordinary visible light at the time of the observations. The map used to create this image is a mosaic of images taken by Cassini's imaging science subsystem cameras on previous flybys of Mimas. The cold side includes the giant Herschel Crater, which is a few degrees warmer than its surroundings. It's not yet known whether Herschel is responsible in some way for the larger region of cold temperatures that surrounds it.

The green grid shows latitudes and longitudes on Mimas at 30-degree intervals.

Cassini took 85 minutes to make the temperature map, as the spacecraft receded from Mimas. During that time, the distance to Mimas increased from 38,000 to 67,000 kilometers (24,000 to 42,000 miles) and the longitude of the center of Mimas' disk increased from 128 degrees west to 161 degrees west, due to the moon's rotation.

Because of this changing geometry, the alignment of the temperatures relative to specific features or coordinates on Mimas is shown only approximately. The temperatures were calculated from the brightness of the moon's infrared heat radiation, measured by CIRS at a wavelength of 12 to 16 microns, and are color coded according to the scale in the lower right of the annotated figure.

The unannotated version shows the imaging science subsystem visible-light mosaic of Mimas from previous flybys on the left. The right-hand image shows the new CIRS temperature data mapped on top of the visible-light image.

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 composite infrared Spectrometer team is based at NASA's Goddard Space Flight Center, Greenbelt, Md., where the instrument was built. 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.

Released: March 29, 2010 (PIA 12867)
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Alliance Member Comments
Harry (Apr 1, 2010 at 5:16 PM):
I totally agree with Red_Dragon. Cassini has been roaming around Saturn for years and is still making astounding discoveries. The Soltice mission is a must unless, cf course, the Pac-Man does not eat Cassini - ha ha! The triangular division is not really that straight since it is projected on a spherical surface. I find it amazing that the thermal gradient is so sharp; 20+ degrees in only a matter of 10s of miles. The impact that formed Hershel surely splattered melted ice. This could have resulted in modifying the surface thermal conductivity while filtering back under the surface. The surface appearance may not have changed appreciably. Planetary physics modeling must be a real blast - ha ha!
mipsandbips (Mar 30, 2010 at 9:14 PM):
The varying degree temp inside the Herschel Crater very possibly holds the key to why the border between the colder and warmer regions form a triangular division. Possibly due to the impact depth of the Herschel Crater where an outer interior core had been breached explaining why there is a warmer region in that section of the crater.
jonathanjay (Mar 29, 2010 at 6:19 PM):
thermal variation almost certainly related to Herschel crater - possibly ejecta. I suspect the 'v' boundary will be shown to be slight variation in ejecta pattern.
JimRinX (Mar 29, 2010 at 4:21 PM):
Well, well. We seem to have a 'too hot' moon problem, don't we?
A theory regarding Enceladus was in New Scientist last week; it specualted that there was some kind of 'inversion' going on, in which deeper, warming bits of the moon 'burst upward' (or something like that) every few million years, bringing deep warmth to the surface, and powering the geysers.
I'm still hopin it's a burried Obilysk - or, at least, a nice interstellar space ship with the engine 'idling', while it waits for us to develope a REAL Space Program.
Red_dragon (Mar 29, 2010 at 2:45 PM):
At the very first (Voyagers), we had a moon that looked like the "Death Star". Now, with Cassini the Death Star has become Pacman's land; it seems Mimas "loves" the best decade ever -the 80's-.
Jokes apart, this is one of the reasons why to have a Solstice mission: to continue unlocking the many secrets the saturnian system has. As sustayne and NeKto, can't wait to see what surprising explanation you'll have to explain that. Excellent work and keep it up!.
sustayne (Mar 29, 2010 at 1:52 PM):
Pacman LIVES! (I can't be the first one to see this here.) Man, I will be hanging on the edge of my seat waiting for ANY plausible science that explains a temp distribution like this one.
NeKto (Mar 29, 2010 at 1:14 PM):
Well here we go, another facinating data set with completely unanticipated information. this certainly warrants further investigation. can't wait to see where this leads.
i'll bet this made for some fun at CICLOPS when it came down. wish i could work there!