Pac-Man moon

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Pac-Man moon

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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.

http://www.nasa.gov/mission_pages/cassi ... 12867.html

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Subtle color differences on Saturn's moon Mimas are apparent in this false-color view of Herschel Crater captured by NASA's Cassini spacecraft during its closest-ever flyby of that moon.

The image shows terrain-dependent color variations, particularly the contrast between the bluish materials in and around Herschel Crater (130 kilometers, or 80 miles, wide) and the greenish cast on older, more heavily cratered terrain elsewhere. The origin of the color differences is not yet understood, but may be caused by subtle differences in the surface composition between the two terrains. False color images from Cassini's previous closest encounter, in 2005, also showed such variations (see PIA06257).

Herschel Crater covers most of the bottom of this image. To create this false-color view, ultraviolet, green and infrared images were combined into a single picture that exaggerates the color differences of terrain on the moon. These data were combined with a high-resolution image taken in visible light to provide the high-resolution information from the clear-filter image and the color information from the ultraviolet, green and infrared filter images.

The natural color of Mimas visible to the human eye may be a uniform gray or yellow color, but this mosaic has been contrast-enhanced and shows differences at other wavelengths of light.

During its closest-ever flyby on Feb. 13, 2010, Cassini came within about 9,500 kilometers (5,900 miles) of Mimas. This view looks toward the northern part of the hemisphere of Mimas that leads in the moon's orbit around Saturn. Mimas is 396 kilometers (246 miles) across. North on Mimas is up and rotated 12 degrees to the left.

http://www.nasa.gov/mission_pages/cassi ... 12572.html
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