The High Resolution Imaging Science Experiment, or HiRISE, run from The University of Arizona, has produced new images that show subtle color differences across the smooth surface of Deimos, the smaller and outer tiny moon of Mars.

The HiRISE camera aboard NASA's Mars Reconnaissance Orbiter captured two images of Deimos using near-infrared, red and blue-green filters on Feb. 21, 2009. The first image was taken in the early afternoon, and the other was taken five hours and 35 minutes later.

The images can be viewed on the HiRISE Web site.

"The subtle color variations – redder in the smoothest areas and less red near fresh impact craters, ridges and high places – are probably the result of exposure to the space environment," said HiRISE principal investigator Alfred McEwen of the UA's Lunar and Planetary Laboratory.

Except for recent impact craters, Deimos' surface is remarkably smooth. The diminutive moon – the tiniest moon of any inner solar system planet – is smooth because it is covered by a blanket of fine rock fragments, or regolith, that was possibly blasted out by impacts and then swept up as the moon traversed its orbit.

Deimos' surface darkens and reddens with prolonged exposure to space environment, McEwen said. The smooth, undisturbed surface is therefore darker and redder, while recent craters and places where regolith has moved downslope are brighter and bluer.

Deimos is 12 kilometers, or about seven-and-one-half miles, in diameter – a distance that can be driven in less than 10 minutes over interstate highway. Like Mars' other moon, Phobos, Deimos is too small for gravity to pull it into a sphere.

HiRISE took color images of Phobos on March 23, 2008. They also are availbale for viewing on the HiRISE Web site.

Both moons are of great interest because they may be rich in water ice and carbon-rich materials, McEwen said.

The HiRISE images are helping scientists understand the origin and evolution of the moons, McEwen said.