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Moonshadows Seen on Mars
By Greg Clark
Staff Writer
posted: 03:21 pm ET
02 November 1999
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mars-moonshadowNewly-released images from the camera aboard the Mars Global Surveyor show the shadow of Mars' tiny moon Phobos cast on the red dust of the planet's surface. 
These images, taken by the spacecraft's Mars Orbiter Camera, show the martian terrain at red wavelengths (left panel), blue wavelengths (center), and in a color composite (right). The shadow in the center is that cast by Phobos. Each of the panels covers an area some 155 miles (250 kilometers) across. No one is certain what causes the dark spots at the bottom of the three craters, though some have suggested that the spots may be dark fields of sand dunes. Solar eclipses on Mars are thousands of times more frequent than on Earth. One of Mars' two small moons casts its shadow across the planet a few times each day. In contrast, a partial or total solar eclipse can be seen from Earth only once or twice a year. The main reason for this frequency is that Mars' satellites are so close to the planet. Even though the irregularly-shaped Phobos is just 8 miles long and 6 miles wide, the moon orbits just 3,700 miles (6,000 kilometers) above Mars' surface. Deimos -- only slightly over half the size of Phobos -- orbits about 12,500 miles from Mars. In contrast, our moon circles Earth at a distance of more than 200,000 miles. An observer on Mars would have a much greater chance of seeing an eclipse, but that viewer would never be able to witness a total solar eclipse. Because of its distance from Mars, the sun appears to be about two-thirds the size it looks from Earth, but Phobos and Deimos are not large enough to block out the whole solar disk. The eclipse images were produced by Malin Space Science Systems/JPL/NASA Below: The culprit, Phobos as seen during the Viking missions. Credit: NASA/JPL. 
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