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Water ice on the planet Mercury: How NASA's Messenger Spacecraft is discovering more about the tiny planet.

Friday, August 2, 2013, 8:29 PM -

It has been two years since Messenger first started orbiting Mercury. Not only is it the first spacecraft to accomplish that feat, but in that time it has successfully imaged 100 percent of the planet's surface. 

Traditionally, Mercury has been known as the very hot, small planet closest to the Sun in our solar system. It has almost no atmosphere, and looks very much like the Moon. But its cratered surface sees huge temperature extremes. By day, Mercury is an inferno with temperatures nearing 430 degrees celsius along the equator. At night, it can plummet to minus 170. 

Water can't exist under those conditions, but it can at the poles. Scientists believe that's where the planet may have abundant stores of water ice. Mercury's axis has only a slight tilt, one-thirtieth of a degree compared to 23 for the Earth. As a result, there are pockets of Mercury's north and south poles that are always dark. 

The idea of water ice on Mercury isn't new, scientists have been suggesting that for decades. In 1991, the Arecibo Radio Telescope in Puerto Rico beamed radar signals toward the planet. The reflected radio waves sent back images of bright patches in the polar regions, typical of water ice. More recently, similar images were taken by the Mercury Dual Imaging System aboard the orbiting Messenger spacecraft. 

Messenger was launched in August 2004, but only formally began collecting data on the planet's geology, magnetic field and chemical composition in April 2011.

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