Much of what we know about Mercury was discovered through flybys of the planet during the Mariner 10 space mission in the mid-1970s. But Mariner photographed only half the planet's surface, and many questions remained. So in 2004, NASA launched a spacecraft called Messenger to Mercury. The newest data from Messenger confirms the existence of water ice hidden in the craters of Mercury's polar regions.
Mercury would seem to be an unlikely place to find ice. Temperatures on the planet's surface can reach a scorching 800 degrees Fahrenheit. However, the tilt of Mercury's axis is almost zero, so there are pockets at the planet's poles that never receive the warmth of the sun. These areas can drop to a very cold minus 280 degrees Fahrenheit. Planetary scientists believed decades ago that water ice might be trapped in those shadowed craters at Mercury's poles.
The theory of water ice on Mercury received support in 1991, when the Arecibo radio telescope in Puerto Rico detected unusually bright radar patches at Mercury's north and south poles. These spots reflected radio waves in the way scientists would expect if there was water ice present. Many of these bright radar patches matched the location of large impact craters mapped by the Mariner 10 spacecraft two decades earlier. Scientists, however, weren't sure if the bright areas detected by Arecibo were actually located in the shadowy places of the craters.
Messenger's arrival at Mercury in 2011 finally unveiled the mystery of the existence of water ice on Mercury. Images from the spacecraft's Dual Imaging System, taken in 2011 and in early 2012, show that the bright radar features at Mercury's poles are indeed within the shadowed regions on the planet's surface.
The new data indicates that water ice in Mercury's polar regions, if spread over an area the size of Washington, D.C., would be more than 2 miles thick.