The hypothesis that Mercury has abundant water ice and other frozen volatile materials in its shadowed polar craters appears to have been confirmed.

NASA announced today that new measurements from its Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft provide “compelling support” for the water ice theory. The space craft measured the excess hydrogen at Mercury’s north pole, the reflectance of Mercury’s polar deposits, and provided detailed models of the surface temperatures, as well as surface temperatures in the planet’s north polar regions. The data has been published today in the journal Science, through the ScienceXpress online pre-publication service.

“For more than 20 years, the jury has been deliberating whether the planet closest to the sun hosts abundant water ice in its permanently shadowed polar regions,” said Sean Solomon, MESSENGER principal investigator and director of Columbia University’s Lamont-Doherty Earth Observatory. “MESSENGER now has supplied a unanimous affirmative verdict.”

The image above highlights the shadowed areas of Mercury’s north polar region taken by MESSENGER (in red) and Earth-based radar (in yellow).

“The new data indicate the 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,” said David Lawrence, lead author of one of three papers and a MESSENGER participating scientist at the Johns Hopkins University Applied Physics Laboratory (APL). “We estimate from our neutron measurements the water ice lies beneath a layer that has much less hydrogen. The surface layer is between 10 and 20 centimeters [4-8 inches] thick,”

Though Mercury’s (short) distance from the sun make it seem an unlikely place to find ice, the planet’s rotational axis tilts less than one degree. This means there are pockets of the planet’s surface that are never hit by sunlight. The idea that these pockets might hold ice is decades old, and the hypothesis came to prominence in 1991 when the Arecibo radio telescope detected radar-bright patches at Mercury’s poles.

(Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory)