Mercury is as cold as ice.
Indeed, the closest planet to the sun possesses a lot of ice, scientists working with NASA's Messenger spacecraft reported Thursday.
Sean Solomon, the principal investigator for Messenger, said there was enough ice there to encase Washington, D.C., in a frozen block 21/2 miles deep.
That is a counterintuitive discovery for a place that also ranks among the hottest in the solar system. At noon at the equator on Mercury, the temperature can hit 800 degrees Fahrenheit.
But near Mercury's poles, deep within craters where the sun never shines, temperatures dip to as cold as minus 370.
"In these planetary bodies, there are hidden places, as it were, that can have interesting things going on," said David Lawrence, a senior scientist at the Johns Hopkins University Applied Physics Laboratory working on the Messenger mission.
The findings appear in a set of three papers published Thursday on the website of the journal Science. The ice could be an intriguing science target for a future robotic lander or even a resource for astronauts in the far future.
Planetary scientists had strong hints of the ice a couple of decades ago when telescopes bounced radio waves off Mercury and the reflections were surprisingly bright. But some researchers suggested the craters could be lined with silicate compounds or sulfur, which might also be highly reflective, and not water ice.
The Messenger spacecraft, which swung into orbit around Mercury in March 2011 and has completed its primary mission, took a closer look by counting particles known as neutrons that are flying off the planet. High-energy cosmic rays slam into the surface, breaking apart atoms, and the debris includes neutrons.
But when a speeding neutron hits a hydrogen atom, which is almost the same weight, it comes to almost a complete stop, just like in billiards when the cue ball transfers its momentum to another ball. Water molecules contain two hydrogen atoms, and thus when Messenger passed over ice-rich areas, the number of neutrons dropped.
The same technique was used to detect frozen water below the surface on Mars and within similar craters on the moon.
The regions on Mercury where the neutrons tapered off corresponded with the bright areas, and computer models based on topographic maps show that the craters are cold enough for water ice to remain there indefinitely.
The neutron number would not have dropped if the bright surfaces were made of sulfur or silicates.
"Water ice is the only candidate we've got that fits all those observations," said Mr. Solomon, who is also director of Columbia's Lamont-Doherty Earth Observatory.
The ice is almost pure water, which indicates that it arrived within the last few tens of millions of years, possibly from a comet that smacked into Mercury. Mr. Solomon said several young craters on the surface of Mercury could be candidates for such an impact.
Not all of the icy regions were bright. In slightly warmer regions, where temperatures exceed minus 280, the ice was covered by a dark layer about half a foot thick. The scientists believe in these places the water ice vaporized, leaving behind other materials that had been trapped, including carbon-based molecules known as organics.
That could be similar to how water and the building blocks of life reached Earth billions of years ago.
The water could also be an intriguing resource for people. Between the scorched equator and the frozen poles, temperatures on Mercury can be temperate, especially a few feet below the surface, where the soil insulates against the temperature swings between day and night -- an ideal location to build a colony.
"People joke about it, but it's not so crazy, really," said David Paige, a professor of geology at UCLA who calculated the crater temperatures.science