Mercury has ice — lots of it, NASA says

Mercury is as cold as ice.

Indeed, Mercury, the closest planet to the sun, possesses a lot of ice, scientists working with NASA’s Messenger spacecraft reported Thursday.

Sean Solomon, 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, in 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 J. Lawrence, a senior scientist at the Johns Hopkins University Applied Physics Laboratory working on the Messenger mission.

The findings appear in three papers Thursday on the website of the journal Science. The ice could be a 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.

The Messenger spacecraft took a closer look by counting neutrons flying off the planet. Highenergy 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. Water molecules contain two hydrogen atoms, so when Messenger passed over ice-rich areas, the number of neutrons dropped.

The regions on Mercury where the neutrons tapered off corresponded with the bright areas, and computer models based on topographic maps show the craters are cold enough for water ice to be 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 Solomon.

The ice is almost pure water, which indicates that it arrived within the past few tens of millions of years, possibly from a comet. In slightly warmer regions, where temperatures exceed minus 280, the ice was covered by a dark layer. 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.