Messenger Detects Water On Mercury
Being the closest planet to the sun, you would think Mercury would be a pretty hot place, but the Messenger probe has detected a massive deposit of frozen water on the surface of the planet:
Mercury is as cold as ice.
Indeed, Mercury, the closest planet to the Sun, possesses a lot of ice — 100 billion to one trillion tons — scientists working with NASA’s Messenger spacecraft reported on Thursday.
Sean C. Solomon, the principal investigator for Messenger, said there was enough ice there to encase Washington, D.C., in a frozen block two and a half 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 J. 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 Web site 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.
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 break 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 as the cue ball in billiards transfers its momentum when it hits another ball. Water molecules contain two hydrogen atoms, and thus when Messenger passed over ice-rich areas, the number of neutrons dropped.
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. Dr. Solomon said several young craters on the surface of Mercury could be candidates for such an impact.
Could there possibly be organic compounds in that frozen water? If a comet was the source of the water as theorized, then it’s certainly a possibility. Perhaps someday we’ll find out.