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How Can There Be Water Ice on Mercury, Which Is Right Next to the Sun?

Megan Garber

A hot planet, NASA just announced, hosts cold -- very cold -- water.

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Ice, ice, baby! This 68-mile-diameter indented crater, located in the north polar region of Mercury, has been shown to harbor water ice. (AP Photo/NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Curiosity may not have found organic matter -- YET -- on Mars. But NASA has made, this afternoon, another big announcement: ice. Yes! Water ice. On Mercury.

The newly observed ice is located in the shadowed region of Mercury's north pole, and is thought to be, the AP reports, somewhere between 1.5 feet and 65 feet deep. The discovery, made by the orbiting probe Messenger, confirms "decades of suspicion" about the presence of ice on the planet.

But wait. If you recall the order of the planets -- the fact that My Very Excellent Mother Just Sent Us Nine Pies, except that now she has rescinded the pies -- you might be thinking, "That's weird. Mercury is the closest planet to the sun." Which means, basically, that NASA has discovered that the planet that receives more sunlight per square meter than any other in the solar system -- the planet that bears the brunt of the sun's energy more directly than any other -- houses ice. Which is ... counterintuitive.

But the cold-ice-on-hot-planet phenomenon isn't as strange as our solar system diagrams might have us believe: Proximity to the sun doesn't fully determine planetary heat. Venus, second rock from the sun, actually takes "hottest planet" honors. And that's because Venus, which has the second-closest orbit to the sun, also has an atmosphere -- something Mercury lacks. And an atmosphere, Universe Today explains -- particularly one, like Venus's, that is enriched with lots of carbon dioxide -- acts like a planetary blanket, trapping heat under its mantle.

Which isn't to say that Mercury isn't hot. Temperatures on the planet can reach a balmy 800 degrees Fahrenheit (427 degrees Celsius). But another thing that Mercury isn't is monolithic. Like Earth, its temperatures vary considerably according to, among other things, their orientation to the sun. And while the side of the planet that faces the sun can be sweltering, the side that faces away from it can drop to as low as 100 Kelvin, or -173 degrees Celsius. And the poles of the planet are permanently shielded from the sun's heat.

Ergo: ice. Lots of it, apparently. Evidence of large swaths of the substance is visible from a latitude of 85 degrees north up to Mercury's north pole, with smaller deposits scattered as far away as 65 degrees north. And there's a chance there's more where that came from: NASA, Messenger instrument scientist Gregory Neumann said today, will direct Messenger's observation toward that area in the coming months -- when, that is, the angle of the sun allows. There's a further good chance that there's ice on the south pole of the planet, too. For now, though, Neumann put it, "there is an ongoing campaign, when the spacecraft permits, to look further northward."

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