This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.


And I'm Melissa Block. If you were standing at the equator today, on Mercury, well, you'd have burned to a crisp. That's not surprising. Mercury is the closest planet to the sun and the surface temperature across most of the planet is hot enough to melt lead. What is surprising is that scientists now say, despite that scorching heat, there is ice on Mercury. NPR's Joe Palca explains.

JOE PALCA, BYLINE: Yes, Mercury is very close to the sun and, yes, most of the planet is extremely toasty. But it's a different story at the north and south poles of Mercury. Almost no sunlight reaches there and if you happen to be at the bottom of a crater near the pole, there's no sunlight at all. And as a result, it's cold - very, very cold. Now, if you were to peer inside one of these craters, you wouldn't see much because, as I said, there's no sunlight.

But if you had a flashlight, it would be a different story.

GREGORY NEUMANN: We have our own flashlight.

PALCA: That's because Gregory Neumann and his colleagues at the Goddard Space Flight Center in Greenbelt, Maryland, put one on NASA's Messenger spacecraft. Messenger has been orbiting Mercury since March 2011. Neumann's flashlight is known formerly as the Mercury Laser Altimeter.

NEUMANN: We can use it to measure reflectance in places where the imagers can't see anything because it's dark.

PALCA: And what they saw was something that looked very much like ice. To tell the truth, Neumann and his colleagues weren't all that surprised. Radar observations from Earth had predicted ice would be at the poles on Mercury and another instrument on Messenger also saw signals consistent with ice so that's what three papers and the current issue of the journal Science have concluded.

There's ice on Mercury. And that raises an interesting question. Where is the water for making the ice coming from? Well, there are a couple possibilities.

NEUMANN: It could be coming from the interior, because every planet contains a little bit of water still in the mantle.

PALCA: But Neumann says that's not likely because scientists can't think of any way that the water trapped in the mantle could make it to the surface.

NEUMANN: Or it could be coming from outside.

PALCA: Outside. What does he mean by outside?

MARIA ZUBER: Mercury gets bombarded periodically by comets.

PALCA: Maria Zuber of the Massachusetts Institute of Technology is another scientist on the Messenger mission. Zuber says comets are sometimes referred to as dirty snowballs, since they're made of organic dirt and frozen water. Both Zuber and Neumann are pretty sure the ice they're seeing on Mercury came from comets. Now, remember, comets are water and dirt.

So not only is water brought to Mercury...

ZUBER: The organics get deposited on the surface, as well.

PALCA: And Zuber's colleague Greg Neumann says Messenger's flashlight was able to detect organics, too.

NEUMANN: They're kind of a carbonaceous, tarry substance that we call goo, to use a technical term.

PALCA: So Mercury has ice and goo. Who knew? Joe Palca, NPR News, Washington.

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