NASA's Messenger probe enabled researchers to find unexpected materials frozen in Mercury's north pole. Scientists think the materials arrived via comets or asteroids that hit millions of years ago.

Despite searing daytime temperatures, Mercury, the planet closest to the sun, has ice and frozen organic materials inside permanently shadowed craters in its north pole,NASA scientists said on Thursday.

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Earth-based telescopes have been compiling evidence for ice on Mercury for 20 years, but the finding of organics was a surprise, say researchers withNASA's MESSENGER spacecraft, the first probe to orbit Mercury.

Both ice and organic materials, which are similar to tar or coal, were believed to have been delivered millions of years ago by comets and asteroids crashing into the planet.

"It's not something we expected to see, but then of course you realize it kind of makes sense because we see this in other places," such as icy bodies in the outer solar system and in the nuclei of comets, planetary scientist David Paige, with theUniversity of California, Los Angeles, told Reuters.

UnlikeNASA's Mars rover Curiosity, which will be sampling rocks and soils to look for organic materials directly, the MESSENGER probe bounceslaserbeams, counts particles, measures gamma rays and collects other data remotely from orbit.

The discoveries of ice and organics, painstakingly pieced together for more than a year, are based on computer models, laboratory experiments and deduction, not direct analysis.

"The explanation that seems to fit all the data is that it's organic material," said lead MESSENGER scientistSean Solomon, withColumbia University in New York.

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NASA surprised to spot ice on Mercury