CAPE CANAVERAL, Florida (Reuters) - 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.

Earth-based telescopes have been compiling evidence for ice on Mercury for 20 years, but the finding of organics was a surprise, say researchers with NASA'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 the University of California, Los Angeles, told Reuters.

Unlike NASA's Mars rover Curiosity, which will be sampling rocks and soils to look for organic materials directly, the MESSENGER probe bounces laser beams, 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 scientist Sean Solomon, with Columbia University in New York.

Added Paige, "It's not just a crazy hypothesis. No one has got anything else that seems to fit all the observations better."

Scientists believe the organic material, which is about twice as dark as most of Mercury's surface, was mixed in with comet- or asteroid-delivered ice eons ago.

The ice vaporized, then re-solidified where it was colder, leaving dark deposits on the surface. Radar imagery shows the dark patches subside at the coldest parts of the crater, where ice can exist on the surface.

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NASA probe reveals organics, ice on Mercury