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In addition, scientists suspect impacts as well as solar and cosmic radiation are triggering chemical reactions in the organic material, turning it about twice as dark as Mercury's surface.

"Is there chemistry going on in some portion of the subsurface where liquid water is present, organic material is present and there’s the equivalent of geothermal energy? I think that's something we need to pursue," Solomon said.

Even if Mercury isn't a good candidate to look for ancient life, the planet may hold clues about how life got started on Earth.

"Finding a place in the inner solar system where some of these same ingredients that may have led to life on Earth are preserved for us is really exciting. That may give us some insights into what may have happened way-the-heck-back there on Earth, where all the evidence is pretty much gone," Paige said.

The discovery also is relevant to the search for habitable planets beyond the solar system.

"It could be that it's not just a question of having planets in a habitable zone. Maybe you need all these other ices and organics out there in the outer solar system to seed the planet, to provide the raw the material," Paige said.

MESSENGER is due to wrap up its two-year mission in March, though scientists are hoping for additional funds from NASA to continue for at least part of a third year.

"The organics are high on the list of follow-on targets," Solomon said.