The north pole of Mercury as seen by MESSENGER during a 2008 flyby. Credit: NASA
NASA dispatched the Curiosity rover to Mars to look for organics, but a probe orbiting Mercury has beaten it to the punch.
New results from the MESSENGER spacecraft not only confirm that the planet closest to the sun has ice inside shaded craters near the north pole, but that a thin layer of very dark organic material seems to be covering a good part of the frozen water.
Both likely arrived via comets or asteroids millions -- or hundreds of millions -- of years ago.
BIG PIC: Dark Shadows Freeze Mercury's North Pole
Unlike Curiosity, which will be directly sampling Martian rocks and soils to look for signs of organic material, MESSENGER bounces laser beams and counts neutrons and gamma rays to collect information remotely about chemical elements on Mercury’s surface.
The information is correlated with detailed topographical and temperature maps, laboratory tests and computer models and then compared with candidate materials to find the best match for the observed conditions.
After years of painstaking work, scientists believe the most likely explanation is that the warmer parts of the shadowed craters contain black patches of organic material overlaying ice. The material, which is somewhat like tar, coal and soot, is believed to be similar to what has been observed on icy bodies in the outer solar system and in the nuclei of comets.
"It could easily have been delivered by comets and asteroids, along with water ice, and may even have darkened in the Mercury environment in response to the intense radiation and space weathering," lead MESSENGER scientist Sean Solomon, with Columbia University, told Discovery News.
"Certainly the darkness of the covering material was a surprise and the explanation that seems to fit all the data is that it’s organic material. It's quite interesting," Solomon said.
While there is no direct evidence the dark patches are organic materials, scientists say it is the only explanation that fits all of the conditions.
"The idea here is that this dark material was somehow cold-trapped along with the water and then the water sublimated (transformed from solid to vapor), leaving this dark deposit behind," planetary scientist David Paige, with the University of California, Los Angeles, told Discovery News.
"It's not just a crazy hypothesis and no one has got anything else that seems to fit all the observations better," Paige said.
PHOTOS: MESSENGER: First Photos from Mercury Orbit
The finding may have implications in the search for life beyond Earth. The chemistry for life, as we know it, requires organics, liquid water and a source of heat. While there is no hard evidence for liquid water on Mercury, it could exist inside the planet beneath the ice.
"It really depends on the thickness of the (ice) deposits and how long some of them have been around. As you go deeper into a planet it warms. Temperature increases with depth," Solomon said.
Over time, a barrage of impacts could have turned the surface, bringing ice to greater depths and closer to the temperatures needed to turn to liquid.
"There might be liquid water beneath some of these permanently shadowed regions," Solomon said.
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.
ANALYSIS: Mercury Not Too Hot For Polar Water Ice?
"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.
The research appears in a trio of papers in this week's Science.
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