After three and a half years in orbit around Mercury the tiny shriveled and pockmarked orb that boasts temperatures in excess of 650 degrees NASAs MESSENGER mission to our solar systems innermost planet is headed for a controlled late March 2015 surface impact.

The spacecraft is getting closer to the planet than weve ever been which helps our [surface] composition measurements a lot, said David Lawrence, a MESSENGER science team member at Johns Hopkins Universitys Applied Physics Lab (APL). Between now and impact

Until the $450 million MESSENGER, Mercury rarely registered as a blip on most researchers radars. But scientists are now in the process of sorting through a plethora of new data about the planet.

Artists rendering of the MESSENGER spacecraft orbiting Mercury (Credit: Wikipedia)

Mercury is an oddball planet of contradictions, said David Blewett, a MESSENGER science team member also at APL, which NASA contracted to build and operate the mission. It has a huge iron core, but the surface rocks are almost totally bereft of iron. Mercury is the planet closest to the Sun, yet it has a polar cap in the form of ice(s) lurking in the eternal darkness of polar craters.

And although Mercury has no apparent current tectonic or volcanic activity; in its distant past, after formation and cooling; the planet did shrink in a manner not unlike a shriveled orange.

Mercurys surface also has hundreds of surface hollows no deeper than a km, says Lawrence, who notes that they cover the planet. You dont see these hollows on any other planet in the solar system, he said. Theyre not craters and they dont look like volcanoes.

Blewett says they have shapes similar to certain depressions found on the South polar cap of Mars Mars, who points out that as the Sun warms the Red Planets surface, these Martian features form from the transformation of carbon dioxide from ice and into a gaseous state. It appears that a similar process is happening on Mercury, says Blewett, but instead of ice being lost, it is solid rock.

As the planet closest to the Sun, however, Lawrence said he would have expected such volatile elements, like sodium, chlorine and sulfur, to have already been baked off the surface. Yet he notes that surprisingly thats not the case.

But compared to the Moon, Mercury doesnt have a lot of compositional surface variability, said Lawrence. In visible light, Mercury is kind of dark gray, bland and uniform.

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NASA's MESSENGER Spacecraft Headed For Mercury Impact