Meteorites from Mercury?

Meteorites from the Moon and Mars give earthbound scientists free rock samples from other worlds. Now Brett Gladman and Jaime Coffey (University of British Columbia, Vancouver) say we should expect a few meteorites from Mercury too.

Taken by Messenger on January 14, 2008, from about 17,000 miles away, this view of a gibbous Mercury shows about half of the area not photographed by Mariner 10 in 1974–75. The heavily cratered landscape is reminiscent of other areas previously seen. The broad circular plain toward upper right, appearing brighter than its surroundings in this red-light view, marks the interior of Caloris basin, a huge impact scar more than 800 miles across.
NASA / JHU-APL / Carnegie Inst. of Washington
Gladman and Coffey conducted computer simulations of what happens after asteroids and comets slam into the innermost planet and kick debris into space. Past studies assumed that rocks knocked off Mercury weren't getting away with much more than its escape velocity of 2.6 miles (4.2 km) per second. That's too slow to climb away from the Sun and make it out to Earth.

But some previous assumptions were wrong, says Gladman, because the collisional circumstances at Mercury are "very different than anywhere else." The Sun's innermost planet speeds through space with a mean velocity of 30 miles (48 km) per second. Furthermore, asteroids and comets crossing Mercury's orbit also travel fast. So impactors strike the planet at speeds 5 to 15 times its escape velocity, and ejecta can rocket off the surface traveling much faster than had been assumed.

The new study, which has been submitted to Meteoritics and Planetary Science, concludes that up to 5% of this high-speed debris from Mercury reaches Earth — a third to a half of the delivery rate of meteorites from Mars. Gladman notes that roughly a half dozen samples of Mercury might already be sitting in meteorite collections worldwide.

Rocks from Mercury? These pieces of the meteorite known as Northwest Africa 2999 are angrites, a rare type that some specialists believe may represent samples of Mercury's surface. The total weight of these is 392 grams (about 14 ounces); the small cube is 1 cm on a side.
But how would an interplanetary prospector recognize that a stone really is from the innermost planet? Some planetary geologists think a rare class of meteorites called angrites might be good candidates, though others disagree. Gladman cautions, "Until you have some kind of ground truth, it's very difficult to make those claims." He says scientists need more information about the composition of Mercury's surface to find matches with suspicious meteorites.

Fortunately, the Messenger spacecraft has begun exploring the planet. Messenger flew past Mercury in January and will go into orbit in 2011. It should provide the data that will confirm or refute candidate meteorites from Mercury.

Ken Croswell is the author of Ten Worlds: Everything That Orbits the Sun (Boyds Mills Press, 2006).