[meteorite-list] Meteorites from Mercury?

[Darren Garrison]

Meteorites from Mercury?
February 15, 2008
by Ken Croswell

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. 

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 should already be sitting in meteorite
collections worldwide. 

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).