During a recent science conference discussing Messenger's results from Mercury, investigator Shoshana Weider (Carnegie Institution of Washington) commented, "Short of landing on the surface, picking up a rock, and bringing it home, the instruments on Messenger that characterize chemistry are the best we're going to get."
Well, Shoshana, you might still get to hold such a rock someday.
According to a 2008 analysis by Brett Gladman and Jaime Coffey (University of British Columbia), chunks of Mercury should be lying somewhere on Earth right now. The dynamicists conclude that 2% to 5% of the debris blasted by impacts off the surface of Mercury at or above escape velocity (2.6 miles per second) should reach Earth within 30 million years.
Their numbers suggest that Mercurian meteorites should be roughly one third as common as those from Mars, for which the count now stands at 60. Gladman conservatively suggests that at least a half dozen stones should be lying around somewhere on terra firma.
Meteorite collectors would value a Mercurian meteorite above all others, likely fetching $5,000 or more per gram, so they've been on the lookout for one. A few years ago, meteoriticists had speculated that the best existing match to Mercury were a rare handful of ancient, basalt-rich stones known as angrites.
But even before Messenger's arrival, ground-based astronomers had concluded that Mercurian surface rocks contained very little iron — strange indeed, given that the innermost planet has an iron core that takes up 80% of its diameter and more than half of its volume!
"At that time," comments geochemist David Blewett (Applied Physics Laboratory), "people were expecting Mercury to have a composition more like a lower-iron version of the lunar highlands. We now know that it's much different than that." After nearly a yearly scrutinizing the planet from orbit, Messenger has confirmed that iron is in short supply at the surface.
Instead, the compositional clues suggest that a Mercurian meteorite would be an igneous rock — or perhaps a fused breccia of different rock types — rich in magnesium and volatile elements (especially sulfur and potassium). This closely matches the composition of another rare meteorite group, the aubrites. Also known as enstatite achondrites, aubrites are igneous rocks dominated by the iron-free mineral enstatite (Mg2Si2O6).
But aubrites aren't from the innermost planet. For one thing, they're too reflective — anything coming from Mercury would be much darker, tinted by some yet-to-be-identified compound that's seen widely in Messenger's images. It might also smell faintly of sulfur, appear heavily shocked, exhibit significant exposure to cosmic rays, and might even be slightly magnetic. Such characteristics would certainly have come to the attention of hunters and collectors, and it's safe to say that none of the world's 40,000 well-documented meteorites are from Mercury.
Yet dynamical probabilities argue otherwise, so why haven't such samples been found? Gladman and Coffey didn't address how chunks of rock might get blasted off the Mercurian surface, only that the high collision velocities of asteroids and comets should make it easy to do so.
Maybe the launch mechanics aren't understood well enough, suggests Jay Melosh, an impact specialist at Purdue University. "Perhaps at the very high speeds required for direct transfer, the fragments are simply too small," he says. "These ejecta have to be launched from the surface very close to the impact point — and perhaps our current models do not give very good results here." However, Messenger finds that big impacts on Mercury are accompanied by clusters of secondary pits, created by tossed-out debris, that are generally much larger — not smaller — than those around comparable lunar craters. "This fact is one of the current big puzzles about the Mercurian cratering record," Melosh concedes.
And so the search goes on for what will almost certainly be the most celebrated meteorite discovery since the finding of stones blasted from surfaces of the Moon and Mars a few decades ago.