When dynamicists run the numbers, it’s at least statistically possible that meteorites should fall to Earth from all over the inner solar system — even from Mercury.
So, spurred by the Messenger orbiter’s close scrutiny of Mercury’s surface, the hunt has been on to find meteorites from the innermost planet. All previous candidates (called angrites and aubrites) are close but imperfect matches to the unique composition found by Messenger on Mercury’s surface: dark igneous rock enriched in magnesium but virtually free of iron.
Now, finally, they might have one in hand.
Weighing just a bit more than 100 g, this is the largest fragment of the meteorite NWA 7325. Note the amazing light-green color of its fusion crust — one of many characteristics hinting that it might be from the planet Mercury. The cube at right is 1 cm square. Click here for a larger view.
The stones’ interiors are full of relatively large and obvious crystals, suggesting that the magma from which they solidified had cooled slowly. The stunning emerald-green color comes from a silicate mineral called diopside that’s infused with chromium. Irving and his team found lots of magnesium and calcium in the suite of silicate minerals, but even more important is what they didn’t find: there’s virtually no iron.
Shoshana Weider (Carnegie Institution of Washington), who’s spent years studying Messenger’s spectra of Mercury, likewise offers a cautionary note. The planet’s surface seems to be rich in the silicate mineral enstatite, which is not obvious in NWA 7325. Also, there shouldn’t be so much calcium. To explain these discrepancies, she and Irving agree that meteorite might have been a deeply buried rock — well below the surface — before a powerful collision sent it flying off into interplanetary space.
There are still many unknowns about these weirdly green space rocks. Tests are under way to determine how long ago they crystallized and how long they were exposed to cosmic rays as they drifted in space before reaching Earth.
Finally, researchers could melt one of the NWA 7325 stones and then let it cool and recrystallize under controlled conditions, to see how closely the result mimics Mercury’s surface composition. “A lot of scientists will want to get their hands on this,” Weider notes.
However, anyone wanting a piece big enough to melt down will likely have to get in line. So far Ralew has donated less than 1 ounce of NWA 7325 for scientific analysis, and he’s got the rest. It’s not inconceivable that bits of this unique find could fetch $5,000 per gram on the sometimes-frenzied meteorite market. For now, at least, he’s holding off offering any of it for sale, to give researchers the chance to run the entire gamut of analytical tests.
“If this rock isn’t from Mercury, it’s still amazing,” Irving notes. It’s from a planet, he says — we just need to figure out which one.