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.
The new Moroccan find is now officially known as Northwest Africa 7325. Ralew sent samples to the laboratory of Anthony Irving (University of Washington), well known for his expertise with unusual meteorites from the Moon, Mars, and elsewhere.Last April, German meteorite dealer Stefan Ralew bought a clutch of 35 small meteorites that had been found a few months earlier in the Moroccan desert. The fragments from a single fall totaled about 12 ounces (354 g). Right away he could see that they were unusual: Their fusion crust, created by flash heating while decelerating in Earth’s atmosphere, was greenish. This was especially evident in the largest, golfball-size piece, weighing just over 100 g. “Green and glassy fusion crusts are known from a few lunar meteorites,” Ralew explains, “but they all don´t have an extreme color as this one.”
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.
Irving, who’ll present his team’s findings at a planetary-science conference next month, is trying to keep his enthusiasm in check. “NWA 7325 is tantalizing, and certainly more consistent with the Messenger results than either angrites or aubrites,” he explains, “but we need a [spacecraft-returned sample] for ‘ground truth’.”
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.
One way to zero in on planetary paternity would be to see if NWA 7325 exhibits remanent magnetism. After all, Mercury has a robust magnetic field that would have been imprinted on any rock as it crystallized. (That said, possible complications might arise from the shock-heating these rocks experienced as they were being ejected into space, or from the strong magnets that Moroccan nomads use when searching for meteorites in the desert.)
A second approach would be to see if the meteorite’s ratios of three magnesium isotopes match what Messenger’s gamma-ray spectrometer is seeing on Mercury. It’s a challenging observation, explains Patrick Peplowski (Applied Physics Laboratory), because the GRS has a magnesium housing. “There does exist the potential to detect different magnesium isotopes, but I expect that the errors on any resulting isotopic ratios would be at the ~5% level,” Peplowski says. “I suspect that this is larger than would be needed to compare to NWA 7325, but I’m not sure.”
(One problem with using magnesium isotopes is that their ratios don’t vary much from place to place in the solar system.)
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.
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