The First-Ever Meteorite from Mercury?

NWA 7325 is a meteorite like no other. Found in Morocco last year, this clutch of small stones looks to be a near-perfect geochemical match to the surface of the innermost planet.

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.

A meteorite from Mercury?
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.
Stefan Ralew / sr-meteorites.de
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 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.

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'."

Interior of meteorite NWA 7325
A polished cut surface of the meteorite NWA 7325 reveals green crystals of the silicate mineral diopside (colored by chromium ions). This meteorite contains abundant magnesium and calcium yet almost no iron — hallmarks of what geochemists believe rocks from Mercury should be like. The cube is 1cm tall. Click here for a larger view.
Stefan Ralew / sr-meteorites.de
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.

6 thoughts on “The First-Ever Meteorite from Mercury?

  1. Henrik

    These three magnesium-rich minerals are known to metamorphose via the simple means of weathering. Forsterite, which together with its iron analog Fayalite constitute Olivine, familiar from Pallasite meteorites or the gemstone variety Peridot, and Enstatite metamorph to serpentines. As there is a calcium analog to Forsterite/Fayalite, I’m not surprised to see a Calcium-Magnesium Olivine in an iron deficient paragenesis. Also, weathering can induce Diopside to change to carbonate, hematite and quartz. And that’s on gentle Earth. Of course the Mercurian surface, subject to fierce solar wind, is going to consist of heavily metamorphed minerals, so good thinking on the part of Dr Weider!

  2. Berkley

    This article does not mention the mineral "olivine", but notes an abundance of diopsidic pyroxene, which is by definition Ca-rich. High Cr therein is consistent with the high Mg/Fe ratio and suggests formation under highly reducing (low oxygen pressure) conditions consistent with what we might expect on a small planet like Mercury with a large metallic core (possibly also owing to loss of lithosphere via collision). Should be interesting to hear how the enstatite-rich (low-Ca) Mercury surface can be reconciled with a diopside-rich (high-Ca) plutonic igneous sample.

  3. Anthony BarreiroAnthony Barreiro

    Although I am ignorant of mineralogy, this does sound like a fascinating problem. I hope these meteorites will be conclusively shown to have come from Mercury! It sounds like Mr. Ralew is cooperating actively and fully with the scientists who can prove that his meteorites are Mercurian. This would be a good business decision if his rocks turn out to have such a rare provenance. But I wonder, is there a generally recognized code of ethics for meteorite dealers? I suppose more meteorites are found because there is a market that will pay people to go look for them. But on the other hand, shouldn’t these rare bits of other worlds be preserved for scientific study rather than sold to the highest bidder to display as curiosities?

  4. Jeff

    Has the testing to date conclusively confirmed that this isn’t the "Holy Grail"; the first extra-solar meteorite?

  5. straydog

    Here’s a question from a non-sicentist: If it IS from Mercury or any planet…How does a rock laying on/in the ground get into outer space ?? volcamo ? I’m asking because I don’t know. – and now here’s a GREAT idea: we should just send another Messenger orbiter back to Mecury to check and see if there are any rocks missing! (OK, just kidding)

  6. Bruce

    I’ll attempt to answer the above questions. Straydog, all the solid surfaces in our solar system show much evidence of impact cratering. With large enough impacts, rocks from the impacted planet can be launched clear into space with speeds exceeding the planet’s escape velocity. The escape velocity on Mercury is only 4.3 km/s, however at Mercury’s distance from the Sun the Sun’s escape velocity is 67.7 km/s. So for this meteorite find to have actually been from Mercury the planet would have had to have been hit by a massive blow from a large asteroid or comet, but the planet’s face does have craters large enough to show that there have been hits big enough to have done the job. Your idea about a volcanic launch mode was thought provoking however. It would take a truly stupendous volcanic blast to eject material into a solar orbit, but on earth there have been super volcanoes. Earth’s escape velocity is 11.2 km/s, so could an earthly super volcanoe blast a piece of earth clear to Mars? Perhaps not impossible, but impacts almost certainly have. Jeff, your question caused me to wonder, how long would it take a stone knocked lose from another stellar system to reach the earth? Suppose a rock was launched out of a system that was only 2 light years ahead of the sun such that the rock and the sun were converging at 200 km/s. At 10 trillion km/ly it would then take 10^11 seconds for our hypothetical rock to make the journey, which is actually only 3169 years. This would imply to me that extra solar meteorites are quite conceivable, so I don’t think Jeff’s question is off base at all.

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