File:NWA 8409 small slice.jpg

Comparing data from the Messenger spacecraft, Professor Anthony Irving of the University of Washington determined that the NWA 7325/8409 meteorite pairing has a composition consistent with that of the planet in our solar system closest to the Sun, Mercury, which Messenger has been orbiting since early 2011. (<a href="https://www.lpi.usra.edu/meetings/lpsc2013/pdf/2164.pdf" rel="noreferrer nofollow">44th Lunar and Planetary Science Conference</a>)

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.

Lead isotopes in the rock were tested for the radioactive decay of uranium by the University of Canberra, Australia, and it was determined that the age of the meteorite is approximately 4.56 billion years. This places it very close to the formation of the planet Mercury itself, or it may also have come from a smaller, Mercury-like proto-planet.

Despite the proximity of Mercury to the Sun's gravitational pull, it has been calculated that detritus thrown up by rocks hitting the planet would be propelled fast enough (faster than 9 km per second) to break free not only of the atmosphere but also of the Sun's gravitational field, and that 2 to 5% of such matter could reach Earth within 30 million years.

"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.