It was found last year in Morocco, but scientists now say this strange, green rock likely originated on our solar system's innermost planet.
The announcement was made earlier this month by meteorite expert Anthony Irving at the 44th annual Lunar and Planetary Science Conference in The Woodlands, Texas. According to him, the pea-green space rock – which has been dubbed "NWA 7325" – is at least 4.56 billion years old, and hails not from Mars, or an asteroid, but from Mercury. If he's right, the discovery would be a first for the field of meteoritics.
"It might be a sample from Mercury, or it might be a sample from a body smaller than Mercury but [which] is like Mercury," Irving said during his presentation. According to him, its chemical composition is different from any Martian meteorite he's ever seen, or any thought to have originated from an asteroid.
NWA 7325 has a lower magnetic intensity — the magnetism passed from a cosmic body's magnetic field into a rock — than any other rock yet found, Irving said. Data sent back from NASA's Messenger spacecraft currently in orbit around Mercury shows that the planet's low magnetism closely resembles that found in NWA 7325, Irving said.
Messenger's observations also provided Irving with further evidence that could support his hypothesis. Scientists familiar with Mercury's geological and chemical composition think that the planet's surface is very low in iron. The meteorite is also low in iron, suggesting that wherever the rock came from, its parent body resembles Mercury.
In a paper describing the rock (pdf), Irving and his colleagues conclude that "ultimately only a sample return from Mercury may provide" a definitive answer as to its origins. But that, unfortunately, could take some time.
Many people are often surprised to learn that Mercury is, historically speaking, an oft-overlooked subject of scientific inquiry. (Messenger is the first spacecraft to ever orbit the planet, and it's only been circling Mercury since 2011.) We've known for a while that Mercury is something of an "oddball planet," to quote David Blewett, one of the scientist for the Messenger mission, a "fascinating, dynamic and complex world." It's awesome to see the Agency's Messenger spacecraft helping meteorite experts nail down the provenance of space rocks here on Earth – but it can only do so much detective work from Mercury's orbit.
Read Irving's paper describing the rock here (pdf).