The solar system can be a violent place. Asteroids frequently crash into each other at speeds of several kilometers per second. Occasionally, those space rocks slam into the surfaces of planets and moons, blasting fragments from the surface of those planets into space.
We’ve found meteorites on Earth before from Mars, and the moon. Could this green piece of stone have come from Mercury? If so, it would be the only time we’ve ever discovered a piece of the tiny world on our planet. To be honest, no one’s entirely sure. Even after analyzing its mineral content, it’s hard to draw a conclusion.
35 pieces of this space rock were collected last year in the Moroccan desert, and sold on to dealers and eventually meteorite collectors. This in itself is not unusual, but when Stefan Ralew from Berlin purchased one of these stones, he realized there was something quite strange about it.
When any serious meteorite collector finds a peculiar specimen, they will often contact Tony Irving at the University of Washington, and this is precisely what Ralew did after acquiring his rather odd piece of space rock.
Irving realized that this meteorite was a particularly interesting variety of space rock. Known as an achondrite (as opposed to the more common chondritic meteorites, and the much more common iron meteorites), meteorites like this one have a dramatic history. Achondrites account for around one in twenty stony meteorites, and they show a variety of internal composition, meaning that they were smashed from the surface of a much larger object, at least 200 kilometers in diameter. This means that achondrites originally formed as part of an asteroid, or a planet, large enough to differentiate internally to form a distinct core and crust. Roughly half of all achondrites discovered are from the asteroid 4-Vesta. But this was not one of them.
The curious little rock then made it’s way to Randy Korotev, a colleague to whom Irving sends achondrites such as the lunar meteorites that Korotev specializes in. But this only seems to have deepened the mystery. “It has very odd chemistry for a meteorite,” Korotev remarked, joking that “if somebody had walked in with this chemical analysis and nothing else I would have told him that it wasn’t a meteorite, just based on the chemistry.”
Meteorite scientists take a detailed look at the chemical composition of meteorites to try and determine where they came from. In particular they look at isotopes of elements such as oxygen, iron, and manganese, which act as fingerprints to be used in finding their origins.
Giving it a catalog designation of Northwest Africa 7325 (NWA 7325), Korotev realized that the chemistry in this stone was very unusual — and its composition was remarkably similar to the data that NASA’s MESSENGER probe is currently sending back from Mercury, our solar system’s innermost planet.
Almost no other meteorites ever found have a chemistry quite like that of NWA 7325. It’s rich in magnesium and low in iron, with oxygen isotope ratios virtually never normally seen. He also found that it was full of chromium-containing silicate compounds, explaining its striking green color. The glass in some bottles also contains chromium for exactly that reason.
But not everything added up. Analyzing NWA 7325, Korotev found a lot more of an aluminum-containing mineral called plagioclase that MESSENGER is seeing on Mercury’s surface. A few other chemical ratios in the rock don’t match Mercury either. While that could possibly be explained by a very large impact smashing into Mercury and excavating NWA 7325 from deep under Mercury’s surface, there was one other problem. This rock looks too old.
NWA 7325 appears to have crystallized (solidified from molten material) around 4.5 billion years ago, which would make it nearly as old as our planet. The data are only preliminary, but if it is indeed this old, it would mean an origin on Mercury was quite unlikely. Assuming Mercurian rocks started to crystallize at the same time as the moon did, this would mean that NWA 7325 is a couple of hundred million years too old to have originated on Mercury.
So there’s evidence both for and against this being a Mercurian stone, and with no examples of any 4.5 billion year old lunar rocks, we have no way of knowing if it could really have left Mercury that long ago. Its origin is still a mystery.
If NWA 7325 really was blasted off the surface of Mercury billions of years ago, it’s in remarkably good condition. The question of how it survived as long as it has is a difficult one to answer. Korotev has a few more ideas of tests to run in order to find the stone’s true origin. One test that he has yet to perform is to examine it for cosmogenic nucleides — elements created in the surface of meteorites as the sun’s solar wind bombards them. This test can give a good estimate for how long the solar wind has been pummeling any piece of space rock, and because Mercury is so close to the sun, high levels of those cosmogenic nucleides would be fairly good evidence for Mercury to be the birthplace of NWA 7325.
For now though, all of the data about this puzzling little achondrite remain inconclusive. We simply have no idea from whence it came. As Korotev explains, “if it’s not from Mercury, then where is it from? That’s really the question.”
Image: A photograph of meteorite Northwest Africa 7325 (NWA 7325), showing its unusual green colouration. The alphabet block to the right has sides of 1cm, for scale. Credit: Stefan Ralew