A meteorite found in the Sahara desert in Morocco may have originated on Earth, before being blasted into space and returning from orbit thousands of years later. If confirmed, this boomerang space rock would be the first of its kind we know about.
Almost all the meteorites we have found come from asteroids, but a tiny fraction are from planetary bodies, such as Mars and the moon. These come from violent impacts that launch debris into space before later falling to Earth. Astronomers have shown that debris from similar impacts on Earth may also have fallen back to its home planet, but we have yet to find a compelling sample.
Now, at the French National Centre for Scientific Research and his colleagues think they may have found a 600-gram meteorite from Earth. The rock, called NWA 13188, has the same chemical make-up as volcanic rock from our planet. It also has a thin layer of melted crust consistent with an impact and contains isotopes of elements that only form when an object has been bombarded with cosmic rays in space.
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鈥淚t鈥檚 a meteorite from the Earth that has spent time in space, between 2000 and a few tens of thousands of years,鈥� Gattacceca told the Goldschmidt geochemistry conference in Lyon, France, on 11 July.
NWA 13188 was originally found in the Sahara desert by a Bedouin group, which scours the landscape for potential meteorites to sell, so its exact location is unknown. It was certified as a meteorite by the Meteoritical Society in 2021, but was classified as 鈥渦ngrouped鈥� because its origin couldn鈥檛 be determined.
When Gattacceca and his team examined the rock, they found it had a similar chemical make-up to rocks on Earth, such as the same oxygen isotope fingerprint, as well as geological structures made at the boundary between tectonic plates, which only Earth has because other planets have no plate tectonics.
They then looked for evidence of irradiation from cosmic rays, which produce elements like helium-3 and beryllium-10 in asteroids. The levels of these elements were lower than in other meteorites, but were still significantly higher than any material found on Earth. This suggests that NWA 13188 spent a brief period outside Earth鈥檚 magnetic field, from a few thousand to possibly up to 100,000 years, says Gattacceca.
But not everyone agrees. 鈥淲hen you鈥檙e claiming extraordinary hypotheses, you need extraordinary evidence to back it up. I am still unconvinced,鈥� says at the Free University of Brussels (VUB) in Belgium.
The lack of a known impact crater, which Gattacceca estimates should be around 20 kilometres wide, counts against it, says Claeys. 鈥淲hen you have an impact crater that young, you would have an impact melt that is still 鈥榟ot and smoking鈥� 鈥� it would be really hard to miss.鈥�
Gattacceca and his team are now trying to determine the age of the meteorite more accurately, using argon and carbon dating, which will help them narrow down a potential source.
There is also a question of whether the object can even be called a meteorite if it comes from Earth and only orbits for a relatively short time. There are many other rocks that reach high in Earth鈥檚 atmosphere from processes like volcanic eruptions, says at the University of Leoben in Austria. 鈥淚f you define an Earth meteorite, you鈥檙e stretching the definition of meteorite.鈥�
Article amended on 18 July 2023
We have corrected the elements produced in asteroids by cosmic rays
