The sale of an extremely rare Mars rock has been slammed by a renowned paleontologist.

The 24-kilogram meteorite, which is the largest known piece of Mars on Earth, was sold for $US5.3 million ($8.1 million) last month to an anonymous bidder in New York.

Known as NWA 16788, the meteorite is massive compared with most Martian meteorites, which tend to be small fragments.

NWA 16788 was discovered in November 2023 in the remote Agadez region of Niger, a poor African nation.

It has been described as a “monumental specimen” that is around 70% larger than the next biggest piece of Mars ever found on Earth.

However, Paul Sereno, paleontologist and founder of Niger Heritage, said the sale of the rock to an anonymous bidder was “brazen”.

“International law says you cannot simply take something that is important to the heritage of a country,” he told the BBC.

“Be it a cultural item, a physical item, a natural item, an extraterrestrial item – out of the country. You know we’ve moved on from colonial times when all this was okay.”

The government of Niger also expressed concern about how the artefact got into the hands of Sotheby’s, the famous auction house where it was sold.

In a statement on X, it “expressed doubts about the legality of its export, raising concerns about possible illicit international trafficking”.

“As with everything we sell, all relevant documentation was in order at each stage of its journey, in accordance with best practice and the requirements of the countries involved,” Sotheby’s said in a statement.

However, it also acknowledged concerns raised by the Nigerien government and others, saying “we are reviewing the information available to us in light of the question raised”.

Concerns have also been raised that the meteorite will not be able to be used for educational purposes if it ends up in private hands.

“It would be a shame if it disappeared into the vault of an oligarch. It belongs in a museum, where it can be studied, and where it can be enjoyed by children and families and the public at large,” Steve Brusatte, a professor of paleontology and evolution at Scotland’s University of Edinburgh, told CNN before the sale.