Are all the Martian metorites found on Earth from here? (Image: NASA/JPL-Caltech/University of Arizona)

Nearly all Martian meteorites found on Earth may come from the same crater on Mars. If so, the claim could overturn our understanding of Martian history, because most pieces of the Red Planet we are able to study in detail would be billions of years older than believed. But the idea has set off a firestorm among meteorite experts, with some unconvinced that the rocks share a common origin.

There are nearly 150 known meteorites from Mars, about 80 per cent of which are in a group called the shergottites, thought to be pieces of the planet’s crust formed from cooling lava. Looking at the way minerals melted and reformed in these rocks, previous studies suggested that shergottites were 600 million years old at most.

But Stephanie Werner at the University of Oslo in Norway and her colleagues believe that the rocks are older because they have all come from the same place: the 55-kilometre-wide Mojave crater on Mars, which was carved out of terrain that is 4.3 billion years old.


To date features on other planets, astronomers use a technique called crater counting, which is based on the rate at which craters of different sizes should accumulate on a planet’s surface. Using this method, Werner’s team calculated that the impact that formed Mojave crater happened around 3 million years ago. That is a puzzle – statistical models suggest such large craters should not have formed so recently in Martian history.

Big baby crater

“These kinds of craters don’t form on Mars any more, so it was very surprising that we found this big, very young crater,” says Werner.

The team turned to the shergottite meteorites. When planetary material is launched into space during an impact, it gets exposed to cosmic rays, and these high-energy particles can alter the rock’s chemical signatures. Based on their cosmic ray exposure, the shergottites must have broken off Mars between 1 and 5 million years ago, putting them in line with the age of Mojave crater.

“That was the strongest hint that these very recently ejected meteorites from Mars could be closely linked to this impact crater,” says Werner. Also, mineral maps from two spacecraft in orbit around Mars show that the rocks in the walls and central region of Mojave crater have similar ingredients to many shergottites.

But if the shergottites are from Mojave, the rocks should be as ancient as the rest of the crater, and certainly not as young as the 600 million years suggested by examining their form. This can be explained by recent measurements of isotopes in the meteorites, says Werner.

Previous work studying ratios of different kinds of lead in shergottites suggested that the rocks are, in fact, between 4.1 and 4.3 billion years old. That would fit with the age of the terrain in Mojave crater, Werner’s team says. With these revised ages, Werner thinks that the famous Allan Hills meteorite, which is 4.1 billion years old and was once claimed to contain fossilised microbes, may have also been sent to Earth by the Mojave impact.

Meteor rift

“This is a fantastic study,” says Audrey Bouvier at the University of Western Ontario, Canada, who studies meteorite isotopes. “I have been fighting this idea of young shergottites for a long time now, because people did not measure the composition carefully enough in the 1980s.”

Harry McSween at the University of Tennessee in Knoxville previously worked with Werner on dating Mojave crater. However, he thinks the team has not made a strong enough case for this location being the source of all shergottites. Groups of these meteorites have different cosmic ray ages and mineral properties, which suggest they came from different craters, he says. “These are separate impact events on Mars that launched each of these homogenous groups of meteorites.”

Both Werner and Bouvier counter that the crater is big enough to account for a variety of rock types. And they think cosmic-ray ages can be unreliable because meteors fragment as they travel through space, exposing fresh surfaces to cosmic bombardment.

“It is a model that can be challenged,” says Kurt Marti at the University of California, San Diego. “There are a number of ad hoc assumptions, like a break-up in space to account for cosmic ray ages, without current experimental support.”

Journal reference: Science, DOI: 10.1126/science.1247282