A glowing fireball descended through the sky over North Africa last July, accompanied by two sonic booms. Observers saw the fireball turn from yellow to green, then split into two parts before one fell to the ground in a valley and the other crashed into a mountain. And then… nothing, for a while.
The rocks that created the fireball had fallen in the desert near the Morocco-Algeria border, in a sparsely inhabited area. They had come from somewhere even more remote, but it took a while for anyone to realise their significance.
By the time Tony Irving, Affiliate Professor at the University of Washington, Seattle heard about the rocks they had been on the ground for several months. “It was probably around October that people who found stones brought them into the towns where there might be someone who would know what they were,” said Irving. Once the rocks were brought in from the desert, people began to connect them to the fireball and piece together what had happened.
By November word got out to the more experienced meteorite dealers in Morocco. One of them sent a sample of the rock to Irving. He analysed it and found that it had come from Mars. “We confirmed it pretty easily,” he said.
Irving and his colleagues kept the discovery quiet for a while. “We did most of our work sort of in secret in a way, talking only to a few colleagues,” he said.
In January this year the meteorite was officially certified as Martian and named Tissint, after a village close to its landing site, by an international meteoritics committee.
Despite the seemingly slow start, Tissint is an impressive discovery. Most Martian meteorites are not seeing falling through the sky, making it almost impossible to know how long they have been lying on the ground before they are discovered. Tissint is only the fifth Martian meteorite in history that people saw fall, and the first in nearly 50 years.
In total, over 12kg of Tissint has been found. It joins sixty other Martian rocks that now reside on Earth.
Tissint was probably ejected from Mars when an asteroid struck the planet, sending rocks from around the impact site hurtling into space. It stayed in space as debris for many years, before being picked up by Earth’s gravity and sent hurtling towards our planet’s surface.
Being able to say for sure that Tissint and the other meteorites came from Mars required some help from the Viking spacecraft, which touched down on the red planet in 1976. Viking measured the proportion of different gases in the Martian atmosphere. When a suspected Martian meteorite was found in 1979, scientists analysed the composition of bubbles of gas trapped within glass veins in the rock. They found that the gas in the bubbles was in the exact same proportions as Viking had measured on the planet itself. Little pockets of Martian atmosphere had travelled across the solar system, trapped inside the rock, and ended up in a lab here on Earth. It was the crucial piece of evidence needed to confirm the rock’s origin.
Not all of the meteorites from Mars have these bubbles of Martian atmosphere trapped inside them. Some fulfil other criteria, such as containing particular minerals. But if the atmospheric link had never been found, it would have been difficult to conclusively say that any of the rocks had come from Mars.
Tissint has plenty of glass pockets, formed when it underwent shock as it was ejected from Mars. No bubbles of gas from the meteorite have been analysed yet, but Irving doesn’t doubt that a match will be found once they are.
For scientists working on Tissint, the next step will be to find out how old it is. Preliminary results show that Tissint was ejected from Mars 1.1 million years ago. This links it to some other Martian meteorites that have been found on Earth, adding weight to the view that the sixty or so Martian rocks we have do not come from sixty different places on Mars. “It’s probably more like seven or eight places that have ejected groups of rock that land randomly [on Earth],” says Irving.
But for now, and until we send a sample return mission to the planet itself, Tissint and the other Martian meteorites are the only rocks we can use to piece together a picture of the red planet. "These are the only samples of rock from Mars that we have, and there are only sixty or so of them,” says Irving. “We need to put the whole forensic puzzle together as best we can."