Tens of thousands of meteorites have been found on Earth, but only a handful are known to be from Mars. By analyzing several of these rare Martian meteorites, which were blasted off the Red Planet’s surface, researchers have now discovered that one of their ancient crystals harbors a secret: Mars was bombarded by at least one large impact—potentially affecting the planet’s habitability—that might have occurred more recently than previously believed.
Pieces of Mars in Africa
Thousands of years ago, humans were treated to a spectacular light show when a cluster of meteors streaked through the atmosphere. After the rocks slammed into northwestern Africa, they lay undisturbed for millennia. Only about a decade ago, when the meteorites were scooped up and thin slices made their way into the hands of scientists, did researchers realize those black stones were something special: pieces of Mars.
The stones are an amalgam of bits of pulverized Martian rock known as regolith. Such material is scientifically valuable because it can record the signatures of long-ago events like giant asteroid impacts, said Aaron Cavosie, a planetary scientist at the Space Science and Technology Centre at Curtin University in Perth, Australia. “It’s truly a big smorgasbord of the early processes on Mars.”
To better understand ancient conditions on Mars—with an eye toward understanding when the planet first might have become habitable—Cavosie and his colleagues focused on grains of a mineral called zircon found within three Martian meteorites recently collected in northwestern Africa.
Long-Lived Recorders of the Past
Zircon is a boon to scientists looking to peer into the past, said Desmond Moser, a geochronologist at Western University in London, Ont., who was not involved in the research. It can be accurately age dated, and it’s hardy enough to survive—and record—a cataclysm like an asteroid impact. “They can be very, very ancient…and yet they can also record these events that they’ve experienced,” said Moser. “It’s the best of both worlds.”
Cavosie and his collaborators analyzed 66 grains of zircon, each measuring just a few tens of micrometers on a side. Using an electron microscope, they zoomed in on the minerals’ crystalline structure. In one grain, they found evidence of atomic rearrangement that had resulted in bits of the crystal growing in a different direction. This feature, called a deformation twin, is the signature of the passage of an intense shock wave. “Literally, the mineral gets compressed like an accordion,” said Cavosie.
Powerful shock waves are associated with large asteroid impacts, and the most intense waves pass through rocks that bear the brunt of the strike, said Cavosie. “On Earth, nearly all occurrences of shock-twinned zircon are found in rocks from the central area of impact sites where pressures are highest,” Cavosie said. Rocks unearthed from the central regions of our planet’s three largest impact structures—Sudbury in southern Canada, Chicxulub in the Yucatán Peninsula, and Vredefort in South Africa—all exhibit shock-twinned zircon.
Finding this same signature in a chunk of Mars suggested that a similarly large impact occurred on the Red Planet, Cavosie and his colleagues concluded. It’s not surprising that Mars has been rocked by impacts throughout its history, but this is the first direct evidence of an impact event, said Cavosie. “Nothing like this has been identified previously.” These results were published in February in Science Advances.
When and Where?
Fundamental questions about this impact—when it occurred, for instance—are tricky to answer, Cavosie and his collaborators conceded. The zircon grain in question is known to have formed 4.45 billion years ago on the basis of uranium-lead dating, but its deformation twin doesn’t come with a time stamp, said Cavosie. “It could have happened any time after 4.45 billion years ago.”
The location of the original asteroid or comet strike also remains mysterious: Large impacts can send rocks flying hundreds to thousands of kilometers, said Cavosie, and this grain might have been blasted off Mars far from its birthplace. “Big impacts vomit shocked zircons everywhere.” And there’s always the possibility of multiple hops across the Red Planet: Subsequent smaller impacts could have launched the tiny grain on additional journeys.
Previous research has suggested that large impacts abated on Mars around 4.48 billion years ago, paving the way for the emergence of habitable conditions. But the discovery of this shock-twinned zircon, the by-product of a powerful shock wave that rippled through Martian regolith no more than 4.45 billion years ago, revises that picture, Cavosie suggested. “Our find of a zircon that was shocked 4.45 billion years ago, or perhaps much later, clearly contravenes their argument.”
—Katherine Kornei (@KatherineKornei), Science Writer