Unique meteorite hints Mars stayed moist for longer

A scorched rock bought in Morocco turned out to be a diamond in the rough. The unusual meteorite may be the first sample of the Red Planet's crust ever to hit Earth, and it suggests that Mars held on to its water for longer than we thought.

The meteorite, dubbed Northwest Africa (NWA) 7034, is strikingly different from the 111 previously discovered Martian meteorites. "You could look at meteorites for the rest of your life and not find another one like this," says Carl Agee of the University of New Mexico in Albuquerque, who was part of a team that has recently analysed NWA 7034. "This is in its own new group."

The most distinctive difference is its mineral content. Previously found meteorites had unearthly oxygen isotopes that marked them as being from another planet, and their volcanic origin made Mars the most likely culprit. But compared to these meteorites, surface rocks studied by Martian rovers and orbiters are much richer in light metals such as potassium and sodium. This suggests the known meteorites came from deeper inside the Red Planet.

"We're watching data coming back from Mars, and everything that comes back doesn't look like the Martian meteorites we have in our collections," says Munir Humayun of Florida State University in Tallahassee, who was not involved in the new study. "That's kind of a bummer."

By contrast, NWA 7034's chemistry closely resembles the rock and soil studied by NASA's Spirit rover. Preliminary measurements from the Curiosity rover, which landed in August 2012, suggest its landing site also has a similar composition.

Drying era

"Finally, it looks as if we have a sample that is very similar to the rocks that the rovers are seeing," Agee says. What's more, the Moroccan meteorite may come from a period in Mars's history when the planet was drying out.

Mars is thought to have once been much warmer, wetter and more hospitable to life. Then it morphed into the dry, cold desert we see today. The oldest known Mars meteorite, called the Allan Hills meteorite, is 4.5 billion years old. The other 110 meteorites are much younger ? 1.5 billion years old at most ? and formed after Mars is thought to have lost its water.

NWA 7034 is 2.1 billion years old, making it the first meteorite that may hail from the transitional era. Intriguingly, it has as much as 30 times more water than previous meteorites locked up in its minerals. "It opens our mind to the possibility that climate change on Mars was more gradual," Agee says. "Maybe it didn't lose its water early on."

Hot deal

The 319.8-gram rock found its way to Agee's lab via an amateur collector named Jay Piatek. He bought it for what turned out to be a knock-down price from a Moroccan meteorite dealer, who recognised its scorched exterior as a sign that it fell from space. "It didn't look like a Martian meteorite, so it didn't have the Martian meteorite value at the time," Agee says, adding that Mars rock can go for $500 to $1000 per gram.

Piatek brought the rock to Agee's lab to find out what it was. "Honestly, I had never seen anything like it. I was baffled, initially," Agee says. "Now, about a year and a half after the first time I set eyes on this thing, we are convinced that it is Martian, a new type, and has important implications for understanding the history of Mars."

Humayun says the results so far are exciting, and that the rock's carbon content could also yield valuable insights once other researchers get their hands on it.

"What's the most exciting thing you would want to do with a rock that comes from the near surface of Mars, especially one that seems to be loaded with water?" he asks. "I would say, what about life?" Agee and colleagues found organic matter in the meteorite, he says, but it will take more work to determine whether it was of Martian or terrestrial origin.

If it's Martian, "that would spark a lot of excitement", he says.

Journal reference: Science, DOI: 10.1126/science.1228858

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