One meteorite from Mars adds weight to the expression “no stone left unturned.” 

Yang Liu, a geologist at NASA‘s Jet Propulsion Laboratory, almost didn’t heed that wisdom when she nearly declined the chance to study the rock. It looked like the most common type of Martian meteorite found on Earth. But something about its texture and minerals made her take a look.

In hindsight, Liu is glad she did. The space rock, known as Teghaza 001, appears to be a piece of Mars that formed more than 4.1 billion years ago, making it one of the oldest Martian meteorites scientists have ever seen.

“This meteorite is really unique,” said Liu, the lead author of the findings, in a statement. “One of the paper’s reviewers called it a Rosetta Stone for Mars.”

The meteorite, found in the Sahara Desert in 2022, preserves a snapshot of Mars as it looked more than 4 billion years ago, perhaps less than 500 million years after the planet formed. Even then, the water locked inside its minerals had already lost much of its lighter hydrogen to space. That chemical fingerprint shows the Red Planet began losing its water almost from the word “go.” The discovery provides context for any debate about life on early Mars: Whatever biology that might have existed, even early on, probably faced a world already drying out.

Every day, about 48.5 tons of space rock and dust strike Earth’s atmosphere, according to NASA. Most of it burns up on the way down, and much of what survives sinks into the oceans, which cover more than 70 percent of the planet. People have found more than 82,000 meteorites, but only about half of one percent of them came from Mars.

Nearly every known Martian meteorite formed from magma, or molten rock, and most of them belong to three main families: shergottites, nakhlites, and chassignites. All three formed less than 2.4 billion years ago. 

A close-up look at a slice extracted from the Martian meteorite, Teghaza 001. Credit: NASA / JPL-Caltech

The nakhlites and chassignites seem closely related through a process called fractional crystallization. As magma cools, certain minerals harden first and separate out, which changes the chemistry of the remaining liquid. Scientists think both groups formed in the same volcanic system.

But Teghaza 001 doesn’t fit any of these groups. Its minerals and chemistry set it apart from every other Martian meteorite. The study appears in the journal Science.

“This is really our first piece of Martian crust from a part of Mars we’ve never had a sample of before,” said Lee Saper, a co-author now based at NASA’s Ames Research Center in California.

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Clues in the rock’s minerals point to a busy water cycle on early Mars, with water moving between rock, ground, and air. The minerals reveal two sources: water that rose from deep inside the planet, and surface water that had already lost much of its lighter hydrogen to space. 

Scientists use geological clues to piece together ancient Mars’ past water history.Credit: NASA illustration

Geologists can tell the two apart because hydrogen comes in a normal form and a heavier form called deuterium. Because the lighter form escapes into space more easily, any water left behind gets richer in deuterium over time. The surface water in Teghaza 001 carries extra deuterium.

From about 4.1 to 3.7 billion years ago, Mars had a fairly dry climate, but one that could have still allowed liquid water on the surface at times, possibly enabling life. Scientists believe the Red Planet’s surface eventually dried out to roughly today’s barren state less than 3 billion years ago.

Thanks to Mars’ lack of shifting tectonic plates, its ground maintains a record of more than 4 billion years of history. That allows scientists to estimate the ages of different parts of the planet’s surface by counting craters: the older the region, the more battle scars it should have. 

Based on the specimen’s minerals and chemistry, scientists trace the rock to one of a handful of locations in Mars’ southern highlands.Credit: NASA / JPL-Caltech / USGS

The mix of minerals matches what NASA’s Mars Reconnaissance Orbiter has seen in the heavily pocked southern highlands of Mars. Scientists pinpointed 13 places on Mars whose minerals and chemistry are consistent with the meteorite.

To date Teghaza 001, scientists used several methods, including one cutting-edge technique that tracks radioactive rubidium, said Francois Tissot, a Caltech scientist and co-author. The rubidium slowly transforms into strontium at a steady rate; thus, the amount of the latter within a sample stands in for a clock. 

“Our hope is to pave the way for dating even more Martian meteorites,” Tissot said in a statement, “or samples returned from other planets, their moons, and asteroids.”