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Rare meteorite from Mars gives us a view into its ancient crust
A meteorite found in the Sahara is a rare piece of Mars’ ancient crust.
Crater counts tell us that most of the Martian surface is more than 3 billion years old, but pieces of Mars we have on Earth tell a different story. The three main classes of Martian meteorites—shergottites, nakhlites, and chassignites—are all 2.4 billion years old or younger. Until now, only two genuinely ancient Martian meteorites were known: Allan Hills 84001, found in Antarctica, and a jumbled rock from northwest Africa nicknamed Black Beauty. Now we have a third one.
A team led by Yang Liu, a planetary scientist at NASA’s Jet Propulsion Laboratory, has shown that a Martian meteorite called Teghaza 001, found in the Sahara Desert in Mali in 2022, formed at least 4.1 billion years ago. “It’s very old and similar to some of the oldest rocks on Earth,” Liu says.
“We don’t fully understand why old Martian rocks are so rare on Earth,” Liu said. Part of the answer may lie in how a piece of Mars gets here. It takes a large impact to launch a rock off the planet, and not every rock can survive that. “If the impact hits into loose powder, it may not get cemented to become a rock and fly off Mars’ gravity,” Liu said. “So the natural impact already acts as a filter. Weak rocks do not make it off Mars.”
“Even to date, none of our collected Martian meteorites are similar to what we found on the surface,” Liu said. The partial exception is Black Beauty, which has a bulk chemistry that matches orbital measurements. But Black Beauty is a mix of soil and many small rock fragments. Teghaza 001 is a single, intact igneous rock.
Teghaza 001 was acquired through the desert meteorite trade, where hunters comb the Sahara for dark stones that stand out against the sand. Its eight stones, weighing just under 800 grams in total, were bought by a private collector, who sent a piece to the University of New Mexico for classification. There, planetary scientists Carl Agee and Karen Ziegler measured its oxygen isotopes. “Each planetary body has a very characteristic oxygen isotope value,” Liu says. Teghaza 001’s values matched those of other Martian meteorites.
At first, the rock was classified as a shergottite, the most common (and young) type of Martian meteorite. But Agee noticed some oddities and reached out to Liu. One of Teghaza’s minerals had been altered on the inside while its outer part stayed pristine, the reverse of how weathering on Earth works. The oxygen isotopes looked a bit strange, and there was some manganese-rich material similar to what Liu had studied in Black Beauty.
“When I got to the sample, I looked around, I said, ‘That’s boring,’” Liu said. “The high-manganese material is actually terrestrial, but I quickly realized the sample is very different, very special.”
Teghaza 001 is made mostly of feldspar, a pale mineral common in the crusts of rocky planets. It has darker pyroxene crystals mixed in; according to Liu, those resemble some rocks exposed in the lunar highlands. Its crystals are rather big, up to a millimeter across, which scientists interpret as an indication it cooled down slowly. “For slow-cooled rock, they tend to have time to grow bigger,” Liu said.
By comparing the crystals with lab experiments and terrestrial rocks, the team estimated Teghaza 001 cooled by roughly 0.01° Celsius per year, indicating it formed from magma that never erupted but instead solidified inside the crust.