A Sahara Meteorite From Mars Is a Rare 4.1-Billion-Year-Old Window Into the Planet's Early Crust

Eight dark stones, together weighing just under 800 grams, turned up in the Sahara in Mali in 2022. Desert meteorite hunters look for exactly this kind of rock, because dark stones stand out against pale sand. A private collector bought the set and sent a piece to the University of New Mexico for classification. It is now known as Teghaza 001, and it appears to be a rare fragment of Mars' ancient crust.

The oddity is in the numbers. Crater counts indicate most of the Martian surface is more than 3 billion years old. Yet the three main classes of Martian meteorites on Earth, the shergottites, nakhlites and chassignites, are all 2.4 billion years old or younger. Only two truly ancient ones were known: Allan Hills 84001, found in Antarctica, and a jumbled northwest African rock nicknamed Black Beauty. A team led by Yang Liu, a planetary scientist at NASA's Jet Propulsion Laboratory, has now shown that Teghaza 001 formed at least 4.1 billion years ago. Liu says it resembles some of the oldest rocks on Earth.

Part of the reason old rocks are scarce may be how rocks reach Earth. It takes a large impact to launch material off Mars, and not everything survives. Liu explained that loose powder may never cement into rock that can escape the planet's gravity, so weak rocks do not make it off. The result is a biased collection. None of the Martian meteorites gathered so far resemble what rovers have found on the surface. Black Beauty comes closest, since its bulk chemistry matches orbital measurements, but it is a mix of soil and many small rock fragments. Teghaza 001 is a single intact igneous rock.

At New Mexico, Carl Agee and Karen Ziegler measured its oxygen isotopes, which match other Martian meteorites. It was first labeled a shergottite, the most common and youngest type. Agee then noticed oddities and contacted Liu: one mineral was altered on the inside while its outer part stayed pristine, the reverse of weathering on Earth. Liu's first impression of the sample was that it was boring, because the manganese-rich material resembled what she had studied in Black Beauty. That material turned out to be terrestrial, and she quickly realized the rock itself was very special.

It is mostly feldspar, with darker pyroxene crystals that Liu says resemble some lunar highland rocks. The crystals reach a millimeter across, which suggests slow cooling. By comparing them with lab experiments and Earth rocks, the team estimated a cooling rate of roughly 0.01° Celsius per year. That points to magma that never erupted and instead solidified inside the crust.

The team dated it several independent ways. Zircon grains gave about 4.11 billion years, and phosphate minerals and rubidium-strontium dating agreed. Liu said the consistency was unexpected. Zircon ages are notoriously hard to reset, and the launch impact, though strong enough to form high-pressure minerals, was not hot enough to do so. Sodium-rich rims on feldspar grains and sodium trapped in the zircons point instead to hot water percolating through the rock soon after it formed. So 4.1 billion years marks its last soaking, and the true crystallization age is older.

The rock is also rich in oxygen-18, a heavier oxygen form, by about two parts per thousand compared with other Martian meteorites. Crystallization or degassing cannot produce a gap that large. Low-temperature water-rock reactions near the surface can, so the team thinks the magma either absorbed crust that had interacted with surface water or reacted with fluids carrying that signature. Liu reads this as active water circulation through the crust, a hydrosphere reacting intensely with it, and says Mars was more dynamic than thought. She argues it fits a wet early Mars and suggests potentially habitable crustal environments.

The interior story is just as notable. The magma came from deep below, and trace-element ratios such as zirconium to titanium differ from every other Martian meteorite. Melting models fit best with a garnet-bearing mantle source, which on Mars is stable only deeper than 160 kilometers. Other meteorites suggest the mantle split into three to five reservoirs after a magma ocean solidified around 4.5 billion years ago, but Teghaza 001 matches none of them.

The team suggests it came from the southern highlands northwest of the Hellas impact basin, though orbital data can narrow a source only to about 100 by 100 meters at best. When it left Mars is unknown. Liu says her team did the minimum needed to show the sample is special, and two or three other groups are studying it. She says samples with careful geological context, like those Perseverance has collected around Jezero Crater, would be the real game-changer, especially from terrain as old as Teghaza 001 or older. The study is published in Science.