An 800-gram Sahara rock is one of the oldest pieces of Mars known

A rose-colored, 800-gram rock called Teghaza 001, picked up in the Sahara in 2022, formed more than 4.1 billion years ago, which makes it one of the oldest pieces of Mars ever found on Earth. NASA-funded scientists dated its minerals and reported the result in Science on October 29. Among roughly 435 known Martian meteorites, it is the first to come from a part of the planet that had never been sampled.

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Its mineral grains also record how Mars’s crust cooled from magma before subsurface water altered it, about 4.1 billion years ago, so the rock itself probably formed earlier still.

Dating a chip smaller than a fingernail

According to NASA’s Jet Propulsion Laboratory, lead author Yang Liu and her colleagues dated the rock using in-situ rubidium-strontium analysis, run by Caltech’s Francois Tissot and his team on single mineral grains from a piece smaller than a fingernail. The method spares the rest of the sample for later study, and zircon crystals inside the rock helped fix the age. Tissot said very few laboratories in the world can perform this kind of age-dating right now.

Identification came first. The University of New Mexico recognized the stone as Martian from its oxygen isotopes, and it initially looked like a shergottite, the most common class of Martian meteorite. The team then matched its minerals and chemistry to measurements from NASA’s Viking landers and the Curiosity and Perseverance rovers.

Older and stranger than its neighbors in the collection

Two other famous Martian meteorites frame the comparison. ALH84001, recovered in Antarctica, is about 4.5 billion years old, and its 1996 fossil claims set off a controversy that a later Carnegie-led study in Science answered by tracing its organic carbon to non-biological water-rock reactions. NASA says ALH84001 is similar in age to Teghaza 001 but made almost entirely of one mineral. Black Beauty, a 320-gram stone found in the Sahara in 2011, is only about 2.1 billion years old and is built from small cemented fragments of volcanic rock. Teghaza 001’s texture and mineral mix set it apart from both.

Not every claim about the stone has been settled. A June study in Science Advances by Saper, Liu and colleagues compared hydrogen isotope ratios in Teghaza 001 with those in two younger meteorites and concluded that Mars was already losing hydrogen to space rapidly by 4.1 billion years ago. Live Science reported that James Day of the Scripps Institution of Oceanography, who was not involved, cautioned that all of those arguments would rest on a single small stone.

The rock’s look and chemistry are part of the puzzle. Live Science describes the stone, which was split into eight pieces and is held by the Maine Mineral and Gem Museum, as resembling granite more than the basalt that covers most of Mars. It is rich in silica, and on Earth granite typically requires plate tectonics, which Mars does not have. Caltech’s Yang Liu and the team behind the earlier preprint posted their age result on April 14, before peer review, and the Science paper described by NASA is the version that has now passed it. The planet itself formed about 4.5 billion years ago, so a stone at least 4.1 billion years old preserves the first few hundred million years of its crust.

Outside researchers have reacted with interest and some surprise. At the Goldschmidt conference in Montreal on July 14, Saper said the meteorite would revolutionize thinking about early Mars, and EarthSky reported that Stanford’s Eva Scheller called the silicon finding very strange because it was not expected. Lydia Hallis of the University of Glasgow noted that the field has doubled its collection of old meteorites with this one, so many laboratories now want a sample.

Thirteen candidate sites in the southern highlands

No one knows exactly where on Mars the rock was blasted from. The researchers identified 13 locations whose mineralogy and chemistry match it, all in the cratered southern highlands that NASA’s Mars Reconnaissance Orbiter has observed. That spacecraft launched on August 12, 2005, reached Mars orbit on March 10, 2006, and carries a camera that can pick out objects the size of a dinner table, which is how candidate terrain can be compared against the meteorite’s chemistry.

Lee Saper, the second author, who did the work as a JPL postdoctoral scholar and now works at NASA Ames Research Center, described the stone as the first piece of Martian crust from a part of Mars never sampled before. Liu said it was only after the team started looking closely that they realized how unique the meteorite is, and that the work has barely begun and that far more remains to be learned from the stone. One of the paper’s reviewers called it a Rosetta Stone for Mars.

Which of the 13 sites, if any, is the true source remains unresolved, and because the dating technique left most of the rock intact, other laboratories can test the ages and the water story on the same stone.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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