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When a rose-colored meteorite was found in the Sahara Desert in 2022, no one knew how old it was or where it came from. Not until NASA-funded scientists age-dated its minerals did they realize the nearly 2-pound (800-gram) rock was a piece of Mars that formed more than 4.1 billion years ago, making it one of the oldest Martian meteorites ever found. Named Teghaza 001 after the salt plain in Mali where it was discovered, the meteorite is one of roughly 435 from the Red Planet.
In a paper published Thursday in Science, researchers offered an initial characterization of Teghaza 001, providing a rare snapshot of Mars' ancient crust and formation. What scientists learn from future studies could help answer many questions, including how water and rocks interacted on ancient Mars. A separate paper published in Science Advances in June detailed the meteorite's water content, offering a look at how water shaped the early planet's surface.
"It was only after we started looking closely that we realized this meteorite is really unique," said the paper's lead author, Yang Liu of NASA's Jet Propulsion Laboratory in Southern California. "One of the paper's reviewers called it a Rosetta Stone for Mars."
Rose-colored stone
What sets Teghaza 001 apart from other Martian meteorites is its texture and age as well as the uniqueness of its igneous, or volcanic, minerals. It is one of the three most ancient Martian meteorites found on Earth. While invaluable, the others provide more limited glimpses of Mars' geological history. They include ALH84001, which is composed almost entirely of one mineral and is similar in age to Teghaza 001. The other meteorite, nicknamed "Black Beauty," formed about 2.1 billion years ago and is composed of small, volcanic rock fragments.
Unlike its ancient peers, Teghaza 001 has a mix of igneous minerals that tells a story about how the planet's crust formed from cooling magma before it was eventually altered by subsurface water. The meteorite's distinctive mineralogy matches what NASA's Mars Reconnaissance Orbiter has observed in the planet's cratered southern highlands. No one will ever know exactly where the meteorite originated on Mars, but the paper identifies several potential locations that match Teghaza 001's mineralogy and chemistry.
"This is really our first piece of Martian crust from a part of Mars we've never had a sample of before," said the new paper's second author, Lee Saper, who conducted the research while at JPL as a postdoctoral scholar. Lee now is a scientist at NASA's Ames Research Center in Silicon Valley. "It records interactions of water and rock and maybe even an imprint of the Martian atmosphere as it was losing water."
Although the Mars meteorites found on Earth differ from one another, they share a common history, having been blasted off the Red Planet's surface by an impact, followed by a trajectory that eventually placed them on Earth and into the hands of scientists. Meteorites are often found by collectors in deserts or snowy plains, where they stand out from the rest of the barren landscape. Scientists can verify a meteorite as Martian by matching its mineral and chemical components to data collected by NASA's Mars missions, including the Viking landers as well as the Curiosity and Perseverance rovers.
Signs of water
When Liu was first asked to look at Teghaza 001, she almost passed on the opportunity.
Researchers at the University of New Mexico had already identified it as Martian based on its oxygen isotopes, which serve as geochemical fingerprints for different planetary bodies. In this case, the researchers matched the meteorite's oxygen isotopes to those seen in other Martian meteorites. Teghaza 001 appeared to be a shergottite which is the most common class of meteorite from the Red Planet. As an expert in planetary samples, Liu has seen plenty of those, but she decided to take a peek, anyway.
At the very least, there were odd textures and unusual minerals she hadn't seen before. Liu and her colleagues discovered zircon crystals in Teghaza 001, which record clues to a rock's formation and can be used by geologists to precisely age-date a sample. Further analysis revealed when Teghaza 001 formed and when certain changes, like water alteration, occurred.
Caltech coauthor Francois Tissot helped the researchers utilize a novel age-dating method pioneered by his team. Compared with bulk-analysis techniques, where a sample is entirely dissolved and analyzed, Tissot's in-situ rubidium-strontium age-dating used a piece of Teghaza 001 that was smaller than a fingernail, analyzing single mineral grains on its surface. Besides its extreme precision and speed, the technique preserves rare samples like Martian meteorites for future studies.
"Right now, we are one of very few labs in the world able to perform this kind of age-dating, and the first to date such an ancient Martian meteorite," Tissot said. "Our hope is to pave the way for dating even more Martian meteorites, or samples returned from other planets, their moons, and asteroids."
When the age-dating results came back, Liu was shocked: This rock had been altered by Martian water sometime around 4.1 billion years ago, suggesting it originally formed even earlier. Clues within the oxygen isotopes, texture, and chemistry of Teghaza 001 minerals point to a much more dynamic water cycle on early Mars.
This year's papers are just the start. Liu and her co-authors are eager to see what the rest of the scientific community can learn from Teghaza 001.
"We've only scratched the surface," Liu said.
