The inventory of meteorites from Mars is astoundingly small: Less than 400 of the known space rocks identified here hail from the Red Planet.
Even smaller is the number of nakhlites and chassignites, two of the three main types of Martian meteorites, of which there are only 35 samples in the world put together, according to The Meteoritical Society. But scientists haven't known much about them, despite having had many of the rocks for decades. James Day, a geologist and geochemist at UC San Diego, wanted to unlock their secrets.
After he and a team of scientists conducted the first comprehensive analysis of this collection, they made an unexpected find — a new Martian rock type — but without the actual rock or any others matching its description on Earth.
The discovery is based on a cryptic geochemical signature embedded within some of the specimens. That distinct rock type is probably Mars' crust, according to the research published in the journal Science Advances.
"It's an indelible fingerprint. There's nothing else it can be. It's literally staring at you in the data," he told Mashable. "We will probably find these rocks on Mars."
NASA estimates about 48.5 tons of ancient meteor material rain down on Earth daily, but much of it vaporizes in the atmosphere or plunges into water, which covers over 70 percent of the planet. People have discovered over 82,000 meteorites, but only about 0.5 percent originated from Mars.
All of the known Martian meteorites came from volcanoes. The nakhlites and chassignites appear to be related to each other through so-called "fractional crystallization," a primary way of causing chemical changes in magmas. Scientists think the two kinds came from the same volcanic system, launched into space after a large meteor crashed into Mars perhaps 11 million years ago. The collision could have blasted rubble out of its atmosphere into space. Over time, some of those bits could have made their way to Earth.
Upon close inspection, the team found a record of the Martian atmosphere in some of the nakhlites — but not all of them. The composition matched the measurements taken by NASA’s Viking landers in the 1970s. The scientists believe the molten nakhlites oozed over the surface of Mars, or just underneath it, melting some of the crust and incorporating it.
"Imagine that you're a Martian, and you have a piece of the Earth in your meteorite collection, and it came from Hawaii. You don't know it came from Hawaii, but you have this rock," Day said. "That rock, when it forms, it melted, it flowed over highly altered rocks, and those highly altered rocks have a signature of the Earth's atmosphere."
Through sulfur isotopes, which are changed in Mars' atmosphere, the team could then infer what those rocks would look like. The rocks, which the team has not named, are basaltic in nature. On Earth, basalts are abundant as the bedrock of the ocean floor and in areas where lava has flowed, such as Hawaii and Iceland.
They are also quite old because the nakhlites themselves are old, formed roughly 1.3 billion years ago. In order for crust rocks to get mixed into the nakhlite material, they would have to be older, Day said.
But why wouldn't any of this Martian crust have landed on Earth in the form of a meteorite? Day believes these rocks probably don't hold their shape well, making it unlikely for pieces to survive a catastrophic meteor impact on Mars. If any chunks did blast into space, they'd be even more unlikely to make the journey all the way to Earth's surface without being destroyed.
"It's an indelible fingerprint. There's nothing else it can be."
In addition to the speculated new rock type, the study provides insight into the Red Planet's internal structure — in some ways similar to Earth's and in other ways quite different. The team suggests Mars has an atmospherically changed upper crust, a complex deeper crust, and a mantle where plumes have penetrated the base of the crust. Early in the planet's evolution, its interior melted to form distinct types of volcanoes, and it seems to have remained that way.
Nakhlites and chassignites formed in a similar fashion to volcanic rocks in Hawaii and Iceland. There, volcanoes press down on the mantle, spurring tectonic forces that create more volcanic activity. But on Earth, plate tectonics have also remixed the reservoirs that feed the volcanoes over time, making the interior more homogenous. That's not the case on Mars, where the reservoirs have stayed distinct.
NASA's Perseverance rover, a car-size lab on six wheels, has been collecting samples from the Jezero crater on Mars since 2021 so they can be brought back for scientific scrutiny. The region, an ancient dried delta, is one where scientists think microscopic organisms might have existed long ago. But the plan to fly rocks and grains to Earth, a complex mission called Mars Sample Return, is in jeopardy. Its rising costs have led to layoffs and warnings of cancellation from Congress. The agency is now making a desperate plea for outside help to save the mission.
Day hopes NASA will find a way to bring those samples home. But if that doesn't happen, there's still much that humans can learn from these meteorites.
"You would think that we would have done everything on these rocks. We haven't," he said. "There's a lot of science still left to be done."
