A meteorite found in Algeria is about 1.273 billion years old, giving scientists a rare sample from a long-missing period in the geological history of Mars. The rock also carries an isotope signature unlike that of other known shergottites, pointing to a portion of deep Mars that may have remained unchanged since the planet formed.
Scientists have found only about 400 meteorites from Mars. Because Martian rocks reach Earth after impacts blast them off the planet, the available samples provide only a limited record of Mars’s geological history.
That gap is especially large among shergottites, the most common group of igneous Martian rocks. Nearly all dated shergottites are younger than 600 million years. The next-oldest characterized examples date to about 2.4 billion years ago.
That leaves an approximately 1.8-billion-year stretch with no shergottite samples available for studying geological activity on Mars.
The newly analyzed meteorite, Northwest Africa 13441, or NWA 13441, falls directly within that missing period. It was discovered in Algeria in 2019 and had not previously been characterized.
Researchers at Boston College obtained a small piece of the meteorite from a colleague at Appalachian State University. They examined both crushed material and a thin section mounted on a glass slide, working with scientists from the Scripps Institution of Oceanography and Britain’s The Open University.
Their first tasks were to confirm that the rock came from Mars, establish when it crystallized and determine how its chemistry compared with other Martian meteorites.
The meteorite is about 1.273 billion years old
Using high-precision radiogenic isotope techniques, the researchers dated NWA 13441 to approximately 1.273 billion years old.
That age places it between the two main groups of dated shergottites. Two other uncommon types of Martian meteorites, chassignites and nakhlites, are also about 1.3 billion to 1.4 billion years old, but they are chemically different from shergottites.
The age of NWA 13441 therefore provides a shergottite sample from a period that had previously lacked one.
“The characteristics of this meteorite were entirely surprising,” said Ethan Baxter, a Boston College professor of earth and environmental sciences and coauthor of the study. “No other Martian meteorite like this is 1.27 billion years old.”
Its neodymium isotopes are unusual
The researchers also found an unusual isotopic feature in the meteorite. They focused on neodymium, a naturally occurring element with seven isotopes.
The neodymium isotopic composition of NWA 13441 matches the composition of the initial solar system. The team reported that this value has not been observed in other shergottites.
That composition is called “chondritic” because it matches the initial solar system value represented by chondrites. Chondrites are meteorites that formed from the earliest aggregates of unmelted rocks in the solar system, about 4.56 billion years ago.
For Baxter and his colleagues, the isotope composition suggests that part of deep Mars remained unaltered by later planetary processes over the billions of years since the planet formed.
Mars formed less than 5 million years after the birth of the solar system, according to the researchers. They also point to the planet’s lack of plate tectonics as a factor that allowed these early-formed compositions to remain pristine.
The team is continuing to analyze the rock
The meteorite’s age and isotope composition provide new constraints on processes that occurred as the early solar system formed and Mars took shape.
The Boston College team is now studying additional isotope systems in NWA 13441. The researchers say those analyses will help them understand how this unusual meteorite relates to other Martian meteorites from early Mars.
The study was published in Geochimica et Cosmochimica Acta.
