Rare Mars meteorite preserves 1.27-billion-year-old clues to planet’s deep interior

Boston College scientists have determined that a newly discovered meteorite from Mars is 1.27 billion years old and derived from a previously unsampled, pristine source on the Red Planet. The study is published in the journal Geochimica et Cosmochimica Acta.

Rocks from Mars must be knocked off the planet by some kind of impact, which has limited the samples available to scientists to approximately 400 Martian meteorites that have been found. Samples from the most common group of igneous Martian rocks, called shergottites, that have been dated are almost entirely geologically young, or less than 600 million years old.

Between that point in time and the next-oldest characterized shergottites, dating back 2.4 billion years, is an approximately 1.8-billion-year gap in understanding the geology and history of Mars. Scientists had no shergottite samples from that period to enrich information about geological activity on Mars, said report coauthor Boston College professor of earth and environmental sciences Ethan Baxter.

A sample from the missing era

Two other rare classes of Martian meteorites, called chassignites and nahklites, have also been dated to 1.3–1.4 billion years ago, but these are chemically distinct from shergottites like this new sample. Baxter and his lab group used cutting-edge radiogenic isotope techniques to determine the meteorite’s crystallization age and composition. The discoveries were unexpected.

“The characteristics of this meteorite were entirely surprising,” said Baxter, who founded the Boston College Center for Isotope Geochemistry, where most of the work was conducted. “No other Martian meteorite like this is 1.27 billion years old.”

The team studied a Martian meteorite referred to as Northwest Africa (NWA) 13441, which was discovered in Algeria in 2019. Baxter’s team at BC obtained a small sample from a colleague at Appalachian State University that was cleaned and crushed, as well as a thin section of NWA 13441 mounted on a glass slide.

The sample had yet to be characterized. Working with colleagues from the Scripps Institution of Oceanography and Britain’s The Open University, Baxter’s team set out to confirm the sample’s suspected Martian origin, determine its crystallization age and analyze its chemical composition compared with other Martian meteorites.

An isotopic signature unlike others

Central to the study was the analysis of neodymium, a naturally occurring but rare element that consists of seven isotopes. The neodymium isotopic composition of the sample matches the value for the initial solar system, unlike that of any other shergottites, the team reported.

The team at Boston College used high-precision techniques to date the sample to approximately 1.273 billion years old, the first such discovery in a nearly 2-billion-year period from which there have been no shergottites to study.

“Martian meteorites of this type are either younger than 600 million years old or approximately 2.4 billion years old,” said coauthor Dylan M. Seal, a BC doctoral candidate. “We dated this sample to be 1.273 billion years old, which fills a roughly 2-billion-year gap for which we had no shergottite samples to provide information about magmatic and volcanic activity on Mars.”

The initial neodymium isotope composition of the sample is the same as that of the initial solar system, represented by another class of meteorites known as chondrites, which are the earliest aggregates of unmelted rocks to form in the solar system 4.56 billion years ago.

This “chondritic” composition for NWA 13441 has never been observed in shergottites and suggests that a portion of deep Mars remained unaltered by subsequent planetary processes in the billions of years since the planet formed, Baxter said.

Mars formed very quickly, Seal noted, less than 5 million years after the birth of the solar system, and it does not have plate tectonics like Earth, allowing these early-formed compositions to remain pristine.

Clues to Mars’s earliest interior

Equally surprising was its chondritic initial isotope composition, which provides important new boundaries on the processes that occurred in the very early solar system as Mars formed, reported Baxter and Seal, along with coauthors Melody Z. Chen, an undergraduate researcher, and former postdoctoral scholar and current Lunar and Planetary Institute scientist Robert W. Nicklas.

Baxter said his Boston College lab continues to study the important meteorite sample with undergraduates and external collaborators.

“Our goal is to analyze additional isotope systems that will help us better understand how this unique sample relates to other Martian meteorites on early Mars,” Baxter said.

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