In 2022, a strange rock was found in the Sahara Desert, and now a new analysis reveals that it’s even more valuable to science than it first appeared. This fragment, weighing about two pounds, came from Mars’ crust. It was likely ejected by an impact, flying through space until it eventually reached Earth. The rock is one of the oldest known samples from our neighboring planet, estimated to be at least 4.1 billion years old. At that time, Earth was a young planet with intense geological activity, likely with oceans but no animals, plants, or any other complex life.

The location is not what makes this finding surprising. The Sahara has long been a kind of natural archive of the history of Earth and the solar system. Among its sands, scientists have found 40-million-year-old fossils and other meteorites. In 2019, for example, a 4.5-billion-year-old fragment was recovered that, according to recent research, may have come from an ancient, now-vanished protoplanet, possibly as large as the moon.

The meteorite, named Teghaza 001, might have remained just another entry in the collection of Martian fragments found on Earth. Although these rocks are extraordinarily useful for studying Mars’ composition and geological history, they often provide limited information about the environmental conditions of its distant past. However, a series of studies published in the journals Science and Science Advances revealed that Teghaza 001 is not like other samples. In addition to being exceptionally old, it has preserved chemical traces of the interaction between materials in the Martian crust and its ancient hydrosphere.

“One of the paper’s reviewers called it a Rosetta Stone for Mars,” Yang Liu of NASA’s Jet Propulsion Laboratory said in a press release.