Most Mars rocks on Earth carved from one crater

OUR Martian visitors may have had  us fooled. Most meteorites from the Red Planet could be from the same crater, which would make them billions of years older than believed. About 80 per cent of meteorites that have fallen to Earth from  Mars belong to a group called the shergottites. Looking at the way minerals melted and reformed in these rocks, previous studies said that shergottites are 600 million years old at most. But Stephanie Werner at the University of Oslo in Norway and her colleagues now say that the rocks have all come from the same place, Mojave crater on Mars, which would make them 4.3 billion years old. When planetary material is launched into space by an impact, it gets exposed to cosmic rays, which alter the rocks’ chemical signature. Based on their cosmic ray exposure, the shergottites must have been launched between 1 and 5 million years ago. Werner’s team calculates that the impact that formed Mojave crater happened about 3 million years ago, so could have dislodged these rocks (Science, doi.org/rtr). Also, mineral maps from spacecraft in orbit show that the rocks in the crater have similar ingredients to shergottites. If all the shergottites are from Mojave, they should be as ancient as the material in the rest of the crater. Werner’s team points to previous work studying the ratios of different kinds of lead in the meteorites, which suggests that the rocks are, in fact, between 4.1 and 4.3 billion years old. Audrey Bouvier at the University of Western Ontario, Canada, is excited about the results. She thinks the shergottites were not dated carefully enough in the past, and the revised ages are a better fit. But Harry McSween at the University of Tennessee in Knoxville says that shergottites fall into subgroups with different cosmic ray ages and mineral properties, which suggests they came from different craters.  Jacob Aron  ■