While the moon has no global magnetic field nowadays, it did have one in the past and researchers believe it lasted at least a billion years longer than previously thought. The Guardian reports: Between 4.25 billion and 3.56 billion years ago, the lunar magnetic field was similar to that of the Earth. The field is thought to have been generated by the churning movement of fluids within the moon's molten core -- a sort of lunar dynamo. But scientists have long puzzled over when the magnetic field disappeared, with previous research unable to tell whether the field had disappeared completely by 3.19 billion years ago or had lingered on in a weaker form. Writing in the journal Science Advances, Sonia Tikoo, a planetary scientist and co-author of the research from Rutgers University, and colleagues from the University of California, Berkeley and Massachusetts Institute of Technology, describe how they set about unpicking the conundrum by analyzing a lunar rock brought back by the Apollo 15 mission in 1971. The sample contains fragments of basalt that had broken off larger rocks. According to a dating technique based on the ratio of different isotopes of argon, the basalt formed from lava flows about 3.3 billion years ago. These fragments are bound together in the sample by a glassy material, which the team say probably formed when some of the basalt melted following a meteorite impact. The researchers dated the formation of the glassy material to between 1 billion and 2.5 billion years ago. Crucially, the impact also melted iron-containing grains within the basalt. These crystalized again within the glassy material as it quickly cooled, capturing a record of the magnetic field of the moon at that time.