When astronauts returned from the moon, they brought back a collection of puzzling, slightly magnetized lunar rocks. The moon is too small for its core to have grown hot enough to churn and create a magnetic field, so researchers have attributed the magnetism to everything from asteroid impacts to measurement errors. Dave Stegman, a graduate student at the University of California at Berkeley, now finds evidence that the true source was a catastrophic episode early in the moon's history. "Basically, the moon burped," he says.


A 3-D model of the early moon shows a giant blob of hot magma (red) rising to the cool lunar surface (blue).
Photograph courtesy of Dave Stegman/University of California at Berkeley.

In scientists' standard model, the moon formed 4.5 billion years ago from the wreckage of an Earth-asteroid impact and has settled down steadily ever since [see "Where Did the Moon Come From?" Discover, February 2003]. Stegman's supercomputer simulations add a wrinkle: As the crust cooled, lighter elements rose to the surface while denser radioactive elements sank, forming a layer around the moon's iron core. Heat from that radioactive blanket built up until it sent a huge belch of magma toward the surface. The rapidly cooling core would then circulate vigorously, creating a short-lived magnetic field that vanished after about 300 million years. The plume might also have spawned the moon's distinctive "seas," actually plains of old lava. Stegman sees the moon as a priceless record of the early stages of planetary development: "Its entire history is laid out right on the surface."