• Dick writes about Earth and planetary science for Science magazine.

Vesta: The Mini-Planet That Could Magnetize

11 October 2012 5:19 pm

Image courtesy of the MIT Paleomagnetism Laboratory and the MIT Experimental Petrology Laboratory

Magnetic recorders. The submillimeter-size dark regions represent fine-grained crystalline material that seems to have recorded a magnetic field on the surface of asteroid Vesta.

Earth has a familiar magnetic field, and little Mercury has an oddly strong one, but could a chunk of rock as small as an asteroid ever have had a magnetic field spawned by the churnings of its own molten metal core? Theorists have had their doubts because the smaller the core, the faster the churning must be. But now, scientists studying a meteorite in the lab report having found signs that the tiny core of the asteroid Vesta once churned hard enough to create a magnetic field. The discovery would give theorists a much-needed, extreme example of a dynamo to ponder.

Vesta researchers have long focused on certain meteorites: rocks that fell to Earth after being blasted off Vesta when other asteroids in the asteroid belt between Mars and Jupiter hit the 525-kilometer-diameter Vesta. (Scientists know the meteorites come from Vesta because the rocks' spectral colors match Vesta's as seen through Earth-based telescopes.) The meteorites' mineralogical composition showed that, like Earth and the other rocky planets but unlike almost all other asteroids, the newly formed Vesta melted and stratified into a rocky crust, a mantle, and an iron.

While orbiting Vesta over the past year, the Dawn spacecraft confirmed the existence of a metallic core, which it gauged at about 220 kilometers in diameter. (Earth's core is 7000 kilometers in diameter.) Could a core as small as Vesta's ever churn fast enough to generate a magnetic field before freezing solid eons ago? Researchers have no way of telling because, as Dawn principal investigator Christopher Russell of the University of California, Los Angeles, recently put it at a meeting, the final Dawn spacecraft "is simpler than I expected it to be." Its initial design included a magnetometer that could have detected a field permanently locked into the asteroid's crust when any early dynamo was active. But cost overruns forced the team to drop the magnetometer.

So Roger Fu of the Massachusetts Institute of Technology in Cambridge and colleagues teased apart the multiple magnetic fields in the meteorite Allan Hills A81001, a piece of vestan crust retrieved from the Antarctic ice. It was a promising candidate because its small crystals—formed when molten crust rapidly cooled—would have faithfully recorded any magnetic field at the surface of Vesta at the time. Fu and colleagues progressively demagnetized samples of the meteorite using an alternating magnetic field or by heating.

Judging by the field remaining after each stage of demagnetization, Fu and colleagues identified a weak magnetic field that could have locked into cooling crust on Vesta, they report online today in Science. In addition, they dated that cooling to about 3.7 billion years ago—a billion years after Vesta formed. That was too late for any dynamo to be active, but it was also too late for sources like the churning gas and dust of the still-forming solar system to be responsible for the 2- to 12-microteslas field identified in the meteorite. (Earth's magnetic field ranges up to 65 microteslas at the surface.)

Instead, the group argues that the meteorite's field is a later rerecording. A dynamo churning in the few million years after Vesta's formation would have imprinted itself on the newly formed crust. Almost a billion years later, a large impact by another asteroid melted some crust that promptly froze to form new crust, including a chunk that became Allan Hills A81001.

The melting erased the original dynamo imprint, but as that rock cooled, the surrounding crustal imprint lingering from the time of an active dynamo would have in turn imprinted itself on the future meteorite. Eons later, a smaller impact would have blasted Allan Hills A81001 off Vesta without erasing the imprinted field. So the group believes they have detected a copy of a recording of a dynamo field that is still enveloping the asteroid.

The finding "probably does suggest a dynamo, but it falls a bit short of proof of a dynamo," says paleomagnetician John Tarduno of the University of Rochester in New York. Being an "echo" of a dynamo weakens the researchers' case, he says. He is looking for the dynamo's initial magnetic imprint on a Vesta meteorite rather than a rerecording, so far without luck.

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