A team of scientists in the US says that the asteroid Vesta probably had a rotating liquid core in its early history. This, the researchers say, created a dynamo that produced a magnetic field strong enough to magnetize the rocks on its surface. As it was previously thought that only larger planets, such as Earth, had dynamos, the work suggests that protoplanets, like asteroids, may be more planet-like than previously thought. The findings could help researchers better understand the early history of the formation of the solar system.

Vesta is one of the most massive asteroids in the solar system, second only to the dwarf planet Ceres. The Dawn mission to study both Vesta and Ceres was launched by NASA in 2007. It entered orbit around Vesta on 16 July 2011 then on 5 September this year it left orbit and it is currently en route to Ceres, where it is scheduled to arrive in February 2015. Vesta is known to have lost about 1% of its mass in a collision that is thought to have occurred a billion years ago. This left a massive impact crater occupying much of Vesta's southern hemisphere and debris from this event has fallen to Earth as howardite–eucrite–diogenite (HED) meteorites. These have been traced back to the asteroid by matching the unique oxygen-isotope "fingerprints", which prove that the meteorites originated from Vesta.

Active cores

It is samples of one of these meteorites – an eucrite meteorite ALHA81001, found in Antartica – that Roger Fu and colleagues from the Paleomagnetism Laboratory at the Massachusetts Institute of Technology in the US have studied. "Our group studies magnetism in rocks – terrestrial rocks, lunar rocks and, now, we have been studying meteorites from Vesta," explains Fu. He points out that the Dawn spacecraft does not have a magnetometer on-board, so the magnetic properties of the asteroid must be inferred from the HED meteorites that are being tested in the lab.

Vesta has a metallic core, which consists of 5–25% of its total planetary mass and is thought to have formed sometime within the first 1–4 million years of the solar system's existence. Via remote sensing, Dawn has made the most accurate measurement yet of Vesta's current size, and has approximated Vesta's core to be about 107–113 km in radius.

Fu and colleagues now say that Vesta's liquid metallic core once had a dynamo. This is the means by which a planetary body creates and maintains a magnetic field via a rotating, convecting and electrically conducting fluid at its core. "If a rock cools in a magnetic field, it records the magnetic properties of the field; so by studying it in the lab, we can deduce the strength of the field," explains Fu. The team did other studies using argon–argon dating and concluded that the meteorite acquired its magnetic signature about 3.69 billion years ago from a surface magnetic field large enough to have been created by an ancient core dynamo. Because this period of magnetization was well before the impact in which the meteorite was created, the team can conclude that the magnetization could not have been induced as a result of the meteorite re-heating on Earth. "When the meteorite formed 3.7 billion years ago, Vesta did not have an active core dynamo. So, this means that there had to be a considerable surface magnetic field on Vesta – to induced magnetism in the rocks – that remained even after the core was not active," says Fu.

Magnetic sunscreen?

In addition, other data show that the surface of Vesta seems to be less space-weathered than expected. If a sample of the Vesta meteorite is exposed to an ionized beam – representing ionized solar wind that causes weathering – the sample changes colour and mineralogical changes are seen too. "But what we actually see on Vesta is different – it is not as weathered. Something is preventing the solar wind from eroding it as much, and there is a good chance that it is the magnetic-field remnant that is doing so," says Fu. He says that the strength of the field required to shield the Vestian surface from the solar wind matches that of the actual field strength deduced from the meteorite. "There is a linear relation between the strength of magnetism in the rocks and the strength of the field they were formed in, so we can calculate it. And the strength of the magnetic field corresponds to the size of the core dynamo, which depends on how big the core is, so it is all linked," says Fu. These findings, combined with the Dawn data, have also provided the final evidence necessary to say that the HED meteorites do originate from Vesta. "There is no lingering doubt now," explains Fu.

In the future, Fu and colleagues will consider magnetism in the early solar system; indeed, they hope to study magnetic fields in protoplanetary discs. "Theorists have not modelled magnetic fields in nebulae in decades, and we would like to do that," says Fu.

The research is published in Science.