Vesta is smallest known planetary object to have generated dynamo

A new study has found evidence that Vesta, the second-most-massive asteroid in the solar system, once harboured a dynamo.

Updated: Oct 12, 2012, 12:37 PM IST

Washington: A new study has found evidence that Vesta, the second-most-massive asteroid in the solar system, once harboured a dynamo -- a molten, swirling mass of conducting fluid generating a magnetic field -- resembling that in much larger planets like Earth.

The findings suggest that asteroids like Vesta may have been more than icy chunks of space debris, said researchers at MIT.

“We’re filling in the story of basically what happened during those first few million years of the solar system, when an entire solar system was dominated by objects like this. These bodies are really like miniature planets,” said Roger Fu, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), and the study’s first author.

Co-author Benjamin Weiss, an associate professor of planetary sciences in EAPS, asserted “Vesta becomes now the smallest known planetary object to have generated a dynamo.”

“You can imagine many asteroids in the early solar system were doing this,” he stated.

Most or all of the planets in the inner solar system are thought to have generated dynamos at some point in their histories. In a dynamo, molten-hot iron flows within the core, generating a magnetic field that may last for millions of years. As a result, the rocks on the surface of a planetary body become magnetized, providing a record of a planet’s early history.

Scientists have attempted to characterize the magnetization of meteorites -- remnants of asteroids that have fallen to Earth -- in order to reconstruct asteroid evolution. But a major challenge has been pinpointing the source of meteorites’ magnetization, which may be formed by any number of processes -- such as plasmas from a meteoroid impact, or more mundane causes, like passing a magnet over a meteorite sample.

Determining that a meteorite’s magnetic field is the result of an early dynamo is therefore a tricky problem.

To solve the problem, Fu and Weiss collaborated with researchers at the University of California at Berkeley, first to determine the magnetization and the age of a meteorite sample, then to check that the observed magnetic field was, in fact, due to an early dynamo.

The group obtained a meteorite sample from Vesta that was originally discovered in Antarctica in 1981. The 50-gram sample, named ALH A81001, retains exceptional magnetic properties that scientists have been examining for years. Fu and his colleagues managed to acquire a one-gram sample of the rock for analysis.

The team first examined the rock’s tiny crystals. When forming in a magnetic field, a rock’s ferromagnetic crystals align in the direction of a background field when the rock is heated. The group measured the alignment of these minerals, or the rock’s magnetic “moment.” The researchers progressively demagnetised the rock until they found the magnetization that they believed to be the oldest remnant of a magnetic field.

The group’s next step was to determine the age of the rock. To do that, UC Berkeley researcher David Shuster analysed the meteorite for evidence of argon. An isotope of argon called argon-40 is produced from the natural decay of potassium-40. A common technique for determining a rock’s age is to heat the rock and measure the amount of argon-40 released: The more argon-40, the older a rock may be. Through this technique, the researchers determined that the Vesta meteorite is 3.7 billion years old.

However, because Vesta formed 4.5 billion years ago, any early dynamo must have decayed by the time the meteorite now known as ALH A81001 formed. So what is the origin of the field that magnetized this rock?

Fu and Weiss believe that an early dynamo likely magnetized the surface of Vesta within the first 100 million years of the asteroid’s history, magnetizing surface rocks that then persisted over billions of years. When ALH A81001 formed 3.7 billion years ago, it would have also become magnetized due to exposure to fields emanating from the surrounding crust.

The finding was published this week in Science.