Why doesn't Eros have a magnetic field?

Posted: May 31, 2001

BOSTON, Mass. - Scientists said this week that they may know the reason why the asteroid Eros appears to lack a measurable magnetic field.

Asteroid Eros as seen by NEAR Shoemaker. Photo: JHU-APL
Two researchers told attendees of the American Geophysical Union's spring meeting in Boston that Eros may contain a large number of tiny, individually-magnetized bodies whose magnetic fields are randomly oriented, effectively canceling each other out.

Eros was the subject of detailed study for over a year by the Near Earth Asteroid Rendezvous (NEAR) spacecraft. The magnetometer on NEAR failed to measure any sign of a global magnetic field for the asteroid down to the limits of the instrument's sensitivity, 1-2 nanotesla. By comparison, the Earth's magnetic field is about 50,000 nanoteslas.

"Eros is a remarkably unmagnetized body," said Mario Acuna, team leader of the magnetic field experiment on NEAR. "It is essentially a non-magnetic object."

The lack of a magnetic field was puzzling because spacecraft flybys of other asteroids, such as Gaspra and Braille, had detected magnetic fields. In addition, most meteorites, including those of the class most closely associated with Eros, are also magnetic.

One possible explanation for the meteorites is that they have been "contaminated" by the Earth's magnetic field, becoming magnetized only after arriving at Earth. To test this, Gunther Kletetschka, a researcher with the Catholic University of America and NASA's Goddard Space Flight Center, exposed chondrules -- tiny droplets of primitive material embedded in the meteorite -- to changes in temperature and magnetic fields similar to what a meteorite might experience when arriving on Earth.

In about half of the cases Kletetschka found that the chondrules took on the magnetization they were exposed to in the experiment. However, in the other half of the cases there was no change in the chondrule's magnetism, indicating that it has a strong remnant magnetism that likely existed since the chondrule was formed billions of years ago, during the formation of the solar system itself.

Since Eros is similar to the class of chondritic meteorites studied in the lab, it likely contains a large number of magnetized chondrules. However, Kletetschka argues that if the chondrules are randomly oriented, they will effectively cancel each other's magnetic fields out, preventing a global magnetic field from forming. This is seen to a limited degree in the lab, where larger samples of chondritic meteorites appear to have weaker magnetization than smaller ones as the randomizing effect begins to take hold.

If true, this implies that Eros has not been modified since the formation of the solar system in any way that would alter the magnetization of the chondrules. "Eros is a very primitive object," said Acuna.

To confirm this, however, would require obtaining samples from the asteroid itself, which is not likely to happen for the foreseeable future. Also required, say scientists, are studies of other asteroids, including follow-up studies of Gaspra and Braille to confirm that they have the magnetic fields detected in previous flybys.