[meteorite-list] Why Doesn't Eros Have A Magnetic Field?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:46:26 2004
Message-ID: <200105311714.KAA03508_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/news/n0105/31eros/

Why doesn't Eros have a magnetic field?
By Jeff Foust
Spaceflight Now
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.

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.
Received on Thu 31 May 2001 01:14:42 PM PDT