Does the asteroid Vesta possess a magnetic field? (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Pierre Vernazza, Observatoire de Paris, LESIA
Tél: 33 1 45 07 74 09
Fax: 33 1 45 07 71 02

25 April 2006

Does the asteroid Vesta possess a magnetic field?

Since 30 years, Vesta, one of the three largest main belt bodies, poses
a problem to the scientists: "Given that its basaltic surface is roughly
similar to the lunar surface, which is intensely space weathered, why is
Vesta not?" Astronomers from the Paris Observatory (LESIA), the
Observatory of Catania and from the CEREGE laboratory brought for the
first time a plausible explanation to this question, suggesting the
presence of a magnetic field on this asteroid!

The Solar Wind (ions and electrons) affects Solar System bodies that are
not protected by an atmosphere or a magnetosphere (e.g. the Moon and
asteroids), altering the optical properties of their soil. This
alteration changes the spectral properties of silicate-rich objects,
inducing progressive darkening and reddening of the solar reflectance
spectra in the UV-Vis-NIR range.

The surface of the asteroid Vesta, one of the three largest main belt
bodies (D = 529 +/- 10 km), is surprisingly pristine. Recent ion
irradiation experiments on pyroxenes have shown significant reddening
and darkening of the collected spectra with progressive irradiation.
Since pyroxene is a major surface component of Vesta as determined by
spectroscopy, a team from the Paris Observatory led by Pierre Vernazza
aimed to test whether the solar wind irradiation alters significantly
the optical properties of the surface of Vesta.

Consequently, an ion irradiation experiment has been performed (at the
Observatory of Catania) on a eucrite meteorite (basalt) called Bereba,
which characterizes well the surface of Vesta, in order to simulate the
solar wind irradiation on this asteroid.

Irradiation of a virgin sample of Bereba (Figure1a), whose spectrum and
albedo are very close to those of Vesta (albedo ~ 0.35), yields a
spectrum that is very similar to the Moon's, in terms of spectral slope
and albedo (Moon's albedo ~ 0.1). It appears that space weathering
affecting the Moon surface minerals left Vesta's surface unaltered.
Moreover, the 6.6x10**15 Ar++/cm2 ion fluence used in this experiment
corresponds to a timescale for the solar wind ions at 2.36 AU (average
heliocentric distance of Vesta) of about 105 years.

This result implies that, if solar wind ions do reach the surface of
Vesta, its reflectance spectrum should be much redder and its albedo
lower. Indeed, this implies that solar wind particles cannot have
reached the asteroid surface. A remnant magnetic field is the most
likely way of forming an obstacle to the solar wind flow resulting in
its diversion. The present data does not enable to distinguish between a
global magnetic field producing a bona fide magnetosphere (Figure 2a)
and a number of uniformly magnetized blocks of crustal material
uniformly magnetized producing several crustal "magnetospheres" (Figure 2b).

Just as Jupiter's magnetic field has been detected by remote sensing via
its radio emission, long before space exploration, the present work
provides a remote detection of Vesta's magnetic field via its color,
opening the way to a novel technique of asteroid exploration.

Reference

Asteroid colors: a novel tool for magnetic field detection? The case of
Vesta
P. Vernazza, R. Brunetto, G. Strazzulla, M. Fulchignoni, P. Rochette, N.
Meyer-Vernet, I. Zouganelis
A&A Letters, 2006, in press.

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/apr06/vesta-f1.gif (25KB)]
a) VIS-NIR (0.4-2.5 um) reflectance spectra of the eucrite meteorite
Bereba before and after irradiation with two different Ar++ ion fluences.
b) The initial reflectance spectrum of Bereba (1) and that obtained at
the highest ion fluence (3) are shown scaled to 1 at 0.7 um, and
compared with the spectrum of Vesta and the spectrum of a small lunar
mare area.

[Figure 2a:
http://www.obspm.fr/actual/nouvelle/apr06/vesta-f2a.jpg (23KB)
Figure 2b:
http://www.obspm.fr/actual/nouvelle/apr06/vesta-f2b.jpg (23KB)]
Two sketches of Vesta's magnetic field. The yellow area represents the
regions directly accessible to the solar wind particles. The white areas
represent the zone protected by the magnetic field. In red are the
magnetic field lines. In brown, Vesta.
a) (left) Vesta protected by a global magnetic field, producing a
magnetosphere akin to the Earth's.
b) (right) Vesta protected by local crustal "magnetospheres". In this
case, the ions can reach the unprotected parts of the surface.