[meteorite-list] Does Asteroid Vesta Possess A Magnetic Field?

[Ron Baalke]

http://www.obspm.fr/actual/nouvelle/apr06/vesta.en.shtml

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.