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 6.6x1015 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 can not have reached the asteroid surface. A remanent 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). 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 6.6x1015 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 can not have reached the asteroid surface. A remanent 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.

Contact

• Pierre Vernazza (Observatoire de Paris, LESIA)