On the Puzzle of Space Weathering Alteration of Basaltic Asteroids

Abstract

The majority of basaltic asteroids are found in the inner main belt, although a few have also been observed in the outer main belt and near-Earth space. These asteroids-referred to as V-types-have surface compositions that resemble that of the 530 km sized asteroid Vesta. Besides the compositional similarity, dynamical evidence also links many V-type asteroids to Vesta. Moreover, Vesta is one of the few asteroids to have been identified as source of specific classes of meteorites, the howardite, eucrite, and diogenite achondrites (HEDs). Despite the general consensus on the outlined scenario, several questions remain unresolved. In particular, it is not clear if the observed spectral diversity among Vesta, V-types, and HEDs is due to space weathering, as is thought to be the case for S-type asteroids. In this Letter, SDSS photometry is used to address the question of whether the spectral diversity among candidate V-types and HEDs can be explained by space weathering. We show that visible spectral slopes of V-types are systematically redder with respect to HEDs, in a similar way to what is found for ordinary chondrite meteorites and S-types. On the assumption that space weathering is responsible for the slope mismatch, we estimated an upper limit for the reddening timescale of about 0.5 Ga. Nevertheless, the observed slope mismatch between HEDs and V-types poses several puzzles to understanding its origin. The implication of our findings is also discussed in light of the Dawn mission to Vesta.

Surface Composition of Vesta: Issues and Integrated Approach

“…The strength of the bands observed for the Vestoids are much closer to that observed in laboratory studies of meteorites (Binzel and Xu 1993;Hiroi et al 1995;Pieters et al 2006). Marchi et al (2010), Moskovitz et al (2010), andDe Sanctis et al (2011b) investigated several of the small Vestoids using earth-based telescopes and found that their visible slopes are generally redder than that of Vesta and systematically redder than HEDs measured in the laboratory. The observed trend for Vestoids (steeper continuum, stronger absorption bands) is opposite that observed for S-type asteroids (steeper continuum, weaker absorption bands), again suggesting that Vesta and its family of small asteroids is distinctive.…”

“…Since the discovery that Vesta's near-infrared reflectance spectrum is closely comparable to laboratory measurements of particulate samples of unweathered HED meteorites (McCord et al 1970), it has been believed that Vesta's surface shows little or no evidence of space weathering alteration. On the other hand, laboratory experiments conducted on HEDs, and pyroxene constituents, suggest that Vesta-like materials should indeed alter under the effects of ion bombardment and micrometeorite impacts in a way similar to that observed for ordinary chondrite-like materials, but at Vesta the alteration timescale is longer (Hiroi and Pieters 1998;Marchi et al 2005;Vernazza et al 2006). The many controlling variables on the rate of changes with space weathering are poorly constrained.…”

Spectral and mineralogical characterization of inner main-belt V-type asteroids

“…Most of them are nonfamily members, according to the Nesvorný et al (2008) definition. The mean orbital parameters, taken from the astdys web site (http://hamilton.dm.unipi.it/astdys/), the absolute magnitude and the diameter, obtained assuming an albedo of 0.3 as in Marchi et al (2010), are provided in Table 1.…”

“…The asteroid spectra were finally normalized to achieve a reflectance value equal to 1 around 1.6 μm. Marchi et al (2010). The family membership has been defined as in Nesvorný et al (2008).…”

“…However, they reported that most of the observed V-ype asteroids show a low abundance of Ca (<10% Wo) with a predominance of diogenitic mineralogies among the non-family members. The spectral diversity between V-type asteroids belonging to the Vesta family and unrelated to it, has been recently addressed by Marchi et al (2010) who found differences in the spectral slopes trends possibly caused by the presence of diogenite-rich asteroids.…”