Space weathering of Vesta and V-type asteroids: new irradiation experiments on HED meteorites

Abstract

Context. In the past few decades, several tens of main belt asteroids have been found to exhibit basaltic surface composition, similar to the one of Vesta and basaltic Howardite, Eucrite, and Diogenite achondrite meteorites (HEDs). Most of these objects (V-types) belong to the Vesta dynamical family. Several questions on the relationship between Vesta, V-types, and HEDs are still unresolved. In particular, Vesta is spectroscopically bluer than most V-types. Aims. To date, it has not yet been understood whether these spectral differences are due to space weathering, similar to what has been observed for OC meteorites and S-type asteroids. To test this hypothesis, ion irradiation experiments were performed on different samples of eucrites to simulate the effects of space weathering on Vesta and V-types by solar wind ions. Methods. Eucrite meteorites were analyzed before and after different stages of ion irradiation by VIS-NIR reflectance spectroscopy (0.4-2.5 μm). We used different ions (Ar + , C + ) with different energies (from 60 to 200 keV) to weather the samples. Results. Ion irradiation was observed to alter the spectral properties of eucrite meteorites, inducing progressive reddening and darkening of the irradiated samples. Different eucrite samples (Bereba and Dar Al Gani 684) show different reddening behaviors. Because they are patchy, reddening also varied between different parts of the same meteorite. Moreover, for both meteorites, irradiation effects are much faster (∼100 times) for C + than for Ar + ions. Conclusions. Comparing the laboratory spectra of eucrites before and after different stages of ion irradiation with those of V-types, it turns out that the slope spread shown by V-type asteroids can be explained by space weathering. These results provide new clues to the connection between Vesta, V-types, and HEDs, and are also useful in the context of the NASA Dawn mission to Vesta.

Optical spectroscopic characterizations of laser irradiated olivine grains

“…Currently, laboratory simulations of space weathering can be categorized into three main types: pulsed laser is used to irradiate analog materials to simulate effects of micrometeorite impacts (e.g., Yamada et al 1999;Sasaki et al 2001;Brunetto et al 2006); proton and ion implantations are used to simulate solar-wind irradiations (e.g., Hapke 1973;Yamada et al 1999;Brucato et al 2004;Marchi et al 2005;Fulvio et al 2012;Miller et al 2014;Kuhlman et al 2015); and thermal treatment is also used to simulate the production of npFe 0 (e.g., Tang et al 2012). Recently, Kohout et al (2014) developed a double heating method to produce npFe 0 in a controlled way.…”

Near-ultraviolet bluing after space weathering of silicates and meteorites

“…After irradiation samples have been removed from the vacuum chamber and further spectral analysis has been obtained ex-situ in a wider spectral range (see e.g., Strazzulla et al, 2005;Fulvio et al, 2012). In this paper we also consider the results obtained after pulsed laser irradiation of silicate samples (Brunetto et al, 2006).…”

Asteroids and Comets

“…Recent observations by the Dawn mission confirmed the mineralogical connection between Vesta, HEDs, and V-type asteroids De Sanctis et al, 2012 and confirmed a highly impacted surface with two large, overlapping basins near the south pole, with the youngest one perhaps just $1 Gy old (Marchi et al, 2012b;Schenk et al, 2012). However, laboratory studies suggest that ion irradiation on HEDs darkens and reddens the samples rapidly, corresponding to a space weathering timescale of 10 5 -10 6 years (Fulvio et al, 2012). The lack of space weathering on Vesta itself is thus a mystery (see summary by Pieters et al, 2006).…”