DETECTION OF WIDESPREAD HYDRATED MATERIALS ON VESTA BY THE VIR IMAGING SPECTROMETER ON BOARD THE DAWN MISSION

Abstract

Water plays a key role in the evolution of terrestrial planets, and notably in the occurrence of Earth's oceans. However, the mechanism by which water has been incorporated into these bodies-including Earth-is still extensively debated. Here we report the detection of widespread 2.8 μm OH absorption bands on the surface of the asteroid Vesta by the VIR imaging spectrometer on board Dawn. These observations are surprising as Vesta is fully differentiated with a basaltic surface. The 2.8 μm OH absorption is distributed across Vesta's surface and shows areas enriched and depleted in hydrated materials. The uneven distribution of hydrated mineral phases is unexpected and indicates ancient processes that differ from those believed to be responsible for OH on other airless bodies, like the Moon. The origin of Vestan OH provides new insight into the delivery of hydrous materials in the main belt and may offer new scenarios on the delivery of hydrous minerals in the inner solar system, suggesting processes that may have played a role in the formation of terrestrial planets.

The geological nature of dark material on Vesta and implications for the subsurface structure

“…Carbonaceous chondrites have a very low albedo and may be considered as the leading candidate for the darkening agents. In addition, dark deposits listed in the VIR catalog show an increased OH absorption feature as the albedo decreases (Palomba et al, 2014), in agreement with the behavior retrieved for the two broad low albedo areas found by De Sanctis et al (2012b). One may assume that low-velocity impacts (O'Brien et al, 2011;Reddy et al, 2012) that are likely to occur on Vesta will have preserved parts of the impactor material that has subsequently been distributed across the surface.…”

“…Russell et al, 2013), that contain clasts of carbonaceous chondrite material within a matrix of pyroxene-rich basaltic material (Cloutis et al, 2013;Herrin et al, 2011;McCoy and Reynolds, 2007). Moreover, the broad correlation between dark material and OH signature, detected by VIR, and H abundance, detected by GRanD, (De Sanctis et al, 2012b;Prettyman et al, 2012) is a further indication of carbonaceous chondrite as darkening agent of the Vestan regolith. Carbonaceous chondritic clasts are the main xenolithic clasts in howardites and exhibit varying degrees of aqueous alteration.…”

“…Another argument for the correlation of dark material and Veneneia is the concentration of H and OH, which corresponds to that of the broader dark areas (Prettyman et al, 2012;McCord et al, 2012;De Sanctis et al, 2012b;Reddy et al, 2012) (Figs. 19 and 20).…”

Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy

“…Analysis of Ne in regolithic howardites indicates that the vestan regolith contains 1-to 100-µg/g H from the solar wind (11). The relatively young Rheasilvia impact basin (59, 60) contains the lowest concentrations of H and OH on Vesta (11,61). If H within Rheasilvia was delivered primarily by the solar wind, then the 250 µg/g lower bound from GRaND is representative of the actual globally averaged [H] in Vesta's upper regolith.…”

Characterization of asteroid analogues by means of emission and reflectance spectroscopy in the 1- to 100-µm spectral range

“…A weak hydration signature at 3 μm was detected from telescopic observations (Rivkin et al 2006;Hasegawa et al 2003), but not confirmed by later observations. The VIR imaging spectrometer on Dawn detected OH (hydroxyl) bands on regions of Vesta's surface (De Sanctis et al 2012). The distribution of the hydration band on Vesta is likely related to differences in the mineralogy of its surface materials and/or the presence of exogenous OH-bearing materials.…”