OH and H2O on Vesta and on the Moon: A comparative analysis

Searching for OH and H2O on Vesta and on the Moon is key [1] because the presence of these molecules at the surface of objects in the inner solar system is fundamental for understanding the formation of water in the early history of the solar system, as well as its implications on the mineralogical, thermodynamical, and geological evolution of the planets [2, 3]. Vesta is mineralogically differentiated, more evolved than most of the asteroids, but not as much as a planet [4, 5]. The surface of Vesta shows very strong pyroxene absorption bands in reflectance spectra [6 - 8], and the howardite, eucrite, diogenite (HED) meteorites that are of Vestan origin are also pyroxene-rich [9 - 13], which strongly suggests that the surface of Vesta is mostly made of basalt. This basaltic-rich surface composition of Vesta is one similarity with the surface of the Moon, where OH and H2O have been detected by imaging spectroscopy in the near-infrared [14 - 16]. On Vesta, the possible origins for OH and H2O are the same as what has been suggested for the Moon [17]: 1) indigenous to the object, 2) in-fall of hydrated exogenous material, and 3) implantation of solar wind protons. The objective of this study is to derive abundance estimates for OH and H2O at the surface of Vesta, and to compare them with abundances found on the Moon. The analysis of OH and H2O at the surface of Vesta is done with the Visible and Infrared mapping spectrometer (VIR) [18] onboard the Dawn orbiter using reflectance spectra between 0.25 and 5.1 μm. We are also analyzing again data from the Moon Mineralogy Mapper (M3) a thermal emission correction based on modeling of brightness temperature as function of the surface roughness [19]. VIR has detected an absorption band at 2.8 μm, attributed to OH [11, 12, 13], in coherent spatial units that have been observed multiple times. Associations with surface morphological features are complex. No correlation with temperature or with illumination suggests that the formation of OH is not likely due to short-term surface processes [17]. On the Moon, feldspathic areas such as highlands show stronger absorption bands of OH and H2O. Vesta is less feldspathic than the Moon, and the strong pyroxene absorption bands indicate that Vesta surface particles are more crystalline, likely with fewer defects than lunar soils, which is less favorable to proton implantation. [1] Pieters et al., 2011, Space Science Rev. 163. [2] Cyr et al., 1998, Icarus 135. [3] Encrenaz et al., 2008, Ann Rev 46. [4] Russell et al., 2007, Earth Moon Planets 101. [5] Russell et al., 2012, Science 336. [6] McCord et al., 1970, Science 168. [7] Gaffey et al., 1997, Icarus 127. [8] De Sanctis M.C. et al., 2012, Science, 336. [9] Binzel R. P. and Shui X., 1993, Science, 260. [10] Bogard & Garrison, 1993, Meteoritics 28. [11] Takeda 1997, M& PS 32. [12] Moskovitz et al., 2010, Icarus 208. [13] De Sanctis et al., 2011, MNRAS 412. [14] Pieters et al., 2009, Science 286. [15] Sunshine et al., 2009, Science 286. [16] Clark et al., 2009, Science 286. [17] McCord et al., 2011, J. Geophys. Res. 116. [18] De Sanctis et al., 2010, Space Science Rev 163. [19] Bandfield, 2009, Icarus 202. [20] McCord et al., 2011, EPSC-DPS ,abstract 567 [21] Combe J.-Ph. et al., 2012, LPSC, 2643, [22] De Sanctis M.C. et al., ApJ, submitted.