Spectroscopic Observations of Asteriod 4 Vesta From 1.9 to 3.5 Micron: Evidence of Hydrated and/or Hydroxylated Minerals

Asteroid 4 Vesta has been done various observations. McCord et al. obtained the first modern extended-visible spectrum of 4 Vesta in the range from 0.3 to 1.1 μm, and showed that the surface of Vesta has a composition similar to that of certain basaltic achondrites. It was noted that visible and near infrared reflectance spectrum of Vesta is unique among the main-belt asteroids larger than 50 km and is closely matched with three subclasses of basaltic achondries: howardites, eucrites, and diogenites (HED). The geometric albedo of Vesta is not contradictory to the visible reflectivities of HED meteorites. Radar observation of Vesta is probably a consequence of Vesta’s relatively strong basaltic surface. The mean density of Vesta (3.9-4.1g/cc) is larger than ones of HED meteorites (3.1-3.4 g/cc). Small asteroids with Vesta-like spectrum are found around Vesta orbit and near-Earth orbit. These researches have strengthened the suggestion that asteroid Vesta is the parent body of the HED meteorites and a differentiated object.

Basaltic rocks formed in a reducing condition such as in vacuum have no hydrated and/or hydroxylated materials, but a result of combining its reflectance spectra in K band and L band regions imply that Vesta has absorption features to originate in OH and/or H2O-bearing and hydrocarbon (Fig. 1). However, the result includes large uncertainty due to non-uniform surface of Vesta and independently obtained each data. In order to confirm whether Vesta has any 3-μm feature or not, simultaneous and continuous spectroscopic observation of Vesta from 2.5 μm to 3.0 μm are required. Therefore, we have conducted such new 3-μm observations of Vesta.