Uv Absroption Features of Asteroid

Introduction: Asteroid (1) Ceres is a unique asteroid , or dwarf planet, yet to be well understood. Classified as G-type [1], Ceres has been generally considered to consist of carbonaceous chondrite material [2]. The weak but complex spectral features in the 3-µm region of the spectrum of Ceres has been known for a long time, but no agreement has been reached on the identification of the specific species. Early studies suggested hydrated and/or ammoniated minerals accounted for the feature [3,4,5]. Recently the discovery of carbonates and iron-rich clays was reported from the 3µm region [6]. A 3-σ detection of water emission off the limb of the north pole of Ceres was reported [7], but never has been repeated. The most recent measurement of the size and shape of Ceres suggested ~25% water in its composition, and a differentiated interior [8]. The evolutionary models suggested liquid water in a mantle beneath the surface in the past, and even possibly today [9]. As one of the two target asteroids of NASA's Dawn mission, scheduled to be orbited in 2015 [10], significant effort has been made to study the composition of Ceres in various spectral regimes. The ultraviolet (UV) spectral region is substantially underexploited in the study of asteroids. Recent imaging photometry of Ceres using Hubble Space Telescope (HST) has drawn our attention to the UV region for Ceres. [11] reported a possible wide and strong UV absorption feature in Ceres' spectrum. In order to confirm the existence of the absorption band and to characterize it, we obtained new images as well as slitless spectra of Ceres from 120 to 200 nm, using the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) onboard HST. We will report the preliminary results of photometric measurements from the far-UV images and discuss the implications. Observations and Data Reduction: Three HST orbits were used to observe Ceres in the far-UV. During each orbit, one 600-sec image of Ceres was obtained through the F125LP filter. The remaining time was used to integrate three slitless spectra of Ceres through the objective prism PR130L, reaching a total exposure time of ~1800 sec. All data were first processed by the data reduction pipeline at the Space Telescope Science Institute. We then measured the total brightness of Ceres in DNs from all exposures, including both images and spectra. Visual inspections show that, within the 200-pixel radius aperture (Ceres being