Program: GS-2006A-C-14

We propose to measure the parent volatile composition of the Jupiter-family comet 73P Schwassmann-Wachmann 3 (SW3). This ecliptic comet split in 1995 and the two brightest fragments were still highly productive on its next apparition (2001). In 2006, SW3 will make an unusually close pass to Earth, presenting an excellent opportunity to characterize the internal volatile composition of the pre-split nucleus. Molecular abundances (or sensitive upper-limits) will be measured for species (H2O, CH3OH, CO, H2CO, HCN, CH4, C2H6, C2H2, NH3, and others). We will characterize the oxidation state of volatile carbon and nitriles in SW-3 and compare it with impact ejecta from 9P Tempel-1. The resolving power offered by Phoenix is critical to this study, because it permits quantifying many spectral lines that are NOT resolved at R ~20,000. This study will provide an important and unique dataset, for comparing the interior composition of SW3 with that of two other ecliptic comets characterized by us with NIRSPEC (2P Encke and 9P Tempel-1). This study can only be done at Gemini South with Phoenix.