The peculiar volatile composition of comet 8P/Tuttle: A contact binary of chemically distinct cometesimals?

We report measurements of eight native (i.e., released directly from the comet nucleus) volatiles (H2O, HCN, CH4, C2H2, C2H6, CO, H2CO, and CH3OH) in comet 8P/Tuttle using NIRSPEC at Keck 2. Comet Tuttle reveals a truly unusual composition, distinct from that of any comet observed to date at infrared wavelengths. The prominent enrichment of methanol relative to water contrasts with the depletions of other molecules, especially C2H2, HCN, and H2CO. We suggest that the nucleus of 8P/Tuttle may contain two cometesimals characterized by distinct volatile composition. The relative abundances C-2/CN, C-2/OH, and CN/OH in 8P/Tuttle (measured at optical/near-UV wavelengths) differ substantially from the mixing ratios of their potential parents (C2H2/HCN, C2H2/H2O, and HCN/H2O) found in this work. Based on this comparison, our results do not support C2H2 and HCN being the principal precursors for respectively C-2 and CN in Tuttle. The peculiar native composition observed in 8P/Tuttle (compared to other comets) provides new strong evidence for chemical diversity in the volatile materials stored in comet nuclei. We discuss the implications of this diversity for expected variations in the deuterium enrichment of water among comets.