The Relationship of HCN, C2H6, & H2O in Comets: A Key Clue to Origins?
Abstract
Background: HCN, C2H6, and H2O are three of the best characterized volatiles in comets. It is often assumed that all three are primary volatiles, native to the nucleus. Here, we compare their properties in 26 comets (9 JFC and 17 Oort-cloud), making 6 points:1. Both HCN and C2H6 are poor proxies for water production. The production rate ratio (Q-ratio) of each trace gas relative to water varies by a factor of six among these comets.2. All 26 comets have Q-ratios HCN/C2H6 > 0.1. In 18 comets the Q-ratios HCN/H2O and C2H6/H2O are correlated, with a mean ratio of 0.33. In 6 comets undergoing complete disruption, this Q-ratio exceeds 0.5.3. Q-ratios HCN/C2H6 are not correlated with Q(H2O), nor are they correlated with dynamical class (Oort cloud vs. JFC).4. The nucleus-centered rotational temperatures measured for H2O and other primary species (C2H6, CH3OH) usually agree within error, but those for HCN are often slightly cooler. Could this mean that HCN is not fully developed in the warm near-nucleus region, and instead is at least in part a product species?5. With its strong dipole moment and H-bonding character, HCN should be linked more strongly in the nuclear ice to other molecules with similar properties (H2O, CH3OH), but instead its spatial release in some comets seems strongly coupled to volatiles that lack a dipole moment and thus do not form H-bonds (methane, ethane). Is HCN produced in part from an apolar precursor?6. ALMA maps of HCN and the dust continuum show a slight displacement in their centroids. Is this the signature of extended production of HCN?HCN as a product species: Points 4-6 suggest that HCN may have a significant distributed source. The astrochemical species ammonium cyanide is a strong candidate for this HCN precursor; at moderately low temperatures (< 200K) NH4CN is a stable solid, but it dissociates into HCN and NH3 when warmed. Disruption could eject macroscopic solid NH4CN into the coma where subsequent warming and release could augment the coma content of NH3 and HCN.Acknowledgments NASA’s Planetary Astronomy and Astrobiology Programs supported this work.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #49
- Pub Date:
- October 2017
- Bibcode:
- 2017DPS....4941419M