Possible sources for methane and C2–C5 organics in the plume of Enceladus

• Published in: Planetary and space science 2012-10, Vol.71 (1), p.73-79
• Main Authors: McKay, C.P., Khare, B.N., Amin, R., Klasson, M., Kral, T.A.
• Format: Article
• Language: English
• Subjects: Acetylene; Amino acids; Carbon monoxide; Enceladus; Hydrocarbons; Low pressure; Methane; Microbiology; Organics; Plume; Plumes; Synthesis
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/j.pss.2012.07.011

We consider six possible sources of CH4 and other lowmass (C2–C5) organics in the plume of Enceladus: three of these sources represent initial endowments of organics: cometary organics, Titan-like tholin, and the Fisher–Tropsch type reactions in the gases from which Enceladus formed. The other three sources represent processes inside Enceladus: water–rock reactions, microbiology, and thermogenesis. We report on new laboratory results for C2 hydrocarbons released by thermogenesis of laboratory tholin and the Fisher–Tropsch type synthesis. Thermal processing of Titan-like tholin produced ratios of CH4/C2H4 and CH4/C2H6 of about two for temperatures up to 450°C and about six for a temperature of 650°C. The low pressure (∼1atm) Fisher–Tropsch type experiments produced CH4/C2H4 of ∼1.5, similar to previous results. C2H2 was not produced by either process. Tests of gas production by four strains of methanogens confirmed the absence of any detectable production of non-methane hydrocarbons. Cometary endowment, the Fisher–Tropsch type synthesis, and Titan-like tholin incorporation could be primary inputs of organics and subsequent thermal processing of any of these all are possible sources of low mass organics in the plume. Biological production and water–rock reactions are an alternative source of CH4. Aqueous reactions with CO and H2 can produce C2–C5 organics even at the low pressures of the interior of Enceladus. If there is a confirmed detection of CO and C2H2 in the plume of Enceladus, this provides an important constraint on sources, as we have identified no process, other than the initial volatile component of cometary organics, which can supply these gases. Precise determination of the relative concentrations of C1–C5 hydrocarbons may provide additional constraints on sources, but a detailed isotopic analysis of C and H in these organics and a search for amino acids constitute the next important steps in resolving the sources of the organics in Enceladus' plume. ► Enceladus could have formed with organics from comets or Titan-like tholin. ► Internal processes include the Fisher–Tropsch synthesis, water–rock reactions, or biology. ► Acetylene provides an important constraint and could only be of cometary origin. ► CO is expected from cometary origin, but could be consumed by internal processes. ► C2H4 could come from comet and could be produced by internal processes.