Ground-based detection of a cloud of methanol from Enceladus: when is a biomarker not a biomarker?

Saturn's moon Enceladus has vents emerging from a sub-surface ocean, offering unique probes into the liquid environment. These vents drain into the larger neutral torus in orbit around Saturn. We present a methanol (CH₃OH) detection observed with IRAM 30-m from 2008 along the line-of-sight through Saturn's E-ring. Additionally, we also present supporting observations from the Herschel public archive of water (ortho-H₂O; 1669.9 GHz) from 2012 at a similar elongation and line-of-sight. The CH₃OH 5(1,1)-4(1,1) transition was detected at 5.9σ confidence. The line has 0.43 km s⁻¹ width and is offset by +8.1 km s⁻¹ in the moon's reference frame. Radiative transfer models allow for gas cloud dimensions from 1750 km up to the telescope beam diameter ~73 000 km. Taking into account the CH₃OH lifetime against solar photodissociation and the redshifted line velocity, there are two possible explanations for the CH₃OH emission: methanol is primarily a secondary product of chemical interactions within the neutral torus that: (1) spreads outward throughout the E-ring or (2) originates from a compact, confined gas cloud lagging Enceladus by several km s⁻¹. We find either scenario to be consistent with significant redshifted H₂O emission (4σ) measured from the Herschel public archive. The measured CH₃OH:H₂O abundance (>0.5%) significantly exceeds the observed abundance in the direct vicinity of the vents (~0.01%), suggesting CH₃OH is likely chemically processed within the gas cloud with methane (CH₄) as its parent species.