Tuesday

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 spectrum of methanol (CH3OH) observed with IRAM 30-m. The moon was observed over a 7h period in 2008 when approaching 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 5.9 sigma confidence. The line has 0.43 km/s width and is offset by 8.1 km/s in the moon’s reference frame. Radiative transfer models allow for gas cloud dimensions from 1750 km up to the telescope beam diameter ~73000 km. However, the CH3OH lifetime against solar photodissociation suggests a length less than 2000 km. Taking into account the velocity offset, our favoured interpretation is a compact, confined gas cloud lagging Enceladus by several km/s. The scale and velocity shift agree reasonably with models from past work, where a part of the ejected gas interacts with Saturn’s magnetosphere. We find this trailing gas cloud to be consistent with significant redshifted H2O emission (4sigma) measured from the Herschel public archive. The measured CH3OH:H2O abundance (greater than 0.5 per cent) significantly exceeds the observed abundance in the direct vicinity of the vents (0.01 per cent), suggesting CH3OH is likely chemically processed within the gas cloud with methane (CH4) as its parent species.