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

Drabek-Maunder, Emily, Greaves, Jane, Fraser, Helen Jane, Clements, David and Alconcel, Leah-Nani 2019. Ground-based detection of a cloud of methanol from Enceladus: When is a biomarker not a biomarker? International Journal of Astrobiology 18 (1) , pp. 25-32. 10.1017/S1473550417000428

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Abstract

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 5.9 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. 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. We find either scenario to be consistent with significant redshifted H2O emission (4 sigma) measured from the Herschel public archive. The measured CH3OH:H2O abundance (> 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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Publisher: Cambridge University Press
ISSN: 1473-5504
Date of First Compliant Deposit: 9 November 2017
Date of Acceptance: 12 October 2017
Last Modified: 29 Jun 2019 15:13
URI: http://orca-mwe.cf.ac.uk/id/eprint/105778

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