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

Published online by Cambridge University Press:  18 December 2017

E. Drabek-Maunder ,
J. Greaves ,
H. J. Fraser ,
D. L. Clements  and
L.-N. Alconcel
E. Drabek-Maunder*
Affiliation:
School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
J. Greaves
Affiliation:
School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK
H. J. Fraser
Affiliation:
School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
D. L. Clements
Affiliation:
Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
L.-N. Alconcel
Affiliation:
Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
*
Author for correspondence: E. Drabek-Maunder, E-mail: Emily.Drabek-Maunder@astro.cf.ac.uk
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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 5.9σ 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.

Keywords

Astrobiologyastrochemistryplanets and satellites: Enceladussubmillimeter:general
Type
Research Article
Information
International Journal of Astrobiology , Volume 18 , Issue 1 , February 2019 , pp. 25 - 32
Copyright
Copyright © Cambridge University Press 2017 

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