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

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Title: Ground-based detection of a cloud of methanol from Enceladus: when is a biomarker not a biomarker?
Authors: Drabek-Maunder, E
Greaves, J
Fraser, HJ
Clements, DL
Alconcel, LN
Item Type: Journal Article
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−1width and is offset by +8.1 km s−1in 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.
Issue Date: 17-Dec-2017
Date of Acceptance: 12-Oct-2017
URI: http://hdl.handle.net/10044/1/60479
DOI: https://dx.doi.org/10.1017/S1473550417000428
ISSN: 1473-5504
Start Page: 1
End Page: 8
Journal / Book Title: International Journal of Astrobiology
Copyright Statement: © 2017 Cambridge University Press. This paper has been accepted for publication and will appear in a revised form, subsequent to peer-review and/or editorial input by Cambridge University Press.
Sponsor/Funder: Imperial College Trust
Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council (STFC)
Funder's Grant Number: N/A
ST/L001314/1
ST/N005317/1
Keywords: 0201 Astronomical And Space Sciences
0403 Geology
Astronomy & Astrophysics
Publication Status: Published online
Online Publication Date: 2017-12-17
Appears in Collections:Physics
Astrophysics
Faculty of Natural Sciences



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