Ground-based detection of a cloud of methanol from Enceladus: when is a biomarker not a biomarker?
File(s)1709.09638.pdf (2.02 MB)
Accepted version
Author(s)
Drabek-Maunder, E
Greaves, J
Fraser, HJ
Clements, DL
Alconcel, LN
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.
Date Issued
2019-02
Date Acceptance
2017-10-12
Citation
International Journal of Astrobiology, 2019, 18 (1), pp.25-32
ISSN
1473-5504
Publisher
Cambridge University Press
Start Page
25
End Page
32
Journal / Book Title
International Journal of Astrobiology
Volume
18
Issue
1
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
Imperial College Trust
Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council (STFC)
Grant Number
N/A
ST/L001314/1
ST/N005317/1
Subjects
Science & Technology
Physical Sciences
Life Sciences & Biomedicine
Astronomy & Astrophysics
Biology
Geosciences, Multidisciplinary
Life Sciences & Biomedicine - Other Topics
Geology
Astrobiology
astrochemistry
planets and satellites:Enceladus
submillimeter:general
CASSINI
SATURN
WATER
ION
0201 Astronomical and Space Sciences
0403 Geology
Astronomy & Astrophysics
Publication Status
Published
Date Publish Online
2017-12-18