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Ground-based Detection of a Cloud of Methanol from Enceladus: When is a Biomarker Not a Biomarker?
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.
E. Drabek-Maunder, J. Greaves, H. J. Fraser, D. L. Clements, L.-N. Alconcel
(Submitted on 27 Sep 2017)
Comments: 16 pages, 4 figures, accepted for publication in the International Journal of Astrobiology (IJA)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1709.09638 [astro-ph.EP] (or arXiv:1709.09638v1 [astro-ph.EP] for this version)
From: Emily Drabek-Maunder
[v1] Wed, 27 Sep 2017 17:21:12 GMT (1879kb,D)