Hostname: page-component-5d59c44645-klj7v Total loading time: 0 Render date: 2024-03-02T18:01:13.540Z Has data issue: false hasContentIssue false

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*
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

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.

Type
Research Article
Copyright
Copyright © Cambridge University Press 2017 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Atreya, SK, Adams, EY, Niemann, HB, Demick-Montelara, JE, Owen, TC, Fulchignoni, M, Ferri, F and Wilson, EH (2006) Titan's methane cycle. Planetary and Space Science 54, 1177.Google Scholar
Bergantini, A, Pilling, S, Nair, BG, Mason, NJ and Fraser, HJ (2014) Processing of analogues of plume fallout in cold regions of Enceladus by energetic electrons. Astronomy and Astrophysics 570, 120.Google Scholar
Bergin, EA and Tafalla, M (2007) Cold dark clouds: the initial conditions for star formation. Annual Review of Astronomy and Astrophysics 45, 339.Google Scholar
Bhardwaj, A, Elsner, RF, Randall Gladstone, G, Cravens, TE, Lisse, CM, Dennerl, K, Branduardi-Raymont, G, Wargelin, BJ, Hunter Waite, J, Robertson, I, Østgaard, N, Beiersdorfer, P, Snowden, SL and Karchenko, V (2007) X-rays from solar system objects Cassidy & Johnson 2010: collisional spreading of Enceladus’ neutral cloud. Planetary and Space Science 55, 1135.Google Scholar
Cassidy, TA and Johnson, RE (2010) Collisional spreading of Enceladus's neutral cloud. Icarus 209, 696.Google Scholar
Coates, AJ, Jones, GH, Lewis, GR, Wellbrock, A, Young, DT, Crary, FJ, Johnson, RE, Cassidy, TA and Hill, TW (2010) Negative ions in the Enceladus plume. Icarus 206, 618.Google Scholar
Cordiner, MA and Charnley, SB (2014) Negative ion chemistry in the coma of comet 1P/Halley. M&PS 49, 21.Google Scholar
Dougherty, MK, Khurana, KK, Neubauer, FM, Russell, CT, Saur, J, Leisner, JS and Burton, ME (2006) Identification of a dynamic atmosphere at Enceladus with the Cassini magnetometer. Science 311, 1406.Google Scholar
Farmer, AJ (2009) Saturn in hot water: viscous evolution of the Enceladus torus. Icarus 202, 280.Google Scholar
Hansen, CJ, Esposito, L, Steward, AIF, Colwell, J, Hendrix, A, Pryor, W, Shemansky, D and West, R (2006) Enceladus' water vapor plume. Science 311, 1422.Google Scholar
Hartogh, P, Lellouch, E, Moreno, R, Bockelée-Morvan, D, Biver, N, Cassidy, T, Rengel, M, Jarchow, C, Cavalié, T, Crovisier, J, Helmich, FP and Kidger, M (2011) Direct detection of the Enceladus water torus with Herschel. Astronomy and Astrophysics 532, L2.Google Scholar
Hedman, MM, Gosmeyer, CM, Nicholson, PD, Sotin, C, Brown, RH, Clark, RN, Baines, KH, Buratti, BJ and Showalter, MR (2013) An observed correlation between plume activity and tidal stresses on Enceladus. Nature 500, 182.Google Scholar
Hodyss, R, Parkinson, CD, Johnson, PV, Stern, JV, Goguen, JD, Yung, YL and Kanik, I (2009) Methanol on Enceladus. Geophysical Research Letters 36, L17103.Google Scholar
Huebner, WF, Keady, JJ and Lyon, SP (1992) Solar photo rates for planetary atmospheres and atmospheric pollutants. Astrophysics and Space Science 195, 1.Google Scholar
Jia, Y-D, Russell, CT, Khurana, KK, Ma, YJ, Najib, D and Gombosi, TI (2010) Interaction of Saturn's magnetosphere and its moons: 2. Shape of the Enceladus plume. Journal of Geophysical Research (Space Physics) 115, A04215.Google Scholar
Johnson, RE, Smith, HT, Tucker, OJ, Liu, M, Burger, MH, Sittler, EC and Tokar, RL (2006) The Enceladus and OH Tori at Saturn. Astrophysical Journal Letters 644, L137.Google Scholar
Jurac, S and Richardson, JD (2005) A self-consistent model of plasma and neutrals at Saturn: neutral cloud morphology. Journal of Geophysical Research (Space Physics) 110, A09220.Google Scholar
Jurac, S, Johnson, RE and Richardson, JD (2001) Saturn's E ring and production of the neutral torus. Icarus 149, 384.Google Scholar
Kargel, JS (2006) Enceladus: cosmic gymnast, volatile miniworld. Science 311, 1389.Google Scholar
Matson, DL, Castillo, JC, Lunine, J and Johnson, TV (2010) Enceladus' plume: compositional evidence for a hot interior. Icarus 187, 569.Google Scholar
McElroy, D, Walsh, C, Markwick, AJ, Cordiner, MA, Smith, K and Millar, TJ (2013) The UMIST database for astrochemistry 2012. Astronomy and Astrophysics 550, 36.Google Scholar
Mincer, TJ and Aicher, AC (2016) Methanol production by a broad phylogenetic array of marine phytoplankton. PLoS ONE 11, e0150820.Google Scholar
Newman, SF, Buratti, BJ, Jaumann, R, Bauer, JM and Momary, TW (2007) Hydrogen peroxide on Enceladus. Astrophysical Journal 670, L143.Google Scholar
Porco, CC, Helfenstein, P, Thomas, PC, Ingersoll, AP, Wisdom, J, West, R, Neukum, G, Denk, T, Wagner, R, Roatsch, T, Kieffer, S, Turtle, E, McEwen, A, Johnson, TV, Rathbun, J, Veverka, J, Wilson, D, Perry, J, Spitale, J, Brahic, A, Burns, JA, Del Genio, AD, Dones, L, Murray, CD and Squyres, S (2006) Cassini observes the active south pole of Enceladus. Science 311, 1393.Google Scholar
Schöier, FL, van der Tak, FFS, van Dishoeck, EF and Black, JH (2005) An atomic and molecular database for analysis of submillimetre line observations. Astronomy and Astrophysics 432, 369.Google Scholar
Shemansky, DE, Matheson, P, Hall, DT, Hu, H-Y and Tripp, TM (1993) Detection of the hydroxyl radical in the Saturn magnetosphere. Nature 363, 329.Google Scholar
Smith, HT, Johnson, RE, Perry, ME, Mitchell, DG, McNutt, RL and Young, DT (2010) Enceladus plume variability and the neutral gas densities in Saturn's magnetosphere. Journal of Geophysical Research (Space Physics) 115, A10252.Google Scholar
Spencer, JR, Pearl, JC, Segura, M, Flasar, FM, Mamoutkine, A, Romani, P, Buratti, BJ, Hendrix, AR, Spilker, LJ and Lopes, RMC (2006) Cassini encounters Enceladus: background and the discovery of a south polar hot spot. Science 311, 1401.Google Scholar
Teolis, BD, Perry, ME, Magee, BA, Westlake, J and Waite, JH (2010) Detection and measurement of ice grains and gas distribution in the Enceladus plume by Cassini’s Ion Neutral Mass Spectrometer. Journal of Geophysical Research 115, A09222.Google Scholar
Tokar, RL, Johnson, RE, Thomsen, MF, Wilson, RJ, Young, DT, Crary, FJ, Coates, AJ, Jones, GH and Paty, CS (2009) Cassini detection of Enceladus' cold water-group plume ionosphere. Geophysical Research Letters 36, L13203.Google Scholar
Waite, JH and Magee, B (2010) Enceladus plume composition. EPSC 5, EPSC2010-305.Google Scholar
Waite, JH, Combi, MR, Ip, W-H, Cravens, TE, McNutt, RL, Kasprzak, W, Yelle, R, Luhmann, J, Niemann, H, Gell, D, Magee, B, Fletcher, G, Lunine, J and Tseng, W-L (2006) Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure. Science 311, 1419.Google Scholar
Waite, JH Jr., Lewis, WS, Magee, BA, Lunine, JI, McKinnon, WB, Glein, CR, Mousis, O, Young, DT, Brockwell, T, Westlake, J, Nguyen, M-J, Teolis, BD, Niemann, HB, McNutt, RL, Perry, M and Ip, W-H (2009) Liquid water on Enceladus from observations of ammonia and 40Ar in the plume. Nature 460, 487.Google Scholar