TABLE 5.1 Examples of Electron Donors or Acceptors for Life in Icy Bodies Both Inferred and Measured in Past Work

Europa Comments Enceladus Comments Titan Comments
Electron Donors (observed)
Organics Detected on surface of Callisto, Ganymede Reference a Expected on the surface. Interior unknown Organic compounds Reference j Size of some known; structures not known Methane and other organics detected in the plume. Organics and methane Reference l and m suggested as a source of energy for organisms by references n and o Surface organics from hydrocarbon cycle
Hydrogen n/a n/a n/a n/a Reference o
Electron Donors (suggested in literature as plausible candidates)
Hydrogen Reference b Reference k
Electron Acceptors (observed)
Oxygen Observed on surface Reference c Interior unknown
CO/CO2 CO2 on surface Reference d Reference j Observed in plume; CO and N2 peaks cannot be differentiated Trace quantities of CO2 observed Reference o
SO2 Reference e
Electron Acceptors (suggested)
Sulfur Sulfate inferred on surface Reference f and g Elemental sulfur also suggested Reference h Interior unknown
Fe3+, other metals Fe3+, other metals Reference b, i Fe3+, other metals depends on connection with silicate core

NOTE: Comments are provided as caveats and context.

a K.P. Hand, C. Chyba, J.C. Priscu, R.W. Carlson, and K.H. Nealson, Astrobiology and the potential for life on Europa, pp. 589-630 in Europa (R. Pappalardo, W. McKinnon, and K. Khurana, eds.), University of Arizona Press, Tucson, Ariz., 2009.

b D. Schulze-Makuch and L.N. Irwin, Energy cycling and hypothetical organisms in Europa’s ocean, Astrobiology 2:105-121, 2002.

c J.R. Spencer and W.M. Calvin, Condensed O2 on Europa and Callisto. Astronomy Journal 124:3400-3403, 2002.

d K.P. Hand, R. Carlson, and C.F. Chyba, Energy, chemical disequilibrium, and geological constraints on Europa, Astrobiology 7:1006-1022, 2007.

e A.L. Lane, R.M. Nelson, and D.L. Matson, Evidence for sulfur implantation in Europa’s UV absorption band, Nature 292:38-39, 1981.

f T.B. McCord, G.B. Hansen, F.P. Fanale, R.W. Carlson, D.L. Matson, T.V. Johnson, W.D. Smythe, J.K. Crowley, P.D. Martin, A. Ocampo, C.A. Hibbitts, J.C. Granahan, and the NIMS Team, Salts on Europa’s surface detected by Galileo’s Near Infrared Mapping Spectrometer, Science 280:1242-1245, 1998.

g R.W. Carlson, R.E. Johnson, and M.S. Anderson, Sulfuric acid on Europa and the radiolytic sulfur cycle, Science 286:97-99, 1999.

h R.W. Carlson, W.M. Calvin, J.B. Dalton, G.B. Hansen, R.L. Hudson, R.E. Johnson, T.B. McCord, and M.H. Moore, Europa’s surface composition, pp. 283-327 in Europa (R. Pappalardo, W. McKinnon, and K. Khurana, eds.), University of Arizona Press, Tucson, Ariz., 2009.

i E.J. Gaidos, K.H. Nealson, and J.L. Kirschvink, Life in ice-covered oceans, Science 284:1631, 1999.

j J.H. Waite, Jr., M.R. Combi, W.-H. Ip, T.E. Cravens, R.L. McNutt, Jr., W. Kasprzak, R. Yelle, J. Luhmann, H. Niemann, D. Gell, B. Magee, G. Fletcher, J. Lunine, and W.-L. Tseng, Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure, Science 311:1419-1422, 2006.



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