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