These places never see sunlight, are buried deep under thick ice crusts and warmed mostly by radioactive decay and tidal forces: subsurface oceans of celestial objects far from their stars – if they have any. Decades ago, they were the domain of science fiction, until such places were hypothesized in our solar system thanks in part to Voyager flybys of Europa in 1979. Shortly after, the idea was popularized when it appeared in Arthur C. Clarke’s Space Odyssey saga. Since then, we learned much more about characteristics of possible subsurface oceans, discovered that they probably exist on more worlds than we dared to expect just a few years ago, and that they’re more fascinating than even SF authors hoped.
My article on the topic of subsurface oceans was published today in Clarkesworld Magazine. I wrote about moons and dwarf planets in our system as well as extrasolar planets; however, the topic is so vast that I couldn’t have possibly covered everything of interest – especially when virtually any piece of information is interesting and thought-provoking. If you’ve read the article and want to go deeper and learn more, you can read some of the following material I’ve used. Many of the scientific papers can be downloaded without any special access (use Google Scholar). The rest should be accessible from most university libraries.
If you don’t want to dig into the scientific articles at first, I can recommend the popular science book Alien Seas: Oceans in Space. It doesn’t deal just with subsurface oceans of icy objects; it concerns nearly any conceivable kind of oceans in a broad sense of the word, in our system as well as in the rest of the galaxy. It’s an excellent introductory read, well-written and an interesting food for thought.
There is plenty of resources about Europa but it’s never a bad way to start with a relatively recent good review. That’s the case of Kargel et al. (2000); very comprehensive information about Europa’s history, geology, characteristics of both the crust and the ocean and its prospects for life can be found there. Specifically conditions for methanogenesis as an energy source for possible life on Europa are discussed in McCollom (1999). More about all three Galilean moons possibly containing bodies of liquid water and consequences of different parameters is to be found in Zimmer et al. (2000) and Spohn and Schubert (2003).
A lot has been published about Saturn’s moons Enceladus and Titan; this is just a tip of the iceberg: Titan’s probable internal structure is described in Tobie et al. (2005). Regarding the tiny Enceladus, Roberts and Nimmo (2007) investigated the long-term stability of its ocean; analysis of ice grains from its geysers in Saturn’s E-ring is present in Postberg et al. (2009); shear heating as a heat source for the ocean is discussed in Nimmo et al. (2007); possible conditions for life in Parkinson et al. (2007); this along with possible biomarkers in McKay et al. (2008).
A paper by Hussmann et al. (2006) dealt with modeling the interior of icy satellites of the giant planets and trans-Neptunian objects. This work represents a turning point of a kind – a subsurface ocean even in very far Kuiper belt bodies like Eris and Sedna (sometimes also considered an inner Oort cloud object) was first officially proposed here. Thermal evolution and possible cryovolcanism of KB objects is also investigated in Desch et al. (2009).
Concerning Pluto, Robuchon and Nimmo (2011) modeled Pluto with several different initial condition sets and proposed what observable features might tell us about the possible presence of the ocean during the New Horizons flyby. Spectroscopy of Pluto, its moon Charon and Neptune’s Triton is described in Protopapa et al. (2007), including the detection of crystalline water ice on Charon’s surface.
A very good overview of possibilities of life in the Solar System, including subsurface oceans, and opportunities of energy cycles and geoindicators of life detection can be found in Schulze-Makuch et al. (2002).
Speaking of even further places, Ehrenreich and Cassan (2006) investigated the possibilities of existence of bodies of liquid water (both surface and subsurface) on extrasolar planets throughout the galaxy. Information specifically about the GJ 667C system can be found in Anglada-Escudé et al. (2012). Exomoons are discussed very well in Scharf (2006).
I hope you enjoyed the Clarkesworld article and this list of resources will be of interest to you. If I’ve managed to ignite even one spark of fascination and curiosity, I’m happy.
3rd April 2014 update: Results from Cassini’s measurements of the gravitational pull of Enceladus suggest a large pocket of liquid water near the south pole, as published in the newest issue of Science (Iess et al. 2014); it adds to the indirect (albeit extremely important) evidence of the moon’s intense cryovolcanism. So – good news! Also, discoveries of three dwarf planets were announced in the last couple of days. 2013 FY27 is might be even larger than Sedna (between 760 and 1500 km compared to about 1000 km) so we can expect quite significant radiogenic heating – according to Hussman et al. (2006) model maybe sufficient for a liquid ocean. Let’s hope for even more amazing discoveries like these.
Anglada-Escudé, G., Arriagada, P., Vogt, S. S., Rivera, E. J., Butler, R. P., Crane, J. D., … & Jenkins, J. S. (2012). A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone. The Astrophysical Journal Letters, 751(1), L16.
Desch, S. J., Cook, J. C., Doggett, T. C., & Porter, S. B. (2009). Thermal evolution of Kuiper belt objects, with implications for cryovolcanism. Icarus,202(2), 694-714.
Ehrenreich, D., & Cassan, A. (2007). Are extrasolar oceans common throughout the Galaxy?. Astronomische Nachrichten, 328(8), 789-792.
Hussmann, H., Sohl, F., & Spohn, T. (2006). Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects. Icarus, 185(1), 258-273.
Kargel, J. S., Kaye, J. Z., Head III, J. W., Marion, G. M., Sassen, R., Crowley, J. K., … & Hogenboom, D. L. (2000). Europa’s crust and ocean: Origin, composition, and the prospects for life. Icarus, 148(1), 226-265.
McCollom, T. M. (1999). Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa. Journal of Geophysical Research: Planets (1991–2012), 104(E12), 30729-30742.
McKay, C. P., Porco, C. C., Altheide, T., Davis, W. L., & Kral, T. A. (2008). The possible origin and persistence of life on Enceladus and detection of biomarkers in the plume. Astrobiology, 8(5), 909-919.
Nimmo, F., Spencer, J. R., Pappalardo, R. T., & Mullen, M. E. (2007). Shear heating as the origin of the plumes and heat flux on Enceladus. Nature,447(7142), 289-291.
Parkinson, C. D., Liang, M. C., Yung, Y. L., & Kirschivnk, J. L. (2008). Habitability of Enceladus: Planetary conditions for life. Origins of Life and Evolution of Biospheres, 38(4), 355-369.
Postberg, F., Kempf, S., Schmidt, J., Brilliantov, N., Beinsen, A., Abel, B., … & Srama, R. (2009). Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature, 459(7250), 1098-1101.
Protopapa, S., Herbst, T., & Böhnhardt, H. (2007). Surface ice spectroscopy of Pluto, Charon and Triton. Messenger, 129, 58-60.
Roberts, J. H., & Nimmo, F. (2008). Tidal heating and the long-term stability of a subsurface ocean on Enceladus. Icarus, 194(2), 675-689.
Robuchon, G., & Nimmo, F. (2011). Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean. Icarus, 216(2), 426-439.
Scharf, C. A. (2006). The potential for tidally heated icy and temperate moons around exoplanets. The Astrophysical Journal, 648(2), 1196.
Schulze-Makuch, D., Irwin, L. N., & Guan, H. (2002). Search parameters for the remote detection of extraterrestrial life. Planetary and Space Science, 50(7), 675-683.
Spohn, T., & Schubert, G. (2003). Oceans in the icy Galilean satellites of Jupiter?. Icarus, 161(2), 456-467.
Tobie, G., Grasset, O., Lunine, J. I., Mocquet, A., & Sotin, C. (2005). Titan’s internal structure inferred from a coupled thermal-orbital model. Icarus, 175(2), 496-502.
Zimmer, C., Khurana, K. K., & Kivelson, M. G. (2000). Subsurface oceans on Europa and Callisto: Constraints from Galileo magnetometer observations.Icarus, 147(2), 329-347.