http://www.sciencemag.org/cgi/content/full/320/5882/1432

Science 13 June 2008:

Vol. 320. no. 5882, pp. 1432 - 1433

PLANETARY SCIENCE:

Enceladus--Oasis or Ice Ball?

Susan W. Kieffer1 Bruce M. Jakosky2

Is it possible for life to exist on Enceladus, the tiny (500 km diameter) icy satellite of Saturn? The Cassini mission found giant gaseous plumes erupting from a tectonically active and warm south polar region. One highly publicized interpretation is that liquid water is present, possibly within tens of meters of the surface (1), or possibly only at depths of tens of kilometers [for example, see (2)]. An antithetical interpretation is that Enceladus is frigid, stiff, thoroughly solid and composed of ice with interstitial gases to great depths (3, 4). However, liquid water is just one of the three environmental conditions that are generally thought to be prerequisites for life (5). There must also be access to the elements out of which complex molecular structures can be constructed--mainly C, H, O, N, S, and--as well as an energy source that can drive metabolism. We examine the range of possible environments on Enceladus that are consistent with the observations in terms of their implications for harboring life.

The instruments on Cassini provide data on gas and surface composition (6-8), thermal emission and radiated power (9), and optical images (1). These data can also provide clues to interior chemistry and conditions as well as surface temperatures and near-surface processes, but are subject to interpretation.

For example, there are competing views as to the physical state of water on Enceladus. One model holds that liquid water is present near the surface and has been dubbed "Cold Faithful" (1) in deference to the plume's apparent similarities to the Old Faithful geyser in Yellowstone National Park. The plume is formed from a reservoir of boiling water. The radiated energy is assumed to be transported to the surface by thermal conduction through the crust. Temperature gradients are high, and liquid water is encountered at shallow depths. In this interpretation, there is a high potential for near-surface extraterrestrial life (10).

In the other model, dubbed "Frigid Faithful" (3), the crust is a frigid mixture of ice and clathrates, a form of ice that stores foreign gas molecules in lattice cages. Fracturing of this solid crust during tectonic activity releases the gases and allows sublimation of cold water vapor. The radiated energy is transported to the surface by advection of gases in cracks in the crust, with condensation onto the walls of fractures. Temperature gradients are low; substantial warming may not occur even to 35 to 100 km (4). No liquid water is present, and there is little potential for life.

Given these two competing models, can a strong argument be made for the presence of liquid water? Water is certainly present in Enceladus, at least as solid and vapor, but the presence of vapor does not necessarily imply that liquid water is present

Even the warmest surface temperatures measured to date are below ~180 K (10). At the moment, the size of the warmest regions can be constrained to be less than a few kilometers, but they are likely to be much smaller (11). Kieffer et al. (3) noted that the entire discharge of a few hundred kilograms per second in the south polar plumes is similar to the discharge of Old Faithful geyser; the vent of Old Faithful is less than 1 m2 in area. Thus, the warmest spots are likely to be very small relative to large adjacent areas that are at 80 K, and so it is difficult to envision liquid water coexisting with such large areas of cold ice. These arguments suggest that any liquid water is "deep," but this depth is not well constrained.

If liquid water is present, does it come into contact with the chemical building blocks needed for life? The elements C, H, O, and possibly N, as well as simple and complex organic molecules, have been observed in gases in the south polar plume or in ices on the surface (12). However, the concept of a liquid-water reservoir runs into problems in terms of the compounds observed in the plume--CO2, CH4, and CO or N2. In the measured abundances, CO2 can exist in solution in liquid water only at depths greater than 20 km, and CO, N2, and CH4 could not be in solution even at the center of this tiny satellite (3). However, in the frigid model, these molecules are readily stored in cages in the clathrates and are released during fracturing associated with the tectonic activity.

It is likely, but by no means certain, that Enceladus has most of the heavier biogenic elements. The elements that are less volatile, including S, P, and Fe, are likely to be present in any rock-containing environment [nitrogen presents a special case (13)]. Although rocky material is not observed on the surface, the bulk density of Enceladus exceeds that of ice by about 60%. It is generally believed that Enceladus is differentiated and has a rocky core. Near-surface liquid could be separated from access to rock by as much as 100 km of ice and clathrate.

Is there a source of energy to drive metabolism? The warm regions at the south pole radiate 3 to 7 GW of power, but organisms cannot use this enormous amount of thermal energy directly. Usable energy may be available in geological environments that bring together the biogenic elements and liquid water in the presence of rock (14), as is seen on Earth and postulated for Mars and Europa (15).

In summary, it has not been conclusively demonstrated that Enceladus meets any of the three key environmental conditions necessary to support life. The range of possible models that can explain the observations--both those invoking liquid water and those excluding it--is still being defined. An important next step will be to create a unified model of conditions and processes at the surface and in the interior that could better constrain the geologic conditions on the "regional" scale of the south polar terrain.

What would it take to resolve this controversy and get a more definitive answer? Even the strongest constraint--measurement of the temperature and size of the warm spots--must be put into context. If the temperatures remain low, then it is likely that the heat advection machine described by the frigid model is at work, although this concept needs further refinement to quantify the relation between the process and the radiated power. However, even if measured temperatures reach 273 K, which would be spectacular and tantalizing for liquid-water models, the challenge remains to explain where the compounds CO2, CH4, CO, or N2 that contain some of the building blocks for life come from, and where the remaining elements are likely to be found.

Future planning of explorations should be guided by the full range of possible models and the uncertainties associated with them. A definitive answer about the potential for life may have to await a follow-on spacecraft mission that can make specific high-resolution observations that could distinguish between the competing models.

References and Notes

1. C. C. Porco et al., Science 311, 1393 (2006).
2. F. Nimmo et al., Nature 447, 289 (2007).
3. S. W. Kieffer et al., Science 314, 1764 (2006).
4. G. Gioia et al., Proc. Natl. Acad. Sci. U.S.A. 104, 13578 (2007).
5. W. Sullivan, J. Baross, Eds., Planets and Life--The Emerging Science of Astrobiology (Cambridge Univ. Press, Cambridge, 2007), pp. 357-387.
6. J. H. Waite Jr. et al., Science 311, 1419 (2006).
7. C. J. Hansen et al., Science 311, 1422 (2006).
8. R. H. Brown et al., Science 311, 1425 (2006).
9. J. R. Spencer et al., Science 311, 1401 (2006).
10. www.jpl.nasa.gov/videos/cassini/Enceladus_Briefing_March_26.mov
11. J. N. Spitale, C. C. Porco, Nature 449, 695 (2007).
12. Initial conclusions were that N2 was present as the molecular weight 28 compound (6); the presence of CO is now the favored interpretation (10), but this ambiguity is still not resolved.
13. Nitrogen poses a potential problem in that, if it is not identified at the surface, it may not be present in appreciable amounts.
14. E. L. Shock, Evol. Biosphere 20, 331 (1990).
15. B. M. Jakosky, E. L. Shock, J. Geophys. Res. 103, 19359 (1998).

10.1126/science.1159702
1Department of Geology, University of Illinois, Urbana, IL 61802, USA. E-mail: skieffer@uiuc.edu

2Laboratory for Atmospheric and Space Physics and Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA. E-mail: bruce.jakosky@lasp.colorado.edu