ASTROBIOLOGYVolume 8, Number 5, 2008© Mary Ann Liebert, Inc.DOI: 10.1089/ast.2008.0265Hypothesis PaperThe Possible Origin and Persistence of Life on Enceladusand Detection of Biomarkers in the PlumeChristopher P. McKay,1Carolyn C. Porco,2Travis Altheide,3Wanda L. Davis,1and Timothy A. Kral3AbstractThe jets of icy particles and water vapor issuing from the south pole of Enceladus are evidence for activity dri-ven by some geophysical energy source. The vapor has also been shown to contain simple organic compounds,and the south polar terrain is bathed in excess heat coming from below. The source of the ice and vapor, andthe mechanisms that accelerate the material into space, remain obscure. However, it is possible that a liquidwater environment exists beneath the south polar cap, which may be conducive to life. Several theories for theorigin of life on Earth would apply to Enceladus. These are (1) origin in an organic-rich mixture, (2) origin inthe redox gradient of a submarine vent, and (3) panspermia. There are three microbial ecosystems on Earththat do not rely on sunlight, oxygen, or organics produced at the surface and, thus, provide analogues for pos-sible ecologies on Enceladus. Two of these ecosystems are found deep in volcanic rock, and the primary pro-ductivity is based on the consumption by methanogens of hydrogen produced by rock reactions with water.The third ecosystem is found deep below the surface in South Africa and is based on sulfur-reducing bacteriaconsuming hydrogen and sulfate, both of which are ultimately produced by radioactive decay. Methane hasbeen detected in the plume of Enceladus and may be biological in origin. An indicator of biological origin maybe the ratio of non-methane hydrocarbons to methane, which is very low (0.001) for biological sources but ishigher (0.1–0.01) for nonbiological sources. Thus, Cassini’s instruments may detect plausible evidence for lifeby analysis of hydrocarbons in the plume during close encounters. Key Words: Life—Enceladus—Biomark-ers—Methanogens—Non-methane hydrocarbons—Ammonia—Origin of life. Astrobiology 8, 909–919.909IntroductionENCELADUS, A SMALL ICY MOON OFSATURN, was thrust ontothe center stage of astrobiology with the discovery by theCassini spacecraft of Enceladus’ dramatic present-day geo-logic activity. The Cassini Imaging Science Subsystem im-ages of Enceladus have revealed about a dozen jets of fineicy particles that emerge from the south polar terrain (SPT)of Enceladus and feed a giant plume extending thousandsof kilometers into space (Porco et al., 2006). Several Cassiniinstruments were able to sample this plume during thespacecraft’s very close flyby in July 2005. In situ measure-ments of the plume found water vapor, simple organic com-pounds, and some level of N2or CO, or both (Waite et al.,2006). Observations at infrared wavelengths by the Com-posite Infrared Spectrometer instrument on Cassini have alsoshown the SPT to be anomalously warm (Spencer et al., 2006),and the comparison of high-resolution images of the SPTwith the highest-resolution thermal measurements haveshown a coincidence between the hottest measured temper-atures in the SPT and the “tiger stripe” fractures that strad-dle the region (Porco et al., 2006; Spencer et al., 2006; Spitaleand Porco, 2007).In this paper, we briefly review the range of geophysicalmodels for the plume on Enceladus. We note that some, butnot all, of these models posit a subsurface liquid water1Space Science Division, NASA Ames Research Center, Moffett Field, California.2Cassini Imaging Central Laboratory for Operations, Space Science Institute, Boulder, Colorado.3Department of Biological Sciences, University of Arkansas, Fayetteville, Arkansas.aquifer. With this caveat understood, we nevertheless focuson such an aquifer and its potential to support an origin oflife and a continuing ecosystem. We base our discussions ofthe possible origin of life on Enceladus on theories for theorigin of life on Earth. For ecosystems on Enceladus, we baseour discussions on specific existing ecosystems on Earth thatsurvive in conditions similar to those that may be present inan aquifer on Enceladus. Many of the issues related to lifeon Enceladus mirror previous discussions of life below theice of Europa, and we make this connection whenever ap-propriate. Finally, we consider how a signature of biologicalprocesses might be present in the plume and detected by theCassini instruments.The Plume of EnceladusEnceladus is a small world: mean radius 252 km, mass1.8  10 5Earth masses, and gravity 0.1 m/s2. It is surpris-ing that such a small world, which is relatively distant fromits planet (orbital distance is 4 times Saturn’s radius), wouldbe one of only three outer Solar System bodies (along withJupiter’s moon Io and Neptune’s moon Triton) where activeeruptions have been observed. An understanding that wouldprovide a physical framework for assessing habitability iscontingent upon answers to the following questions: (1)What is the source of energy that powers the activity on Ence-ladus? (2) Does the plume originate from a subsurfaceaquifer? (3) What is the source of CH4and N2in the plume?(4) If there is a subsurface liquid aquifer, how long has it per-sisted? (5) Is the concentration of solutes in such a liquidaquifer consistent with habitability? We consider each ofthese questions here.Several models have been proposed to explain the activ-ity on Enceladus and the associated 6 GW of heat (Spenceret al., 2006). Some models assume the heat is generatedwithin a silicate core (Castillo et al., 2007; Collins and Good-man, 2007). Others assume heating within an outer ice shell(Nimmo et al., 2007). All require heat production at least inpart by tidal flexing due to the forced eccentricity of Ence-ladus by Dione, as the observed heat output far exceeds thatproduced by radiogenic heating (Porco et al., 2006). How-ever, recent models show that the current rate of heat pro-duction observed at Enceladus is inconsistent with a steady-state orbital eccentricity (Meyer and Wisdom, 2007). Thisresult implies that the current warm state of Enceladus isprobably the result of a higher eccentricity in the past, thoughit is presently uncertain how that might have happened.There is not enough data to guide the models, and the sourceof the heating on Enceladus remains obscure.The source of the H2O ice and vapor in the plume is alsouncertain. Initial work suggested a shallow