44 Volume 33, I s sue 2 SKEPTICAL INQUIRER

NASA’s Cassini spacecraft continues its close flybysof Saturn’s moon Enceladus, sampling the enor-mous plumes of water vapor and icy particles being

ejected hundreds of miles into space. The most recent flybywas in October 2008; the next is planned for November2009. The prospect of near-surface liquid water on anotherplanetary body has never been so promising. Of coursewater in and of itself is not what makes this discovery soexciting: it’s the underlying potential of a habitat conduciveto supporting life. Not since the Mars Viking missions of the1970s has the talk of possible life on another planet buzzedso prolifically. But is this buzz justified? Healthy skepticismabounds as the search goes forward.

The Potential Habitable Zoneon Saturn’s Moon Enceladus

Recent discoveries from NASA’s Cassini spacecraft have led many scientists to believe that numerousmicroorganisms found on Earth could potentially survive on Saturn’s moon Enceladus.

FELIX WASIAK

SI M-A 2009 pgs 1/21/09 5:03 PM Page 44

SKEPTICAL INQUIRER March / Apr i l 2009 45

“Enceladus represents one of the likely, if there is such athing, places in our solar system other than the Earth where wemight find biological activity,” said Andrew J. Dombard, vis-iting associate research professor, department of Earth andEnvironmental Sciences, University of Illinois at Chicago.

“Personally, I don’t think there is life anywhere in this solarsystem apart from the Earth, but I do think it’s very likelythere is life in other solar systems,” said Francis Nimmo, asso-ciate professor, department of Earth and Planetary Sciences,University of California, Santa Cruz.

Dombard and Nimmo both served on the Science Defini-tion Team for NASA’s Enceladus Flagship Mission ConceptStudy (NASA Goddardd Space Flight Center 2007).

Many scientists now believe an “Earth-like” planet is notnecessary for potential life elsewhere. The so-called “habitable-zone,” the distance a planet would have to be from the Sun toensure life wouldn’t get too hot or too cold and the Sun’senergy would provide the energy for life through photosyn-thesis, is being extended. Once thought to only lie somewherebetween Earth’s closest neighboring planets Venus and Mars,several potential micro-habitable zones have now been identi-fied in our solar system. Many scientists now believe thatnumerous microorganisms found on Earth would do just fineon several of the moons orbiting Jupiter and Saturn. It is notonly discoveries in space that have fueled this paradigm shift—in the 1970s scientists discovered life deep in our oceans,where sunlight is unable to reach, that is not dependent onenergy derived from our sun. Instead, it depends on hot, nutri-ent-rich water emanating from hydrothermal vents. Ratherthan utilizing photosynthesis, these organisms rely onchemosynthesis to survive.

How life on Earth originated is not known; however, thegeneral theory is that billions of years ago the Earth had theingredients, the environment, and most important, the timefor life to form. The time—the primary limiting factor in lab-oratory experiments—spanned hundreds of millions of years,during which complex chemistry occurred until finally, in justthe right way and under the right environmental conditions,the chemistry came together and voila: life. A very basic sys-tem of organic molecules was created within a rudimentarymembrane, capable of following a template to replicate andcatalyzing other reactions necessary for maintenance (Lunine2005). Early life would most probably have obtained energythrough chemosynthesis, with the more complex photosyn-thesis evolving later.

A vivid imagination is not necessary when contemplatingwhat sort of life is possible on Enceladus—one need only lookat the earthly examples known as “extremophiles.” These arebacteria capable of living in extreme environmental conditionsof temperature, pH, radiation, pressure, salinity, dryness,

Cassini image of Enceladus shows a bizarre mixture of softened craters and complex, fractured terrains. The tiger stripes and other fractures demonstrate a geologically active world. (Photo by NASA-JPL)

Felix Wasiak is a doctoral student in space and planetary sciencesat the University of Arkansas. He is currently on sabbatical fromNorthrop Grumman Space Technology, following a twenty-five-year career as an aerospace engineer, to pursue his interests inplanetary sciences. He is a member of the Fayetteville Freethinkersin Fayetteville, Arkansas. E-mail: fwasiak@uark.edu.

SI M-A 2009 pgs 1/21/09 5:04 PM Page 45

46 Volume 33, I s sue 2 SKEPTICAL INQUIRER

absence of oxygen, absence of sunlight, and chemicals. Earthis teeming with life, from deep within its crust to high in theatmosphere, and the variety of that life is greater than we couldever imagine. Indeed, to many extremophile microorganismson Earth Enceladus might be like a balmy vacation resortwhere life is easy and a party is never far away.

Were the conditions on Enceladus right for life to arise atsome point? Perhaps it doesn’t matter. The earliest evidence oflife on Earth is found in biogenic matter in sedimentary rocksdating 3.85 billion years ago (Lunine 2005). Since that time,the Earth has been subjected to countless meteor impacts, thelargest of which are capable of thrusting material from Earthinto space. While much of this debris falls back, some can

escape Earth’s gravitation. Certain bacteria and spores buriedwithin this rock could lie dormant for eons in space, with therock acting as a shield against solar radiation and cosmic rays.Should this rock subsequently strike a planetary body such asEnceladus, it could “seed” this new world. Is life generatedfrom seeding more probable than life forming spontaneously,and what is the probability of neither occurring? Rocks fromEarth certainly have been thrust into space, and these rockscertainly have had bacteria within them. Whether any of theserocks then made it to the distant reaches of the solar systemand subsequently seeded new worlds is one of the big ques-tions NASA would like to answer.

Other planetary bodies, such as Jupiter’s moon Europa, arethought to have subsurface liquid water, so why all the fussabout Enceladus? Remotely drilling through hundreds (orthousands) of meters of ice on a distant planet is all but animpossible task, if not technically than certainly financially.The plumes on Enceladus offer the ability to sample water

through conventional means without complicated drilling sce-narios (NASA Goddard Space Flight Center 2007).

Enceladus is geologically active, and one of only three outersolar system bodies where volcanic eruptions have beenobserved; the other two are Jupiter’s moon Io and Neptune’smoon Triton. The eruptions occur from fissures commonlyknown as the “tiger stripes” in the south polar terrain.Enceladus is a relatively small moon with a diameter of512 !494!489 kilometers (318!307!304 miles). Due toits distance from the Sun it receives very little solar energy, andhas an estimated mean surface temperature of –198° Celsius(–324° Fahrenheit). Heat acquired during Enceladus’s initialformation 4.6 billion years ago would have cooled off long ago

on a body this small. So where does the energy to power thisgeological activity come from? Many models developed byplanetary scientists explain the current state of affairs. Theyinclude tidal heating from Saturn’s intense gravity and radioac-tive decay in Enceladus’s silicate core.

Tidal heating is a phenomenon similar to the ocean tideson Earth, which are caused by the Moon’s (and to a lesserextent, the Sun’s) gravitational pull on the Earth’s oceans.Enceladus experiences a much greater gravitational force fromthe great mass of Saturn than Earth does from its Moon, andthis force bulges, cracks, and shifts Enceladus’s surface as itorbits Saturn. The resulting frictional forces create heat.

“If you have a crack and you’re shearing it back and forthon itself you can dissipate heat, I mean the same way that youcan warm your hands by rubbing them together. It’s the sameprinciple,” Dombard said.

Accounting for the approximately six gigawatts of poweremanating from the southern polar region has been a chal-

The Cassini spacecraft swings in orbit around Saturn.

NASA

SI M-A 2009 pgs 1/21/09 5:04 PM Page 46

SKEPTICAL INQUIRER March / Apr i l 2009 47

lenge. One model suggests the heat required to power theplumes is based on shear heating by tidally driven lateral“strike-slip” fault motion. The energy generated by shearingwould cause heating and thus induce sublimation—transitionfrom solid to gas without the intermediate liquid stage—which occurs at relatively low temperatures on Enceladus dueto a lack of atmospheric pressure. The vapor produced by thisheating escapes through cracks opened by tidal stresses, result-ing in the observed plumes. The tidal displacements requiredsuggest a subsurface ocean, either global or possibly localized,decoupling the thick ice shell from the silicate interior, thusallowing sufficient flexing to generate the energy seen inCassini observations (Nimmo et al. 2007). Of course, modelscan only go so far with so much data; a future mission couldhelp resolve many ambiguities.

“For the orbiter it would be very nice to have a radar, because

a radar is the best way of seeing through the ice shell, potentiallyto the ocean below,” Nimmo said. “Also it’s good at detectingpockets of water, even if there is not a continuous layer of water,so that would help tell us what the mechanism is that’s actuallycausing the tiger stripes and the vapor plumes. For a ground-based instrument it would be very interesting to have a seis-mometer, because that might tell you something about howdeep the fractures are, and whether they’re really active and gen-erating strike-slip motion, because that’s what we think.”

“I would love to get a lander down on the surface to do insitu chemical analysis and geophysical techniques—seismologyto try to pin down the nature of the ice shell,” Dombard said.

Another model suggests that short-lived radiogenic iso-topes, especially aluminum 26 and iron 60, were present at thetime of formation, and this initial heat “pulse” led to meltingin the silicate core, resulting in volcanism. Tidal heating hasmaintained partially molten regions over the long term.(Castillo-Rogez et al. 2007).

The instruments onboard the Cassini spacecraft have takenplume composition measurements. The plume is 91 percentwater, about 4 percent nitrogen, 3.2 percent carbon dioxide,and 1.6 percent methane (NASA-JPL News Release 2008).Trace amounts of organic material besides methane also havebeen detected. While non-biological processes produce numer-ous organic compounds that are found throughout the solarsystem, their detection is always encouraging when consideringthe potential for life, as all life on Earth is based on carbon.

So what should a future flagship mission to Enceladus

entail? According to NASA’s Enceladus Flagship MissionConcept Study (NASA Goddard Space Flight Center 2007),the greatest scientific payoff would come from an Enceladusorbiter and lander. How much priority should be given toinstrumentation for the detection of life?

“Assessing the biological potential was our number one pri-ority for the Enceladus Flagship (Mission Concept Study),”Dombard said. “That’s why we want to fly through theplumes, that’s why we want to get down to the surface and doin situ chemical analysis looking for amino acids and otherchemical compounds.”

“The people who are interested in astrobiology would reallylike to get hold of a sample of the plume material becausethat’s likely coming from inside the satellite, and so it gives youyour best chance of sampling the satellite interior, so beingable to measure the composition of the material that’s coming

off the tiger stripes I think is very important, whether or notyou actually want to be inside a tiger stripe, I think that’s lessobvious, but getting a sample of the plume material is obvi-ously very important,” Nimmo said.

No one knows if life does actually exist beyond Earth. Wedo know there are life forms on Earth capable of surviving invaried and harsh conditions, and there are environments onplanetary bodies such as Enceladus that appear capable of sup-porting such life. Many scientists remain open to the possibil-ity and are doing their best to answer the question—they con-tinue to do science. !References Castillo-Rogez, J. C., D. L. Matson, S. D. Vance, A. G. Davies, and Torrence

V. Johnson. 2007. The early history of Enceladus; setting the scene fortoday’s activity. Lunar and Planetary Science; XXXVIII; Papers Submittedto the Lunar and Planetary Science Conference 38, Abstract 2265.

Domard, A. 2008. Telephone interview by Felix Wasiak.Lunine, Jonathan Irving. 2005. Astrobiology: A Multidisciplinary Approach. San

Franciso: Pearson Addison Wesley. NASA-JPL News Release. 2008. Cassini tastes organic material at Saturn’s

geyser moon. March 28. Available online at www.jpl.nasa.gov/news/news.cfm?release=2008-050.

NASA Goddard Space Flight Center. 2007. Enceladus flagship mission con-cept study. August 29. Available online at www.lpi.usra.edu/opag/Enceladus_Public_Report.pdf.

Nimmo, F. 2008. Telephone Interview by Felix Wasiak.Nimmo, F., J. R. Spencer, R. T. Pappalardo, and M. E. Mullen. 2007. Shear

heating as the origin of the plumes and heat flux on Enceladus. Nature447(7142), 289–91.

Spencer, John, and David Grinspoon. 2007. Planetary science: InsideEnceladus. Nature 445(7126), 376–7.

A vivid imagination is not necessary when contemplating what sort of life is possible

on Enceladus—one need only look at the earthly examples known as “extremophiles.”

SI M-A 2009 pgs 1/21/09 5:05 PM Page 47