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Enceladus: Making the Case for Life

Thoughts on Enceladus as a home to life have kept astrobiological debate lively, an unexpected but welcome development from the Cassini mission. The interest is understandable: Cassini has shown us plumes that seem to be the result of some kind of geothermal venting, with liquid water and geothermal energy sources all possible drivers for the formation of life. We don’t exactly know what’s going on here, but the possibility of a hydrological cycle — liquid, solid, gas — has kept theorists active, as witness a research note by Christopher Parkinson (Caltech) and team.

The early Earth serves as a possible model for life elsewhere. With photosynthesis not available, life would depend on abiotic sources of chemical energy. It’s believed this would have come in the form of oxidation-reduction processes driven by factors like hydrothermal activity, impacts, electrical discharges, or solar ultraviolet radiation. Organics may have been synthesized from inorganic molecules near submarine hydrothermal vents. In similar ways, the authors believe, Enceladus may offer energy-generating reactions that create conditions favorable for life.

I’ve never seen the case for this moon made quite so emphatically. From the paper (the italics are mine):

The combination of a hydrological cycle, chemical redox gradient and geochemical cycle give favorable conditions for life on Enceladus. To our knowledge, these conditions are not duplicated anywhere else in our solar system except our planet. Compared to Mars, Titan and Europa, Enceledus is the only other object in our solar system that appears to satisfy the conditions for maintaining life at present, even if the ability of life to evolve there is uncertain.

Plumes on Enceladus

Image: Ice geysers erupt on Enceladus, bright and shiny inner moon of Saturn. Shown in this false-color image, a backlit view of the moon’s southern limb, the majestic, icy plumes were discovered by instruments on the Cassini spacecraft during close encounters with Enceladus in November of 2005. Eight source locations for these geysers have now been identified along substantial surface fractures in the moon’s south polar region. Researchers suspect the geysers arise from near-surface pockets of liquid water with temperatures near 273 kelvins (0 degrees C). That’s hot when compared to the distant moon’s surface temperature of 73 kelvins (-200 degrees C). The cryovolcanism is a dramatic sign that tiny, 500km-diameter Enceladus is surprisingly active. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA.

What about those other habitats we’ve been considering? The paper continues:

Mars may have had a hydrological cycle in its early history, but there is no evidence that one exists today. Titan may be a repository of pre-biotic organic chemicals, but the conditions do not appear favorable for the development of life. Europa currently may have a hydrological cycle, but it may be a closed chemical system that will eliminate any chemical redox gradient in a geologically short time. Presently Enceladus is the most exciting object in the solar system for the search of extant life.

And nowhere else have I seen the suggestion that concludes this interesting paragraph:

We have compelling evidence supporting the view that Enceladus has active hydrological, chemical and geochemical cycles, which are essential ingredients for originating and sustaining life. Planetary protection issues aside, if life does not yet exist on Enceladus, artificial introduction of terrestrial life to this environment would be an interesting, and most likely successful experiment.

Here’s interesting fodder for a science fiction story. If you could introduce life into an environment ready for it, what sort of life would you choose? Enceladus, like any other body on which this was attempted, would pose its own formidable constraints, but the broader question of whether life ought to be introduced into such environments is open to considerable philosophical debate. Just what constitutes planetary protection?

Of course, Enceladus offers us plenty of work before we ever reach the stage of attempting such a thing. For one thing, we’d like to know whether those intriguing plumes are transient or long-term. We’ll need information about the presence of oxidants in this environment, along with information on surface properties to study impact erosion and resurfacing. And we need a world of information about the chemical evolution of organics in ice in the presence of energetic particles.

The list could continue, but I turn you over to the paper, which is Parkinson et al., “Enceladus: Cassini observations and implications for the search for life,” in Astronomy & Astrophysics 463 (2007), pp. 353-357 (available online).

Comments on this entry are closed.

  • tacitus January 19, 2008, 15:59

    One intriguing thought. Cassini is scheduled to fly through the Enceladus plumes later on this year (barely above the moon’s surface!) and may well get a sniff of their chemical content.

    Now, while I doubt the Cassini is equipped to detect the signature of life from the encounter, the possibility of directly sampling water that might have once harbored life without having to go to all that bother of landing on the moon’s surface has got to be a factor in any future missions to Saturn. Right?

  • andy January 19, 2008, 20:02

    I thought the clathrate model was in vogue at the moment: evidence is currently pointing against there being a liquid water ocean. The paper linked doesn’t mention the clathrate model at all as far as I can tell.

  • Administrator January 19, 2008, 20:25

    The clathrate vogue may not be as pronounced as either of us thought. In any case, I see no clathrate discussion in this paper either.

  • James M. Essig January 19, 2008, 21:59

    Hi tacitus, andy, and Paul;

    The possibility of life on the moon Enceladus is very intriguing. Other obvious potential locations for life are Europa and perhaps even Titan. As I am sure much of the readership may be aware, Europa might have vast salt water or brine oceans below the surface ice which is believed to be on the rough order of magnitude of 1 kilometer thick. I would be thrilled if by chance, aquatic organisms such as fish or other animal life forms existed in this subterrainian aquatic environment and which were detected by a submersable that melted its way through the ice sheet. Perhaps Titan might have some sort of life form based on liquid hydrogenous compounds or derived from such that are gaseous at STP on Earth such as methane, ethane, and the like. Perhaps solid body structures could be composed of frozen hydrocarbons that are normally liquid at STP on Earth.

    At the very least, these moons could be vaste resources for exothermically nuclear fusionable fuels for effecient fusion powered electric propulsion systems to power manned missions throughout our local interstellar neighboorhood. It is interesting that NASA is developing hybrid chemical rockets that can run on a variety of liquid, or gaseous fuels, for powering manned interplanetary space craft. What a source of fuel for planetary space space passenger liners, commercial cargo vessels, and military/security vessels that would ply the solar system and proximate settlements.

    Granted that the hydrogenic gas combustion powered vessels would need oxidizers such as oxygen to burn these limited Isp fuels, perhaps it is possible that new forms of super high energy density chemical fuels could be developed to bridge the gap until fission or fusion powered rockets could be fielded. I do not know if this story is true or not, but a high school friend once told me he heard of a supposed rumour that some person accidently discovered a hydrocarbon based fuel during the early 20th century that had an energy density 100 times greater than desiel but that the formula was not made known for what ever reasons. This could simply be a science myth, however, I have always been intriguid by the possibility the chemists might cook up some form of chemical fuels with mass specific energy densities of anywhere from 1/2 to 2 orders of magnitude greater than H2/LOX chemical fuel. What such fuels would be, I am not sure.

    Thanks;

    Your Friend Jim

  • MaDeR January 20, 2008, 6:41

    I think that IF there are really conditions favorable for life on Enceladus, and these conditions has been for long (geologically speaking) time, then life would be here already by means of interplanetary panspermia. In other words, this life would be imported, not native variety. Most probable source? Of course, Earth.

  • andy January 20, 2008, 7:38

    Ah, this paper is apparently from February last year (so says NASA ADS)… IIRC the gradual shift towards the clathrate model has occurred since that time.

  • Administrator January 20, 2008, 8:22

    Those wanting more information on clathrates and the evolving model for Enceladus can start with this link:

    https://centauri-dreams.org/?p=1403

    to get a look at a completely different vision of this moon.

  • Adam January 21, 2008, 2:50

    Hi All

    James, anything stronger than hdyrocarbon chemistry, but less powerful than fission reactions, would be of interest as a non-nuclear thermonuclear trigger. A series of imploding shells of the stuff would make a nice thermonuclear trigger and thus the imperative to make (and keep) it secret would be high.

    MaDeR, panspermia from Earth to Enceladus has the same problem as panspermia to Europa – too deep down the gravitational hole. Any meteoroid carrying bacteria would impact too fast for survival. Titan, with its deep atmosphere, is the only outer planet moon that life-bearing meteorites would arrive safely. Anywhere else and they vaporise.

    Not so sure about the redox chemistry thing on Europa – the stream of oxidants from the surface should help matters. Enceladus seems a long shot for retaining a viable ecosystem.

  • Richard Greenberg January 21, 2008, 14:05

    Note that the Parkinson et al. paper was written over a year and a half ago, and there has been plenty more research done interpreting the data on Enceladus since then. Considerable uncertainty remains about the physical conditions and dominant processes there.

    Regarding Europa, the paper mentions the production of oxidants at the surface and the severe constraints on life in the ocean, assuming that the ocean is isolated from the oxidants and the surface. However, there are probably very strong connections between the ocean and the surface of Europa. Continual cracking of the ice through to the ocean, separation of plates of crust, daily tidal working of the cracks, thermally driven melt-through events, etc., all allow oxidants to find their way to the ocean. These processes plausibly make the crust as well as the ocean quite habitable. For more detail, see my book “Europa the Ocean Moon: Search for an Alien Biosphere”, and/or my various other publications.

    Richard Greenberg

  • Administrator January 21, 2008, 14:41

    Considerable uncertainty is right, and it’s clear we’ll be trying to figure out Enceladus for some time to come. Meanwhile, Richard’s 2005 Europa book from Springer comes highly recommended:

    http://www.amazon.com/Europa-Ocean-Moon-Biosphere-Geophysical/dp/B000PY3UMG/ref=sr_1_11?ie=UTF8&s=books&qid=1200944202&sr=8-11

    and I’m curious about his thoughts on when we’ll see a dedicated Europa mission. Not as soon as most of us would wish, I’m sure.

  • David Langlois January 22, 2008, 8:24

    Does continuous loss of water from Enceladus imply that its submerged ocean is becoming saltier over time? How would its salinity differ from Europa’s which shows no hydrothermal venting thus far? Prospecting for life should in any event be easier at Enceladus due to its spraying its insides all over space. Solar energy, if a factor in the hypothesized biospheres at all, is in abundance in inversely proportional to their distance from the Sun, apparently putting Enceladus at the disadvantage. Again, abundance of light at the “surface” of the water should be greater on Enceladus, due to the relative thinness of the ice. Or am I missing something?

  • actionforspace.com January 22, 2008, 15:44

    Tell the candidates about the importance of discovering more about Enceladus!

    http://www.actionforspace.com is where you can demand of the next candidates to fund space exploration.

    Put your money where your mouth is and speak out for space exploration.

  • ljk January 23, 2008, 9:11

    My predicted response from most if not all of the 2008
    Presidential candidates when asking them to support a
    mission to Enceladus.

    “Encelawhat??”

  • Edg Duveyoung February 3, 2008, 11:29

    Wow! This research shows “possiblity of life” on most of the outer orbs in our solar system.

    http://discovermagazine.com/2008/feb/did-life-evolve-in-ice

    Who’d-a-thunk?

    Edg

  • Adam February 4, 2008, 3:57

    Hi Edg

    Well spotted. Fascinating research. Makes me wonder just where life did get started, since Earth is often thought to have been very hot even after the Late Heavy Bombardment. Some lines of evidence indicate it was near boiling point for hundreds of millions of years.

  • ljk February 6, 2008, 0:01

    Saturn Has a ‘Giant Sponge’

    One of Saturn’s rings does housecleaning, soaking up material gushing from the fountains on Saturn’s tiny ice moon Enceladus, according to new observations from the Cassini spacecraft.

    “Saturn’s A-ring and Enceladus are separated by 100,000 kilometers (62,000 miles), yet there’s a physical connection between the two,” says William Farrell of NASA’s Goddard Space Flight Center in Greenbelt, Md. “Prior to Cassini, it was believed that the two bodies were separate and distinct entities, but Cassini’s unique observations indicate that Enceladus is actually delivering a portion of its mass directly to the outer edge of the A-ring.” Farrell is lead author of a paper on this discovery that appeared in Geophysical Research Letters January 23.

    This is the latest surprising phenomenon associated with the ice geysers of Enceladus to be discovered or confirmed by Cassini scientists. Earlier, the geysers were found to be responsible for the content of the E-ring. Next, the whole magnetic environment of Saturn was found to be weighed down by the material spewing from Enceladus, which becomes plasma — a gas of electrically charged particles. Now, Cassini scientists confirm that the plasma, which creates a donut-shaped cloud around Saturn, is being snatched by Saturn’s A-ring, which acts like a giant sponge where the plasma is absorbed.

    Shot from Enceladus’ interior, the gas particles become electrically charged (ionized) by sunlight and collisions with other atoms and electrons. Once electrically charged, the particles feel magnetic force and are swept into the space around Saturn dominated by the planet’s powerful magnetic field. There, they are trapped by Saturn’s magnetic field lines, bouncing back and forth from pole to pole. The fun ends, however, if their bouncing path carries them inward toward Saturn to the A-ring. There they stick, in essence becoming part of the ring. “Once they get to the outer A-ring, they are stuck,” says Farrell.

    “This is an example of how Saturn’s rings mitigate the overall radiation environment around the planet, sponging up low- and high-energy particles,” says Farrell. By contrast, Jupiter has no dense rings to soak up high-energy particles, so that planet’s extremely high radiation environment persists.

    The Cassini observations confirm a prediction by John Richardson and Slobodan Jurac of the Massachusetts Institute of Technology. In the early 1990’s, Hubble Space Telescope observations revealed the presence of a large body of water-related molecules in orbit about 240,000 kilometers (almost 150,000 miles) from Saturn. Richardson and Jurac modeled this water cloud and demonstrated it could migrate inward to the A-ring. “We relied on their predictions to help us interpret our data,” said Farrell. “They predicted it, and we were seeing it.”

    At the time of their prediction, the source of the water cloud was unknown. The source was not identified until 2005 when Cassini discovered the stunning geysers emitted from Enceladus.

    Data for the discovery that Saturn’s A-ring acts like a sponge were collected in July 2004 when Cassini arrived in orbit around Saturn, making its closest flyby over the A-ring. “We skimmed over the top of that ring fairly close,” said Farrell.

    Hot spots on the inside wall of the plasma donut — the part colliding with the A-ring — were emitting radio signals. These signals behaved as a sort of natural radio beacon, indicating the local plasma density at the inner edge of the donut. The signals were detected by Cassini’s Radio and Plasma Wave instrument. The team used these signals to monitor the density of the plasma (the higher the frequency, the greater the density) and hence witness the change in gas density with time.

    “As we approached the A-ring, the frequency dropped, implying that the plasma density was going down because it was being absorbed by the ring,” said Farrell. “What really drove this home was what happened to the signal when we passed over a gap in the rings, called the Cassini division. There, the frequency went higher, implying that the plasma density was going up because plasma was leaking through the gap.”

    The research was funded by NASA through the Cassini-Huygens project. Cassini-Huygens is an international collaboration among NASA, the European Space Agency, and the Italian Space Agency. The Cassini orbiter was built and is managed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

    For more information about Cassini, visit:

    http://saturn.jpl.nasa.gov/home/index.cfm

    Written by: Bill Steigerwald, Goddard Space Flight Center

    Media contacts: Carolina Martinez (JPL) 818-354-9382 and Bill Steigerwald (Goddard)

    301-286-5017

  • ljk February 6, 2008, 21:39

    Case builds for water on Saturn moon

    by Staff Writers

    Paris (AFP) Feb 6, 2008

    Astrophysicists in Germany say they can add evidence to bolster theories that water, one of the precious ingredients for life, exists on the Saturnian moon Enceladus.

    A tiny satellite measuring just 504 kilometres (315 miles) across, Enceladus has become one of the most fiercely debated objects in the Solar System, thanks to close-up pictures taken by the US probe Cassini.

    Enceladus has a brilliant white shell of ice that is untouched except for some strange-looking grooves and impacts from space rocks.

    Cassini revealed plumes of water vapour that gush from surface stripes near its south pole, shooting crystal jets upwards for hundreds of kilometres (miles) into space.

    Fuelling discussion about the origin of these strange “cryo-volcanoes” is the fact that icy particles of dust are also mixed in with the eruptions, but beguilingly travel far slower than the vapour.

    A team led by Juergen Schmidt of the University of Potsdam, near Berlin, say they can now answer at least this part of the mystery.

    Full article here:

    http://www.saturndaily.com/reports/Case_builds_for_water_on_Saturn_moon_999.html

  • ljk February 22, 2008, 12:57

    Slow dust in Enceladus’ plume from condensation and
    wall collisions in tiger stripe fractures

    Publication date: 07 Feb 2008

    Authors: Schmidt, J. et al.

    Journal: Nature
    Volume: 451
    Issue: 7179
    Page: 685-688
    Year: 2008

    Copyright: Nature Publishing Group

    One of the spectacular discoveries of the Cassini spacecraft
    was the plume of water vapour and icy particles (dust)
    originating near the south pole of Saturn’s moon Enceladus.
    The data imply considerably smaller velocities for the grains
    than for the vapour, which has been difficult to understand.
    The gas and dust are too dilute in the plume to interact, so
    the difference must arise below the surface.

    Here we report a model for grain condensation and growth
    in channels of variable width. We show that repeated wall
    collisions of grains, with re-acceleration by the gas, induce
    an effective friction, offering a natural explanation for the
    reduced grain velocity. We derive particle speed and size
    distributions that reproduce the observed and inferred
    properties of the dust plume.

    The gas seems to form near the triple point of water;
    gas densities corresponding to sublimation from ice at
    temperatures less than 260 K are generally too low to
    support the measured particle fluxes.

    This in turn suggests liquid water below Enceladus’ south pole.

    Link to Publication

    http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42339