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Enceladus, Europa and Life

Having looked at Titan and Europa yesterday, we can complete our outer planetary trifecta with a stop at Enceladus, lately of great interest as an active and possibly life-bearing moon. One hot spot detected there by Cassini is ejecting plumes of ice and vapor above the arid world in a cloud so fine that, according to William McKinnon (Washington University, St. Louis), the result is like a smoke made of ice, its particles about one-thousandth of a millimeter across. Enceladus is clearly a geologically active world, far from the inert desolation once expected.

Enceladus from Cassini

All of which makes the Saturnian moon intriguing in the extreme when you start wondering about the presence of water and the possibilities of life. But McKinnon is quick to dash that hope when asked bluntly whether there is evidence for a subterranean ocean:

“I don’t think so,” McKinnon said. “The strongest piece of evidence against that is measurements made from Earth of the plume don’t show any sodium. If the source of the plumes were fresh water like on Earth, the plumes would contain enough detectable sodium. Fresh water flows through rocks and on streambeds, and so it picks up bits of mineral chemistry. The emerging view is that there’s not obvious evidence for a subterranean ocean in contact with rock, no boiling or venting.”

Image: Enceladus as seen by Cassini. Investigating its hot spots is again on the orbiter’s agenda in March. Credit: NASA/JPL/Space Science Institute.

We thus wind up with what is still the most obvious explanation for the plumes of Enceladus, the tides that cause crustal motion in a set of fault lines near the south pole. On its next pass, in March of this year, Cassini will go again into the plume and examine the venting area in the infrared, along with the cracks, impact craters and various fissures found in this intriguing area. We should also wind up with better measurements of the gases and vapors issuing from this activity.

The tidal motion here is interesting. Enceladus is in dynamic resonance with Dione, so that every time the more distant Dione orbits Saturn, Enceladus makes two revolutions. The continuous squeeze from Saturn does interesting things to the surface, with the temperature at the hot spot at least 100 degrees warmer than the temperature at the actual poles. And the same kind of tidal mechanisms help power Io’s volcanism and put Europa under continual forces that re-shape its surface.

The same American Geophysical Union meeting where McKinnon delivered his Enceladus presentation in December was the venue for his comment that an ocean on Europa is now considered all but a certainty. Radar sounding by a future Europa orbiter could tell us much more about the depth of the ice shell covering that body of water. So we continue to turn to the moons of the outer planets as possible sites for life, with the odds on Enceladus (never high) perhaps fading but Europa more in the picture than ever. Nor (and I hope David Grinspoon is still with us!) should we count out exotic possibilities on Titan itself.

Comments on this entry are closed.

  • Todd January 10, 2008, 22:41

    Isn’t the slight deviation from a circular orbit the key in a resonance situation with two or more moons? Enceladus has a slightly higher eccentricity than Dione or Tethys. But those other moons, and possibly Rhea as well, may still have some surprises due to their deviations from a perfect circular orbit.

    What I find so interesting is that whales and dolphins might find far, far more friendly extraterrestrial playgrounds than human beings. What might that say about the possibilities for life in the universe?

  • Adam January 11, 2008, 2:28

    Hi Paul

    Europa should be an ESA focus in their astrobio program because Mars is so dominated by NASA’s efforts, but NASA seems bent on under-funding Europa studies thus providing a big opportunity for real break-thru science by Europe.

  • Administrator January 11, 2008, 9:20

    Todd, these are interesting speculations about Dione, Tethys and Rhea, and we’ll doubtless be finding some surprises on these moons as Cassini keeps up its work. Your notion about extraterrestrial habitats more hospitable for whales and dolphins is indeed something to ponder, given the possibility of sub-surface oceans on other moons as well as Europa (I believe Callisto is still in the mix). Europa is a tough mission (and Adam, I can see Gregory Benford’s point about doing Mars first), but the opportunities for a Europa orbiter as a next step seem profound, and of course from there we’ll learn much about the other Galilean moons.

  • ljk January 11, 2008, 10:52

    There was an article circa 1980 about sending dolphins
    to Europa to explore its oceans. Just a few logistical
    problems, but the very idea has its appeal.

    Hey, if we are planning to send humans to places
    like Mars that require bringing one’s own environment,
    why not dolphins who will at least have a huge ocean
    to swim around in.

  • andy January 11, 2008, 16:12

    Enceladus has a slightly higher eccentricity than Dione or Tethys. But those other moons, and possibly Rhea as well, may still have some surprises due to their deviations from a perfect circular orbit.

    Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects – Rhea seems to be a possibility.

    Going beyond the Saturn system, Titania, Oberon, Triton, Pluto, Eris (2003 UB313), Sedna and Orcus (2004 DW) seem to be worth a look…

    Whether such small objects could provide enough energy sources to drive a biosphere, I don’t know.

  • Todd January 12, 2008, 11:19

    Thanks for responding directly, Paul.

    The Europa mission will indeed have challenges. Do you know if they’ve started looking at mission design yet? That’d be a darned interesting discussion. It seems they will have to use multiple gravity assists with the moons. I wonder if a Messenger-like mission will be utilized: repeated fly-bys of Ganymede, Callisto, and Europa to get just the right approach and speed for the orbit burn.

    That approach stage would finally give us significant data on the Galilean moons to match the quality/quantity we’re getting with Cassini. At least a year or two of satellite encounters (or ion propulsion activity) followed by an orbit stage. The main challenge there would be radiation. The Io neighborhood was tough on Galileo in the extended mission, and even at Europa’s distance they’d have to devote some weight to protecting electronics.

    A Titan orbiter almost seems easier, given that aero-braking is part of the equation there.

    That abstract is interesting, Andy. We didn’t get much of a look at the Uranian moons, did we?–just enough to tantalize. A Uranus orbit mission to study satellites would be even tougher than Europa: the distance, the lighter masses of those satellites (it would be like doing a Cassini without any Titan gravity assists) and the once-every-forty years alignment of the orbital plane. Still, I bet there are interesting surprises there. I probably won’t live to see another mission.

  • Administrator January 12, 2008, 13:24

    Todd, you’re right, Uranus poses challenges, but I hope you’re wrong about not living to see another mission — if you’ll see it, I’ll see it, as I think we’re about the same age! We certainly have much to learn about the moons there. And you’re certainly right about the radiation issues around Jupiter, though recent work suggests it’s less intense at Europa than previously believed. Mission design will be fascinating to watch, and I’ll see what I can find out about ongoing studies.

  • ljk February 20, 2008, 0:48

    Mysterious Creatures Found in Antarctica

    Feb 19 05:00 PM US/Eastern

    Giant ‘Sea Spiders’ Captured on Video in Antarctic Waters

    SYDNEY, Australia (AP) – Scientists investigating the icy waters of Antarctica said Tuesday they have collected mysterious creatures including giant sea spiders and huge worms in the murky depths.

    Australian experts taking part in an international program to take a census of marine life in the ocean at the far south of the world collected specimens from up to 6,500 feet beneath the surface, and said many may never have been seen before.

    Some of the animals far under the sea grow to unusually large sizes, a phenomenon called gigantism that scientists still do not fully understand.

    “Gigantism is very common in Antarctic waters,” Martin Riddle, the Australian Antarctic Division scientist who led the expedition, said in a statement. “We have collected huge worms, giant crustaceans and sea spiders the size of dinner plates.”

    Full article here:


  • ljk April 30, 2008, 9:39

    Jupiter’s Europa – Will It Provide the 1st Proof of Extraterrestrial Life?

    To quote:

    With Jupiter being the largest planet in the solar system, its tidal stresses on Europa create enough heat to keep the water on Europa in a liquid state. More than just water is needed to support life. Tides also play a role in providing for life. Ocean tides on Europa are much greater in size than Earth’s with heights reaching 500 meters (more than 1,600 feet). Even the shape of the moon is stretched along the equator due to Jupiter’s pull on the waters below the icy surface.

    The mixing of substances needed to support life is also driven by tides. Stable environments are also necessary for life to flourish. Europa, whose orbit around Jupiter is in-sync with its rotation, is able to keep the same face towards the gas giant for thousands of years. The ocean is interacting with the surface, according to Greenberg, and “there is a possible biosphere that extends from way below the surface to just above the crust.”

    “The real key to life on Europa,” Greenburg adds, “is the permeability of the ice crust. There is strong evidence that the ocean below the ice is connected to the surface through cracks and melting, at various times and places. As a result, the biosphere, if there is one, includes not just the liquid water ocean, but it extends through the ice up to the surface where there is access to oxidants, organic compounds, and light for photosynthesis. The physical setting provides a variety of potentially habitable and evolving niches. If there is life there, it would not necessarily be restricted to microorganisms.”

    The future NASA DepthX mission to Europa, scheduled for 2019, is a mushroom-shaped machine, an underwater hydrobot that ‘thinks’ for itself. DepthX is currently undergoing tests in one of the world’s deepest flooded cave systems -the El Zacaton cave complex in Mexico- to simulate penetrating the Europa’s ice-covered seas. The next version of the machine will be tested in Lake Vostok, a deep ice-covered lake in the Antarctic. The craft sent to Europa would use nuclear power to melt through the 10km of ice that cover the moon’s ocean. The mission will be one of the most complex ever attempted by the American space agency NASA.

    Full article here:


  • ljk May 10, 2008, 9:04


    The sea floor off the coast of Eureka, California, is home to a
    diverse assemblage of microbes that scavenge methane from cold
    deep-sea vents. Researchers at the California Institute of
    Technology have developed a technique to directly capture these
    cells, lending insight into the diverse symbiotic partnerships that
    evolved among different species in an extreme environment. The
    community’s interconnected metabolism sheds light on how the
    anaerobic microbes, which consume nearly 80 percent of the methane
    leaked from marine sediments, limit oceanic emissions of this potent
    greenhouse gas.

    Details: http://mr.caltech.edu/media/Press_Releases/PR13141.html

  • ljk May 15, 2008, 10:39

    Pole Shift on Europa?

    Written by Nancy Atkinson

    Curved features on Jupiter’s moon Europa may indicate that its poles have wandered by almost 90°, a new study reports. Researchers believe the drastic shift in Europa’s rotational axis was likely a result of the build-up of thick ice at the poles.

    “A spinning body is most stable with its mass farthest from its spin axis,” says Isamu Matsuyama of the Carnegie Institution’s Department of Terrestrial Magnetism. “On Europa, variations in the thickness of its outer shell caused a mass imbalance, so the rotation axis reoriented to a new stable state.” An extreme shift like this also suggests the existence of an internal liquid ocean beneath the icy crust.

    The research team used images from the Voyager, Galileo, and New Horizons spacecraft to map several large arc-shaped depressions that extend more than 500 kilometers across Europa’s surface. With a radius of about 1500 kilometers, Europa is slightly smaller than the Earth’s moon.

    By comparing the pattern of the depressions with fractures that would result from stresses caused by a shift in Europa’s rotational axis, the researchers determined that the axis had shifted by approximately 80°. The previous axis of rotation is now located about 10° from the present equator.

    Full article and images here:


  • ljk May 29, 2008, 15:12

    Bacteria “Feed” on Earth’s Ocean-Bottom Crust

    “Seafloor bacteria on ocean-bottom rocks are more abundant
    and diverse than previously thought, appearing to “feed” on the
    planet’s oceanic crust, according to results of a study reported
    in this week’s issue of the journal Nature.”


  • ljk June 16, 2008, 14:15

    NASA Astrobiology Research: Astronaut, Scientists Explore
    Lake to Learn More About Life

    “The Pavilion Lake Research Project (PLRP) science team
    will conduct rigorous scientific research of life in extreme
    environments from June 23 to July 3, 2008 using DeepWorker
    submersibles to study towering, freshwater reef-like structures
    built by bacteria, along the bottom of the lake.

    While gathering their underwater samples, scientists and
    astronauts hope to learn collection techniques of interest to
    future lunar and space exploration missions.”


  • ljk October 8, 2008, 22:19

    October 07, 2008

    Will Jupiter’s Europa Provide the 1st Proof of Extraterrestrial Life?

    “It is critical to know what to look for in the search for life in the solar system. The search so far has focused on Earth-like life because that’s all we know, but life that may have originated elsewhere could be unrecognizable compared with life here. Advances throughout the last decade in biology and biochemistry show that the basic requirements for life might not be as concrete as we thought.”

    John Baross, Professor of Oceanography and the Astrobiology Program at the University of Washington.

    As the New Horizons mission -currently en route for the newly christened dwarf planet Pluto and its three moons, Charon, Nix and Hydra- made its way past Jupiter in the early half of this year it was able to take detailed scans and imaging of our solar system’s largest planet. New Horizons zipped by Jupiter, making its closest pass on February 28, and used the gaseous planet’s considerable gravity to slingshot itself toward Pluto.

    “The Jupiter encounter was successful beyond our wildest dreams,” says New Horizons Principal Investigator Alan Stern, of NASA Headquarters. “Not only did it prove out our spacecraft and put it on course to reach Pluto in 2015, it was a chance for us to take sophisticated instruments to places in the Jovian system where other spacecraft couldn’t go, and to return important data that adds tremendously to our understanding of Jupiter and its moons, rings and atmosphere.”

    The New Horizons spacecraft, also snapped images of the tiny rings encircling Jupiter, studied a huge, swirling storm and explored the planet’s long magnetic tail.

    The New Horizon flyby at its closest came within 3 giga-meters of the planet and made important discoveries focusing on Jovian weather. Heat-induced lightning strikes were found occurring in Jupiter’s
    Polar Regions, the first polar strikes observed anywhere else other than earth.

    New Horizon also made the single most detailed analysis of waves that traverse the entire width of the planet. These waves indicate the presence of massive storms below the levels of ammonia that make up the clouds obscuring much of Jupiter’s surface.

    But while in the neighborhood, the fastest spacecraft ever launched from Earth aimed its cameras and sensors at Jupiter and its four largest moons, making about 700 observations.

    Due to New Horizon’s primary purpose for visiting Jupiter – a gravitational speed bump – it was out of range to properly focus on the 60 Jovian moons. However, as it is designed to focus on dim targets, the teams were able to acquire some data from Jupiter’s four largest moons, including Europa, thought by many planetary scientists as the Solar Systems best bet for the first discovery of extraterrestrial life.

    The recent publication of the National Research Council of its report, Limits of Organic Life in Planetary Systems, urged NASA to expand its search for life in the Solar System to include non-carbon forms of life. On the microbial level, most serious astronomers and scientists have little doubt that life will be found within the next decade within our Solar System in the Martian soil, or in the methane seas of Saturn’s moon, Titan, or the turbid seas of Jupiter’s smallest moon, Europa.

    Europa might not only sustain, but foster life, according to the research of University of Arizona’s Richard Greenberg, a professor of planetary sciences and member of the Imaging Team for NASA’s Galileo Jupiter-orbiter spacecraft.

    Full article here:


    To quote:

    The future NASA DepthX mission to Europa, scheduled for 2019, is a mushroom-shaped machine, an underwater hydrobot that ‘thinks’ for itself. DepthX is currently undergoing tests in one of the world’s deepest flooded cave systems -the El Zacaton cave complex in Mexico- to simulate penetrating the Europa’s ice-covered seas. The next version of the machine will be tested in Lake Vostok, a deep ice-covered lake in the Antarctic. The craft sent to Europa would use nuclear power to melt through the 10km of ice that cover the moon’s ocean. The mission will be one of the most complex ever attempted by the American space agency NASA.