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Heating Up Enceladus

How to explain the water vapor and ice blown off in the form of geyser-like jets from Enceladus? It’s a question we need to answer, because we’re learning just how interesting the icy moons of gas giants can be, with the potential for biological activity far from the Sun.

In the case of Enceladus, though, the average global thickness of the ice is thought to be 20 to 25 kilometers. What has thinned the ice at the south pole, where warm fractures expel mineral-rich water into space? An unusual amount of heat is demanded to sustain this ongoing activity, along with a mechanism to explain what is happening within the moon.

The heat in question is likely the result of friction, according to a new study published in Nature Astronomy. The work of Gaël Choblet (University of Nantes, France) and colleagues, the investigation involved modeling by Cassini researchers in Europe and the U.S., tapping the abundant data that the Saturn orbiter returned to Earth in its 13 years at the planet. It was Cassini that first showed us the jets of water vapor and simple organics.

Image: Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. The tiger stripes are fissures that spray icy particles, water vapor and organic compounds. More than 30 individual jets of different sizes can be seen in this image and more than 20 of them had not been identified before. At least one jet spouting prominently in previous images now appears less powerful. This mosaic was created from two high-resolution images that were captured by the narrow-angle camera when NASA’s Cassini spacecraft flew past Enceladus and through the jets on Nov. 21, 2009. Credit: NASA/JPL/Space Science Institute.

The authors find evidence in the Cassini data that hydrothermal activity takes place on the seafloor under Enceladus’ ice. Rock grains thought to be the result of this activity show chemistry occurring at temperatures of at least 90 degrees Celsius. What’s intriguing here is that Choblet and team believe the energy required to produce these temperatures exceeds what could be expected from the decay of radioactive elements within Enceladus. Says Choblet:

“Where Enceladus gets the sustained power to remain active has always been a bit of a mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy.”

The result: An Enceladus whose core is relatively loose and rocky, with 20 to 30 percent empty space. The orbiting moon would experience tidal forces that cause rock in the relatively porous core to flex and abrade against adjacent rock, generating the required heat. This internal scenario also accounts for the rise of heated water from the ocean, moving upward and interacting chemically with the rocks. From the paper’s abstract:

Water transport in the tidally heated permeable core results in hot narrow upwellings with temperatures exceeding 363 K, characterized by powerful (1–5 GW) hotspots at the seafloor, particularly at the south pole. The release of heat in narrow regions favours intense interaction between water and rock, and the transport of hydrothermal products from the core to the plume sources. We are thus able to explain the main global characteristics of Enceladus: global ocean, strong dissipation, reduced ice-shell thickness at the south pole and seafloor activity.

If Choblet’s models are correct, we should expect maximum activity from this process at the moon’s poles, with mineral-rich water thinning the ice shell to as little as 1 kilometer at the south pole, and being expelled into space through fractures in the ice. This analysis would explain why Enceladus has thinner ice at the south pole, but it does not explain why north and south poles are so utterly different. Unlike the south pole, Enceladus’ north pole is cratered and ancient.

The circulation of the global ocean over millions of years would mean that the entire volume of the ocean passes through the moon’s core, an amount estimated to equal 2 percent of the volume of Earth’s oceans. The new study goes past earlier work in modeling tidal friction at a higher level of complexity — earlier models found that tidal heating would be insufficient to keep the ocean liquid. The porous core model thus accounts for the plumes we see today, which may result from thinner ice in the south to begin with, causing runaway heating over time.

The paper is Choblet et al., “Powering prolonged hydrothermal activity inside Enceladus,” Nature Astronomy 6 November 2017 (abstract).

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{ 14 comments… add one }
  • DJ Kaplan November 8, 2017, 15:08

    The probability is so high that there is some sort of life wiggling around in that water… I’d think it would be an irresistible scientific target. When is the launch date for the new probe?

  • John walker November 8, 2017, 18:00

    Choblet and Tobie calculate that this frictional heating could maintain a liquid ocean for Gyrs. Be a shame if Enceladus were as has been speculated only a few 10s of Myrs old.

    • John walker November 9, 2017, 5:49

      From the perspective of bio-evolutionary potential.

    • andy November 10, 2017, 13:52

      Since that was published, there’s been further investigation into whether the moons could have been re-formed after a giant collision, see Hyodo & Charnoz “Dynamical Evolution of the Debris Disk after a Satellite Catastrophic Disruption around Saturn“. It looks like it does hold up, but would not be able to explain the formation of the rings because the disc produced by the collision does not spread out very much before re-accreting into new satellites. Ring formation may still be possible by disrupting an inner satellite similar to Mimas, according to this paper.

      Gas giant satellite systems are starting to look like dangerously violent environments, even in old planetary systems!

  • Andrew Palfreyman November 9, 2017, 8:39

    The political climate at NASA in recent years seems to favour staying away from the oceans of these Moons. The Planetary Protection Police seem to be calling the shots here. So no penetrators, no drilling down or melting down, and no submarines for these moons anytime soon. Simply getting a lander on Europa Clipper was apparently a battle, even though it can only mess around on the surface.

    (A similar story obtains with Mars. Despite umpteen satellites, probes and rovers, there has been no subterranean excavation and in situ biological analysis anywhere near where subsurface water is likely to be found.)

    • DJ Kaplan November 9, 2017, 13:15

      Do you sense that they simply want to ensure that germs from Earth don’t contaminate any experiments (and spacecraft can be sterilized) or do they want to avoid discovering life?

      • Andrei November 10, 2017, 3:56

        Sterilization cost a lot more, and have not done to save money.
        Current Mars rover go far over budget and again had to get less sterilization – full treatment so no life present is impossible only that very little remain.
        So the rover of most resent mission have to avoid and keep distance to possible waterspring if it ever find one.

  • ljk November 9, 2017, 9:40

    Enceladus may not be the only Saturnian moon with geysers and therefore a subsurface ocean:

    https://www.space.com/20481-dione-moon.html

  • ljk November 9, 2017, 9:42

    Nov. 2, 2017

    Picture This SELFI: NASA Advances Instrument to Study the Plumes of Enceladus

    NASA scientists and engineers have conceived and plan to build an ambitious submillimeter-wave or radio instrument to study the composition of geysers spewing water vapor and icy particles from the south pole of Saturn’s small moon, Enceladus.

    The team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, recently received support to advance technologies needed for the Submillimeter Enceladus Life Fundamentals Instrument, or SELFI. This remote-sensing instrument represents a significant improvement over the current state-of-the-art in submillimeter-wavelength devices, said SELFI Principal Investigator Gordon Chin.

    SELFI is being designed to measure traces of chemicals in the plumes of water vapor and icy particles that emanate from fissures, also known as tiger stripes, on Enceladus, Saturn’s sixth largest moon. By studying the plumes, scientists believe they can extrapolate the composition of the ocean that lies beneath the moon’s icy crust and its potential to host extraterrestrial life.

    Full article here:

    https://www.nasa.gov/feature/goddard/2017/picture-this-selfi-nasa-advances-instrument-to-study-the-plumes-of-Enceladus

  • ljk November 10, 2017, 17:23

    Once again it is Russian billionaire Yuri Milner to the rescue:

    https://www.geekwire.com/2017/yuri-milner-enceladus-breakthrough-paradise-papers-russian/

    To quote:

    Milner said he and other interested parties (including Porco) have been discussing what it would take to find a “smoking gun” for biochemical activity at Enceladus.

    “We formed a little workshop around this idea: Can we design a low-cost, privately funded mission to Enceladus which can be launched relatively soon, and that can look more thoroughly at those plumes, try to see what’s going on there?” he said.

    NASA and the European Space Agency have been considering several proposals for Enceladus missions, including ELF, ELSAH and E2T. But Milner said it could take as long as 10 years to get those more expensive projects off the ground. He wants to send something through Enceladus’ plumes sooner.

    Milner told GeekWire that the time frame for the mission he has in mind is still up in the air. The mission’s potential price tag is also yet to be determined, although he said he’s sure it’ll be cheaper than a full-blown NASA mission. For now, the only expense is the cost of doing a feasibility study.

    “And if I am funding the study, I can do something there,” Milner said.

  • Michael Fidler November 14, 2017, 23:01

    WHEN WATER MET IRON DEEP INSIDE THE EARTH, IT MIGHT HAVE CREATED CONDITIONS FOR LIFE.
    “Reservoirs of oxygen-rich iron between the Earth’s core and mantle could have played a major role in Earth’s history, including the breakup of supercontinents, drastic changes in Earth’s atmospheric makeup, and the creation of life, according to recent work from an international research team published in National Science Review.”

    https://www.astrobio.net/also-in-news/water-met-iron-deep-inside-earth-might-created-conditions-life/

    https://www.astrobio.net/wp-content/uploads/2017/11/PRMaoWhenIronMetWater-800×371.jpg

  • ljk November 15, 2017, 16:28

    Russian Billionaire Wants to Launch a Private Mission to Enceladus

    By Jay Bennett

    November 14, 2017

    The two most exciting places in the solar system may be Jupiter’s moon Europa and Saturn’s moon Enceladus. The watery worlds are widely considered the best bets to find extraterrestrial life. NASA is now building a spacecraft called Europa Clipper to launch to Jupiter’s moon in the early 2020s. Enceladus, however, does not yet have its own mission on the books.

    Yuri Milner wants to change that. The Russian billionaire venture capitalist and amateur physicist is the man behind the Breakthrough Starshot mission to send a nano-spacecraft to the closest star, Proxima Centauri, and an initiative called Breakthrough Listen to use powerful radio telescopes to search for signals from intelligent extraterrestrial civilizations. Now Milner’s Breakthrough Initiatives has set its sights on Enceladus.

    Full article here:

    http://www.popularmechanics.com/space/solar-system/a13579385/breakthrough-initiatives-launch-private-mission-enceladus/

    To quote:

    “We formed a sort of little workshop around this idea: Can we design a low-cost, privately funded mission to Enceladus which can be launched relatively soon?” Milner said at an inaugural international space summit called “A New Space Age” put on by the Economist magazine in Seattle, as reported by Space.com. If Milner is serious about launching a spacecraft to Enceladus, it would be a historic feat as the first privately funded mission to the outer solar system. (If it launched today, it would be the very first private interplanetary mission at all.)

    The Russian tech magnate went on to say he wanted a spacecraft “that can look more thoroughly at those plumes and try to see what’s going there ahead of a more expensive mission that NASA is considering right now, which might take maybe 10 years to launch.”

    Details about the Breakthrough Initiatives’ proposed mission, such as a timeline or list of spacecraft instrumentation, are currently lacking. But given the vast resources of the organization, perhaps Yuri Milner and co. and can do something never done before—send a spacecraft beyond the asteroid belt without the help of a national government. And if they go, they will go in search of an answer to the most profound question in science: Is there life beyond Earth?

  • Michael Fidler November 20, 2017, 10:40

    Holographic Imaging Could Be Used to Detect Signs of Life in Space.

    “A technique called digital holographic microscopy, which uses lasers to record 3-D images, may be our best bet for spotting extraterrestrial microbes.”
    “To study the motion of potential microbes from Enceladus’s plumes, Nadeau proposes using an instrument called a digital holographic microscope that has been modified specifically for astrobiology.”

    http://www.caltech.edu/news/holographic-imaging-could-be-used-detect-signs-life-space-78931

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