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No Heat Signature of Europan ‘Plumes’

One of the youngest surface features on Europa draws attention because of its possible connection with what lies beneath the Jovian moon’s ice. The dark center of Pwyll crater, visible in the image below, is some 40 kilometers across, with a central peak reaching about 600 meters. At issue is the terrain resulting from the impact causing the crater. An impact perhaps 20 million years ago seems to have blown water and ice across the Europan surface. Evidence of a possible plume from Europa’s ocean in this area is the subject of continuing work.

The bright terrain around the crater suggests water ice, and note, too that the Pwyll impact left ejecta rays as far as the Conamara Chaos region 1000 kilometers to its north. Conamara Chaos features themselves have been studied extensively for terrain suggestive of melting and refreezing ice. We saw recently how Xianzhe Jia (University of Michigan), working with the SETI Institute’s Melissa McGrath, used data from the Galileo mission to support later Hubble evidence of possible plumes on Europa (see Galileo Evidence for Plumes on Europa).

Image: This enhanced color image of the region surrounding the young impact crater Pwyll on Jupiter’s moon Europa was produced by combining low resolution color data with a higher resolution mosaic of images obtained on December 19, 1996 by the Solid State Imaging (CCD) system aboard NASA’s Galileo spacecraft. This region is on the trailing hemisphere of the satellite, centered at 11 degrees South and 276 degrees West, and is about 1240 kilometers across. North is toward the top of the image, and the sun illuminates the surface from the east. The 26 kilometer diameter impact crater Pwyll, just below the center of the image, is thought to be one of the youngest features on the surface of Europa. The diameter of the central dark spot, ejecta blasted from beneath Europa’s surface, is approximately 40 kilometers, and bright white rays extend for over a thousand kilometers in all directions from the impact site. Credit: NASA/JPL-Caltech.

Plumes on Europa would be a momentous discovery, one reason the elusive evidence for their existence is proving so controversial. For if we had active plumes venting seawater from the deep Europan ocean, we could study their composition without the need for landing and drilling through the ice. Europa, like Enceladus, could prove a target for astrobiologically-focused missions in the near future, and indeed, the two researchers I cited above are both associated with the Europa Clipper science team, which will perform multiple flybys of the moon.

This morning we learn that Julie Rathbun (Planetary Science Institute) has dug into the evidence for Europan plumes in a talk titled “A closer look at Galileo Thermal data from possible plume sources near Pwyll, Europa,” with news that makes these plumes more unlikely. For what we would expect at Europa is similar to what we see at Enceladus, the signature of hotspots that flag the energy source driving the plume activity. Similar hotspots can be found on Earth at geysers like Yellowstone and its associated hot springs. But no Europan hotspots can be found.

Rathbun states the issue concisely, so let me just quote her on this:

“We searched through the available Galileo thermal data at the locations proposed as the sites of potential plumes. Reanalysis of temperature data from the Galileo mission does not show anything special in the locations where plumes have possibly been observed. There are no hotspot signatures at either of the sites.”

Such plumes may exist, but appear only rarely. Rathbun continues:

“This is surprising because the Enceladus plumes have a clear thermal signature at their site of origin, so this suggests that either the Europa plumes are very different, or the plumes are only occasional, or that they don’t exist, or that their thermal signature is too small to have been detected by current data.”

Rathbun presented these findings at a Division for Planetary Sciences press conference held at the American Astronomical Society 50th annual meeting in Knoxville, TN. Plume research is ongoing, and bear in mind that the possible plume in the area near Pwyll is not the only one being considered. There is evidence in the Xianzhe Jia and Melissa McGrath work for plume activity about 1,000 kilometers northeast of the first site. Indeed, Jia and McGrath found that magnetic perturbations found by Galileo during its E12 flyby were consistent with a rising plume.

So the jury is still out. If we can get Europa Clipper off, perhaps as early as 2022, its flybys could prove conclusive one way or another. While we wait, analysis and simulations of the interactions between possible plumes and the plasma environment around Europa continue. We have only two Galileo flybys of the Moon containing magnetometer data that came closer than 400 kilometers from the surface, where we might expect a plasma and magnetic field signature if plumes exist, but Europa Clipper will ramp up the magnetic and thermal dataflow considerably.

The Xianzhe Jia paper is “Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures,” published online at Nature Astronomy 14 May 2018 (abstract).

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{ 37 comments… add one }
  • ljk October 23, 2018, 13:24

    What does this mean for the thickness of the surface ice? What does this mean for the potential for life in that alien global ocean? Are the two connected with the rarity (?) of active plumes?

    The reddish material in all those surface cracks – are they organic? Are they the remains of aquatic native life? Does this mean the ice is not that thick, or that this is from a buildup over a very long time?

    Yes, we need a mission dedicated to Europa and soon. The nice part as we enter the 2020s is that we may not longer have to rely solely on NASA for this to happen.

    https://spacenews.com/europa-or-enceladus-if-nasa-switches-from-sls-to-falcon-heavy-it-wont-have-to-choose/

    • Antonio October 24, 2018, 5:35

      I highly doubt NASA will choose wisely. It can’t dejustify development of the Senate Launch System.

    • john walker October 24, 2018, 17:30

      As I wrote further down, the reddish hues are thought to be salt (NaCl)deposits. Colored by the effects of radiation.

      • ljk October 25, 2018, 10:39

        Yes, but we should still check for Europan algae and fish, just the same.

  • Robert Walker October 23, 2018, 15:09

    I think it’s not too surprising it’s different. The Europan crust is much thicker, it’s remarkable if there is a connection to the sea below. And they aren’t erupting continuously, we already know that, at least not the ones we can detect from Earth. The Enceladus geysers erupt continuously, every photo of them showed multiple eruptions in progress.

    The geysers could also come from near subsurface water from rising plumes of hot ice that turn to liquid as they reach the surface. If it is say a near surface lake, then you’d have huge ice floes the size of Manhatten turning over, splitting, crumbling, rotating, so lots of potential for dynamic processes that can lead to a geyser stopping and starting again in the same location.

    • ljk October 23, 2018, 17:04

      I thought the experts were favoring a thin ice crust Europa these days?

      https://www.centauri-dreams.org/2011/04/12/europa-thin-ice-and-contamination/

      • ljk October 25, 2018, 9:05

        They are much too invested in their SLS/Orion program and feel if they back out now, then they will be conceding to the young, fast upstarts, especially SpaceX. It is a reusable rocket and CubeSat world now, and NASA is still doing the equivalent of building mainframe computers when everyone wants and expects laptops and pads.

        Like IBM when the computer world changed to faster, better, cheaper, NASA has to change with the times or they will become irrelevant. Having the U.S. Government holding the purse strings doesn’t help, either.

        • ljk October 25, 2018, 9:05

          The above comment was meant to respond to Antonio’s regarding the state of NASA.

        • Alex Tolley October 25, 2018, 11:42

          Not to support the SLS decision by Nasa (and Congress), but mainframes are still widely used. 9 Mainframe Statistics That May Surprise You. From what I read, Nasa still adheres to the approach that was used by Apollo – a big rocket that can complete the task.

          They are not entirely wrong, as SpaceX’ BFR takes that approach in spades. Where they are wrong is still going for performance and reusability be damned. With budget constraints and excessive costs, Nasa may be trapped in this approach.

          I strongly agree with your major point. “Smaller, cheaper, faster” is where the action is. CubeSats are one path down this road. I expect more, especially when the cost of access to space finally does decline.

          My one concern is that like the causes of AGW, the ease of access to space will precipitate the Kessler syndrome before we think this through.

        • Antonio October 25, 2018, 15:49

          The problem is not SLS size (indeed, a heavy lift rocket is much needed) but its delays and cost overruns, due to its cost-plus type of contracts.

          • ljk October 26, 2018, 8:58

            I am certainly not against heavy boosters. My main concern is that they are taking way too long with the SLS and pinning so many of the agency’s plans and hopes on it.

            From what I have read the SLS won’t even be ready for its first real TEST launches until 2022! Meanwhile Elon Musk has already successfully test flown his Falcon Heavy rocket earlier this year – and with his hot red sports car as payload to boot, which is going past the orbit of Mars.

            I am not asking NASA to pull some kind of stunt just for the sake of looking hip and cool, but they are rapidly becoming granddad’s space agency in the eyes of today’s culture.

    • Andrei October 26, 2018, 12:56

      Yes the ice on Europa is thick, with possible reservoirs of water in between. The chaotic terrain is thought to be a sign of such reservoirs.
      This report is one such, though it is optimistic about finding life in one such even then – I remain unconvinced.
      Finding nothing from a plume originating from such a reservoir or from the reservoir itself might be a false negative result, so assessing from how deep a possible plume come from – will be necessary.
      So Europa might not be easy world to reveal its secrets, and those who favor Enceladus for a space mission where complex organics already been found perhaps have a good point.

  • john walker October 23, 2018, 15:55

    It has never been clear to me why there should be a thermal anomaly at Pwell. The very fact that a 26 km crater exists there indicates that the ice is likely very thick at that spot. Otherwise the impactor would have penetrated the crust. There was a paper published 2008 in the Journal of the Meteoritical Society by Cox et al relating to impact events on Europa which came to the following conclusion,
    “Firstorder impacts—into thick ice or at low impact energy—produce craters. Second-order impacts punch through the ice, making holes that resemble raft-free chaos areas. Third-order impacts—into thinnest ice or at highest energy—produce large irregular raft-filled zones similar to platy chaos.”

  • Alex Tolley October 23, 2018, 16:36

    Do plumes require underlying heat to create them, or is that just one mechanism? On Earth we see short ejections of water in response to geography and water motion, as well as volcanic plumes, albeit much smaller than volcanic ones. Could tidal forces on the overlying ice create transient pressures that literally squeeze the water out of vents be sufficient, or are those forces either too small or those forces would generate enough heat to be sensed?

    While the phenomenon would be fascinating in its own right and looking for signs of life well worth being part of a Europa mission, I suspect funding is based far too heavily on the [flawed IMO] logic of: Earth has undersea vents that team with life, and since Europa has water and possible volcanic undersea vents, we can/should hope to find life there. I would make a plea that we do not do the post-Viking approach to look for life on Mars, using sensors that only look for peripheral and proxy indicators, like organics and gas emissions. Let’s be far more aggressive and accept failure if an instrument turns up nothing. Perhaps low-cost probes coupled woth cheap launch costs might allow this approach to be tried, rather than the one-off, very expensive probes, where the failure of a result for each instrument is not an option.

    • Antonio October 24, 2018, 5:45

      ” Let’s be far more aggressive and accept failure if an instrument turns up nothing.”

      I agreed with you until this part. I thought that that “far more aggressive” part would mean… well… a really aggressive approach, trying to penetrate the ice and search directly for life, instead of the ” using sensors that only look for peripheral and proxy indicators” you mentioned just before :)

      • Alex Tolley October 24, 2018, 11:49

        Perhaps I should have used “high-risk/high-reward” rather than “aggressive”. I do not favor costly programs unless there is no other way to do something. If launch costs are low, as are small, dedicated probes, then the logical sequence is to start out with a lot of cheap probes looking for easy, but direct signs of life. A swarm that both sampled the plumes (if present), impacting the ground where organics are suspected would be the first wave. The swarm provides redundancy and allows for cheaper manufacture, and assumes some will fail. If any of the results look really promising, then I would go with a swarm that could penetrate the ice at thin points. Again, the swarm would do different things, as simply as possible. They may not even need to be submarines in the sense that they could travel long distances from their entry point. Something more like a pressure resistant box with limited hydrodynamic steering as it descends the water column might be sufficient (Heat sensors would target it towards any hot spots).

        By keeping the probes simple, they could be mass manufactured to reduce costs and have short development life cycles. SciFi movies move plots along by having new technology developed at wartime rates or even faster. In reality, space technology develops at such slow rates to meet the high specification bar that it takes ages and huge costs to get much going. A key issue is reduced launch costs to make this approach work.

        • Antonio October 24, 2018, 17:00

          Well, it’s quite strange that you consider “high risk” sending cheap probes and “direct signs of life” organic compounds in the plumes or surface.

          • Alex Tolley October 25, 2018, 11:50

            Are you familiar with science funding? Most funding is for experiments that should produce results, whether or not those results are particularly interesting. That is “low risk – low reward”. The NIH has been criticized for this approach as more adventurous experiments that might not work at all are denied funding, yet if they do, could offer breakthroughs. Those experiments needn’t be hugely costly either.

            • Antonio October 25, 2018, 15:53

              Yeah, I know governments preference for low risk – low reward, but I never talked about that but about your strange definition of high risk.

  • ljk October 23, 2018, 17:02

    If the Europan ocean floor is sixty miles below the ice surface, as I have read, then perhaps we should be amazed if there are any detectable active geysers at all.

    The ones at Enceladus must be much closer to its surface. And if Saturn is not tugging on its moon as much as Jupiter is with Europa, then what mechanism is making Enceladus so much more active in that regard?

    Of course there could be lots of active hydrothermal vents and geysers on Europa, but we cannot detect them due to the depth of its ocean. Again, the reddish material in the fissures all over its icy face could be material from those geological features.

    Or is some of that material from Io? Its volcanoes erupt sulfur over 200 kilometers into space.

  • Andrew Palfreyman October 23, 2018, 20:00

    No microscope, wasted mission. Ditto Mars excavation.

    • ljk October 24, 2018, 12:14

      Seeing how NASA outright rejected any objects that looked like fossils imaged by their Mars Rover microscopes, I have to wonder how much better they would fare on Europa and other worlds?

      I understand scientific caution and that having the actual samples to examine in a lab on Earth would be best, but there were images of objects on Mars that if they were not fossils, I would like to know what they could be. And no, I am not talking about the nonsense of seeing vehicle parts and Elvis.

      • Antonio October 24, 2018, 17:03

        Well, they already did that with Allan Hills meteorite, so no wonder they did the same for a simple rover, with much more limited technology.

        • ljk October 25, 2018, 10:44

          ALH84001 and its “microfossils” show that when it comes to identifying alien life, we still have a long way to go. As for that specific example, keep in mind that part of it was due to getting NASA funding for its revived Mars exploration missions. Note the timing: 1996, just before the first US rover was about to head to the Red Planet. Nothing wrong with that, just making a note.

          • Antonio October 25, 2018, 15:58

            For me, ALH84001 lines of evidence are quite compelling. The first ones weren’t so much compeling, but when more solid evidences were found, NASA simply ignored them. Anyway, that would be a long discussion and quite OT here.

  • Karl Pestell October 23, 2018, 22:13

    Possibly being over-simplistic here as there are so many models of ice thickness etc, but the definite facts are.. Enceladus is 500km across; Europa is 3000km across, and is 480 times as massive. Doesn’t that very strongly suggest that even if you get plumes at Enceladus, you won’t get them at Europa? Does it suggest that if you were to find plumes on Europa, they’d be uninteresting ones from transient subsurface lakes unconnected to the mighty ocean far beneath? And therefore doesn’t it suggest that with current technology we should go to Enceladus and sample its plumes as boring into Europa is way beyond our current capabilities? Plus if we go to Saturn instead of Jupiter, the planet won’t try to fry our probes. (I really think Europa is the best chance for life elsewhere in the Solar System, but I suspect it will be very, very hard to detect.)

    • ljk October 24, 2018, 12:17

      We do not need to drill or blast our way into Europa’s ocean if we want to see what is in it. Just land and examine the reddish material along all of its surface fissures. Most of it must surely come from the ocean below.

      And yes, a submarine explorer would be wonderful on Europa. I am sure it can be done, especially if the ice is not that thick.

    • Antonio October 24, 2018, 17:10

      Not so costly. People simply are too conservative. Put a small nuclear reactor in the probe, let it sink throught the ice and voila, you have your probe in the ocean.

      • ljk October 25, 2018, 10:47

        Just make sure that nuclear reactor somehow never leaks or otherwise contaminates the area. Expense alone might keep such a plan from happening, but the environmental considerations will play a big factor, too.

        Maybe we will start life on Europa that way. While the moon is bathed in lethal radiation on its surface thanks to Jupiter, ice does a very good job as a radiation shield.

        • Antonio October 25, 2018, 16:05

          I think a nuclear reactor can be easily sterilized. It’s basically metals and ceramics, so you can heat it enough to destroy any terrestrial life.

          • ljk October 26, 2018, 11:53

            I was talking about the radiation produced by the reactor itself. I am certain that any probes sent to Europa will be sterilized as much as possible.

    • john walker October 24, 2018, 17:46

      Good points. Although one could conjecture that putative Europan hydrothermal emissions may be orders of magnitude larger than those of Enceladus, the thermal dispersion in such a deep water table will undoubtedly weaken pressure at the underside of the ice crust. But I think more critical than oceanic depth is the thickness of the crust itself. I have not read any description of a geyser like mechanism to transport primordial oceanic water to the surface of Europa if the crust thickness exceeds 20km. At that thickness only very large impactors will stir things up.

  • Infinite123Lifer October 24, 2018, 13:05

    I had read somewhere that the reddish tint on the surface of Europa might actually come from Io? Not sure how that idea ever played out.

    • john walker October 24, 2018, 17:27

      Lab studies subsequently have demonstrated that NaCl under heavy radiation turns red. Thus the conjecture that the red lines on Europa are salt.

      • ljk October 25, 2018, 10:48

        Check for Europan fish just the same. And as Freeman Dyson said in 1997, we should also check for them circling the moon.

  • Gary Wilson October 24, 2018, 13:55

    Surely the cost of investigating Europa effectively prohibits going there. The ice must be extremely thick if a crater of the size of Pwyll exists there. Ice plumes must be quite rare if present at all. Pragmatically speaking the likelihood of penetrating the ice seems extremely remote. Enceladus is surely the place to send a lander. Smaller body, thinner ice cover, far more plumes. As an aside I have read that SLS will cost 1.5-2.5 billion per launch. Surely it should be scrapped? That price means very few launches per year. It is antiquated technology leading us in the wrong direction. The Spacex (and other private companies) approach which leads to reduced launch costs is the only sensible route to leaner, more efficient space exploration.

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