≡ Menu

Galileo Evidence for Plumes on Europa

Once again we take advantage of older databanks to tease out new information. Europa is the case in point this morning, with Galileo data — magnetic field and plasma wave observations from 1997 — being brought in as evidence for a water vapor plume rising from the surface. The Galileo flyby took the craft closer than 400 kilometers, where a brief spike in plasma density was detected along with a concurrent decrease in magnetic field magnitude and a bend in the direction of the field. The data are consistent with a plume interacting with Jupiter’s plasma outflow, and support earlier Hubble indications of possible plumes on the moon.

We have no firm measurement of the thickness of Europa’s ice, but if we are to get to that fascinating ocean now known to exist below the surface, we have to contend with penetrating it. Plumes from the interior would be helpful indeed, as Enceladus showed us at Saturn. Flying a spacecraft through a plume lets us measure its ingredients and sample the ocean below. So this new work, which has just been published in Nature Astronomy, offers the prospect of plume sampling via close flyby for upcoming missions like Europa Clipper.

Image: Artist’s illustration of Jupiter and Europa (in the foreground) with the Galileo spacecraft after its pass through a plume erupting from Europa’s surface. A new computer simulation gives us an idea of how the magnetic field interacted with a plume. The magnetic field lines (depicted in blue) show how the plume interacts with the ambient flow of Jovian plasma. The red colors on the lines show more dense areas of plasma. Credit: NASA/JPL-Caltech/Univ. of Michigan.

As it happens, lead author Xianzhe Jia (University of Michigan) is co-investigator for two instruments designed for Europa Clipper, and it was another member of the Europa Clipper science team, Melissa McGrath (SETI Institute) who prompted the deeper dive into Galileo data, inspired by the intriguing observations from Hubble. The space telescope was operating at the far edge of its capabilities, leaving the prospect of Europan plumes ambiguous, but the location of one of the Hubble ‘plumes’ turned out to be useful. Says Jia:

“One of the locations she mentioned rang a bell. Galileo actually did a flyby of that location, and it was the closest one we ever had. We realized we had to go back. We needed to see whether there was anything in the data that could tell us whether or not there was a plume.”

What Hubble observed at the site appeared to rise some 200 kilometers above the surface of Europa, consistent with the Galileo data. As the paper notes, Galileo acquired magnetomer data on eight targeted passes of Europa during its eight years in Jovian orbit, but only two of these passes came closer to the surface than 400 kilometers, a height at which a plume might give off both a plasma and magnetic field signature. As the paper notes:

Both passes crossed the trailing hemisphere of Europa (E12 near the equator and E26 at high southern latitude) and recorded short time-duration, large-amplitude perturbations accompanied by a sharp decrease of the field magnitude near closest approach. A previous study of the potential effect of plumes on Europa’s plasma interaction suggested that the magnetic perturbations during the E26 flyby could be associated with atmospheric inhomogeneity resulting from a plume. Here, we show that a localized signature in the MAG data acquired on Galileo’s closest encounter with Europa (E12 flyby) is fully consistent with the perturbations expected if the spacecraft crossed a plume rising above the nearby surface.

Image: This is Figure 3 from the paper. Caption: Location of the modeled plume on Europa’s surface. a,b, Perspectives from a longitude of 270° W (a) and from above the northern pole (b). The surface features shown on the sphere were extracted from a global map of Europa compiled by the United States Geological Survey. The cyan iso-surface of constant neutral density illustrates the shape and location of the modelled plume, and the magenta traces show the Galileo trajectory. The green ellipse in a indicates the location of a putative plume imaged by Hubble. Credit: Jia et al.

Certainly the experience we gained with Cassini at Enceladus has helped to investigate this phenomenon, for we know that material within the plumes, once ionized, has effects on the magnetic field similar to the bend in the field observed in the Galileo data. The spacecraft’s plasma wave spectrometer likewise proved useful, examining the interactions caused by charged particles above the Europan surface, and clearly demonstrating a spike in intensity.

Using 3D modeling tools developed by his team, Jia was able to simulate the interactions of plasma with bodies like Europa, producing simulated plumes that matched the Galileo data and were consistent with what Hubble had revealed.

“There now seem to be too many lines of evidence to dismiss plumes at Europa,” says Robert Pappalardo, Europa Clipper project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This result makes the plumes seem to be much more real and, for me, is a tipping point. These are no longer uncertain blips on a faraway image.”

We can hope that Europa Clipper will get off as early as mid-2022, for its low-altitude flybys should allow us to sample dust particles and frozen liquid, a glimpse into the moon’s dark interior. As you would imagine, these findings have the Europa mission team examining orbital trajectories that will allow such measurements to be taken. The Europan plumes certainly aren’t as visible as the plumes of Enceladus, but if what Galileo encountered really was a plume during its eight-year mission, then plumes must be occurring on relatively short timescales.

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


{ 13 comments… add one }
  • ljk May 15, 2018, 15:32

    In this NASA Watch piece on the Europa water plume finding:


    There was this item in the comment thread:

    “Jia presented this work at last December’s American Geophysical Union conference. So it isn’t exactly secret or embargoed. (The AGU has abstracts on their web page, if you care to search for it.) Personally, I think the results are highly ambiguous. The data from Galileo’s E12 encounter could be consistent with a plume crossing. But the data also show that E12 was highly atypical and lots of things were happening at the same time. It’s almost like flying over a city during a hurricane and saying an observed updraft is due to a power plant’s smokestack. It could be. But it could also be something else. Who knows?”

  • J. Jason Wentworth May 16, 2018, 6:03

    I wonder if such plumes from Europa could be periodic. If they’re caused by undersea volcanic eruptions in that area, maybe the eruptions only occur–or are only strong enough to go through the ice crust–when tidal interaction-caused heating is at its strongest, and:

    Unlike Io, which experiences a powerful gravitational tug o’ war between Jupiter and the three other Galilean moons that orbit farther out, Europa’s tidal heating is milder, and might require Ganymede and Callisto to line up beyond it (with Io and Jupiter on the other, inner side) to cause enough internal activity to temporarily breach Europa’s ice crust. (Jupiter’s radio emissions–the ones emanating from above the planet–also exhibit a periodicity, which is governed by Io’s position in its orbit, at which times it “completes the circuit” to trigger the associated current flows that generate those radio emissions.)

    • ljk May 16, 2018, 9:43

      Good points. Has anyone checked where the other Galilean moons were positioned when the space probe flew by Europa in 1997 to catch that event?

      • J. Jason Wentworth May 16, 2018, 13:31

        Thank you. I don’t know myself, but I’m sure the positions of all of the Galilean moons’ positions at the time are documented somewhere. That information is probably in the Galileo data set, although it might take some digging to find it (I imagine their positions–and maybe *all* of the moons’ positions–were kept current for the team members, so that they could plan–or take advantage of fortuitous opportunities for–imaging sequences and instrument scans of the various Jovian moons.) Also:

        If worst came to worst, ephemerides for the moons at those times are available outside of NASA and JPL. The U.S. Naval Observatory and the Royal Observatory, Greenwich maintain good ephemerides of the Galilean moons because they serve as a back-up “clock in the sky” for time-keeping, so that mariners could, if necessary in emergencies, use their positions to determine the time, and thus their longitude, in the middle of the ocean.

  • Alex Tolley May 16, 2018, 9:41

    What are the consequences if a sample mission shows the plumes are devoid of any biosignatures? Would this dampen the search for life on Enceladus or other solar system bodies? I hope this doesn’t mean that we get perennial “teaser” missions as Nasa seems to have done ever since the failure of the Mars Viking landers to detect life. OTOH, maybe the search for life shifts away from solar system missions to interstellar observations.

    • ljk May 16, 2018, 11:20

      Other than never sampling the plume, I don’t think we are going to have much choice in the matter since it is obviously easier and cheaper to get a bit of the Europan ocean that way than using a lander with a drill device.

      Otherwise I share your sentiments.

    • J. Jason Wentworth May 16, 2018, 12:56

      I don’t know…but there is, I think, a way to ensure that microbes are found (and unambiguously) in plumes, if there are any to be found:

      The Wolf Trap, developed by microbiologist Wolf Vishniac of the University of Rochester, a friend of Carl Sagan’s (see: http://www.google.com/search?source=hp&ei=jU78WvbtHYfEjwPn2qWoDg&q=Wolf+Trap+Wolf+Vishniac&oq=Wolf+Trap+Wolf+Vishniac&gs_l=psy-ab.12…6639.26798.0.28630.…1.1.64.psy-ab..0.22.2477…0j0i131k1j0i22i30k1j33i22i29i30k1j33i160k1.0.F6HMhaeYTIc ), was the simplest microbe detection instrument ever devised. It had only one, simple requirement (unlike the other life-detection instruments that flew aboard the two Viking landers)–that any microbes present, when given the liquid growth medium–would *grow*, causing increasing turbidity (cloudiness) in the liquid, and making it increasingly acidic. Both of these changes would be detected and measured electrically (a calibrated light source/photocell combination would measure the turbidity). The Wolf Trap was originally selected for Viking, but was deleted when the project’s budget was cut. Vishniac developed it after an astronomer at a scientific convention he attended in the 1950s, who anticipated space probes in the foreseeable future, expressed amazement that biologists hadn’t developed a simple, remotely-operable instrument that could detect microbes on other worlds. Also:

      One of the three biology instruments that did fly on the Viking Landers, Gilbert Levin’s Labeled Release experiment (see: http://www.google.com/search?ei=IVX8WtnCEY7GjwO19rGADg&q=Gil+Levin+Labeled+Release+Experiment&oq=Gil+Levin+Labeled+Release+Experiment&gs_l=psy-ab.12..33i160k1.2525.21092.0.25091.…1.1.64.psy-ab..0.34.3904…0j0i67k1j0i131k1j0i131i67k1j0i10k1j0i22i30k1j33i21k1.0.qfI6K7Inhzo ), could have definitively answered the question of life on Mars had it, too, not been a victim of the budget axe that NASA is quite used to being struck by at intervals. Levin’s instrument was originally designed to offer not one but two growth media to the hoped-for microscopic Martians, with both chiralities–that is, one broth would contain “left-handed” amino acid molecules, while the amino acids in the other broth would be right-handed. This would not only have ruled out false positives (both landers’ labeled release experiments got positive results, which–Levin is convinced–were due to life) caused by “exotic” inorganic soil chemistry, but it would also–if only the “right-handed” broth gave positive results–have eliminated aboriginal microbes brought from Earth, perhaps by ‘Earth impact splashed’ meteorites or the earlier, failed Soviet landers. Unfortunately, budget reductions required his experiment to be pared down, and only the left-handed growth medium was used, and:

      I think it’s time that we started flying properly-outfitted life detection instruments (which could be much smaller, lighter, and more reliable today, enabling smaller landers–and rovers–to carry them). Outer planet orbiters (and even comet probes) could also carry such instruments, with collection “scoops” optimized for gathering plume samples from Europa, Enceladus, and other such ice/liquid/slush moons. All probes equipped with life detection instruments could also carry mass spectrometers for detecting and identifying organic compounds in the samples. In addition:

      After the Wolf Trap was dropped from Viking in 1971, Vishniac remained on the project’s biology team, and he decided to search for microbes in the dry (antarctic desert) valleys of Antarctica, probably the most Mars-like areas on the Earth. Earlier researchers had found no microorganisms there and concluded that the valleys were sterile, but Vishniac wasn’t so sure. He emplaced little microbe collection stations at selected sites, which were recovered after about a month. A special, exceptionally thorough microbe scoring method that he developed did detect microbes; moreover, many of them–including a species of yeast found by his widow Helen, after his accidental death in a cliff fall there on December 10, 1973–proved to be indigenous to Antarctica (many scientists had thought that any microbes found in the dry valleys would have to have been blown there from elsewhere by the winds), and:

      The Wolf Trap, incidentally, was also selected for deletion from Viking because it used much more water than the other biology experiments, and since the Mariner 9 Mars orbiter data had–erroneously, as we now know–suggested that Mars was bone-dry (with no liquid water even around the polar caps, with the water ice and carbon dioxide ice believed to all sublime directly into gases [we now know that liquid water occurs underground in places]), it was thought that the Wolf Trap’s larger amounts of introduced water would drown the little Martians, if any. As horses say, “My foresight has limited depth perception just ahead of my nose, but my hindsight’s 20/20!” :-)

      • J. Jason Wentworth May 16, 2018, 14:22

        Another possible–although it’s admittedly a long shot–opportunity for finding microbes might come as a result of asteroid or comet (particularly short-period comet) missions. It could happen like this:

        Iain Nicolson, in his and Andrew Farmer’s (the illustrator) book “The Road to the Stars” in 1978, covered a proposed NASA asteroid exploration probe. The ion-drive vehicle, derived from JPL’s solar electric Halley’s Comet probe (which itself was based on Boeing’s SEPS [Solar Electric Propulsion Stage] for carrying single or multiple spacecraft payloads to desired drop-off points), was equipped with rolls of sticky nylon rope, which had small weighted harpoons at the ends of the ropes, and:

        The Gulliver life-detection instrument (which was also originally proposed to fly on the Viking landers, as separately-landing, capsule-housed instruments [rather like the Deep Space 2 penetrator probes that flew aboard the failed Mars Polar Lander]) also had retractable sticky-string soil samplers, which were attached to large, low-velocity bullets fired from three equidistantly-mounted guns inside the capsules.) In addition:

        Standing off a few tens of meters from the surfaces of small asteroids without landing on them, the sticky nylon ropes from the ion-drive asteroid probe would collect samples of the target asteroids’ surfaces for return to the Earth (to Earth orbit, where a Space Shuttle [or today, a Dragon V2 or Soyuz, or even the ISS] would be used to retrieve the samples for transportation to the Earth’s surface). Short-period comets (some asteroids with more eccentric orbits are actually extinct comets) could also be visited by such a probe, and samples of their surfaces could be collected via sticky ropes, too. It is possible that microbes (and certainly, interesting organic molecules) could be collected in this way, particularly if microbes reside in shaded clefts, and/or in–and/or on the surfaces of–ice under the surface dust (such ice-residing microbes have been found on the Earth)

      • Alex Tolley May 18, 2018, 15:28

        Growing microbes is not trivial. Most species on Earth have not been cultured. Of course, we only need 1 species to be culturable, so that improves the odds of finding one living example, but it may be that the conditions we use for culture are just inadequate and we come up empty, despite the presence of microbes.

  • ljk May 16, 2018, 17:09

    Something else about Europa that also happened 21 years ago:

    Freeman Dyson said we should be looking for native fish from that moon, not on Europa itself but in nearby space…


    The quote:

    To land a spacecraft on Europa, with the heavy equipment needed to penetrate the ice and explore the ocean directly, would be a formidable undertaking. A direct search for life in Europa’s ocean would today be prohibitively expensive. But just as asteroid and comet impacts on Mars have given us an easier way to look for evidence of life on that planet, impacts on Europa give us an easier way to look for evidence of life there. Every time a major impact occurs on Europa, a vast quantity of water is splashed from the ocean into the space around Jupiter. Some of the water evaporates, and some condenses into snow. Creatures living in the water far enough from the impact have a chance of being splashed intact into space and quickly freeze-dried. Therefore, an easy way to look for evidence of life in Europa’s ocean is to look for freeze-dried fish in the ring of space debris orbiting Jupiter. Sending a spacecraft to visit and survey Jupiter’s ring would be far less expensive than sending a submarine to visit and survey Europa’s ocean. Even if we did not find freeze-dried fish in Jupiter’s ring, we might find other surprises — freeze-dried seaweed, or a freeze-dried sea monster.

    Freeze-dried fish orbiting Jupiter is a fanciful notion, but nature in the biological realm has a tendency to be fanciful. Nature is usually more imaginative than we are. Nobody in Europe ever imagined a bird of paradise or a duck-billed platypus before it was discovered by explorers. Even after the platypus was discovered and a specimen brought to London, several learned experts declared it to be a fake. Many of nature’s most beautiful creations might be dismissed as wildly improbable if they were not known to exist. When we are exploring the universe and looking for evidence of life, either we may look for things that are probable but hard to detect or we may look for things that are improbable but easy to detect. In deciding what to look for, detectability is at least as useful a criterion as probability. Primitive organisms such as bacteria and algae hidden underground may be more probable, but freeze-dried fish in orbit are more detectable. To have the best chance of success, we should keep our eyes open for all possibilities.

  • ljk May 16, 2018, 17:58
  • ljk May 21, 2018, 13:15

    Our aquatic universe

    We know that the universe is awash with watery moons and planets. How can we pinpoint which of them could support life?

    The largest and deepest body of water known to us has never been sailed upon; it has no islands or shores, no wind-churned waves, no sunlight-silvered surface. This dark ocean can’t be found on any map of Earth – it’s more than 300 million miles away, on Europa, one of at least 69 moons that orbit Jupiter.

    Data from the Galileo spacecraft, which flew past Europa 11 times between 1995 and 2003, revealed that an immense salty sea lies beneath this moon’s smooth icy surface. Estimated to be 60 miles deep – about eight times the maximum depth of the Pacific – it has two to three times as much water as all of Earth’s oceans combined.

    And Europa isn’t some singularly soggy outlier. At least two additional Jovian moons – Ganymede and Callisto – have subsurface oceans. Titan and Mimas, which orbit Saturn, probably do, too. And there’s no doubt that another Saturnian moon, Enceladus, harbours water beneath its frozen crust, probably a volume comparable to the Great Lakes.

    Astonishing and irrefutable evidence for Enceladus’s briny deep came in 2005 when the Cassini space probe captured images of geysers spouting ice and water vapour hundreds of miles into space. Cassini even flew right through the geysers in October 2015, skimming within 30 miles of the moon’s surface to sample their contents.

    To say that the abundance and ubiquity of liquid water in the outer solar system completely upended scientists’ expectations doesn’t do justice to the discoveries. Before the revelations provided by Cassini, Galileo and other probes, the consensus was stark: the moons around Jupiter and Saturn would look much like our own or those of Mars – rocky, crater-pocked wastelands utterly hostile to life.

    ‘Nobody expected that there were subsurface oceans,’ says Seth Shostak, an astronomer with the SETI Institute in Mountain View, California. ‘It extends our concept of habitability and where you might find life to worlds that we hadn’t considered before. We always assumed that it had to be on a planet. I reckon there are seven other places in our solar system where we have reason to think there might be life – at least the conditions for life. Seven! And most of them are moons!’

    Full article here:


Leave a Comment