Dark Material in Fractures on Europa’s Surface Is Sea Salt, New Research Suggests

It has been estimated that Jupiter's moon Europa has an ocean which is larger than all of Earth's oceans - combined. Image Credit: NASA / JPL
The darker-colored material within the fractures and elsewhere on Europa’s surface might be sea salt brought up from the ocean below. Image Credit: NASA/JPL/Ted Stryk

For over a decade, scientists have been curious about the long fractures on Europa’s icy surface and the darker-colored material they contain, as well as other relatively young geological features which are also coated with the mystery dark stuff. Now, researchers have come up with an explanation which not only provides an answer, but suggests that the moon’s subsurface ocean is able to interact with the surface as well as the rocky interior: the dark material is sea salt. Plus, a proposed squid-like robotic probe might actually explore that alien salty ocean one day. …

“If it’s just salt from the ocean below, that would be a simple and elegant solution for what the dark, mysterious material is,” said research lead Kevin Hand, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Close-up of salt grains discoloured by radiation after the "Europa-in-a can" test. Photo Credit: NASA/JPL-Caltech
Close-up of salt grains discolored by radiation after the “Europa-in-a can” test. Photo Credit: NASA/JPL-Caltech

Based on other research, Europa’s ocean is thought to be salty, like oceans on Earth. Also like on Earth, it is thought to be in contact with the rocky interior, which could provide mineral nutrients for any possible life forms in the ocean. Water reaching the surface through cracks or possible plumes could deposit such salts on the surface, providing clues to potential habitability deep below.

“We have many questions about Europa, the most important and most difficult to answer being is there life? Research like this is important because it focuses on questions we can definitively answer, like whether or not Europa is inhabitable,” said Curt Niebur, Outer Planets Program scientist at NASA Headquarters in Washington. “Once we have those answers, we can tackle the bigger question about life in the ocean beneath Europa’s ice shell.”

The new study was just accepted for publication in the journal Geophysical Research Letters.

Previous studies, which used data from the Galileo spacecraft, suggested that the dark discolorations were due to sulfur and magnesium-containing compounds. The compounds would be altered by the harsh radiation from Jupiter’s magnetic field hitting Europa’s surface. The new experiments, however, indicate that those compounds might only explain the colors in the older parts of the affected terrain; sea salts would better explain the colors in the younger regions.

The "Europa-in-a-can" at the NASA/JPL laboratory, which recreates the near vacuum and intense radiation conditions on Europa's surface. Photo Credit: NASA/JPL-Caltech
The “Europa-in-a-can” at the NASA/JPL laboratory, which recreates the near vacuum and intense radiation conditions on Europa’s surface. Photo Credit: NASA/JPL-Caltech

To test this, Hand and his co-author Robert Carlson created a simulated patch of Europa’s surface in their laboratory. They tested different kinds of material, collecting the spectra, like chemical fingerprints, for each one.

As Hand explained: “We call it our ‘Europa-in-a-can’. The lab setup mimics conditions on Europa’s surface in terms of temperature, pressure and radiation exposure. The spectra of these materials can then be compared to those collected by spacecraft and telescopes.”

In these tests, they used samples of a common salt (sodium chloride) along with other mixtures of salts and water. The vacuum chamber recreated the freezing surface of Europa at minus 280 degrees Fahrenheit (minus 173 Celsius). An electron beam simulated the intense radiation hitting the surface. The samples, which started off white in color, gradually changed to a darker yellowish-brown, just like the darker material on Europa itself. Analysis of the spectra showed a strong similarity to the colors in the fractures on Europa’s surface. Also, the longer the samples were exposed to the radiation, the darker they became, just like what is seen on Europa. Those variations in color and darkness could help scientists determine how old various features on the moon are.

“This work tells us the chemical signature of radiation-baked sodium chloride is a compelling match to spacecraft data for Europa’s mystery material,” Hand said.

Image Credit: NASA/Cornell University/NSF
The “robotic squid” mission proposal which would explore Europa’s subsurface ocean. Image Credit: NASA/Cornell University/NSF

More detailed analysis will probably have to wait for a return mission to Europa, however, such as the Europa Clipper now on the drawing boards. The concept includes multiple flybys of the moon to learn more about its surface and the subsurface ocean and the potential for life. A possible lander is also being proposed as part of the mission, which could potentially examine some of the darker material up close if it could land in one of those areas. Orbiting Europa would be more expensive, but even a Europa Clipper-type spacecraft could conduct much more detailed analysis of the dark material than any telescope on Earth.

But there is also another more ambitious mission being contemplated: NASA has just selected 15 proposals for study under Phase I of the NASA Innovative Advanced Concepts (NIAC) program, and one of these is a “robotic squid” or aquatic rover which could explore the subsurface ocean of Europa or other similar moons like Enceladus.

From the press release:

“One of the selected proposals calls for the use of a soft-robotic rover for missions that can’t be accomplished with conventional power systems. This rover would resemble an eel with a short antenna on its back that harvests power from locally changing magnetic fields. The goal is to enable amphibious exploration of gas-giant moons like Europa.”

Tentacle-like structures would serve as electrodynamic “power scavengers” to harvest power from locally changing magnetic fields in the ocean.

How cool would that be? Being able to explore an alien ocean in-situ would be one of the most incredible space missions ever, and could directly sample and analyze the seawater, including, of course, searching for possible evidence of life of some kind—even if just microscopic—swimming in those waters. The ocean could still be studied remotely from a spacecraft, but nothing would be better than actually being in that ocean, swimming around like a fish as it were, and seeing it up close for the first time ever.

All of the 15 mission proposals can be seen here, and more information about Europa is available here.


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  1. “-nothing would be better than actually being in that ocean, swimming around like a fish as it were, and seeing it up close for the first time ever.”

    That is going to be the new Human Space Flight exploration holy grail- Human-crewed submersibles like Alvin exploring these ocean moons. The screaming cheap of NewSpace sycophants that dominates the media makes such proposals….. unpopular right now.

    I despise NewSpace and reject their LEO tourist business plan as the worst thing that has ever happened to space exploration.

    The steps to having an Alvin clone exploring Europa and several other moons are simple enough:

    1. A Super Heavy Lift Vehicle like SLS.

    2. The ice on the Moon as water-for-spaceship-cosmic-ray-shields.

    3. Nuclear Pulse Propulsion. The only place to light off bombs being outside the magnetosphere. And the only place to acquire cosmic ray shielding, assemble, test, and launch nuclear missions being the vicinity of the Moon.

    As for the NewSpace mob screaming cheap, generating revenue for space exploration also starts with going back to the Moon.


    • Gary,
      After watching Europa Report there is a certain irony about what might actually ” be out there” vs what we plan for. I think the life forms will be far more advanced and even larger by adapting to such a hostile environment. I would expect this to be a robotic mission.

      How much will this cost if we just use machines rather than a manned sub. I am thinking less than $15B and that might be high but well worth it. This might come sooner than we think as SpaceX and Musk has indicated that he will soon have a F9 Heavy plus a cargo configured Dragon version 2 that will deliver robots to anywhere in the solar system, again that is robotically speaking not a manned mission.

      Wouldn’t that be a better scenario at first, by letting the machines take the risk?

      • “-he will soon have a F9 Heavy plus a cargo configured Dragon version 2 that will deliver robots to anywhere in the solar system-”

        Your name really describes what you just did by replying to my comment with a SpaceX commercial. The Muskiiah is the worst thing that has ever happened to space exploration.

      • While the Falcon Heavy will be able to deliver large payloads to low-earth orbit (LEO), it’s lack of a high-performance second stage limits its ability for interplanetary payloads. In fact, this limitation means that until such a second stage is certified for the Falcon Heavy, at a C3=0 of ~12 mT a Delta IV Heavy and Atlas V 551 can deliver about 3 mT more payload beyond earth orbit. SpaceX is currently working on such a second stage, but I confess that I haven’t kept up with its testing at Stennis, which got underway not too long ago.

        To get a very large payload, such as a Europa lander with a robotic submersible, or a Pluto orbiter will require something like SLS Block I that will, according to the latest info from Todd May, deliver about 85 mT to LEO and an approx. C3=0 of 18-20mT. An SLS Block II will come in at well over 130 mT LEO and a C3=0 of 40-50 mT, with that gap caused by architectural choices in both the second stage and advanced stage. Never-the-less, such a massive payload capacity opens the door to more persistent robotic and eventually human exploration beyond the earth-moon system.

        • Another feature of the funky falcon faux heavy is the caveat that it cannot really lift what they say it can- not without the miracle of “propellent crossfeed.” So even that wonderful payload to LEO is not what they say until they make that system work- and transferring those propellents is another KISS principle failure to add to those 27 little bad ideas.

          One of the great passages concerning space exploration is the famous quote by Richard Feynman:

          “NASA owes it to the citizens from whom it asks support to be frank, honest, and informative. . . .For a successful technology, reality must take precedence over public relations, for nature cannot be fooled.”

          The first part about NASA’s obligation usually get’s left out. Human Space Flight to places like Europa are actually perfectly doable right now with present technology. The problem is that technology, atomic bomb propulsion, is politically unacceptable and verboten to talk about. The Orion Launch Abort System makes transporting the bomb pits an acceptable evolution but nobody can say a word about this. Likewise the massive cosmic ray shielding can only come from the same place outside the magnetosphere where the bombs necessary to push it around can be assembled and tested- the Moon. And the only way to get to the Moon with meaningful payloads is the Super Heavy Lift Vehicle of which only one is presently being developed on planet Earth- the SLS.

          NASA is not informing the citizens.

        • Jim, what do you think of the possibility that Blue Origin may supply high-performance upper stages to SpaceX. It would seem that a business deal and a little integration might change the relative capabilities fairly quickly.

          • John,

            Apologies for the late response.

            I think the chances of Blue Origin working with SpaceX are zero. There’s the personal aspect; Musk has made derogatory comments about Blue Origin in the past that were unnecessary. From the business side, Blue Origin won’t work with SpaceX because they are competitors in a very limited market space, so Blue Origin has every incentive to work against SpaceX.

            • Jim,
              Fair enough. Theoretical capabilities do tend to take a back seat to realities on the ground.

              • I’ll make a prediction that is worth every penny paid for it. In 10 years Blue Origin will be the leader in commercial space commerce in total dollars and more importantly profit margin. But then, what else would be expected from Bezos?

                Let’s circle up in 5 and 10 years and see how things shake-out.

                • I’ll put up a steak dinner against nothing that you’re wrong, not that I’m choosing someone else as winner, just playing the odds.:-)

                  That’s my way of saying that you might be right, but there are a lot of yards left to the goal posts, and a lot of games left in the season. I don’t mean that SpaceX will lead, or even ULA, just uncertainty. I’m predicting some surprises, not which ones, just surprises.

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