Dark material in fractures on Europa’s surface is sea salt, new research suggests

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
The darker-coloured 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-coloured 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 discoloured 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 discolourations 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 colours in the older parts of the affected terrain; sea salts would better explain the colours in the younger regions.

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.

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

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 colour, 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 colours 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 colour 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.

The “robotic squid” mission proposal which would explore Europa’s subsurface ocean. 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.

This article was first published on AmericaSpace.

1 Comment

Thoughts? Leave a Reply!