Ocean Worlds: Enceladus Looks Increasingly Habitable, and Europa’s Ocean Under the Ice More Accessible to Sample

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NASA’s Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. (NASA/JPL-Caltech)

It wasn’t that long ago that Enceladus, one of 53 moons of Saturn, was viewed as a kind of ho-hum object of no great importance.  It was clearly frozen and situated in a magnetic field maelstrom caused by the giant planet nearby and those saturnine rings.

That view was significantly modified in 2005 when scientists first detected signs of the icy plumes coming out of the bottom of the planet.  What followed was the discovery of warm fractures (the tiger stripes) near the moon’s south pole, numerous flybys and fly-throughs with the spacecraft Cassini, and by 2015 the announcement that the moon had a global ocean under its ice.

Now the Enceladus story has taken another decisive turn with the announcement that measurements taken during Cassini’s final fly-through captured the presence of molecular hydrogen.

To planetary and Earth scientists, that particular hydrogen presence quite clearly means that the water shooting out from Enceladus is coming from an interaction between water and warmed rock minerals at the bottom of the moon’s ocean– and possibly from within hydrothermal vents.

These chimney-like hydrothermal vents at the bottom of our oceans — coupled with a chemical mixture of elements and compounds similar to what has been detected in the plumes — are known on Earth as prime breeding grounds for life.  One important reason why is that the hydrogen and hydrogen compounds produced in these settings are a source of energy, or food, for microbes.

A logical conclusion of these findings:  the odds that Enceladus harbors forms of simple life have increased significantly.

To be clear, this is no discovery of extraterrestrial life. But it is an important step in the astrobiological quest to find life beyond Earth.

“The key here is that Enceladus can produce fuel that could be used by biology,” said Mary Voytek, NASA’s senior scientist for astrobiology, referring to the detection of hydrogen.

 

This graphic illustrates how scientists on NASA’s Cassini mission think water interacts with rock at the bottom of the ocean of Saturn’s icy moon Enceladus, producing hydrogen gas (H2). It remains unclear whether the interactions are taking place in hydrothermal vents or more diffusely across the ocean. (NASA)

“So now on this moon we have many of the components associated with life — water, a source of energy and many of the important chemical building blocks.  Nothing coming from Cassini will tell is if there is biology there, but we definitely have found another important piece of evidence of possible habitability.”

The finding of molecular hydrogen (H2 rather a single hydrogen atom) in the Enceladus plumes was described in a Science paper lead by authors Hunter Waite and Christopher Glein of the Southwest Research Institute, headquartered in San Antonio.

They went through a number of possible sources of the hydrogen and then concluded that the clearly most likely one was that chemical interaction of cool water and hot rocks — both heated by tidal forces in the complex Saturn system — at the bottom of the global ocean.

“We previously thought that the water was heated but now we have evidence that the rocks are as well,” Waite told me.  “And the evidence suggests that the rock is quite porous, which means that water is seeping through on a large scale and producing these chemical interactions that have a byproduct of hydrogen.”

The moon Enceladus is the sixth largest in the Saturn system. This image was taken by Cassini in 2008. (NASA/JPL-Caltech, Space Science Institute.)

He said that the process could be taking place in and around those chimney-like hydrothermal vents,  or it could be more diffuse across the ocean floor.  The vent scenario, he said, was “easier to envision.”

What’s more, he said, the conditions during this water-rock interaction are favorable for the production of the gas methane, which has been detected in the Enceladus plume.

This is another tantalizing part of the Enceladus plume story because the earliest lifeforms on Earth are thought to have both consumed and expelled that gas.  At this point, however, Waite said there is no way to determine how the methane was formed, which would be a key finding if and when it is made.

“Our results leave us agnostic on the presence of life,” he said. “We don’t have enough information for that.”

“But we now can make a strong case that we have a very habitable environment on this moon.” It’s such a strong case, he said, that it would be almost as scientifically interesting to not find life there than to detect it.

One of the more interesting remaining puzzles is why the hydrogen is present in the plume in such unexpectedly substantial (though initially difficult to detect) amounts.  If there was a large microbial community under the ice, then it could plausibly be argued that there wouldn’t be so much hydrogen left if they were consuming it.

The possibilities:  Waite said that it could mean there is just a lot of “food” being produced for potential microbes to survive on in the ocean, or that other factors limit the microbe population size.  Or, of course, it could mean that there are no microbes at all to consume the hydrogen food.

Astronomers have twice found evidence of a plume of water vapor coming from the same location on Europa. Both plumes, photographed in UV light by Hubble, were seen in silhouette as the moon passed in front of Jupiter. (NASA/ESA/STScI/USGS)

News of the Enceladus discovery came on the same day that other researchers announced that strong evidence of detecting a similar plume on Jupiter’s moon Europa using the Hubble Space Telescope.

This was not the first plume seen on that larger moon of Jupiter, but is perhaps the most important because it appeared to be was spitting out water vapor in the same location as an earlier plume.  In other words, it may well be the site of a consistently or frequently appearing geyser.

“The plumes on Enceladus are associated with hotter regions,” said William Sparks of the Space Telescope Science Institute. “So after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly,”

Sparks led the Hubble plume studies in both 2014 and 2016, and their paper was published in The Astrophysical Journal.  He said he was quite confident, though not completely confident of the result because of the limits of the Hubble resolution.  A 100 percent confirmation, he said, will take more observations.

Since Europa has long been seen as a strong candidate for harboring extraterrestrial life, this is extraordinarily good news for those hoping to test that hypothesis.  Now, rather than devising a way to blast through miles of ice to get to Europa’s large, salty and billions-of-years-old ocean, scientists can potentially learn about the composition of water by studying the plume — as has happened at Enceladus.

As their paper concluded, “If borne out with future observations, these indications of an active Europan surface, with potential access to liquid water at depth, bolster the case for Europa’s potential habitability and for future sampling of erupted material by spacecraft.”

This is particularly exciting since NASA is actively developing a mission to Europa that would orbit the moon and could target the plume area for study.

NASA teams have also proposed a Europa lander — a mission that was rejected by the Trump administration in its budget proposals.  But discovery of  what might be a regularly-spurting plume just might change the equation.

The plumes of Enceladus originate in the long tiger stripe fractures of the south polar region pictured here. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

The news about both Enceladus and Europa illustrates well the process by which the search for life beyond Earth — astrobiology — moves forward.

Like few other disciplines, astrobiology needs expertise coming from a broad range of fields, from astrophysicists, geochemists, biochemists, geologists, and more.

Hunter Waite, for instance, trained as an atmospheric  scientists and now builds mass spectrometers for spacecraft such as Cassini,  operates them in flight, and analyzes and reports the data.  He is something of a “plume” expert as well, and will follow up his team leading work on Enceladus as principal investigator of the Europa mass spectrometer that surely will investigate that other moon’s new-found plumes. (The Europa mission, called the Europa Clipper, is loosely scheduled to launch in 2022.)

His colleague, Christopher Glein, is a geochemist.  And the leader of the Europa plume-spotting team, William Sparks, is an astronomer.

Mary Voytek, NASA senior scientist for astrobiology.  (NASA)

Each discipline focuses on a part of the larger system that might, or might not, be habitable.  No single scientists or discipline of scientists is capable of detecting extraterrestrial life.

This has long been the view of NASA’s Voytek, who views astrobiology as a kind of very long-term scientific full-court press.

She is wary of overselling discoveries that involve the search for life beyond Earth and the origin of life here, saying that they sometimes are well-meaning “science fiction” more than science.

However, the Enceladus findings in particular have her excited.  A lot of questions remain, such as whether the water with molecular hydrogen is coming from a hydrothermal vent or across the ocean floor, and whether the amount of methane detected in the plume increases or decreases the likelihood of life on the ocean floor.

But her conclusion: “I think this puts Enceladus into a different category and definitely higher up on the index of habitability.”  Any potential life, she said, would almost surely be microbial, though it might be larger “if we get lucky.”

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More Evidence of Water Plumes On Europa Increases Confidence That They’re For Real

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 Figure 2: This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The Hubble data were taken on January 26, 2014. The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions. Credits: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center Image comparison of 2014 transit and 2012 Europa aurora observations

This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The Hubble data were taken on January 2014, and appear to show plumes that spit out as much as 125 miles.  The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions. (NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center)

Europa is a moon no bigger than our own and is covered by deep layers of ice, but it brings with it a world of promise.  Science fiction master and sometimes space visionary Arthur C. Clarke, after all,  named it as the most likely spot in our solar system to harbor life, and wrote a “2001: A Space Odyssey”  follow-up based in part on that premise.

Many in the planetary science and astrobiology communities are similarly inclined and have supported a specifically Europa mission geared to learning more about what is generally considered to be a large ocean beneath that ice.

Along the way, Europa became the only object deemed by Congress to be an obligatory NASA destination, and formal plans for such a voyage have been under way — however slowly — for several years.  Formal development of the “Europa Clipper” flyby project began last year, after a half decade of conceptual work.

The logic for the flyby got a major boost on Monday when a team using the Hubble Space Telescope reported that they had most likely detected plumes of water erupting out of Europa on three separate occasions.

Because of the difficulty of the observation — and the fact that plumes were found on 3 out of 10 passes — nobody was willing to claim that the finding was definitive.  But coupled with an earlier identification of a Europa plume by a different team using a different technique, the probability that the plumes are real is getting pretty high.

And if there really are plumes of water vapor or ice crystals being pushed through Europa’s thick surface of ice, then the implications for the search for signs of habitability and of life on Europa are enormous.

“Europa is surely one of the most compelling astrobiological targets in solar system with its apparent saline oceans,” said William Sparks, an astronomer with Space Telescope Science Institute in Baltimore and lead author of the Europa paper, to be published in The Astrophysical Journal.

“And now there’s the real possibility that we can explore for organics or even signs of life without drilling into the ice.”  In other words, the plumes could allow NASA to collect material once in the oceans of Europa via a flyby, rather than requiring a landing and a subsequent piercing through miles of ice.

Bizarre features on Europa’s icy surface suggest a warm interior. This view of the surface and shows a color image set within a larger mosaic of low-resolution monochrome images. The Galileo spacecraft was able to survey only a small fraction of Europa's surface in color at high resolution; a future mission would include a high-resolution imaging capability to capture a much larger part of the moon's surface. (NASA/JPL-Caltech)
Beautiful and mysterious features on Europa’s icy surface suggest a warm interior and hint at the geology that could allow water to break through a weak point and erupt as a plume. This view of the surface and shows a color image set within a larger mosaic of low-resolution monochrome images. The Galileo spacecraft was able to survey only a small fraction of Europa’s surface in color at high resolution; a future mission would include a high-resolution imaging capability to capture a much larger part of the moon’s surface. (NASA/JPL-Caltech)

The findings make it increasingly likely that there are at least two moons in our solar system that have spurting plumes of water vapor.  Saturn’s moon Enceladus definitely has plumes — the Cassini spacecraft traveled through one of them, collecting data — and now there’s good reason to conclude Europa does as well.  This raises the possibility that icy water worlds around the galaxy are spouting out their insides, and some day they might be measurable as well.

Sparks and other scientists on a NASA teleconference repeatedly emphasized, however, that more needed to be done before the presence of Europa plumes could be deemed conclusive.

“These are challenging observations pushing the limits of Hubble,” he said of their work, which used the Space Telescope Imaging Spectrograph to capture far ultraviolet radiation.

To get their results they used the “transit” method of observing Europa, a technique pioneered by exoplanet astronomers working to understand the compositions of the atmospheres of planets far, far away.  That scientists studying possible Europan plumes and methane compositions around distant planets demonstrates just how much cross pollination exists between astronomy, planetary science, astrobiology and exoplanet research have become.

“The atmosphere of an extrasolar planet blocks some of the starlight that is behind it,” Sparks explained. “If there is a thin atmosphere around Europa, it has the potential to block some of the light of Jupiter, and we could see it as a silhouette. And so we were looking for absorption features around the limb of Europa as it transited the smooth face of Jupiter.”

The results were collected over a 15 month period, largely in 2014.  It took several years to process the information and check and recheck the instruments and methodologies to limit the possibility of a false positive.

The findings support earlier evidence for water plumes on Europa detected by a team led by Lorenz Roth of Southwest Research Institute in San Antonio.  They concluded that water vapor had erupted from south polar region of Europa and reached more than 100 miles into space.

Although both teams used Hubble’s Space Telescope Imaging Spectrograph (STIS) instrument, each used an independent method to arrive at the same conclusion.

 

Europa orbits Jupiter every 3 and a half days, and on every orbit it passes in front of the planet. That choreography raises the possibility of plumes being seen as silhouettes absorbing the background light of Jupiter. (A. Field; STScI)
Europa orbits Jupiter every 3 and a half days, and on every orbit it passes in front of the planet. That choreography raises the possibility of plumes being seen as silhouettes absorbing the background light of Jupiter. (A. Field; STScI)

Europa is cold — very cold.  The surface temperature at the equator never rises above minus 260 degrees Fahrenheit. At the poles of the moon, the temperature never rises above minus 370 F.

So how can there be a large ocean beneath the ice to begin with?

The answer involves heat produced by “flexing” of Europa based on its eccentric orbit and the gravitational pull of Jupiter.

This is how a NASA article described it:

“As Europa’s distance from Jupiter changes over the course of its orbit, Jupiter’s gravitational attraction changes, since gravitational force is dependent on distance.  As a result, the tidal bulge raised by Jupiter goes up and down. This flexing causes heating of Europa’s subsurface.”

But Europa, like Earth and every other celestial body, has also been on the receiving end over the eons of material from asteroids, comets, smaller planets and even deep space.  In addition to basic elements and compounds, Europa likely had organic material — the building blocks of life — delivered to it as well.

What’s more, the moon no doubt contains long-lived radioactive isotopes brought to it after formation. The decay of these isotopes would produce radiogenic heating, and may play a role in keeping the Europan ocean wet.

Results from prior missions strongly suggest that this water on Europa is salty, adding to the prospects for life.  And in 2016, a study suggested that Europa produces 10 times more oxygen than hydrogen, which is similar to the ratio on Earth.

 

cryovolcanoesn in europa
Scenario for getting water to Europa’s surface. Artist’s conception of ridges and fractures of Europa.  The ice cover is estimated to be between 6 and 13 miles thick. (NASA/JPL-Caltech)

So, that there may well be a habitable ocean between the ice mantle and rocky insides of Europa is well accepted.  How might some of that water rise up to the surface and ultimately erupt as a plume?

Britney Schmidt, assistant professor at the School of Earth and Atmospheric Sciences at Georgia Institute of Technology in Atlanta, said it has long been hypothesized that vents open at times between the surface and the water below — if not the oceans, then perhaps some lakes closer to the surface.

This is hypothesized to be most likely in chaos terrain, where features such as ridges, cracks, and plains (be they of ice or rock) appear jumbled and enmeshed with one another.  Europa is covered with this kind of geology.

Schmidt, who has studied Europa extensively, described the upwellings as “cryovolcanoes” — small fingers of liquid water pushed up to the surface by the intense pressure of miles of ice above these oceans.

While the presumptive plume activity appears to be most common near the south pole, it is also found elsewhere, including one detection near the equator.

 

NASA hopes the Europan Clipper will fly in the early or mid 2020s and will search for signs of habitability. It is expected to circle the moon for three years. (NASA/JPL-Caltech)
NASA hopes the Europan Clipper will fly in the early or mid 2020s and will search for signs of habitability. It is expected to circle the moon for three years. (NASA/JPL-Caltech)

Europa has been on NASA’s radar screen for some time.  The National Research Council’s once-a-decade review for planetary science, issued in 2013, made exploration of Europa the agency’s highest-priority mission in that arena. The agency generally follows the NRC recommendations.

As currently conceived, the mission would send a probe and lander to the moon, to be launched in the 2020s. Funding for the estimated $2.3 billion project remains a major issue, but last year NASA asked scientists and engineers to propose designs for instruments. Thirty-three proposals came in and nine were selected.

As it is being developed, the mission would send a solar-powered spacecraft into a long, looping orbit around Jupiter to then perform three-years of repeated close flybys of Europa. In total, the mission would perform 45 flybys at altitudes ranging from 16 miles to 1,700 miles (25 kilometers to 2,700 kilometers).

But later action in Congress — where Rep. John Culberson, a Texas Republican who represents the area including NASA headquarters,  has pushed hard for a major Europa mission — added a lander to the project, to be launched two years after the Clipper.   Culberson, chair of the House subcommittee that funds NASA, strongly believes that NASA will find the first evidence for life beyond Earth in the ocean beneath the thick surface ice.

 Rep. John Culberson, R-Texas, is chairman of the House Subcommittee on Commerce, Justice, Science, which oversees NASA funding. Culberson is a strong proponent of missions to Europa in search of extraterrestrial life. (Photo By Tom Williams/CQ Roll Call)
Rep. John Culberson, R-Texas, chairman of the House Subcommittee on Commerce, Justice, Science, is an active proponent of missions to Europa to search of extraterrestrial life. His subcommittee oversees NASA funding. (Photo By Tom Williams/CQ Roll Call)

The selected science instruments includes cameras and spectrometers to produce high-resolution images of Europa’s surface and determine its composition. An ice penetrating radar will determine the thickness of the moon’s icy shell and search for subsurface lakes similar to those beneath Antarctica. The mission also will carry a magnetometer to measure strength and direction of the moon’s magnetic field, which will allow scientists to determine the depth and salinity of its ocean.

A thermal instrument will scour Europa’s frozen surface in search of recent eruptions of warmer water, while additional instruments will search for evidence of water and tiny particles in the moon’s thin atmosphere.

Curt Niebur, project scientist for the Europa mission, was on the teleconference call as well, and described the increasing possibility that the Clipper will find Europan plumes as enormously exciting and important.  He said that the spacecraft will have the ability to change course and head to an area experiencing a plume — should one erupt.

He also said the vehicle could potentially pass through a plume like Cassini did with one of the plumes of Enceladus.  Those plumes are larger than anything tentatively detected on Europa and are more predictable in their timing and locations.

But who knows what might be possible?

“We found out a lot about Enceladus by flying through the plume and sampling plume material,” Niebur said.  “We would kill to have that kind of information for Europa.”

 

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