A Global Ocean Beneath Europa’s Icy Shell

In the cold and distant realm of the majestic giant planets, the beautifully banded behemoth Jupiter stands out in the crowd as the true “King of Planets”. Jupiter is larger than all of the other major planets of our Solar System combined, and it is orbited by a mysterious myriad of mostly icy moons. However, it is most famous for its quartet of Galilean moons that were discovered by Galileo Galilei in 1610, using only a primitive telescope–one of the first to be used for astronomical purposes. Slightly smaller than Earth’s own large lunar companion, Europa is one of the bewitching Galilean moons of Jupiter–along with Io, Ganymede, and Callisto. Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s. This icy moon-world that displays a shattered, cracked, and chaotic jumbled crust has long been suspected of containing an ocean of life-loving liquid water beneath its frozen shell–and the existence of liquid water is necessary for the emergence of life as we know it. In September 2016, astronomers using NASA’s Hubble Space Telescope (HST) announced that they have imaged what may be water vapor plumes shooting off the frigid surface of Europa–and this finding strengthens other HST observations indicating that this icy moon-world erupts with high altitude water vapor plumes.

These new observations are important because they increase the possibility that missions to Europa may succeed in sampling some of its ocean without having to drill through miles of ice.

“Europa’s ocean is considered to be one of the most promising places that could potentially harbor life in the Solar System. These plumes, if they do indeed exist, may provide another way to sample Europa’s subsurface,” noted Dr. Geoff Yoder in a September 26, 2016 Hubblesite Press Release. Dr. Yoder is acting associate administrator for NASA’s Science Mission Directorate in Washington.

The team of planetary scientists estimate that the water plumes rise to approximately 125 miles before, presumably, showering material back down onto Europa’s surface. Europa’s enormous subsurface global ocean contains twice as much water as Earth’s oceans. However, Europa’s sloshing ocean of liquid water is protected by its shell of extremely hard, frigid ice of as yet unknown thickness. The plumes provide planetary scientists with a gift of sorts, because they enable them to collect samples originating from beneath the surface without having to land or drill through the shell of ice.

Bewitching, Bewildering Moon-World

Europa is believed to harbor a rocky mantle and an iron core, as well as its subsurface ocean of salt water that flows beneath its icy shell. It is sufficiently far from the melting heat of our Sun to keep its ocean’s surface globally frozen over. Along Europa’s numerous fractures, and in splotchy regions across its cracked surface, there is a mysterious dark reddish-brown material whose composition has not yet been identified–but it may hold a treasure trove of clues concerning the distant moon’s potential as a habitable world.

Galileo discovered Europa in January 1610, along with the other three of Galilean moons, that have been named in his honor. Galileo observed the quartet of large Jovian moons when he was star-gazing up into the dark, clear winter sky above Padua, using a small and primitive “spyglass”. Historically, this accomplishment represents the first time a moon had been spotted orbiting a planet other than Earth. Both Ganymede and Callisto are composed of rock and ice, and Ganymede is the largest moon in our entire Solar System. Io, in marked contrast, is a small moon whose hell-like surface has been likened to a “pepperoni pizza”–splotched, scarred, and pockmarked by fiery, erupting volcanoes, and richly endowed with sulfur. Galileo made the first reported observation of Io and Europa on January 7, 1610. However, in that initial observation, Galileo could not separate Io and Europa because of the low magnification of his telescope–so the duo of little moons were recorded as a single point of light. The next day, January 8, 1610, Io and Europa were seen for the very first time as separate little moons in their own right, as Galileo continued his observations of the Jovian system. The four moons were also possibly discovered independently by Simon Marius.

For decades, mysterious jumbled regions of ice disruption on Europa, designated the chaos terrains for obvious reasons, were viewed as exotic areas of unknown origin–and at the time were considered to be unique in our Solar System. However, it is currently thought that the chaos terrains were formed by the sloshing movements of Europa’s global water ocean swirling beneath its icy shell.

Europa circles its gas-giant planet every 3.5 terrestrial days, and it is locked by gravity to Jupiter in such a way that the same hemisphere of the moon always faces its enormous parent-planet. Because Europa’s orbit around Jupiter is elliptical, its distance from its planet varies. This causes tides that mercilessly and repeatedly both relax and stretch the little moon’s unfortunate surface. The tides are the result of Jupiter’s gravity that is slightly stronger on the near side of Europa than on the far side–and the magnitude of this continual variation changes as Europa wanders on its orbit around its planet. Tidal flexing is the result of tides that provide energy to Europa’s shell of ice, and this creates the linear slashing fractures that scar its intriguing surface. If Europa’s ocean does exist, the tides can potentially create volcanic or hydrothermal activity on the seafloor, and in this way provide nutrients that could conceivably result in an ocean that is habitable. The currently most-favored model suggests that heat resulting from this tidal flexing keeps Europa’s subsurface ocean liquid and drives ice movement comparable to plate tectonics on Earth, absorbing chemicals from the surface into the hidden ocean beneath. In addition, sea salt from the subsurface ocean may be coating some of the geological features observed on Europa’s surface. This indicates that the ocean is interacting with the seafloor.

Launched in 1989, NASA’s Galileo mission to the Jovian system has provided most of the current data collected on Europa. The Galileo mission ended on September 21, 2003, when the spacecraft was deliberately forced to crash down into the crushing Jovian atmosphere. NASA’s ongoing Juno mission was launched on August 5, 2011, and entered orbit around Jupiter on July 5, 2016. Juno will search for clues about how Jupiter formed. Galileo and Juno are the only two spacecraft so far to have gone into Jupiter-orbit.

No spacecraft has as yet landed on little Europa, but its captivating collection of characteristics have resulted in several ambitious exploration proposals. Although Europa was visited by twin sister spacecraft–Pioneer 10 and Pioneer 11–in the 1970s, and the duo of Voyagers back in 1979, these early missions dispatched back to scientists on Earth only some dim, grainy images. However, as flawed as these early images were, they successfully unveiled enough about Europa to make it an intriguing target for further study. Pale yellow icy plains could be seen in the Voyager pictures, and they were also mottled with mysterious red and brown areas. Long fractures could also be seen, and they reached thousands of miles over Europa’s frozen, slashed crust. Similar features seen on Earth suggest features such as deep canyons and high mountains. But nothing higher than a few kilometers could be observed on this bewitching little moon. Europa, in fact, has one of the smoothest surfaces in our entire Solar System. There are also only a small number of craters pock-marking Europa’s frozen surface. This suggests that the moon’s surface is young–probably no more than 40 to 90 million years old, which is quite youthful on geological timescales.

In addition, the HST is responsible for detecting the very interesting water vapor plumes, that resemble those observed on Enceladus, a small icy moon of the ringed-gas-giant planet, Saturn. The erupting plumes of Enceladus are thought to be the result of erupting cryogeysers (water geysers). Indeed, Europa is particularly interesting because it is among the bodies of our Solar System that potentially could contain a great amount of liquid water, along with the geologic activity that could cause the exchange of chemicals from the surface with the watery environment under the ice. For this reason, Europa is one of the most promising worlds in our Solar System to hunt for signs of life.

One of the most important measurements made by the Galileo spacecraft showed how Jupiter’s magnetic field was disrupted in the space surrounding Europa. This measurement strongly indicates that a special kind of magnetic field is being induced inside the little moon by a deep layer of an electrically conductive fluid sloshing and swirling around under Europa’s icy shell. Because of Europa’s frozen composition, astronomers believe the most promising material to form this particular magnetic signature is a global ocean of subsurface salty water.

A Global Ocean Beneath Europa’s Ice Shell

The planetary scientists, who used the HST to spot the water vapor plumes shooting out from Europa’s frozen surface, were led by Dr. William Sparks of the Space Telescope Science Institute (STScl) in Baltimore, Maryland. The team observed the finger-like water vapor projections while observing Europa’s limb as the little moon floated in front of the face of its huge, banded parent-planet.

The original goal of the team’s observing proposal was to determine whether Europa has an extended, thin atmosphere or exosphere. The scientists used the same observing technique that spots atmospheres around extrasolar planets that orbit other stars beyond our own Sun. The scientists had come to the realization that if there really was water vapor erupting from Europa’s surface, this observational technique would be the method of choice for them to use to detect it.

“The atmosphere of an extrasolar planet blocks some of the starlight that is behind it. 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,” Dr. Sparks explained in the September 26, 2016 Hubblesite Press Release.

In ten separate events that spanned 15 months, the planetary scientists observed Europa floating in front of the banded face of Jupiter. They were able to detect possible plumes of water vapor shooting out from Europa’s surface on three of these ten observing events.

This new research lends support to earlier signs of water plumes on Europa. In 2012, a team led by Dr. Lorenz Roth of the Southwest Research Center in San Antonio, Texas, announced that they had spotted signs of water vapor erupting from the frozen, icy south polar region of this strange moon. The plumes that Dr. Roth’s team detected soared more than 100 miles up into space from Europa’s surface. Even though both teams of planetary scientists used HST’s Imaging Spectrograph (STIS) instrument, each used a completely different method to reach the same conclusion.

“When we calculate in a completely different way the amount of material that would be needed to create these absorption features, it’s pretty similar to what Roth and his team found. The estimates for the masses are similar, the estimates for the height of the plumes are similar. The latitude of two of the plume candidates we see corresponds to their earlier work,” Dr. Sparks continued to explain in the Hubblesite Press Release.

However, the two teams have not yet simultaneously detected the plumes using their independent techniques. Observations so far have indicated that the plumes could be highly variable in nature. This means that they may be sporadically erupting some of the time–and then dying down. For instance, observations conducted by Dr. Roth’s team within a week of one of the detections by Dr. Spark’s team failed to spot any plumes.

If confirmed, Europa would be the second moon in our Solar System known to erupt water vapor plumes. It was in 2005 that NASA’s Cassini orbiter, that is currently observing the Saturn system, spotted similar jets of dust and water vapor shooting off Enceladus.

Planetary scientists may use the infrared vision of the upcoming James Webb Space Telescope (JWST), that is scheduled to launch in 2018, in order to confirm the existence of plume activity or venting on Europa. NASA also is in the process of developing a mission to Europa that will carry instruments that could confirm the existence of plumes, and then study them from close range during a series of flybys.

Hubble’s unique capabilities enabled it to capture these plumes, once again demonstrating Hubble’s ability to make observations it was never designed to make. This observation opens up a world of possibilities, and we look forward to future missions–such as the James Webb Space Telescope–to follow-up on this exciting discovery,” Dr. Paul Hertz noted in the September 26, 2016 Hubblesite Press Release. Dr. Hertz is director of the Astrophysics Division at NASA Headquarters in Washington, D.C.

The research by Dr. Sparks and his colleagues is published in the September 29, 2016 issue of The Astrophysical Journal.