In the cold twilight of our Solar System's outer limits, the banded behemoth Jupiter reigns supreme. Jupiter is by far the largest of our Sun's planetary offspring – and it is an enormous world that may well have become a binary stellar companion to our Sun, making life on Earth impossible. But, fortunately for us, Jupiter just missed attaining sufficient mass to light its nuclear-fusing stellar fires, and instead began to shrink. Among Jupiter's many moons, the icy little moon-world named Europa stands out in the crowd, and sings an intriguing sirens' song to astronomers searching for possible life existing beyond Earth. That is because this frozen little moon, beneath its cracked, shattered, and chaotic crust of ice, may hide a sloshing global ocean of life-sustaining liquid water. Where liquid water exists, there is the possibility – though by no means the promise – that life as we know it may also exist. In May 2018, a team of scientists announced that after re-examining data from an old mission, they had gained new insights indicating that Europa may have the right ingredients to support life. The data provide strong independent evidence that the moon-world's subsurface liquid water reservoir may be revealing its presence by venting plumes of water vapor above Europa's cracked icy shell.
Data gathered by NASA's Galileo spacecraft back in 1997 were re-examined by scientists using new and more advanced supercomputer models to solve an intriguing mystery. This puzzle centers around a strange localized band in Europa's magnetic field that has gone unexplained until now. Earlier ultraviolet images taken by NASA's Hubble Space Telescope (HST) in 2012 hinted at the presence of plumes, but this new study used data that had been gathered much closer to the source, and it is considered to be strong corroborating support for the existence of water vapor plumes shooting out from Europa's cracked eggshell-like icy crust. The new findings appear in the May 14, 2018 issue of the journal Nature.
The study was led by Dr. Kevin Xianzhe Jia, a space physicist at the University of Michigan in Ann Arbor and lead author of the Nature article. Dr. Jia is also a co-investigator for two instruments that will make the long and treacherous journey aboard the upcoming Europa Clipper. The planned Europa Clipper is designed to explore Europa's potential habitability.
"The data were there, but we needed sophisticated modeling to make sense of the observation," Jia explained in a May 14, 2018 NASA Jet Propulsion Laboratory (JPL) Press Release. The JPL is located in Pasadena, California.
Jupiter's Icy And Mysterious Water-Moon
Galileo Galilei discovered Europa in January 1610, along with three other Jovian moons . This quartet of large moons – Io, Europa, Ganymede and Callisto – were collectively designated Galilean moons in honor of their discoverer.
On one clear, starlit winter night, Galileo climbed to the roof of his house in Padua, and discovered the four moons that bear his name. He did this using only a small and primitive "spyglass" – one of the first telescopes to be used for astronomical purposes. Historically, this also marked the first time that a moon had been detected in orbit around a planet other than our own. Both Ganymede and Callisto are composed of ice and rock, and Ganymede has the distinction of being the largest moon in our Solar System. Io differs from its three Galilean siblings because it is a small moon with some hell-like attributes. Volcanic little Io's colorful surface has been compared to a "pepperoni pizza" – scarred, splotched, and pockmarked by erupting, fierce and fiery volcanoes. It is also well-endowed with sulfur.
Galileo made the first reported detection of both Io and Europa on January 7, 1610. However, during that first observation of the two moons, he could not separate Io and Europa into two distinct entities. This is because of the low magnification abilities of his primitive telescope. For this reason, the duo appeared to be only a solitary, single point of light. The next day, Io and Europa were observed for the first time as separate small, distant moons in their own right, as Galileo continued his study of the Jovian system. The quartet of Galilean moons may have been discovered independently by the German astronomer Simon Marius (1573-1625).
The moon-world, Europa, is thought to contain a rocky mantle and an iron core – as well as its subsurface ocean of salt water flowing beneath its icy shell. Europa is far enough from the melting heat of our Star to keep its subsurface ocean's surface frozen over, thus creating its shattered icy crust. There is also a mysterious dark reddish-brown material appearing along Europa's numerous fractures – as well as splotchy regions scarring its surface. This dark reddish-brown material has not yet been identified – but it may eventually reveal a surprise package full of clues about this distant moon's potential as a habitable little world.
At the time of their discovery, jumbled regions of ice disruption on Europa, appropriately called the chaos terrains , were considered to be exotic areas of mysterious origins. However, it is currently hypothesized that the chaos terrains were formed as the result of the sloshing motions of Europa's global subsurface ocean of liquid water churning beneath its icy shell.
Europa orbits Jupiter every 3.5 days, and it is bound gravitationally to its parent-planet in such a way that it always shows it the same face. Also, because Europa's orbit around Jupiter is elliptical, its distance from Jupiter varies according to where it is in its orbit. This causes tides that repeatedly both stretch and then relax the icy moon's shattered surface ( tidal flexing) . The tides are the result of Jupiter's intense gravitational influence on Europa. This is because Jupiter's gravitational pull is more powerful on Europa's near side than on its far side. To make things even worse, the magnitude of this relentless variation changes as Europa travels around in its orbit around Jupiter. Tidal flexing results from the tides that contribute energy to Europa's icy crust, and this creates the long slashing fractures that tear through its exotic surface. The Europan tides can potentially create volcanic or hydrothermal activity on the seafloor – which can provide precious nutrients to possible life-forms floating around in the salt water, thus making the ocean habitable. Currently, the favored scientific model proposes that heat resulting from tidal flexing keeps Europa's subsurface ocean in its life-sustaining liquid phase, and drives ice movement that is similar to plate tectonics on Earth – thus absorbing chemicals from the surface into the well-hidden ocean churning around beneath Europa's icy shell. Furthermore, sea salt formed in Europa's subsurface ocean may be coating some of the geological features that have been observed on Europa's surface. This indicates that the ocean is interacting with the seafloor. Such an interaction could also play an important role in the potential emergence of life-forms on this moon-world.
Based on the small number of observable craters pockmarking Europa's icy shell, the surface is thought to be young – only about 40 to 90 million years old. Heavily cratered surfaces are older than surfaces that show comparatively little cratering. This is because geological activity has erased the older craters, thus repeatedly providing the world with a fresh new surface. By comparison, Callisto – another of the quartet of Galilean moons –has a surface that is estimated to be a few billion years old because it is heavily scarred by craters that resurfacing has not erased.
NASA's Galileo spacecraft to the Jupiter system was launched in 1989, and it has provided most of the current data gathered about Europa. The Galileo mission ended on September 21, 2003, when the spacecraft was deliberately forced to crash down into the crushing atmosphere of the giant planet that it had been observing successfully for so long. NASA's ongoing Juno spacecraft was launched on August 5, 2011, and entered orbit around Jupiter on July 5, 2016. Juno will hunt for clues about how Jupiter itself was born. Both Galileo and Juno are the only spacecraft so far to have gone into Jupiter orbit.
No spacecraft has yet landed on icy and mysterious Europa. However, because of its many intriguing characteristics and bewitching possibilities, this will soon change. Perhaps swimming around in Europa's subsurface sea, there are tidbits of life that can finally answer the profound question of whether or not we are alone in the Cosmos. Astronomers will begin observing Europa anew with the Europa Clipper mission, scheduled for launch in the 2020s. According to plans, Europa Clipper will reach Jupiter several years after being launched, and would then try to observe whether the frozen little moon-world could possess conditions suitable for life. The Europa Clipper spacecraft is radiation-tolerant, and it will accomplish 45 flybys of Europa at altitudes varying from 1,675 miles to 16 miles from a looping, long orbit around Jupiter.
Europa Clipper's instruments will include cameras and spectrometers to obtain high-resolution images of Europa's exotic crust and determine its composition. An ice-penetrating radar will also determine the thickness of Europa's cracked icy shell and go on the hunt for subsurface lakes that resemble those on Earth's Antarctica. The mission will also carry a magnetometer to measure both the direction and strength of Europa's magnetic field. This measurement will help astronomers determine both the salinity and depth of Europa's subsurface global ocean.
Even though Europa was visited by the duo of sister spacecraft– Pioneer 10 and Pioneer 11– in the 1970s, as well as by the twin Voyagers back in 1979, these early missions of space exploration sent only a handful of grainy, dim images back to astronomers on Earth. However, even though these early pictures were not up to today's standards, they succeeded in successfully unveiling enough about Europa to make it a interesting interesting target for later study. Icy plains of pale yellow could be seen on the images sent back to Earth by Voyager , and they also showed those mysterious reddish-brown regions of unknown composition. The long fractures slashing through Europa's icy shell could be observed, and they extended thousands of miles over the moon's cracked and frozen surface. Similar features on Earth indicate the existence of mountains and deep canyons. However, nothing higher than a few kilometers could be seen on this cracked icy moon. Indeed, Europa's smooth, youthful crust is one of the smoothest surfaces in our Solar System.
The HST is responsible for detecting the first hint of water vapor plumes shooting out from the surface of Europa. These plumes bear a striking similarity to those observed on the icy water-moon, Enceladus of Saturn. The water plumes of Enceladus are the result of erupting cryovolanoes (water geysers).
Europa is a particularly interesting moon-world because it is among the bodies inhabiting our Sun's family that could potentially host life as we know it. The icy moon likely contains large quantities of liquid water, and the possibility of life in Europa's seas is enhanced because its geologic activity could result in the exchange of chemicals from the surface with the ocean hidden under the ice.
Everything Old Is New Again
Dr. Jia's team explored the old Galileo data following a presentation given by Dr. Melissa McGrath of the SETI Institute in Mountain View, California. Dr. McGrath, a member of the Europa Clipper science team, delivered her presentation to fellow team scientists, paying special attention to other HST observations of Europa.
"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, "Dr. Jia commented in the May 14, 2018 JPL Press Release.
At the time Galileo made its 1997 flyby above Europa's surface, the team of Galileo astronomers did not even suspect that the spacecraft might be grazing a water-vapor plume shooting out from the icy crust of the moon. The flyby carried the spacecraft only about 124 miles above Europa's surface. Today, Dr. Jia and his team believe that Galileo's path, during that flyby, was a lucky one.
When the scientists studied the information collected during that flyby almost a generation ago, they found that high-resolution magnetometer data had uncovered something weird. Drawing on what astronomers had already learned from exploring the plumes erupting from Enceladus of Saturn – that material in the plumes becomes ionized and leaves behind a characteristic blip in the magnetic field – they were prepared, and knew what to look for. There it was –a short-lived localized bend in the magnetic field that had never been explained.
Galileo had been equipped with a powerful Plasma Wave Spectrometer capable of measuring plasma waves resulting from charged particles in gases contained in Europa's atmosphere. Dr. Jia's team pulled that data as well. This newly-acquired information also strengthened the theory of the existence of a plume of water vapor.
However, numbers by themselves could not tell the whole story. Dr. Jia layered the magnetometry and plasma wave signatures into new 3D modeling that had been created by his team at the University of Michigan (Ann Arbor). This model simulated the interactions of plasma with Solar System bodies. The last necessary ingredient was the data derived from HST that showed the dimensions of the possible plumes ..
What emerged from the model, with a simulated plume, was a match to the magnetic field and plasma signatures the team of scientists had obtained from the old Galileo data.
"There now seem to be too many lines of evidence to dismiss plumes on Europa. 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," noted Dr. Robert Pappalardo in the May 14, 2018 JPL Press Release. Dr. Pappalardo is a Europa Clipper project scientist at the JPL.
The new findings are good news for the Europa Clipper mission , which may launch as early as 2022. From its orbit around Jupiter, Europa Clipper will soar close to the icy little moon in speedy, low-altitude flybys. If the plumes are really hurling out tattle-tale water vapor from Europa's ocean or global subsurface lakes, Europa Clipper could take samples of the frozen liquid and dust particles. The mission team is currently devising potential orbital paths, and the new research will certainly aid in those discussions.