With its smooth, bright surface of water ice slashed by crisscrossed long, linear fractures, Jupiter's moon Europa is a strange world that is only slightly smaller than Earth's Moon. Europa is believed to have a rocky mantle, an iron core, and an ocean of salt water hidden beneath its frozen, icy shell--it is so far from the fiery heat of our Star, the Sun, that its ocean's surface is globally frozen over. Along Europa's numerous fractures, and in splotchy regions across its cracked-eggshell-like surface, is a mysterious dark reddish-brown material whose composition has not been determined, but that may hold precious clues concerning the distant icy moon's potential as a habitable world. In May 2015, it was announced that NASA research has finally shed light on this mystery--laboratory experiments suggest that the dark material coating some geological features on Europa's strange, cracked surface is likely sea salt from a subsurface ocean, discolored by exposure to radiation.
The existence of sea salt staining Europa's frozen crust indicates that the global ocean is interacting with its rocky seafloor. This type of interaction represents an important consideration in determining whether the icy, distant little moon-world could potentially harbor life.
The study describing this new research will appear in the journal Geophysical Research Letters. It is available online.
"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 habitable. Once we have those answers, we can tackle the bigger question about life in the ocean beneath Europa's ice shell," noted Dr. Curt Niebur in a May 12, 2015 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Niebur is an Outer Planets Program scientist at NASA Headquarters in Washington D.C. The JPL is in Pasadena, California.
A Bewitching, Icy Moon-World
Discovered on January 8, 1610 by Galileo Galilei, Europa is a frigid, fascinating little moon-world. The discovery of Europa, along with the three other Galilean moons, was the first time a moon was detected orbiting a planet other than Earth.
Beneath a shattered, tortured, splotchy crust of ice, there likely exists a gigantic, global ocean on this little moon--the sixth largest moon in our Solar System. Despite its small size, very few bodies have bewitched astronomers as much as this jumbled, cracked, strange small world. This is because where liquid water exists, life as we know it may also exist.
Europa is named for the daughter of Agenor in ancient Greek mythology, who was abducted by Zeus (the Greek equivalent of the Roman Jupiter), who had taken the shape of a white bull. Europa was so enchanted by the gentle beast that she decorated it with flowers and rode around on its back. Of course, Zeus took advantage of the situation, and rode away with her into the ocean to the island of Crete, where he transformed himself back into his real shape after his metamorphosis.
Europa is one of the quartet of large Galilean moons circling the gas-giant planet Jupiter. The four moons--Io, Europa, Ganymede, and Callisto--are collectively dubbed the Galilean moons in honor of their discoverer, who spotted them when he was peering up into the dark, starlit, winter night sky over Padua with his small, primitive "spyglass"--one of the first telescopes to be used for astronomical purposes. Both Ganymede and Callisto are icy-rocky moon-worlds, and Ganymede is the largest moon in our entire Solar System. Io, however, is a small hell-like sphere, scarred and pockmarked by fiery volcanoes, and abundantly splattered with sulfur.
For decades, bizarre jumbled areas of ice disruption on Europa, appropriately designated the chaos terrains, were weird regions of undetermined, mysterious origin. Indeed, bewildering regions like the chaos terrains have not been seen anywhere else in our Solar System. However, it is now thought that these bizarre chaos terrains were formed by the motions of a sloshing global ocean swirling beneath Europa's icy shell.
Europa orbits its planet every 3.5 days and is locked by gravity to Jupiter in such a way that the same hemisphere of the little moon always faces its parent planet. Because Europa's orbit is slightly elliptical (out of round), its distance from Jupiter varies. This creates tides that stretch and relax its tormented surface. The tides occur because Jupiter's gravity is slightly stronger on the near side of Europa than on the far side, and the magnitude of this variation alters as Europa travels on its orbit around its planet. Flexing resulting from the tides provides energy to Europa's icy shell, thus forming the linear fractures that slash its surface. If Europa's ocean really does exist, the tides can possibly also create volcanic or hydrothermal activity on the seafloor, and in this way supply precious nutrients that could make the ocean a comfortable abode for life.
There are only a small number of craters pock-marking Europa's frozen surface. This indicates that the moon's surface is probably no more than 40 to 90 million years old--which, by geological standards, is quite young. Although Europa was visited by a duo of sister spacecraft--Pioneer 10 and Pioneer 11--back in the early 1970s, and the twin Voyagers in 1979, these early visits sent back to Earth only some grainy, dim pictures. However, as flawed as they were, these first images revealed enough about this intriguing little moon-world to make it an object of fascination. Icy plains of pale yellow could be seen in the Voyager images, and they were also mottled with strange red and brown regions. Long fractures could also be seen, and they extended for thousands of miles over the cracked, frozen crust. On Earth, similar fractures indicate features like tall mountains and deep canyons. However, nothing higher than a few kilometers could be seen on the little moon-world. Indeed, Europa revealed one of the smoothest surfaces in our entire Solar System!
NASA's highly successful Galileo mission, which explored the Jupiter system from 1995 to 2003, made a number of flybys above Europa. It gathered the closest images to date of the little moon's fractured surface, observing weird pits and domes that indicated that the ice could be slowly turning over, or convecting, as a result of heat coming from below. In addition, and of particular interest, was the strange chaos terrain which showed bewildering blocky, broken landscapes blanketed by the mysterious reddish material. Planetary scientists, studying Galileo mission data, theorized that the chaos regions must be areas where geologic activity has disrupted the otherwise very smooth surface of Europa. In 2011, the intriguing idea was proposed that the chaos terrains are regions where the surface has collapsed above lens-shaped lakes situated within the ice.
In 2013, NASA announced that they had attained surprising evidence from astronomers using the Hubble Space Telescope that Europa might actively be venting plumes of water out into space. This discovery stimulated great excitement among scientists, because it provided evidence that the little moon-world is still geologically active today. The plumes could be studied by future missions in a way similar to the Cassini spacecraft's flights through the plumes of Saturn's moon Enceladus.
Europa is particularly intriguing because it is among the bodies of our Sun's family that has a potentially immense amount of liquid water, along with geologic activity that could enable the exchange of chemicals from the surface with the watery environment sloshing around beneath the ice. For this reason, Europa could well be the most promising world in our Solar System to search for signs of present-day life.
One of the most important measurements made by the Galileo mission revealed how Jupiter's magnetic field was disrupted in the space surrounding Europa. This important measurement strongly suggests that a special type of magnetic field is being induced within the icy little moon by a deep layer of some electrically conductive fluid swirling around beneath Europa's frozen shell. Because of Europa's icy make-up, astronomers think the most promising material to create this particular magnetic signature is a global ocean of salty water.
Future missions to Europa will likely try to confirm the presence of its global subsurface salt water ocean. Also of great interest is the composition of the reddish material on Europa's icy crust. Astronomers would like to find out if this material holds precious clues to the composition of the ocean and whether material is cycling between the surface and the interior.
Europa In A Can
For over a decade, astronomers have wondered about the compositon of the strange dark material that coats the long, linear features on Europa's crust of ice. Its association with youthful terrains indicates that the material has erupted from the interior of the moon. However, limited data has been available, and the mysterious material's composition has remained undetermined.
"If it's just salt from the ocean below, that would be a simple and elegant solution for what the dark, mysterious material is," noted research lead Dr. Kevin Hand in the May 12, 2015 JPL Press Release. Dr. Hand is a planetary scientist at the JPL.
It is well-known that Europa is continually showered with radiation created by Jupiter's strong magnetic field. Electrons and ions smash into the icy moon's frozen crust with the power of a particle accelerator. Theories devised to explain the mysterious nature of the dark material include the idea that this radiation is a probable star-player in the drama that creates it.
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