Europa is a fascinating, enticing little world–a mysterious, bewitching moon of ice that circles the gas-giant planet Jupiter in the cold outer regions of our Solar System. Many planetary scientists think that there is an enormous global ocean of liquid water churning beneath the cracked and jumbled icy crust of this distant moon-world–and where liquid water exists, there is always the possibility, though not the promise, of life as we know it to exist as well. In September 2014, two separate teams of planetary scientists released some additional fascinating revelations about this mysterious moon-world. One study indicates that scientists have found evidence of plate tectonics on Europa–representing the first sign of this type of surface-shifting geological activity on a world other than Earth. The second study reveals that the huge plumes of water vapor seen erupting from the surface of Europa, during earlier observations, seem to have disappeared–and astronomers are not sure why!
Europa is one of the four large Galilean moons of the planet Jupiter, named for their discoverer, Galileo Galilei, who spotted the quartet when he was observing the dark night sky above his home in Padua with his “spyglass”–one of the first, primitive telescopes used to observe celestial bodies. The other three Galilean moons, the sisters of Europa, are Io, Ganymede, and Callisto. Ganymede is the largest moon in our Sun’s family, and it is an icy-rocky moon-world that many scientists think also harbors a subsurface sea of life-loving liquid water. Io, on the other hand, is a small, fiery sphere that bears a beguiling resemblance to a pepperoni pizza. It is also hell-like; scarred by volcanoes, and lavishly smeared with streaks of yellow sulfur.
The bizarre, jumbled regions of ice-disruption on the surface of Europa, termed the chaos terrains, puzzled planetary scientists for years. The bewildering origins of the chaos terrains were difficult to explain, and similar regions were observed on no other body in our Solar System. However, most planetary scientists now think that these weird terrains formed as a result of the sloshing, churning subsurface sea of liquid saltwater beneath Europa’s cracked, icy shell.
Even though Europa was visited by both the Pioneer 10 and Pioneer 11 spacecraft back in the early 1970s, and the sister Voyagers at the end of that same decade, these early visits produced only some very grainy, fuzzy, and dim images. However, these old pictures did show enough about the ice-crusted moon to tantalize astronomers. Pale yellow icy plains that were mottled with red and brown regions were seen in the old Voyager pictures. In addition, strange, long cracks that extended for literally thousands of miles over the cracked icy crust of Europa enticed astronomers to learn more about this frozen little world. On Earth, similar cracks usually imply features like deep canyons and tall mountains. However, nothing taller than a few kilometers was spied on Europa–one of the smoothest worlds in our Sun’s entire family.
NASA’s Galileo spacecraft (1989-2003) managed to get a better look at Europa during a flyby on September 7, 1996. Galileo observed Europa’s tantalizing surface much more closely than the Pioneers and Voyagers had years earlier. The Galileo images revealed a weird surface crust that has been likened to broken glass, stuck back together with an icy glue–that was slithering up to the surface from below.
The most detailed images of Europa reveal still more evidence of icy slush that forms a subsurface mush beneath the moon’s surface shell. Europa is smaller than Earth’s own Moon, and its frigid surface temperature–characteristic of bodies in the outer realm of our Solar System–could readily freeze a water ocean solid over a time span of merely several million years. However, the warmth resulting from a game of tug-of-war between Europa and Jupiter, as well as with other moons, keeps Europa’s subsurface ocean in liquid form. This process, which is termed tidal heating, refers to a process by which the gravitational pulls of a nearby object–or objects–bend, stretch, contract, and expand another object–causing it to heat up. This is why Europa is much more balmy than its great distance from our fiery Star would otherwise render it.
Planetary scientists have long had clear visual evidence that Europa’s icy shell is expanding. However, they were unable to spot areas where the old crust was destroyed in order to make room for fresh, new terrain. While studying images of Europa taken by Galileo back in the early 2000s, planetary geologists Dr. Simon Kattenhorn, of the University of Idaho, Moscow, and Dr. Louise Prockter of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland, detected some bizarre geological boundaries.
“We have been puzzled for years as to how all this new terrain could have formed, but we couldn’t figure out how it was accommodated. We finally think we’ve found the answer,” Dr. Prockter commented in a September 8, 2014 JHUAPL Press Release.
Plate tectonics is the term used for the scientific theory that our own planet’s outer layer is composed of plates–or blocks–that move. This movement would account for why mountains and volcanoes form on Earth–as well as why earthquakes occur.
The surface of Europa looks like a cracked egg-shell–it is scarred by cracks and ridges that torment its icy crust. Surface blocks are recognized to have shifted in the same way blocks of our planet’s own outer ground layer, on either side of the San Andreas Fault, slide past each other in earthquake-prone California. Many regions of Europa’s frozen surface display signs of extension, where bands that are miles wide are created as the surface is torn apart and fresh, new ice from the underlying shell oozes up into the newly formed space–a process that is similar to seafloor spreading on our planet.
On our own planet, as new surface material is created at mid-ocean ridges, the old material is destroyed at subduction zones. Subduction zones are areas where two tectonic plates meet together and then overlap as one is pushed under the other. However, in spite of the degree of extension that was obvious on Europa’s icy crust, planetary scientists were unable to determine how the surface could accommodate the abundance of new material.
Planetary scientists who observe Europa frequently reconstruct that moon’s frozen surface blocks into their original configurations–much in the same manner as a jigsaw puzzle–in order to get a better picture of what the surface may have been like before the disruption occurred. When Dr. Kattenhorn and Dr. Prockter rearranged the icy surface in the images, they found that approximately 7,700 square miles of the moon’s surface was missing in its high northern latitudes.
Additional evidence indicated that the missing terrain had slid beneath a second surface plate. This particular scenario is frequently seen on Earth at plate-tectonic boundaries. Dr. Kattenhorn and Dr. Prockter observed ice volcanoes (cryovolcanoes) on the overriding plate, that may have been caused by the melting and absorption of the slab of ice as it sunk beneath the surface, as well as an absence of mountains at the subduction zone. This suggests that material was forced into the interior, rather than smashed up as the two plates collided into each other.
The planetary scientists think that the subduction area was absorbed into Europa’s icy, frozen shell, which may be as much as 20 miles thick–instead of tearing through it into Europa’s subsurface liquid water ocean. On Europa’s relatively youthful surface–which is only about 40 to 90 million years old, on average–planetary scientists have observed signs of material sliding up from beneath the frozen shell but, until now, no process had been discovered for shifting material back into the shell, and possibly into the immense global ocean sloshing around beneath the ice.
“Europa may be more Earth-like than we imagined, if it has a global plate tectonic system. Not only does this discovery make it one of the most geologically interesting bodies in the Solar System, it also implies two-way communication between the exterior and interior–a way to move material from the surface into the ocean–a process which has significant implications for Europa’s potential as a habitable world,” Dr. Kattenhorn explained in the September 8, 2014 JHUAPL Press Release.
The team’s results appear in the September 7, 2014 online edition of the journal Nature.
“Europa continues to reveal itself as a dynamic world with compelling similarities to our own planet Earth. Studying Europa addresses fundamental questions about this potential habitable icy moon and the search for life beyond Earth,” Dr. Curt Niebur noted in the September 8, 2014 JHUAPL Press Release. Dr. Niebur is Outer Planets program scientist at NASA Headquarters in Washington.
Giant Geysers Gone Missing
In December 2013, planetary scientists using NASA’s Hubble Space Telescope (HST) announced that they had detected signs of giant geysers erupting into space from Europa’s south polar region. The discovery triggered much excitement among planetary scientists, as it implied that a robotic flyby probe could possibly sample Europa’s subsurface ocean of liquid water without ever having to touch down on its icy surface.
Alas, follow-up HST observations conducted in January and February of 2014 revealed no signs of the blasting geyser plumes, which were estimated to extend approximately 125 miles into space.
The mystery of the missing geysers presents, of course, an intriguing puzzle. However, there are several explanations that may account for the strange case of the missing plumes. For example, Europa’s geysers may be sporadic, erupting more in the manner of volcanoes on Earth than the plumes blasting off constantly from the south pole of one of the ringed-planet Saturn’s moons–icy and fascinating Enceladus, which also contains a subsurface liquid water ocean like Europa. Another possibility is that Europa’s plumes may only be visible to HST’s instrument at special times.
“It could be just the way that we use the auroral emissions coming from those plumes at the UV [ultraviolet] wavelengths of light that we use with Hubble,” discovery team member Dr. Kurt Retherford noted in the September 5, 2014 Space.com. Dr. Retherford is of the Southwest Research Institute in San Antonio, Texas.
“These things depend on Jupiter’s plasma environment. Maybe there were just a lot of particles, atoms, getting excited by electrons and ions in Europa’s atmosphere, more so than at other times, and [they] just lit up the plumes more than they usually do,” he added.
Another possibility is that the plumes may just be too small to detect, according to Dr. Retherford. However, there is also the nagging possibility that the plumes really do not exist at all. The discovery of the plumes by HST, based primarily on observations that the space telescope made back in December 2012, may be just an artifact or misinterpretation of some sort.
Dr. Retherford and his team are going to hunt for the missing plumes again soon. They will turn HST’s observing eyes on Europa from November 2014 through April 2015, in a more comprehensive effort to confirm the existence of the water-vapor spitting geysers and to characterize their behavior.
“The question is the variability aspect of the plumes. Why do we see them in some observation sets and not others?” Dr. Retherford noted in the September 5, 2014 Space.com.