What lies under the ice, deep in Europa’s alien waters? This has been one of the most burning questions among planetary scientists and space enthusiasts alike, ever since NASA’s Galileo mission helped to establish during the late 1990s that Jupiter’s intriguing icy moon hosts a vast underground ocean of liquid water. Even though a definitive answer to this question is still several decades away, waiting for a future dedicated lander that will finally scoop up and study Europa’s icy sediments up-close, scientists have nevertheless made great progress in their understanding of the distant moon through the years by utilising some of the most advanced ground-based telescopes in operation today. A new study from a team of planetary scientists in the U.S. offers more credence to this hypothesis by showing that the moon’s famous surface “chaos terrain,” where the underground liquid ocean is thought to come into contact with the icy surface, features a distinct chemical composition that is unique on the entire moon, suggesting that it is composed of underground material that has risen to the surface. These regions on Europa offer a unique opportunity to study its internal makeup and can be considered as prime targets for a future robotic lander mission.
With a diameter slightly smaller than that of the Earth’s Moon, Europa is believed to have a layered internal structure, much like the Solar System’s terrestrial planets. A long list of evidence have shown that above the layers of an iron core and a rocky mantle lays a vast ocean that covers the entire moon, an ocean composed entirely of salty water ice. The images that were beamed back by NASA’s Voyager 1 and Galileo missions, which flew by the Jupiter system in the late 1970s and 1990s respectively, revealed that Europa had a very smooth and young surface that was mostly free of craters and impact features. Most importantly, these close-up images showed that the distant moon’s surface exhibits some very interesting characteristics that have intrigued planetary scientists ever since: It is crisscrossed by a vast network of chaotic red-colored ridges which span the entire globe, resembling more than anything else the fractured sea ice that is seen on Earth’s arctic regions. It is believed that these ridges are cracks on Europa’s icy surface, caused be the intense tidal forces that are exerted from the massive nearby Jupiter and the orbital resonances with the other Galilean moons, Io, Ganymede, and Callisto.
These fascinating geologic features have made Europa one of the prime targets in the search for extraterrestrial life in our own Solar System, alongside Mars and Saturn’s moons Titan and Enceladus. Yet, contrary to the latter’s infamous south polar geysers which spew water vapor and organic compounds from the moon’s interior to heights of hundreds of km above its surface, Europa’s liquid ocean is instead firmly trapped under an 100-km-thick layer of ice. The exploration of Europa’s underground ocean with a dedicated robotic submarine mission that would drill through all of this surface ice has been in the wish list of every planetary scientist and space enthusiast for decades, but such a prospect seems far removed into the future, at least for now. So where does that leaves us in terms of further studying Europa’s potentially habitable environment? In the absence of in-situ images and measurements from a dedicated orbital mission to this fascinating faraway moon, scientists have to rely on ground- and space-based telescopes in order to glean as much data as possible from a distance of half a billion km away.
In their latest research, a team of astronomers comprised of Patrick Fischer, Mike Brown, and Kevin Hand from NASA’s Jet Propulsion Laboratory in California, who have been studying the icy worlds of the outer Solar System for more than a decade, conducted a detailed analysis of spectroscopic observations of Europa that had been made with the OSIRIS near-infrared spectrograph on the 10-m Keck II telescope in Hawaii, in the hopes of better identifying the mineralogical composition of the moon’s interior. “We have known for a long time that Europa’s fresh icy surface, which is covered with cracks and ridges and transform faults, is the external signature of a vast internal salty ocean,” says Brown.
The team analysed a total of 1,600 spectroscopic observations of Europa that were taken with OSIRIS back in September 2011, in order to match the different spectra to specific locations on the moon’s surface as accurately as possible. To that end, the researchers employed a novel algorithm which allowed them to group the different spectra together according to the different absorption lines that they contained, which corresponded to different chemical elements on the surface of Europa. They then correlated these spectral observations to a topographic map of the icy moon that had been obtained by the Galileo mission in the 1990s. The results of their study revealed a set of three distinct groups of mineralogical compositions on Europa, one of which had never been observed before. More specifically, the first group was located on Europa’s trailing hemisphere which faces away from Jupiter and was dominated by water ice, as would be expected since the latter is the dominant compound on the moon’s surface. The second group was located at the polar regions of the leading hemisphere which faces toward Jupiter and was found to be rich in oxygen and ionised sulfur—again an expected finding, since the source of the latter is mainly the violent volcanic eruptions of neighboring Io. Yet, what caught the researchers’ eye was the third group, which not only differed greatly from the other two in chemical composition, but it also lacked elements like magnesium and other chemical compounds like hydrated sulfate minerals that were expected to be present in great abundances on Europa. Furthermore, the third group was found to overlay Europa’s trademark “chaos terrain” features, where subsurface material is thought to constantly come into contact with the moon’s surface terrain.
“The geographic distribution of each component is a strong indication of its identity,” write the researchers in their study, which has been accepted for publication at The Astronomical Journal. “The distribution of component 1 is globally symmetric, centered on the trailing hemisphere equator. This is consistent with the known patterns of enhanced irradiation and bombardment from the Jovian magnetosphere. This strongly suggests that component 1 is exogenous, or initially endogenous but heavily altered by exogenous processes. Component 2 dominates the leading hemisphere polar regions, consistent with the known distribution of pure water ice or deposited water ice frost. Unlike components 1 and 2, the distribution of component 3 is not globally symmetric, nor is it consistent with regions known to be compositionally distinct. The shapes of the regions dominated by component 3 are revealing; they are markedly similar to geologic units of chaos [terrain features]…Our interpretation is that component 3 contains the greatest fraction of salts derived from the subsurface, possibly a brine or evaporite deposit.”
“I was looking at the maps of the third grouping of spectra, and I noticed that it generally matched the chaos regions mapped with images from Galileo,” says Fischer. “It was a stunning moment. The most important result of this research was understanding that these materials are native to Europa, because they are clearly related to areas with recent geological activity.”
Even though it was determined that the third group lacked the expected spectral signatures of hydrated sulfate minerals, their mineralogical composition nevertheless remains unknown for the time being. “Unique identification has been difficult,” comments Brown. “We think we might be looking at salts left over after a large amount of ocean water flowed out onto the surface and then evaporated away. They may be like the large salt flats in the desert regions of the world, in which the chemical composition of the salt reflects whatever materials were dissolved in the water before it evaporated.”
Determining the mineralogical composition of Europa’s “chaos terrain” is crucial to gaining a better understanding of the chemical composition and the geophysical processes that take place on the underground liquid water ocean itself. This in turn could help to shed much light on Europa’s potentially habitable environment that may be hiding under the ice. Such an understanding is most likely to come from a future robotic surface sampling mission, if and whenever that finally becomes a reality. Even though the such a prospect represents major fiscal and engineering challenges at present, the rewards would far outweigh the expenditure. “If you had to suggest an area on Europa where ocean water had recently melted through and dumped its chemicals on the surface, this would be it,” says Brown. “If we can someday sample and catalog the chemistry found there, we may learn something of what’s happening on the ocean floor of Europa and maybe even find organic compounds, and that would be very exciting.”
Indeed, it’s difficult to think of a better investment of financial resources than that of answering some of the greatest questions of astrobiology through space exploration.