With the recent exciting discoveries by NASA’s Curiosity rover of methane and other more complex organic compounds on Mars, the Red Planet has once again entered the spotlight as one of the best places in the Solar System where life could have taken hold and possibly even be present today. Nevertheless, Mars isn’t alone in the list of astrobiologically interesting environments in our corner of the galaxy, with Jupiter’s moon Europa equally capturing the attention and imagination of the scientific community and the general public alike, with its potential for harboring a habitable environment. Further evidence for the latter were presented last year, when scientists had announced that the Hubble Space Telescope had observed what were thought to be water plumes erupting off the moon’s surface, indicating that Europa might exhibit a geophysically active environment. However exciting these findings may have been, they seem to contradict the results of a new study which has found no signs of Europa’s alleged plumes, while also casting doubt on the extent of the moon’s present geophysical activity.
With a diameter of 3,120 km, Europa is one of the largest moons in the Solar System. Even though it is located five times farther from the Sun than Earth, Europa has long been regarded as one of the best places to harbor life. A series of compelling and tantalising evidence have shown that above the moon’s rocky mantle lays a global underground ocean, which is probably composed entirely of salty water ice. The upper layers of this ocean comprise Europa’s surface which remains frozen solid, while exhibiting temperatures that never rise above a frigid -160 degrees Celsius.
Observations with the Hubble Space Telescope during the 1990s had also revealed the presence of an extremely tenuous atmosphere that had a surface pressure of approximately one hundred billionth that of Earth’s and consisted for the most part of molecular oxygen. Scientists had long thought that the origin of this faint atmosphere is the moon’s surface ice layer. Europa, along with the other Galilean moons Io, Ganymede, and Callisto, is located inside Jupiter’s vast magnetosphere which traps and accelerates charged particles along its magnetic field lines to very high speeds, creating radiation belts of very high intensity, while producing auroras on the massive planet and its Galilean moons, including Europa. As the latter completes an orbit around Jupiter every 3.5 days, it sweeps across this intense magnetic field dragging its magnetic field lines along while causing the moon’s surface to be constantly bombarded with the charged particles that are trapped inside the planet’s radiation belts. This in turn causes the water ice molecules on the surface to be broken down through photolysis into their chemical constituents, releasing hydrogen and oxygen ions. The former being much lighter eventually escape into space, leaving the latter to form a tenuous atmospheric layer above the surface.
Additional observations with Hubble had also revealed an excess of oxygen and hydrogen ions in ultraviolet wavelengths, which seemed to be concentrated over the moon’s south polar regions. Further analysis led scientists to conclude that these chemical fingerprints were indicative of water plumes that were venting water ice from the moon’s interior out into space, which was subsequently broken apart by the intense radiation around Jupiter, creating the observed hydrogen and oxygen ion signatures. If confirmed, the phenomenon would be similar to what had already been observed on Enceladus, one of Saturn’s icy moons. NASA’s Cassini spacecraft, which has been exploring the Saturn system since 2004, directly imaged a series of fountain-like eruptions on Enceladus’ surface that were venting water ice and various organic compounds high into space, while constantly replenishing Saturn’s faint E ring with fresh material. Now a new study, which was recently published at The Astrophysical Journal by a research team led by Dr. Donald Shemansky, co-investigator of Cassini’s Ultraviolet Imaging Spectrograph Subsystem, or UVIS, at Space Environment Technologies in Pasadena, Calif., comes to draw a somewhat different picture. By examining a set of archival Cassini data that had been collected as the spacecraft flew by Jupiter in 2001 on its way to Saturn, the researchers detected no evidence whatsoever of plume activity on Europa, while also suggesting that previous findings regarding the moon’s atmospheric density and content may have been misinterpreted.
More precisely, Cassini’s findings showed that the plasma environment around Europa was dominated by heavy concentrations of sulfur ions, which in turn originated from the large amounts of sulfur dioxide that the volcanically active nearby moon Io spews into Jupiter’s magnetosphere every second. Furthermore, the amount of oxygen ions that Cassini’s UVIS instrument recorded coming from Europa amounted to no more than 25 percent of the total ion population on the moon’s vicinity—approximately 40 times lower than previous estimates. “Deep exposures using the Cassini UVIS extreme ultraviolet spectrograph have revealed a low-density plasma at the Europa orbit, composed mainly of ions originating in the Io plasma torus,” write the researchers in the study. “This is a predictable finding because the radial electron density distribution measured previously using the Voyager and Galileo missions produced similar densities and temperatures to the values in the present analysis. No neutral atomic emissions were detected, and upper limits from approximate plasma analysis and the emission results indicate a mean neutral density of 0.003 g per cubic centimeter in [Europa’s] plasma sheet. There is an indication that Europa contributes oxygen to the plasma … estimated to be a factor of 40 below [previous theoretical predictions].”
“Our work shows that researchers have been overestimating the density of Europa’s atmosphere by quite a bit,” said Shemansky in a statement, during the presentation of his team’s findings at the American Geophysical Union’s recent fall meeting in San Francisco.
According to Shemansky’s team, these new estimates on the density of Europa’s atmosphere also call the hypothesis of the moon’s active water plumes into question. For instance, if the latter had been actively venting water ice into space during the spacecraft’s flyby of the Jupiter system, then the plasma torus around Europa should have been dominated by oxygen ions instead. Since that wasn’t the case, this indicates that Europa might exhibit far less geophysical activity than previously thought. In addition, a water ice plume activity would also help to lower the overall temperature of the plasma around Europa. Yet, what Cassini observed was the presence of a hot plasma instead. “Given the stochastic remote sensing evidence, mass loading at the Europa orbit is not consistent with a geophysically active body,” conclude the researchers in the study. “The overall impact of the present work on the assessment of the level of geophysical activity is to indicate that the evidence for vents releasing gas into the atmosphere is at best tenuous.”
“We found no evidence for water near Europa, even though we have readily detected it as it erupts in the plumes of Enceladus,” said in a statement during the AGU meeting Dr. Larry Esposito, a professor of Astrophysical and Planetary Sciences at the University of Colorado and Principal Investigator for Cassini’s UVIS instrument.
Nevertheless, the absence of evidence for plume activity on the Cassini data doesn’t necessarily mean the evidence of absence of the plumes per se. One possibility could be that Europa’s plumes exhibit an intermittent activity and were shut off during Cassini’s flyby, or it could be that they weren’t active enough at the time to be detected in the first place. “It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus,” says Dr. Amanda Hendrix, a Senior Scientist at the Planetary Science Institute in Pasadena, Calif., and co-investigator for UVIS. “If eruptive activity was occurring at the time of Cassini’s flyby, it was at a level too low to be detectable by UVIS.”
Whatever the case, the mystery can only be solved by more detailed, in-situ observations from a dedicated Europa mission. “Europa is a complex, amazing world, and understanding it is challenging given the limited observations we have,” says Curt Niebur, Program Scientist for NASA’s New Frontiers program, in Washington, D.C. “Studies like this make the most of the data we have and help guide the kinds of science investigations NASA should pursue in the future.” Such a prospect for a Europa mission, which has long been the desire of both NASA and the planetary science community alike, has made a step forward this year toward eventually becoming a reality. As part of the omnibus appropriations bill for FY2015, which was passed by Congress earlier this month, NASA has been granted among other things an amount of $100 million specifically for the formulation of a Europa mission concept.
Although encouraging, whether this ongoing political support will finally lead to a planetary mission clearing the launch pad on the way to this distant and enchanting moon in the outer Solar System remains to be seen.