Fountains of Enceladus
by Peter Jekel
There is one know not what sweet mystery about this sea whose gently awful stirrings seem to speak of some hidden soul beneath.
When William Herschel turned his new telescope toward the heavens in 1789, he saw something that he had seen before, but now it took on new meaning; the same small non-descript dot first observed two years previously turned out to be a moon of Saturn. He dubbed the new moon, Enceladus, named after a Titan giant of Greek mythology, on a suggestion by his son, John Herschel in his publication, Results of Astronomical Observations at the Cape of Good Hope.
The moon continued to be a point of light in the sky for almost two hundred years with astronomers able to describe its orbital mechanics, mass, density and albedo, but little else. Note that Enceladus has the highest albedo (the proportion of the incident light or radiation reflected by a surface, typically that of a planet or moon) in the solar system at greater than 0.9, which is explainable if the surface is made up of fresh clean water ice. It would take spacecraft flying in the vicinity of the moon years later to verify this. Confirmation of the albedo reading came with the Voyager spacecraft launched in 1977. Voyager 1 passed within 202,000 kilometers of its surface and revealed a highly reflective surface of water ice with a marked absence of impact craters. It also appeared to be embedded within the Saturn rings, specifically, the densest part of Ring E.
On August 26, 1981, almost two hundred years to the day of Herschel’s initial discovery on August 28, 1789, Voyager 2 passed within 87,010 kilometers of the surface of the moon. The visuals from the spacecraft confirmed the findings of the predecessor Voyager craft, but showed some more details of the surface. It was found to be relatively heavily cratered in the far north to a more gradual cratering near the equator to the smooth plains in the south; the craft, however, did not see what would later be revealed to be the most interesting thing, though, the south polar region. The appearance of the moon was in sharp contrast to its neighbour moon, Mimas, which is heavily cratered throughout. At a diameter of only five hundred kilometers, about half the size of earth’s moon, Enceladus would remain an enigma, since it appeared too small to generate the heat necessary to erase the violence of being a member of the solar system.
In addition to answering numerous questions about the moon and raising even more, the Voyagers performed perhaps their important task. They helped to whet the appetite of NASA, the European Space Agency and the Italian Space Agency to launch Cassini in 1997, probably the most sophisticated robotic craft ever launched. After the teasing findings of the Voyagers, Enceladus was certainly on the agenda for Cassini to explore. It would take almost seven years, though, before Cassini’s potential was fully realized, when it pulled into the Saturnian system.
With Cassini’s sophisticated instrumentation, Enceladus was giving up her secrets, secrets which were nothing short of astounding. It was found to be not only geologically active, one of only three moons in the solar system known to be volcanic, which include the much larger Jovian moon, Io and Triton in Neptune’s system, but Enceladus was also blessed with organic compounds, precursors of life, and possibly underground caverns of liquid water or even an ocean. All of the key ingredients of life as we know it were to be found on this tiny moon: an energy source, organics and water. The small dot that Herschel took two years to realize what he had seen has now moved to the forefront of the fledgling science of astrobiology.
Science fiction, which oftentimes appears ahead of science with their speculations, was caught unawares by the findings of Cassini with respect to Enceladus. There is not a lot of fictional literature out there about Enceladus unlike its sister moon, Titan, for example. Most novels or stories about Enceladus have used the moon as a backdrop to their stories, not as a central theme. Most of the stories are also set prior to Cassini’s amazing discoveries too. Some of the novels include Grant Callin’s Saturnalia (1986) and Charles Pellegrino’s Dust (1999). Paul McAuley’s The Quiet War is a space opera that was published in 2009 and so has more scientific data to draw on in its description of Enceladus; the story takes place on multiple worlds of the outer solar system with Enceladus being just one of them.
A Buck Rogers (1932-1947) radio play featured Enceladus as a background to a story, being depicted with an atmosphere and a humanoid lifeform. Perhaps the most interesting speculation with respect to Enceladus is a television program by History Channel’s Life After People series. In one episode, the Cassini spacecraft, with no human intervention in her mission, crashes to the surface of Enceladus where bacteria on the craft begin to inhabit their new home, and to evolve into truly extraterrestrial life.
This lack of fictional literature is in sharp contrast to the fiction set on Europa, a similar world to Enceladus in many ways, but in many ways different as well. Arthur C. Clarke, in 2010: Odyssey Two and 2061: Odyssey Three, both speculated on life on Europa that was evolutionarily kickstarted by alien beings. Other stories that have speculated about life on Europa include Europa Strike by Ian Douglas, Ilium by Dan Simmons, and again The Quiet War by Paul McAuley.
Enceladus is truly untapped territory for a science fiction writer even though it has many advantages over Europa in terms of potential for life. Enceladus, for example, does not experience the intense radiation of Jupiter. This not only makes the likelihood of life on Enceladus more likely, but the environment far less stressful on would-be explorers and their instrumentation. Unlike Enceladus, Europa also does not have active vents on its icy surface, thus making sampling of underground liquid water all that much more difficult to do.
So here we have a world that has all of the necessary ingredients of life as we know it and most interesting of all, that may be the most accessible extraterrestrial world with the potential to harbour some form of life. How is that possible on a world as tiny and seemingly insignificant as Enceladus?
First it has an energy source that is enough to make it volcanically active. In January 2005, cameras aboard the Cassini spacecraft took a picture of what appeared to be a flare in the south polar region of the moon. There was hesitancy amongst the Cassini scientists to release the photograph at first because sometimes such flares are often artifacts of the cameras in the clarity of outer space. The thought that an announcement of an active flare that would later prove to be nothing more than a camera artifact was foremost in the minds of the Cassini scientists.
Fortunately, the sophisticated instrumentation aboard Cassini did not disappoint. In February and March, 2005, during a couple of flybys along the equator and slightly above, the careful Cassini scientists were closer to verification that there were indeed plumes being ejected from Enceladus through analysis of data from the magnetometer. However, the cautious team continued their silence. There had to be more evidence.
In July 2005, the team moved Cassini into a near flyby to 175 kilometers above the moon’s surface that took the spacecraft under the moon so that it could see the heretofore unexplored south polar region. The terrain was roughly circular in shape, had no impact craters at all, and was etched by deep parallel cracks now known as “tiger stripes,” because of their resemblance to the big cats’ camouflage. These “tiger stripes” run about 130 kilometers and are evenly spaced terminating in hook-like endings. In between the stripes, the landscape is a bright plain that is marked by fine grooves. The entire region is surrounded by concentric mountains and valleys.
During the flyover, a dust analyzer detected tiny particles coming from the “tiger stripes” while other instruments detected water vapour along with carbon dioxide, nitrogen and methane. A thermal infrared imager aboard Cassini went even further and found that temperatures along the fractures were as high as -93 degrees Celsius, far higher than would be expected if solar heating was the only source of heat. In November of 2005, the team concluded that they were looking at a plume that stretched several hundred kilometers above the moon’s south pole. There were indeed fountains on Enceladus. Further analysis showed that most of the shower of particles would fall back to the moon’s surface, but many also fed the E ring of Saturn; it is estimated that about ten percent of the material that is ejected from Enceladus makes its way into the E-ring, with the rest falling back to the moon,
During another flyover in March of 2006, when the spacecraft was taken to within 25 kilometers of the surface, the instruments captured evidence of water vapour, nitrogen, carbon dioxide, methane and more complex organic molecules including acetylene, ethane, benzene, formaldehyde, propane and hydrogen cyanide as well as other organic chemicals in the plumes of Enceladus.
With all of the intriguing findings, scientists were still looking for the source of heat for the tiny world. One set of research presented at the annual meeting of the European Geosciences Union suggested that Enceladus may have a core of molten rock that reached temperatures of over one thousand degrees Celsius.
There are other factors which could be contributing energy to Enceladuas to explain its activity. The “tiger stripes” of the south polar region are aligned at 45 degrees to Saturn in keeping with the orientation of tidal forces. Ice slabs move along the cracks against one another generate heat through friction. Such a form of heating need not be distributed evenly over the surface either, which is in keeping with Enceladus' activity in the south and relative inactivity in the north. Tidal forces, however, over time should reach an equilibrium but the Enceladus system shows no “off switch.” To explain this one has to look outside of the moon itself to its relationship to its neighbour, the moon Dione. Dione forces Enceladus into a continual elliptical orbit rather than settling into a circular orbit as would be expected over time due to a 2:1 orbital resonance; Enceladus completes two orbits of Saturn for every one of Dione’s, thereby continuing to supply energy to Enceladus through gravitational boosts.
The energy is still not enough to explain the observations, though. There has to be another source. One model suggests that there is a zone of weakness at the south pole that focuses the tidal energy over time. The model’s success is dependent on the material that lies below. It is encouraging when it comes to the possibility of life. In the model, ice in the zone must be warm, almost at the melting point, and there must be a liquid layer between the overlying ice and a rocky core. The idea of subterranean water reservoirs or a subsurface sea is encouraging to astrobiologists. It also explains what is being observed from a geological perspective in the south polar region of Enceladus.
A warmer subsurface would also explain the spreading centre of the south polar region that is being observed. Ice would melt along the deep cracks and the meltwater would further enhance the heating rate. A self-sustaining subterranean water reservoir is born. Liquid water would also account for the eruptions on Enceladus. A model has been developed that shows that the partial freezing of an underground reservoir of water would increase its pressure and force liquid up which would explain the plumes.
Further evidence of underground water comes from the fact that sodium has been detected in the plumes as was found during one of Cassini’s flybys. When a mass of water is in constant contact with surrounding rocks, it will naturally dissolve some of the minerals in those rocks--water is known as the universal solvent. The question has been raised as to why no salt has ever been detected from earthbound telescopes since sodium is easily detected from telescopes on earth. Perhaps the fact that Cassini only found that sodium made up less than two percent of the mass of the plumes may be a factor. Another possible cause of the sodium anomaly is the possibility that the salt is bound to the water ice, further hiding its presence. Whatever the reasoning, there apparently is salt present in the plumes of Enceladus.
To further add weight to the idea of a liquid subterranean water reservoir under the ice, in the south polar region of Enceladus ammonia was found by a Cassini flyby in July and October 2008. Ammonia was found by the craft’s Ion and Neutral Mass Spectrometer. Ammonia mixed with water can act as an antifreeze. Mixed with ammonia, water can stay liquid at temperatures as low as -143 degrees Celsius. With temperatures near the fractures being higher than that, the possibility of liquid water is augmented. Whether the water reservoirs are pockets beneath the icy crust of Enceladus or an ocean surrounding the rocky core of Enceladus is still up for debate and will need further study. One thing that astronomers and geologists generally agree to is that the plumes are not like the geysers found in Yellowstone National Park, but more like steady water and ice jets being fed from underground water reservoirs.
More answers to questions will come as the Cassini craft continues its work in the Saturnian system revealing more mysteries about the beautiful planet and its retinue of moons. It must be frustrating, however, to scientists involved with the project, to have come so close to finding life on another world only to be limited by the technology. As truly remarkable as the technology of Cassini is, the final answer as to whether Enceladus is an abode of life would be found if we send a craft to the world and sample the subterranean caverns of liquid water that are speculated to be there. Maybe one day we will glimpse the world beneath Enceladus. As with its Voyager predecessors, Cassini has whetted the appetites of planetary scientists and astrobiologists. All we can do now is wait.