Before Voyager 1 flew past Enceladus in November 1980, it was nothing more than a star-like point of light shining feebly around magnitude +11.7. Not much could be gleaned from that spark, so it was generally thought that Enceladus would look like nothing more than a cratered moon. In the frozen depths of the Solar System the assumption was that the moons of the outer planets were too small to retain their heat from formation and would freeze fast with no geological activity. Basically the moons would all be boring except for possibly Titan and Triton. Things began to change when Voyager 1 flew past Jupiter in March 1979 and discovered its moon Io, the most volcanically active world known. When it comes to Solar System exploration, it is not wise to make assumptions, as truth can often be more bizarre than the imagination.
The Voyager missions made many discoveries. Saturn proved exciting with Titan even more so, but its featureless, smog-shrouded globe was a big disappointment. Another moon, Enceladus, ended up looking like a strange world that defied explanation and sort of stole the show as scientists pondered over its strange appearance. Voyager 1 revealed it to be a moon that is unusually smooth and bright. Enceladus reflects nearly 90% of the sunlight that falls on it. This is better than pure snow and meant that Enceladus was covered with fresh crystals from an unknown process. At the time, scientists speculated that Enceladus might have geysers that were active in recent geological history, or presently. Both Voyagers 1 & 2 had state of the art cameras for that time, but when they flew by, neither detected any plumes or vapors emanating from Enceladus or anywhere along its orbit. Even though Enceladus is only 311 miles in diameter, it was thought that the strong gravitational tug between Saturn and the other moons, especially Titan, could create tides and internal heating. The recent discovery of active volcanoes at Io reinforced the theory. Photos of Enceladus showed evidence of a geological past of soft or slushy ice flowing across the surface, which later solidified. There were very few craters on those ice flows that indicated that that area of Enceladus was created in recent geological times. Large grooves rake the southern polar region and many craters are truncated where flowing ice partially obliterated them. A huge part of Enceladus, especially the southern hemisphere is covered with young ice flows and bright, icy deposits. Enceladus, Titan, Saturn, its rings, and the other moons were so fascinating and left so many unanswered questions that an orbiter mission to Saturn was proposed and approved. In spite of the expected over $1 billion cost, it was important to return to Saturn for several years in order to make crucial observations and measurements.
In the end, the Cassini orbiter mission, born in the late 1980’s and launched on October 15, 1997 on an adventure of exploration, cost $3.3 billion. It arrived at Saturn on June 30, 2004 and deployed the Huygens probe, which landed on Titan on January 14, 2005 for a historic first. Cassini bristled with the most sophisticated cameras and detectors; there would be no way any geyser would escape detection. If there were really geysers, they would be found. Indeed it was not long after the Titan landing that Cassini flew past Enceladus and looked back to make one of the most important discoveries of its mission: active geysers! They truly did exist and they were huge and plentiful! Several jets of water ice particles around 100 miles high were seen spewing from the south polar region. The sources of the geysers were along a series of fissures that cut deep into Enceladus, which are now known as “Tiger Stripes” for their appearance. Cassini was able to map the temperature of the fissures on later flybys and found that they were about -120ºF, much warmer than the overall -330ºF surface; therefore Enceladus has a warm interior as theorized. Cassini flew as close as 12 miles from the surface in March 2008! It flew through the plumes without harm and took some excellent measurements of the vapor and particles. It turns out that these plumes are pure water ice that is not tainted with ammonia or methane. The water ice contains nitrogen, carbon, and organic compounds, all of which are found in Earth’s water and essential to life. Enceladus may hold a vast reservoir or ocean of liquid water beneath its icy crust.
Europa was always fascinating because of the possibility of an ocean of water beneath the crust. The challenge is getting access to the water since the icy crust is quite thick, which makes Europa orbiter and lander missions risky and expensive. Europa also has the added problem that it orbits within the dangerous radiation belts of Jupiter. By contrast, Enceladus allows scientists easy access to its water, which Cassini already discovered by flying through the plumes. Thus, there has been a push for a return mission to Saturn that would focus on Enceladus. Several missions have been drafted with one creating a spacecraft that would go into orbit around Enceladus. This is not as difficult as it sounds since it would use the gravity of the various moons to ease itself into orbit. One plan is for a launch on January 28, 2023 that would arrive at Saturn on July 29, 2031. It would then make around a dozen flybys of Titan to shrink its orbit enough where the next inner moon, Rhea would take over. Then after about a dozen orbits the next inner moon Dione would take over for about another dozen orbits and finally the spacecraft would swing past the next inner moon Tethys several times to finally put it into orbit around Enceladus for at least six months to maybe a year.
The trajectory sounds torturous, but is necessary because going directly into orbit around Enceladus from Earth would require far too much fuel and presently there is no way to launch such a heavy spacecraft. An added bonus would be a few years of opportunity to study the close encounters of many of the moons. Upon reaching orbit around Enceladus, the spacecraft would directly sample and analyze the water, determine how much water is emitted, how powerful and warm the geysers are, determine the gravity and geology of the fissures and how the tidal heating works. Since Enceladus is so small and orbits so close to Saturn, the orbiter will encounter difficulty maintaining a stable orbit around Enceladus and would be required to use its rockets to make periodic corrections. Eventually it would run out of fuel and be crash into Enceladus far away from the fissures to avoid the slightest risk of contaminating the fissures with any earthly microbes that may have survived. The mission would end in 2036 and would cost about $1.6 billion, actually half the cost of Cassini. Another version of the mission has a small probe landing near a fissure, but such a mission is risky and costly, possibly over $2 billion, because the lander must be sterilized of any earthly microbes. A sample return mission where a spacecraft would fly through a plume and capture some particles using sticky areogel is also under consideration, but costly because sterilization is required.
Enceladus has earned enough respect where it will be among one of the prime targets for future, advanced missions. However, its distance of nearly one billion miles and the long flight times involved will leave Cassini as the only source of Enceladus data for decades to come.