This finally turned the core into what we know it to be now, an arid silicate core surrounded by a thick layer of ice. Between the two, the chemical reaction keeps a layer of liquid water. At the same time, Enceladus continually lost mass this way and shrank to its current diameter of 500 kilometers. The core has been gradually cooling and probably today is minimally warmer than the ocean, and possibly even cooler.
Other ice moons, by the way, have followed a different path. Mimas, for example, is quite large, but does not have a true core. It still resembles the dirty snowball that it was when it came into being. Scientists speculate that at the beginning, this moon contained less rocky material. Therefore, there were not enough radionuclides to heat the interior and to press the ice outward.
The Exploration of Enceladus
The first one to gaze on Enceladus was the British-German astronomer and musician Frederick William Herschel, who in 1789 focused what was then the largest telescope in the world (1.2 meters) on the ringed planet Saturn. The name of the moon was derived from the giant Enkelados (Latin: Enceladus) in Greek mythology. This was actually a mistake, because Enkelados, as one of the giants, never joined with the Titans in their war against the gods (whose leader was Kronos, called Saturn by the Romans). The giants, including Enkelados, rebelled later, after Zeus had locked away the Titans (the sons of the ancestral mother Gaia) in the underworld.
For almost 200 years Enceladus remained an unremarkable if unusually bright spot in the sky. Due to its closeness to Saturn and Saturn’s rings, which were so much brighter, the moon was hard to observe. Voyager 1 was the first object created by humans to pay it a visit. On November 12, 1980, this probe flew past it at a distance of 202,000 kilometers. The pictures taken then already showed that Enceladus had a very young face that showed no deep craters. On August 26, 1981, Voyager 2 came even closer, as close as 87,010 kilometers, and thus provided images with considerably better resolution. These pictures excited scientists. How could such a cold, small moon have such differently formed areas, some of which could only be a few million years old?
The answer finally was provided in 2004 by the Cassini probe, which had been sent by NASA and ESA, and which had already placed a lander on the larger moon Titan. Afterward, Cassini followed an orbit around Saturn that often swung very close to Enceladus. At first, fly-bys at distances down to 1,500 kilometers had been planned, but after it was discovered that water vapor shot into space from the region around the South Pole, planners decided to take an even closer look at this moon. Cassini sometimes came as close as 25 kilometers from the surface of Enceladus, and it was able to send spectacular photos. On October 28, 2015, it even managed to fly through a geyser eruption. During this, the probe analyzed the composition of the ejected material, which was so dense that it measurably slowed Cassini’s rate of travel.
Currently, no space agency on Earth has specific plans for another visit to Enceladus. At the beginning of the 21st century, ESA discussed the concept of a Titan-Enceladus mission named TandEM. This was merged in 2009 with a similar concept at NASA and became the Titan Saturn System Mission (TSSM). For budgetary reasons, TSSM was canceled in favor of EJSM (Europa Jupiter System Mission, now often called Europa Clipper), which was proposed to explore the ice moon Europa orbiting Jupiter, scheduled to launch in the 2020s.
Time will tell whether that was a smart decision—Europa orbits within the radiation belt of Jupiter. While Cassini was able to provide data from Saturn and Enceladus for a decade, the Europa Clipper probably won’t even last a year.
Parallel to that, ESA is considering the mission JUICE (JUpiter ICy moons Explorer). The plan is to have a probe arrive near Jupiter in 2030 and explore the ice moons Ganymede, Callisto, and Europa for three years. They, at minimum, expect to discover an ocean on Europa similar to that on Enceladus, though the ice layer is considerably thicker there.
So far, ELF (Enceladus Life Finder) only exists as a proposal. It should be launched no sooner than 2021 and the goal is to hunt for traces of life, such as amino acids or fatty acids, in the water vapor of the geysers. For this purpose, the probe would make several flights through geyser eruptions. Parallel to that, NASA and JAXA are jointly developing LIFE (Life Investigation For Enceladus), a probe that could take samples from the geysers and bring them back to Earth. Both projects applied in 2015 for one of five spots in NASA’s Discovery Program (in which each mission cannot cost more than 450 million dollars). Yet when the decision was made in the summer of 2016, neither of the two missions was selected.
Yet there is some hope for the future. The lower-priced Discovery missions focus on using only proven technologies. ELF and LIFE would have used solar energy, which is rather optimistic at that distance from the sun. The Europa mission (which also uses solar energy) could now demonstrate that this technology has matured.
Furthermore, NASA announced in early 2016 that it would also accept proposals explicitly for expeditions to Titan and Enceladus for its New Frontiers program. This program (of which the New Horizons probe to Pluto was a part) accepts innovative technologies—and such proposed missions can be more expensive, in the two to three billion dollar range. In 2017, NASA selected a proposed mission to Enceladus as the recipient of technology funds in preparation for future mission competitions. Therefore, it is not improbable that ELF might be revived.
Flying to Enceladus with a Crew?
Currently, humanity lacks the technological capability of sending a manned spaceship to Saturn and its moons—at least if one looks at it realistically. A robot probe can take as much time as it needs and can do without many things humans absolutely need—life support (oxygen, water, food,) gravity, protection against radiation, a return ticket. While it might be possible in principle to build a spaceship for a Saturn mission with today’s technology, it would be very, very expensive.
The Juno probe, which weighed almost four tons and entered an orbit around Jupiter in the fall of 2016, cost about a billion dollars. A manned spaceship would be at least ten times heavier and more complex. After all, it would have to accelerate for the return flight and then decelerate again. Therefore, it does not just need twice as much fuel, but several times as much. Considering all these requirements, it might cost 100 billion dollars or more. Juno, the fastest probe so far, took five years to reach Jupiter—and Saturn is twice as far from us. Therefore, the crew would spend 20 years on this mission.