Brandon Morris - The Enceladus Mission - Hard Science Fiction

We also do not yet know how thick the ice layer is. Models resulted in a thickness of 50 to 80 kilometers. Somewhere in the ice or below it, as measurements of the orbital movements of Enceladus have indicated, there must be a liquid layer. Enceladus ‘wobbles’ a bit on its path, like a spinning raw egg. The moon therefore can be compared to a husked coconut with the addition of a large core—a hard, thick shell and, below it, a more or less nutritious liquid, and finally an even harder, indigestible core.

The ocean under the ice may extend only below the South Pole (up to 50 or 60 degrees southern latitude), or around the entire moon. The first model seems to be the most likely one to most researchers. Then the ice crust would be 30 to 40 kilometers thick, but significantly thinner near the South Pole. French scientists have calculated that it might be only five kilometers thick at the pole.

The ocean itself might have a depth of about ten kilometers, and at the bottom the pressure would reach between 28 and 45 bars. That corresponds to the water pressure one would experience on Earth at a depth of 300 to 400 meters. Other models assume a water depth of 30 to 40 kilometers. For comparison, the average ocean depth on Earth is 3.7 kilometers.

There is no doubt about the existence of the Tiger Stripes. In the roughly one kilometer deep by nine meters wide Baghdad Sulcus, the Cassini probe measured a temperature of minus 75 degrees Celsius. That is not actually warm enough for liquid water to exist. It is assumed, therefore, that the surface is covered by fresh, cold snow which lowers the measured temperature.

Water jets constantly shoot up out of the Tiger Stripes, and through this process Enceladus loses 150 to 200 kilograms of water per second. In its existence, it must have lost up to a fifth of its mass and at least three-quarters of its original water content.

Infrared measurements near the South Pole showed this area to be considerably warmer than its surroundings. At this distance from the sun, minus 200 degrees Celsius should be expected, but the average temperature is 15 degrees warmer. That does not sound like much, but it means a heat output of 4.7 gigawatts is emitted. That is twice the output of the power stations at the Hoover Dam.

Where does that heat come from? Currently, there is no definitive explanation as to how the necessary heat is generated. It is probably a combination of several factors. First of all, Enceladus is under the influence of mighty Saturn. This moon is not completely homogenous (of a uniform structure), so that the gravitational pull of the planet acts with different force on different areas, strongly massaging Enceladus, as it were. This causes friction, and friction generates heat. However, this so-called tidal heat would not suffice to keep the ocean liquid, even considering that the ice crust acts as an insulating layer.

Besides physical forces, chemical ones could be another important factor. At the interface between ocean and rocky core, saltwater meets stone. This causes a reaction called serpentinization. The water reacts with the silicates, giving off energy. Per reaction quantity of 1 mol, enough heat is generated to melt 11 mol of water ice. During its history, this could have led to a chain reaction. It would have been enough if water reacted with silicates at one location. Then this reaction could have spread all over Enceladus. The composition of the water vapor jets from the cryovolcanoes on Enceladus suggests this must have happened at some time.

Finally, a certain percent of the heat could also come from the decay of long-lived radioactive substances in the core.

Enceladus was probably born at the same time as Saturn. At a distance of 9.5 astronomical units from the sun, the protoplanetary nebula cooled off more quickly than in the inner solar system, near the hot primal sun, where water more likely existed in liquid form or water vapor. Furthermore, the lighter elements predominated here—hence the creation of gas planets rather than rocky planets.

Once the temperature had fallen enough, first the firmer and then the more volatile compounds condensed down to water vapor, which froze into ice crystals. When particles met, they merged into larger clumps, which in turn combined into even bigger pieces. This finally created planetesimals, or minute planets, which were still undifferentiated. This means they had neither core nor crust, and that rock and ice were still randomly mixed.

At the very beginning, these pieces still contained a larger quantity of radioactive nuclides. These heated the interior of the future moon, which then had a diameter of 600 kilometers, instead of its present-day 500, and they baked the individual pieces more firmly together. The ice warmed up so that Enceladus could contract with the help of its own gravity, like pulling a coat more tightly around itself. Back then, the moon must have shrunk by about 20 kilometers. At some point, the interior temperature must have risen so much that the still widely-dispersed ice began to melt, and the hidden ocean came into being. The first serpentinization reactions started. This changed the properties of the silicates in such a way that the remaining water was pressed outward, where it froze again. When the core temperature finally reached 450 degrees, the reverse reaction to serpentinization set in.

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 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?