Nevertheless, such a hike would be exhausting, and just because you weigh less does not mean you can quickly accelerate to a high speed. Your so-called inertial mass plays an important role in this, and that is not different from what you would have on Earth.
Coconut with Core
Relatively early, astronomers realized Enceladus could not be a pure ice moon. Considering its size, it is too heavy for that. At a density of 1.61 grams per cubic centimeter—water weighs only one gram per cubic centimeter—it is third among the Saturn moons in this aspect. Inside it, there must be a dense rocky core. Earth’s moon, for comparison, has a density of 3.3 grams per cubic centimeter, but water ice is relatively rare there.
Yet compared to the ‘blue planet’ Earth, Enceladus has quite a bit of water. If all the water on Earth were formed into a ball, it would have a diameter of 1,384 kilometers (Earth’s diameter: 12,740 kilometers). If all the ice on Enceladus were formed into a ball, it would be almost 400 kilometers in diameter (and the total diameter of Enceladus is 504 kilometers). To put it differently, billions of years ago, when Earth—which then was dry—received its water, several bodies the size of Enceladus must have crashed into it.
The rocky core of Enceladus probably accounts for half of its mass, and has a diameter of 300 to 340 kilometers. It probably consists of materials rich in silicon (silicates), similar to the crust and mantle of Earth.
Scientists cannot agree on how high the percentage of short- and long-lived radioactive substances was and is. Their decay offers a mechanism that allows a celestial body to create heat long after it came into being. It was assumed earlier that on Earth this radioactivity was the precondition for all life. Actually, though, the heat of Earth’s core is a remainder from the early period of the solar system. The core not only releases heat to the mantle, but additional energy is released when previously liquid material crystalizes—heat of crystallization. A compression of material sets in which releases additional energy as the gradually solidifying inner core slowly shrinks.
The rocky core of Enceladus does not play the same role, but heat rising from it may lead to a melting of ice.
The Realm of Water and Ice
Above the rocky core comes the realm of water and ice. Ice is not always the same, because it possesses various phases that differ in their physical properties. It is not exactly known which phases occur on Enceladus. The decisive factors are pressure and temperature, but the admixture of other substances can also change the properties of the ice. For instance, traces of ammonia would lower the freezing point—water in one place could be liquid even though elsewhere it would have frozen. However, such traces have yet to be found on Enceladus. It is likely that the majority of its ice layer consists of ‘normal’ ice as we know it from Earth; this is Ice I.
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.
Hot Stripes
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.
The Birth of the Moon
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.