Ice Moon 1 The Enceladus Mission

by Brandon Q Morris

  For one orbit around Saturn, Enceladus needs one Earth day, plus an additional eight hours and 53 minutes. It orbits at a velocity of 12.64 kilometers per second. Therefore, Enceladus is twelve times as fast as our moon orbiting Mother Earth. The reason for this is not the laziness of Earth’s moon, but rather the much stronger pull of Saturn on Enceladus. If Enceladus were as slow as our moon, it would have ceased to exist a long time ago. Our moon, on the other hand, would have quickly escaped the vicinity of Earth if it orbited as fast as Enceladus.

  By now you know how slowly Enceladus rotates. As it always shows Saturn the same side, it finishes exactly one rotation during an orbital period. The axis around which Enceladus rotates is exactly perpendicular to its orbital plane. Therefore, Saturn can always be seen at the same location in the sky over Enceladus. The rotational axis of Earth, on the other hand, is tilted toward its orbital plane around the sun—otherwise, Earth would have no seasons.

  White and Cold: The Surface

  There is a simple reason for the fact, as mentioned in the introduction, that Enceladus reflects light so well. The moon is completely covered with ice, perfectly normal ice as we all know it, i.e. water ice. This reflects light even better than freshly fallen snow on Earth. Enceladus is therefore often called an ‘ice moon,’ even though that is not literally true, as we will see in the next section.

  This fact has some practical consequences for anyone traveling to Enceladus. You might know this from clear winter days—light reflected from the surface cannot warm the surface. Not only is this moon already at an enormous distance from the sun (more than 1.4 billion kilometers), but the high albedo (reflectivity) causes it to be even colder on Enceladus than the distance from the sun would make it. On the surface of its sibling, Dione, which has an albedo of 55 percent, i.e. much darker, the temperatures can reach minus 187 degrees Celsius, whereas on Enceladus the ‘warmest’ is only minus 200 degrees.

  Even though the landscape on the surface is completely white, there are some variations. Plains (planitia, plural form planitiae) alternate with ridges 500 to 2,000 meters high (dorsum, plural dorsa), and both can contain trenches (sulcus, plural sulci), depressions (fossa, plural fossae) and cliffs (rupe, plural rupes), or be covered to some extent with craters.

  If you try to find your way around Enceladus with a map, you might get the impression you are in the Arabian Nights. This is because the International Astronomical Union (IAU) decided to use geographical names from this famous work of world literature for this moon—except for the craters, which are named after protagonists from these stories.

  Geologically speaking, the plains on Enceladus are very young. You can see that they—for example, the Sarandib Plain—contain far fewer craters than one would expect on a moon. It is estimated that many of its plains are less than a couple of hundred million years old. The impact craters, as the photos taken by the Cassini probe show, are in various stages of aging. While the climate on Earth makes craters wear away, and on Earth’s moon they are destroyed by new impacts, on Enceladus shifting of the ice erodes the craters.

  The trenches and canyons are witnesses to this activity, and they can be up to 200 kilometers long, 5 to 10 kilometers wide, and up to 1,000 meters deep. They often cut through other geological features, so one must assume that they are relatively new phenomena. On Earth, rocky continental plates collide, but on Enceladus it seems to be ice plates. This sometimes creates cliffs up to 1,000 meters high.

  Chaos in the South

  The area around the South Pole of Enceladus plays a special role. Even in the pictures taken by Voyager 2, scientists discovered a chaotic terrain, a mixture of very different terrains. Since the Cassini probe sent its spectacular images, we know a lot more. The area reaching up to 60 degrees southern latitude (about the position of Tierra del Fuego on Earth) is characterized by fractures and cliffs, but it contains even fewer craters than the rest of the moon. The surface must therefore be far younger. Scientists estimate it to be an average of 500,000 years old. Geologically speaking, that is very young.

  From above, the center of this region looks particularly chaotic. Besides the fractures and plates, there are also giant ice boulders measuring 10 to 100 meters. The area is dominated by four fractures with a depth of up to 300 meters, the so-called Tiger Stripes, each of which is several hundred kilometers long. Pictures show that the ice at their edges has a considerably different composition from the ice on the regular plains. Organic compounds have been found here.

  The Tiger Stripes, which are up to 25 degrees Celsius warmer than their surroundings, form the source of the famous Enceladus Geysers. The entire world saw the photos of them taken by Cassini. Across almost the entire length of the stripes, large amounts of crystallizing water vapor are shot into space at high speeds, between 400 and 1250 meters per second (m/s). Part of it falls back on the moon as snow, part of it replenishes the material of the E Ring. As the escape velocity on Enceladus is below 240 m/s (860 km/h), an outgassing into space is perfectly possible.

  The activity of the geysers changes periodically. It is suspected the Tiger Stripes are squeezed by the gravity of Saturn when the moon approaches the planet, which increases the pressure at which the material is ejected, and reduces its quantity.

  The Cassini probe even managed to fly directly through a geyser plume. Therefore we know these consist primarily of rapidly freezing water vapor, but also include percentages of methane and carbon dioxide, as well as simple-to-more-complex organic molecules. The composition resembles that of a comet. How these compounds could have been created will be explained next.

  A Great View

  Due to its low gravity, Enceladus does not possess a true atmosphere. The disadvantage of this fact is that a spaceship could not use the braking effect of the atmosphere during landing.

  However, near the South Pole, enough of the geyser eruptions remain so that traces of an atmosphere have been detected, comprised of 91 percent water vapor, 4 percent nitrogen, 3.2 percent carbon dioxide, and 1.7 percent methane.

  An astronaut who has just landed on Enceladus’ surface might look up to the sky first. It would be completely black, as the moon has no atmosphere to speak of. No clouds will obscure the sun, which appears at 3.5 minutes of arc, just one ninth of the size we are used to on Earth.

  Saturn can only be seen from the side of the moon that faces the planet. Here it appears in the sky at a height dependent on the geographical latitude of the observer’s current position. Therefore, at the equator, Saturn shines vertically above you, but the closer you get to the poles, the lower the planet sits above the horizon. It is always impressive, though, as its disc with a diameter of 60 degrees is about 120 times the size of Earth’s moon in our night sky.

  Unfortunately, a space tourist would not get a good view of the rings of Saturn. After all, these surround Saturn in the same plane as the moon. Therefore, you are looking directly at their (very narrow) edge and will only see them as a line. Depending on the position of the sun, though, the shadows of the rings may be seen upon the planet.

  If during your visit to Enceladus you experience a moonrise, don’t worry. You are not confused—you just saw the inner moon Minas, which moves past Saturn every 72 hours and has an apparent size in the sky like that of the Earth’s moon. Tethys, on the other hand, appears to be twice as big, though you could only observe this outer moon from the side of Enceladus facing away from Saturn.

  Other of Saturn’s moons appear in the sky as star-like objects, or cannot even be detected with the naked eye.

  Hiking on Enceladus

  Let’s say you are not satisfied with just looking at the moonscape facing Saturn, but want to also explore the other side of the moon. No problem. The low gravity lets you almost float. If you weigh 86 kilograms, you would weigh only 1 kilogram on Enceladus using a spring scale—a beam balance would indicate 86 kilograms, as it compares weights. Even with a heavy spacesuit, this would not add up to more than 2 kilograms.

 
That does not mean you can jump 40 times higher than on your home planet. For one thing, the space suit is cumbersome. On Earth, no one can jump in a spacesuit. On our moon, you could jump to a height of about two meters in a spacesuit, though no human astronaut has attempted so high a jump yet. On Enceladus you could perform a 20-meter jump (even 40 meters without a spacesuit)—though that is not recommended. The issue is one of safety. After all, you return to the ground with the speed you jumped up with. A spacesuit ought to withstand that, but the risk is simply too great.

  A hike on Enceladus is rather like a spacewalk. Outside there is a vacuum—almost. Therefore, the preparations should resemble those of an EVA in space. The fact that there is no atmosphere is actually rather fortunate. At minus 200 degrees Celsius your suit will cool off much faster in an environment filled with some kind of air than by just giving off heat as radiation.

  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.

  Realm of Water and Ice

  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, with a diameter of 300 to 340 kilometers. And 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 coming 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.

  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 sphere with a hard, thick shell and, below it, a more or less nutritious liquid, and within that liquid is 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.

  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