The New Biography of Enceladus
Introduction
Astronomers noticed relatively late that Enceladus would make a fascinating travel destination. It had been discovered in 1789 by the German-British astronomer William Herschel as the sixth moon of the planet Saturn. At first sight, it acted as would be expected from a moon of its size. It was only the photos taken by the Voyager probes in the 1980s that changed this perception. During its fly-by on August 26th, 1982, Voyager 2 sent spectacular images of the snow-covered surface, the network of craters and of deep fissures in the ice. These also showed that Enceladus was uncommonly bright, as it reflected 99 percent of the incoming sunlight.
The photos spurred imagination among astronomers in several ways. For one thing, the photos showed large plains without any craters, which indicated they must be relatively fresh. Therefore, there must be processes in the interior of the moon that renewed the surface. The high albedo (reflectivity), the highest of any known in our solar system, can also only be explained by a snow-covered surface that is refreshed at regular intervals. But if there was no atmosphere, where would the snow come from?
And then there was the mysterious E Ring of Saturn, which the Allegheny Observatory of the University of Pittsburgh first photographed in 1966. Spectroscopic analysis showed that it consisted mostly of small ice crystals—and there were other aspects of this ring that were exceptional. Enceladus moves around Saturn along the inner edge of the E Ring, exactly where the ring has its greatest density, which quickly made this moon the suspected cause for the unique properties of the E Ring.
On July 14, 2005, the Cassini probe sent by NASA and ESA caught it red-handed. It photographed clouds of frozen water vapor above the surface of Enceladus, which was covered by relatively warm fissures. Two years later, Cassini provided the first photos of the geysers near the South Pole shooting water from the interior of the moon into space. A portion of that water seemed to provide new material for the E Ring, while the rest fell back down on the moon and made it shine in the whitest of whites.
What other secrets does Enceladus hide? Follow me to an icy world, which might harbor unknown forms of life.
The Orbit of Enceladus around Saturn
Enceladus is the sixth largest moon of the ringed planet Saturn, and it was also the sixth Saturn moon to be discovered. Today, a total of 62 Saturn moons are known. Counted from the center of the Saturn system, Enceladus is the fourteenth moon, though in the classical numbering system, it was the second. Therefore, it received the designation Saturn II by the International Astronomical Union. With an average diameter of 505 kilometers—about the distance from New York to Pittsburgh as the crow flies—Enceladus is considerably smaller than Earth’s moon, which has a diameter of 3,475 kilometers, though among all the moons of the solar system, it still ranks as the seventeenth largest. It is the ‘average’ diameter, because the gravitational pull of Saturn flattens this moon slightly, by about three percent. This shape is called an ellipsoid.
Enceladus cannot be observed from Earth with the naked eye. Its apparent brightness is at 11.8 apparent magnitude, and the human eye can see an object—under ideal conditions—up to a value of 6.
Its orbit around the planet is an almost perfect circle. The average orbital radius lies at about 238,000 kilometers. This puts Enceladus very close to its mother planet, which will be important for understanding the processes in its interior. The orbit of Earth’s moon is about 50 percent larger. In addition, Saturn is huge compared to Earth. The distance from Enceladus to the ‘surface’ of the planet is only slightly below 180,000 kilometers, and Saturn has about 95 times the mass of Earth, and therefore 95 times the gravitational pull.
This strong gravity has several effects on Enceladus, including on its orbit. Over millions of years, it caused the moon to always face the planet with the same side, something called ‘captured rotation.’ If you land on the back side of Enceladus you will never see Saturn, while from the planet’s perspective you could only see the ‘front side’ of Enceladus. This is also the relationship between Earth and its moon.
Like almost all of the moons of Saturn, as well as the rings, Enceladus orbits Saturn in a plane parallel to the equator of the planet. This is at an angle of about 27 degrees to the plane in which the planets move around the sun—the ecliptic plane. The orbit of Enceladus is also influenced by its siblings, which sometimes get very close to it. The orbit of Pallene, for example, the next moon closer to Saturn, is approximately 26,000 kilometers away, while the next moon further out, Tethys, has a distance of almost 57,000 kilometers. The tidal forces created by this interaction compel the moons into a kind of cosmic ballet. Dione, which is more than twice as large, is in a 2:1 orbital resonance with Enceladus. This means that for every two orbital periods Enceladus completes, Dione will finish one. With Mimas, which is further inward and a bit smaller, Enceladus is linked in a 3:2 resonance. And with the already mentioned moon Tethys, which is twice its size, Enceladus has agreed on a 4:3 orbital resonance.
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.