Brandon Morris - The Enceladus Mission - Hard Science Fiction

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.

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 could obscure this 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.

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.

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.

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.