A Liquid Ocean in the Strangest Place: Enceladus

The Southern hemisphere of Saturn's moon Enceladus photographed by the Cassini space probe. (NASA)

On August 28, 1789, Sir William Herschel focused his 47 inch (1.2-meter) telescope on a small dot in the sky near the planet Saturn. Observing its motion, Herschel concluded that this tiny object was an undiscovered moon of the ringed planet. This little moon, which would later be named Enceladus, has become of great interest to scientists in recent years because it seems to be one of the few locations in our solar system, outside planet Earth, that might harbor life.

Enceladus, with a diameter of 310 miles (500km), is the sixth-largest moon of Saturn and orbits the planet every 33 hours at an average distance of 147,909 miles (238,037 km) . Like most moons of Saturn it is named for one of the titans in greek mythology. This makes sense as the leader of the titans was Cronus (whose Roman name was Saturn). Now Enceladus and the other titan-named moons of Saturn follow their king eternally along his orbit.

A Shiny Mystery

Astronomers have been intrigued by Enceladus because, unlike most moons, it reflects 90% of the light that hits it, making it one of the brightest objects in the solar system. It took many years for them to figure out just why this happened, but when the Voyager 2 space probe took pictures of the moon as it passed by it in 1981, the mystery was solved. Scientists realized that Enceladus had a much smoother surface that they expected. Most moons are scarred with huge craters which are remnants of the early solar system when collisions with meteors and comets were common. The largest crater on this moon, however, is only 22 miles wide. This makes scientists suspect that the moon is geologically active and has recently resurfaced itself with fresh, clean ice. It is this smooth ice that reflects the light off of Enceladus' surface so well. The fact that there is so much recently formed ice on this moon makes them also suspect that there is a significant amount of liquid water under a frozen crust of ice.

Seven Quick Facts
What it is: The 6th largest moon of the planet Saturn.
Discovered: August 28, 1789, by Sir William Herschel.
Size: 310 miles (500km) in diameter.
Orbit 147,909 miles (238,037 km) in the planet's E-ring:
Brightness: Its surface of fresh ice makes the moon the shiniest thing in the solar system, reflecting 90% of the light that falls on it.
Activity: The moon has geysers that shoot liquid water into space.
Other: The moon is believed to have a liquid water ocean under its icy crust and may be the only place that might be able to sustain life in the solar system besides Earth.

While Enceladus' surface doesn't have very many big craters, it does have a number of extensive linear cracks. This would seem to be consistent with a surface that was geologically active and remaking itself.

Geysers Erupt into Space

The idea that there was liquid water on the moon was confirmed when the Cassini space probe flew by it in 2005. The probe observed over a hundred geysers erupting on the surface of the planet along a set of fissures with a "tiger-striped" pattern (a process called cryovolcanism). Some of the water vapor from these geysers gives the moon a thin atmosphere that most moons of this size do not have. The rest of it seems to contribute to the material in the "E-ring" of Saturn. The E-ring is one of the outermost rings of Saturn and is composed of tiny ice crystals, unlike the inter-rings which are mostly made of larger chucks of rock. Enceladus orbits in the center of the E-ring.

Studies from Cassini checking the density of the moon in different areas suggest that the source of this cryovolcanism is probably an ocean about the size of Lake Superior and around 10 km (6.2 miles) deep under Enceladus' southern hemisphere.

How does Enceladus manage to have a liquid ocean in the freezing depths of space? Scientists believe that the powerful gravity of Saturn causes the rocky core of the moon to stretch and heat up. This, along with some radioactively in the core makes heat that then raises the temperature under the surface ice enough to melt it. At times the crust of the moon, especially near its south pole, fractures under stress and some of the sub-surface water of the moon is ejected creating the geysers observed by the Cassini probe.

Cassini was able to determine that mixed into the moon's salty water was nitrogen (in the form of ammonia), along with nutrients and organic molecules, including trace amounts of simple hydrocarbons such as methane, propane, acetylene and formaldehyde. The presence of these materials along with a source of heat raises the possibility that microbial life may have formed in the moon's underground ocean.

Scientists believe life needs three things to form: first a source of energy, second, a rich chemical background and finally, a consistent source of liquid water. Enceladus is the only place in the solar system, besides Earth that we know for sure has these things (scientists also suspect Jupiter's moon Europa might have them, too, but the evidence isn't there yet).

Does that mean that Enceladus has some kind of life in its cold ocean? Until scientists figure out how to send a probe that can land and drill down to that sea to take samples, we probably won't know. Until that time what might be swimming in this tiny, shiny moon's, underground ocean will remain a tantalizing mystery.

The geysers erupting from the "Tiger Stripes" of the southern hemisphere. (NASA)

Copyright Lee Krystek 2015. All Rights Reserved.



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