Does One of Saturn’s Moons Harbor Life?

by Callie Hood

Image courtesy of NASA/JPL-Caltech and Space Science Institute

Are we alone in the universe? This simple question leads most of the current research in astronomy as technology allows us to see farther into the universe than ever before in search of potential cosmic neighbors.  The basic requirements for life as we know it, mainly liquid water and organic materials, are not exactly common in the observable universe. However, astronomers at NASA have discovered evidence for an ocean encased by miles-deep ice under the surface of Enceladus, one of the many moons of Saturn, using the Cassini-Huygens robotic space probe.

Equipped with over eighteen different instruments, the Cassini spacecraft left Earth in 1997 to explore the mysteries of Saturn and its natural satellites. As an unmanned probe, Cassini mainly functions to take pictures and measurements of the various objects in Saturn’s area.

In 2005, the Cassini probe sent pictures of a large plume of water vapor spurting off from the South Pole of Enceladus.  Similar to the ash clouds emitted by erupting volcanoes, these plumes propel water and other materials into space. Astronomers predicted that these propulsions may have originated in a reservoir of liquid water underneath the moon’s surface, but did not have any other evidence at the time to support this hypothesis.

An image of the plumes of Enceladus, courtesy of NASA/JPL-Caltech and Space Science Institute

Less than ten years later, Dr.Luciano Less and his colleagues have used the Cassini probe to map out the gravitational field of Enceladus. The moon’s gravity pulls on the probe in various amounts depending on the probe’s position, producing changes in Cassini’s speed. Astronomers could determine these changes in speed by taking advantage of the common Doppler Effect, the change in a wave’s frequency due to changes in the velocity of the source of the wave. For example, this effect can be seen when the siren on a car sounds higher when moving towards you and lower when driving away.  The radio waves sent out by Cassini experienced similar changes, allowing the authors of the study to calculate the gravitational field around the moon.

Less and his colleagues expected there to be a smaller gravitational field at Enceladus’s south pole, as there is a depression in the moon’s surface at that point. However, the field measured by Cassini decreased less than expected in this region, implying that something with more mass than the ice on the surface was making up for the depression. As liquid water is slightly denser than its frozen counterpart, meaning that a certain amount of water is more massive than the same volume of ice. An ocean of liquid water underneath the crust of Enceladus would satisfy the strange data observed during the study.  The authors of the study propose the existence of a 30 or 40 km shell of ice that surrounds such a sea under the surface to explain Cassini’s observations.

The discovery of such a large reservoir of liquid water within Enceladus makes it one of the only other candidates for life other than Earth in our solar system. Enceladus’s ocean would touch silicate within the moon, allowing for elements vital for life to interact with liquid water, providing a favorable breeding ground for the compounds we currently recognize as integral to life. Only one other object in the solar system, Jupiter’s moon Europa, presents a similar set of conditions.

The best part about Enceladus’s possible habitability, however, is the relatively easy access to this water through its spectacular plumes off the moon’s surface. Whereas Europa’s subsurface ocean is thought to be encased in even thicker ice than that of Enceladus, the plumes of Enceladus allow Cassini to take a closer look at which elements exist on the moon. In 2006, Cassini detected organic material within the plumes.  These building blocks of biochemistry make-up all lifeforms we see on Earth.

These organic materials and the presence of liquid water underneath the surface provide one of the best places we have to look for extraterrestrial life within the solar system. However, true insight into the mysteries of Enceladus would benefit from a mission designed solely to explore for life. Congress already approved the funding for a study of such a mission to Europa, meaning Enceladus’s ocean would have to be on the backburner for now.

Despite this obstacle, Cassini still has a few things to learn about Enceladus. It is scheduled to return to the moon’s plumes before the end of its mission in 2017, possibly even bringing a sample back to Earth for future study. Until then, we can only guess at what kinds of life could hide underneath the icy surface of Enceladus.

Further Reading:
Follow the Plume: The Habitability of Enceladus
10 NASA Images of Planets like Earth

Reference: Less, L., et al. “The Gravity Field and Interior Structure of Enceladus.” Science 344.6179 (2014): 78-80. Web.
doi: 10.1126/science.1250551