Out­-of­-this­-world: An Ocean on Enceladus

Enceladus, a small moon only 300 miles (500 km) across. Credit: NASA/JPL/Space Science Institute

Measurements from the Cassini spacecraft have found a body of liquid water the size of a great lake on Saturn’s moon Enceladus.

“There are two main results. The first one is that Cassini data are consistent with the existence of a large water reservoir underneath the icy surface of Enceladus,” said Dr. Luciano Less, a Cassini team member at the Sapienza University of Rome, Italy. “The second important result is that Enceladus is a differentiated body. There are two layers-an external icy layer of 30-40 km, and an internal rocky core made up of silicates.”

Speculations about the abundance of water on Enceladus have been ongoing since plumes were discovered jetting out of its south pole in 2005. Debates centered around whether the water jets were a local phenomenon, resulting from friction between surface ice, or proof of a large subsurface supply, such as a lake or ocean.

In 2011, Cassini flew through a plume. It discovered salts and organic compounds in the dust. This led to the possibility that Enceladus might contain all of the ingredients necessary for life, if an ocean could be proven to exist.

This latest evidence strongly indicating the existence of an ocean on Enceladus comes from multiple visual and gravitational inspections of the poles between 2010 and 2012. Visually, Cassini noted a one kilometer depression in the ice at the south pole. Less ice should imply less mass and therefore less gravity in that vicinity. At the same time, Cassini noted that more gravity was present in the region than could be accounted for by just ice-especially ice that wasn’t there. The best way to explain both the gravitational measurements and the depression was by the presence of a material denser than ice: namely, water.

David Stevenson from the California Institute of Technology said that the findings should be understood as a series of logical inferences.

The water vapor jets from Enceladus’ south pole may originate from its sub­polar ocean. Credit: NASA/JPL/SSI; Mosaic: Emily Lakdawalla

“When you interpret data like this, gravity of course doesn’t tell you what type of material. What you do is make use of all information that you have,” said Stevenson, “We have a lot of information, we know in the outer solar system, the outer bodies are rock and ice, indeed we are assuming that those are the two things that matter most, and you have to explain the density of the body, and the internal structure with that simplification.”

In summing up the interpretations of Cassini’s findings, Less said, “Water can easily explain the observations.”

Based on the observations, certain constraints have been applied to the ocean’s size, width and depth. The presumed water reservoir would need to be the size of Lake Superior, or larger, to exert the observed gravitational effect. The water layer would need to be located approximately 30-40 km beneath the ice shell, and be 8-10 km thick. These estimates mean that this small ocean likely covers Enceladus’ entire south pole region, and possibly extends to at least 50 degrees latitude. Though it is impossible to know for certain from the current measurements, the ocean may reach past the moon’s equator toward the north pole, or even extend globally.

However far up it extends, a reservoir of liquid water is only possible on Enceladus because of tidal heating, which presents another interesting consideration. Saturn’s gravitational tides and those of Dione, a nearby moon, tug on Enceladus during its orbit. Both Enceladus and Dione are thought to be slowly drifting away from Saturn. In the process of migrating, there may have been times when tidal forces on Enceladus were stronger, and other times when they were appreciably less forceful. In other words, the mechanism generating the ocean may wax and wane. Therefore, it is extremely difficult to know how long this body of water has existed. While we see evidence of a 8-10 km deep ocean on Enceladus today, in the past the water supply might have been many more kilometers deep, or many less. The southern reservoir itself may have been in place for hundreds of millions of years, or it may come and go as part of a cyclical process over the eons.

Even if the reservoir is relatively young on the scale of geologic time, there’s still a chance for life. Jonathan Lunine from Cornell University explained how a pool of stable water in contact with a silica core, like that hypothesized to exist inside Enceladus, can lead to habitability.

The new model of the structure of Enceladus, showing the southern ocean and rocky silicate core. Credit: NASA/JPL and Cal Tech

“There appears to be a liquid water ocean, which has to be sandwiched between the ice and the rocky core,” said Lunine, “In that configuration, you would have liquid water circulating into the rock, leaching elements like phosphorus, sulphur, potassium, sodium, out of the rock into the liquid itself, providing two things. One, elements that life needs to build certain kind of molecules to store and transfer energy. [Two], the potential chemical gradient between places where water is coming out of the rock and the regions adjacent to it.”

This unexpected finding of an ocean on a small moon so far from the Sun raises a distinct possibility: that there are more oceans on more moons, each with a chance for life.

“Before Cassini [sic], few people really expected this level of activity on the tiny moon that’s Enceladus,” said Less. “The main implication is that there are potentially habitable environments in the Solar System in places which are completely unexpected.”

With our current technology, we cannot directly observe Enceladus’ water, its rocky core, or the exchange of energy between the two. However, we can detect Enceladus’ missing mass of ice. We can observe its excess gravity at the poles using radio waves. With our understanding of the materials that make up icy moons in that part of the Solar System, we have built a model that fits the observations made by Cassini over the last decade. Out of all the data, a familiar picture resolves.

“The configuration that we’re seeing makes the base of [Enceladus’] ocean very much like the base of our own ocean on the Earth,” said Lunine.

When asked what our response should be to this titanic discovery on a tiny world, Lunine said, “We need to be more aggressive in getting the next spacecraft out there.”

Funding by JPL