With a diameter of 500 km, Enceladus is a small icy moon of Saturn. Images taken by NASA’s Cassini spacecraft show large plumes of water vapour and ice erupting from the south-polar region on Enceladus. The source of these plumes, or geysers, is believed to be an ocean of liquid water beneath Enceladus’ icy crust. Tidal interactions between Enceladus and Dione (another moon of Saturn) generate the heat necessary to keep this body of water in a liquid state. A study done in 2011 found that Enceladus’ south-polar region pumps out an estimated 15.8 gigawatts of endogenic heat. This amount of heat is sufficient to maintain an ocean of liquid water under a thermally conductive icy crust.
Figure 1: Saturn’s moon Enceladus, covered in snow and ice, resembles a perfectly packed snowball in this image from Cassini. Credit: NASA/JPL-Caltech/Space Science Institute.
From 2010 to 2012, Cassini performed 3 close flybys of Enceladus that allowed for ultra-precise radio tracking of the spacecraft from Earth using the giant ground antennas of NASA’s Deep Space Network. For these close flybys, Cassini flew within 100 km of Enceladus’ surface, twice above the southern hemisphere and once over the northern hemisphere. During each flyby, the spacecraft’s velocity is perturbed by small but measurable amounts that depend on variations in the gravity field of Enceladus.
In a new study published in the April 4 issue of the journal Science, a team of researches used the Doppler data from the ultra-precise tracking measurements to map out Enceladus’ gravity field. “The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun,” said Sami Asmar of NASA’s Jet Propulsion Laboratory in Pasadena, California, a co-author of the paper. “As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we’re trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the Solar System.”
What the team found is the presence of a negative mass anomaly at Enceladus’ south-polar region. A negative mass anomaly means the area contains less mass than would be expected for a perfectly spherical body. Although a negative mass anomaly makes sense since Enceladus’ south-polar region is depressed by a depth of ~1 km, the observed negative mass anomaly turned out to the significantly smaller than expected. As a result, there must be “extra” mass beneath the surface to account for the smaller than expected negative mass anomaly.
The team’s calculations suggest that the presence of a subsurface ocean of liquid water, which is 8 percent denser than the surrounding ice, is the only reasonable explanation. In the model, the ocean is ~10 km think and lies beneath a shell of ice 30 to 40 km thick. The ocean extends from the pole to roughly 50° south latitude and its thickness diminishes toward the lower southern latitudes. Nevertheless, the current data does not rule out the possibility of a global ocean. Furthermore, the ocean is believed to be in direct contact with a rocky seafloor.
Figure 2: Enceladus’s gravity disturbances mapped onto a reference ellipsoid. The negative mass anomaly at the south-polar region is clearly indicated. Credit: L. Iess et al. (2014).
Figure 3: This diagram illustrates the possible interior of Saturn’s moon Enceladus based on a gravity investigation by NASA’s Cassini spacecraft and NASA’s Deep Space Network, reported in April 2014. The gravity measurements suggest an icy outer shell and a low density, rocky core with a regional water ocean sandwiched in between at high southern latitudes. Credit: NASA/JPL-Caltech.
This study is the first time gravity measurements were used to infer the presence of an ocean on another world. Enceladus’ subsurface ocean is probably the source of its geysers. Along with water vapour and ice, these geysers also spill out organic molecules. In the Solar System, other worlds such as Europa and Ganymede also harbour subsurface oceans of liquid water. However, Enceladus and Europa are the only ones with subsurface oceans that are in direct contact with rocky seafloors, allowing their oceans to play host to a wide range of complex chemical reactions that are conducive for life, just as in Earth's oceans. This makes Enceladus and Europa amongst the best destinations in the Solar System to search for the presence of life.
L. Iess et al., “The Gravity Field and Interior Structure of Enceladus”, Science 344, 78 (2014)