If you look at a diagram of the Cassini spacecraft, the most obvious feature is the high-gain antenna used for communication with Earth. But it also has a second, critical function, as part of the RADAR instrument that gave us our first glimpses of the surface of one of Saturn’s moons, Titan. In 2011, during Cassini’s closest flyby of the moon Enceladus, RADAR got a close look at the moon’s south pole, where geysers spew water into space.
At the same time, the instrument took a measurement of the temperature in the area. An analysis of all that data suggests that the subsurface ocean powering the geysers is very close to the surface at the South Pole, only a few miles deep. And it can be kept close to the surface through a self-reinforcing behavior, even as individual geysers go on and off.
RADAR was built to image Saturn’s largest moon, Titan, which has a surface that is constantly obscured by its hydrocarbon-rich atmosphere. It succeeded spectacularly, finding entire oceans of liquid methane. During planning, it probably wasn’t even clear that a small moon like Enceladus would have many interesting surface features. But the discovery of geysers spewing water from large crevices at the moon’s south pole drew a lot of attention to Enceladus and led to a very close flyby.
The Enceladus data came from an orbit that occurred in late 2011, when Cassini passed within 500 km of Enceladus at a time when the South Pole was in complete darkness. While the probe was not aimed to directly image the fissures of active geysers, RADAR swept over a portion of the surface within about 50 km of them. This provided a detailed picture of the area’s geography and an indication of its temperature, which was read using a frequency band (2.2 cm wavelength) that picks up thermal emissions.
The measurements showed two things. The first is that there appears to be an internal heat source in space: “the measured [temperature] values were too high to be caused by the simple thermal re-radiation of the light absorbed at the surface and require a buried heat source.” The second is that the heat was localized at an altitude as great as one kilometer. the entire moon is about 250 km, the equivalent of a 25 km difference on Earth.
In fact, one of the hot wounds is very similar to those further south that are the source of the geysers. The authors suggest that this could be an inactive geyser source, one that is currently shut down.
Based on the amount of heat reaching the surface, the team estimates that liquid water is present at shallow depths, perhaps 2 km below the surface ice.
Combined, the data paints a picture where Enceladus’ south pole is geologically active, with several vents that are dormant and perhaps reactivated. In fact, the physics are such that a region of thin crust on a moon of this size bends more under the gravity it experiences and thus generates more tidal heat. So the South Pole may be a self-reinforcing feature that continually spawns new areas of active geology.
Nature Astronomy2017. DOI: 10.1038/s41550-017-0063 (About DOIs).