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, acting 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 pass over the moon Enceladus, RADAR got a good look at the moon’s south pole, where geysers spew water into space.
At the same time, the instrument took a measure of the temperature in the area. An analysis of all that data suggests that the subsurface ocean that powers the geysers is very close to the surface at the south pole, only a few kilometers deep. And it may be kept close to the surface by a self-reinforcing behavior, even as individual geysers turn on and shut off.
RADAR was built to image Saturn’s largest moon, Titan, which has a surface that is perpetually obscured by its hydrocarbon-rich atmosphere. It succeeded spectacularly, finding entire oceans of liquid methane. During the planning, it probably wasn’t even clear that a small moon like Enceladus would have much in the way of interesting surface features. But the discovery of geysers that spew water out of large fissures at the moon’s south pole focused a lot of attention on Enceladus and led to a very close flyby.
The Enceladus data came from an orbit that occurred in late 2011, when Cassini passed within 500km of Enceladus at a time where the southern pole was in complete darkness. While the probe wasn’t oriented to directly image the fissures of active geysers, RADAR swept across a slice of the surface within about 50km of them. This provided a detailed image of the geography of the area and an indication of the temperature, which was read using a frequency band (2.2cm wavelength) that picks up thermal emissions.
The measurements showed two things. The first is that there seems to be an internal source of heat in the area: “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. The biggest sources were slices of jagged terrain, with changes in elevation as large as a kilometer. For context, the radius of the entire moon is about 250km; it’s the equivalent of a 25km difference on Earth.
In fact, one of the warm gashes looks very much like the ones farther south that are the source of the geysers. The authors suggest that this might 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 as little as 2km beneath the surface ice.
Combined, the data paints a picture where the south pole of Enceladus is geologically active, with different vents going dormant and perhaps reactivating. In fact, the physics is such that an area of thin crust on a moon this size flexes more under the gravitational stresses it experiences and thus generates more tidal heat. So the south pole may be a self-reinforcing feature that continually produces new areas of active geology.
This article and images was originally posted on Ars Technica
By JOHN TIMMER