New hot spots on Enceladus

Ever since the Cassini spacecraft arrived at Saturn in 2004, its 22 flybys of Enceladus have offered up a series of increasingly tantalizing observations of the icy moon. (See the article by John Spencer, Physics Today, November 2011, page 38 .) Four parallel cracks at the moon’s south pole, dubbed “tiger stripes,” appear anomalously warm in IR images, and they spew jets of salty ice and water vapor into space. Taken together, those signs point to a liquid water ocean beneath the frozen outer shell.

Now Alice Le Gall (LATMOS/University of Versailles–Saint Quentin, France) and her collaborators have found that the Enceladean ocean may be closer to the surface than previously thought: Instead of tens of kilometers, the south polar ice could be just 2 km thick. That’s thin enough that ground-penetrating radar on a hypothetical future orbiter could see right through it.

The conclusion is based on data from Cassini‘s radar. Developed primarily to image the surface of the moon Titan by sending pulses through its optically opaque atmosphere, the radar can also operate passively to detect thermal radiation in the microwave regime. The longer wavelength means that radar is sensitive to lower temperatures and greater depths than IR detectors are, but at the cost of much poorer spatial resolution.

Cassini‘s radar and IR instruments can’t operate simultaneously, and the latter usually took priority during flybys of Enceladus. In fact, the radar collected spatially resolved passive data of the south polar region for just a minute and a half during a single flyby. But the results of that observation, shown as the colored stripe in the figure, reveal hot spots, shown in red and orange, in areas where the IR images show no sign of any excess heat.

Enceladus is heated by tidal deformations from Saturn’s gravity; a thin ice crust deforms more than a thick one, so it generates more frictional heat. When Le Gall and company modeled the process to account for the newly discovered hot spots, they inferred a crustal thickness of just a few kilometers at the south pole.

Cassini will end its mission in a few months, and it will make no more visits to Enceladus during its remaining time. Further insight into the moon’s secrets will have to wait until the next mission to Saturn, which has yet to be planned. (A. Le Gall et al., Nat. Astron. 1, 0063, 2017 .)