How an icy moon got its stripes

In his speech at the Planetary Exploration symposium, planetary scientist and geophysicist Dr. Bert Vermeersen (Aerospace Engineering) offered a new perspective on Enceladus’ stripes. AE’s study association Leonardo da Vinci organized the event on March 4.

Vermeersen argued that space missions Galileo and Cassini-Huygens have rejuvenated our view of the planetary moons around Jupiter, Saturn, Neptune and Uranus. Both the Voyager missions provided us with stunning up-close images of moons like Io and Europa next to Jupiter and Titan and Enceladus spinning around Saturn.

Instead of dead boring rocks, planetary moons are now regarded as prime candidates for extraterrestrial life. Vermeersen argues that in a number of cases at least there is water (underneath the ice), together with mineral building blocks from volcanism and energy dissipation from tidal movements. That sounds like a recipe for life. The JUICE (Jupiter Icy Moons Explorer) mission, to be launched in 2022 and to arrive by 2030, is partly motivated by this quest for ET life.

Meanwhile another planetary feature has grasped his attention: the tiger stripes on Saturn’s tiny icy moon Enceladus. With a diameter of 500 kilometres, the icy moon is hardly bigger than the Netherlands. Planetary scientists have drawn attention to the strikingly different surfaces of the tiny moon. One side is crater-marked and white, while the other half is smooth and adorned with a tiger-stripe pattern.

The Cassini mission has filmed vapours and ice plumes emerging from the tiger stripes. So the suggestion that there is fluid water underneath Enceladus’ icy surface isn’t that far-fetched. The eruption process is thought to be powered by tidal movements. These however do not explain the periodicity in the blue stripes.

 

Vermeersen offers another explanation for Eneceladus’ tiger stripes, namely underwater waves. These underwater waves occur in layered fluids and are driven by varying gravitational forces. Underwater waves have been shown to move oblique through fluids. Another strange thing: phase propagation is perpendicular to the particle oscillations. The phenomenon is explained by Prof. Leo Maas (IMAU and NIOZ) and Dr. Jeroen Hazewinkel (Shell) in the Dutch physics magazine Nederlands Tijdschrift voor Natuurkunde (November 2012).

Laboratorium experiments at Royal NIOZ at Texel, featuring a glass tank with stratified salt water that was shaken horizontally, have shown that repeated reflections of underwater waves result in attractors or hot spots where things are happening: where water solved matters emerge at the surface.

Vermeersen suspects that what we see as stripes at Saturn’s icy little moon could in fact be the attractors of the underwater waves under the ice crust.

→ Bert Vermeersen, Onderwatergolven in IJsmanen, NTvN, November 2012