Did Enceladus experience true polar wander?
Enceladus is a water ice-covered moon of Saturn famous for geyser-like plume eruptions at its south pole.
It is important as a world considered to have a subsurface liquid water ocean with the ingredients for life. Enceladus is also mysterious since it is not clear why Enceladus appears to be so geologically active despite being tiny (500 km across) and therefore unlikely to retain significant heat from its formation or the decay of radioactive elements in its core, what keeps rocky planets like Earth warm enough to have geologic activity. Why does it matter? Many reasons, but one is answering the question of whether there could be life on Enceladus, something I, at least, and many others, would like to know. Life requires a heat source, so understanding where Enceladus is getting its heat can provide clues how long and to what degree the icy moon’s subsurface ocean could sustain life.
The main hypothesis for how icy moons like Enceladus retain their heat is through tidal dissipation. Because planetary bodies orbit each other in ellipses, the gravity of Saturn on Enceladus will be different at different points in its orbit because the distance of Enceladus from Saturn will vary over the course of its orbit. This varying gravitational pull on Enceladus by Saturn causes Enceladus to be stretched and squeezed, generating heat.
The proposed current scenario is that this tidal heating partially melted the ice layer on Enceladus leading to a subsurface ocean of liquid water. Furthermore, this heat could also have melted rock in the rocky core of Enceladus leading to volcanic activity at the ocean floor in the form of hydrothermal vents. Hydrothermal vents are usually crevices where nearby molten rock superheats water in the pore space within the surface rock. When the hot water in the pore space of the rock mixes with the cold open ocean water, unique and complex chemical reactions take place that create energy that life can use. There are also cold hydrothermal vents with different complex chemical reactions that can also be used as an energy source by life. Hydrothermal vents are found on Earth’s ocean floor and the chemical environment of hydrothermal vents make them prime habitats for microbial life. In fact, hydrothermal vents are one of the places where astrobiologists propose that life may have first formed on early Earth.
The Cassini spacecraft orbited Saturn from 2004 to 2017 and took samples of the plume material coming from the interior of Enceladus and analysis of the samples revealed evidence of hydrothermal activity. Although neither this nor discovering compounds essential to life are by themselves evidence of life on Enceladus, it is exciting to find a potential habitable environment on an icy moon in the outer solar system. What is mysterious about Enceladus is that despite clear evidence of Enceladus having a heat source driving activity on a rocky seafloor and sustaining a liquid subsurface water ocean, Enceladus’s orbital eccentricity is almost zero, meaning the tidal heating should be minimal. In fact, tidal heating models show that Enceladus should be frozen solid. If it is not coming from current tidal heating, where is it coming from?
One possibility is that the tidal heating happened in the very recent geologic past, as in the past ~500 million years (a very short time for a geologist). Gravitational interaction between Enceladus and the other moons of Saturn could have caused Enceladus’s eccentricity to temporarily increase, leading to more intense tidal heating. What we are seeing now could just be the result of the leftover heat from that period. What evidence do we have of Enceladus having experienced significant changes in its orbit around Saturn?
There is some evidence that Enceladus experienced what is called true polar wander (TPW) in which the axis of rotation of a planetary body shifts over time. For example, on Earth, the current axis of rotation goes through Antarctica and the Arctic Ocean. TPW would be if that axis of rotation were to shift so that now the geographic north pole around which Earth was rotating around, say New York City. TPW would represent further evidence that Enceladus has gone through significant changes in its motion around Saturn which could also have involved changes in its orbital eccentricity.
Both stress from a high orbital eccentricity and stress from TPW would lead to geologic features forming on the surface of Enceladus. This is because as the ice shell is under stress the cold upper part of the ice shell near the surface will start to fracture brittlely. This creates the formation of ridges and canyons on the surface of Enceladus.
How do we test whether TPW occurred on Enceladus? One way is to use tidal stress models which predict where the stress would occur on the surface of Enceladus and compare that model to the locations of actual features on the surface of Enceladus. Currently, this is a project that I am working with a colleague to test whether TPW played a role in the geological resurfacing of Enceladus.
The involvement of TPW would mean that the history of Enceladus is more interesting than previously thought. It would also support changes in Enceladus’s orbit which could also have involved changes in the orbital eccentricity of Enceladus leading to intense tidal heating, resulting in a subsurface ocean with hydrothermal vents.