Measurements of the moon’s rotational activity indicate a slight wobble, similar to the natural motion of a living egg when we spin it on a table.
Impression of geologists’ potential understanding of Enceladus based on Cassini’s gravity investigation. (NASA/JPL-Caltech)
This is the latest finding in a series of studies on the geological activity of the six largest moons of Saturn. The existence of water near the surface, potentially a large lake or ocean on Enceladus, first became suspected when Cassini captured plumes of water vapor, icy crystals, and simple organic molecules escaping from fissures in the moon’s southern polar region. However, changes in the moon’s gravitational forces measured by Cassini suggested that deep beneath Enceladus’ icy surface, there could be a large ocean.
This discovery seems to confirm suspicions of an ocean based on independent analyses of the moon’s wobbling or rocking phenomenon in its natural rotational motion, as measured by the Cassini spacecraft since its arrival in the Saturn system and subsequent exploration of multiple moons of the planet since 2004. To measure the wobbling motion of Enceladus, scientists compared numerous images captured over several years of a specific feature on the moon, such as distinct craters. With hundreds of such images, they were able to measure and conclude that the moon exhibits a slight wobble during its rotation.
According to NASA, the cause of this wobbling is partly due to Enceladus’ slightly irregular shape and partly due to its elliptical orbit around Saturn. The combination of these factors creates an uneven gravitational pull, causing Enceladus to undergo unusual variations similar to the motion of the Moon around the Earth. The question arises as to what causes this gentle wobbling and raises the suspicion of whether it is a completely solid object or not.
By processing a series of computer models, NASA scientists believe that the intensity of Enceladus’ wobbling is influenced by a layer of ice crust and a rocky core separated by a layer of liquid. This creates a greater imbalance as the moon rotates, similar to the difference between spinning a hard-boiled egg and a raw egg, with the raw egg being disrupted by its unbalanced yolk as it rotates.
Matthew Tiscareno, a scientist from the SETI Institute in Mountain View, California, said, “If the surface and core were tightly connected, the core would provide enough dead weight to make the wobbling intensity smaller than what we observe. Thus, it demonstrates the necessity of a liquid layer separating the surface and core of Enceladus.”
Scientists are currently trying to determine what causes Enceladus to behave unlike a frozen rock. One possibility is the tidal heating from Saturn. To help find the answer, the Cassini spacecraft is scheduled to survey one of the polar regions at an altitude of about 49 kilometers on October 28th.