Tidal forces pinpointed as catalyst for Enceladus' eruptions

Cassini image of Enceladus' southern poll(Credit: NASA/JPL-Caltech/Space Science Institute)

Researchers from the University of Chicago and Princeton University have generated a new computer model that successfully simulates the mechanism driving impressive geyser eruptions observed taking place on the Saturnian moon Enceladus. The geysers have been active since Cassini first observed the phenomena in 2005, and were likely erupting long before the probe entered orbit around Saturn.

Enceladus has served as the focal point of repeated observation over the course of Cassini's prolonged mission characterizing Saturn and her moons. Yet, in spite of the generous attention paid to the moon, the mechanism that drives of the icy body's impressive geysers has remained a mystery.

These geysers spew forth vast quantities of frost and vapor from vast rents, or "tiger stripes" in the moon's south polar region. Furthermore, the subsurface ocean believed to be buried beneath Enceladus' icy exterior is considered one of the most likely places in our solar system to discover the presence of alien life.

In an attempt to gain a better understanding of the materials cast out by Enceladus impressive cryovolcanoes, the Cassini spacecraft dived through one of Enceladus' plumes last year in the search of clues as to whether the environment beneath the surface of the seemingly barren moon is hospitable to life.

The constant nature of the eruptions has given rise to a number of questions. For example, how have the geysers managed to operate continuously for over a decade without being sealed at least temporarily by a build-up of frost particles choking the entrances of the shafts?

Another anomaly has been observed wherein eruptions fail to achieve their peak activity until roughly five hours after the expected time, based on tidal response models. One theory attempted to explain away the lag by asserting that Enceladus boasted a spongy shell that took longer than anticipated to react to the tidal pressures exerted on the icy body by the nearby gas giant.

The researchers created a computer model of Enceladus, complete with a series of parallel slots located at the observed eruption sites that extended from the surface of the moon down to its underground ocean.

This model was then subjected to the tidal forces generated by Saturn's gravity as it interacts with the moon's interior. Tidal pumping in the slots causes turbulence, heating the water contained within. However, the activity did not occur all at once in the new model, with the key variable in eruption timing proving to be the diameter of the separate parallel shafts.

The influence of the gas giant's gravity on water in narrow shafts led to an eruption up to eight hours after the predicted peak level of activity, while wider shafts respond much faster. The sweet spot in between these two extremes creates an eruption with a five-hour delay, explaining the lag observed by the Cassini spacecraft.

According to the researchers, their model could be tested against the new data collected by Cassini during its recent flyby of Enceladus. If the heat driving the plumes is in fact being generated from deep within the vents by tidal pumping, then the surface of the south polar region between the vents would register as cold in the Cassini data.

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