In 2005 Saturn’s moon Enceladus was discovered to be an active world with water ice particle geysers at its south pole. The driver of this activity on a moon so small remains a mystery. One possible explanation has been suggested by Dr. Francis Nimmo, a planetary scientist from the University of California Santa Cruz. Nimmo visited the University of Arizona Lunar and Planetary Laboratory to present his model on Thursday, September 18, 2007 for the department’s weekly Planetary Sciences Colloquium. The talk was based on his recent publication [reprint PDF] “Shear heating as the origin of the plumes and heat flux on Enceladus” in Nature.
While the old view of the outer solar system as a cold and dead region devoid of most geological activity has eroded with surprising discoveries since the 1980’s, one bias persisted until 2005: only large moons in the outer solar system could potentially be active. Below a particular diameter, moons were not believed to be able to produce sufficient internal heating to drive surface activity. With a diameter of only 252 kilometers (157 miles), Enceladus was expected to have frozen solid long ago. The discover of geysers and a thin oxygen atmosphere at Enceladus proved that this was not the case.
Enceladus’ geysers do not appear to be a global phenomena, however. They are localized to the moon’s south pole, apparently along fractures referred to as “Tiger Stripes.” Taking into consideration the moon’s size, possible internal structures, orbital eccentricity, and other characteristics Nimmo has been working to develop a model that can explain the localized nature of this activity.
An important feature of his model is reorientation theory, where a mass such as a diapir rising to the surface from the mantle can reorient a body, driving the mass’ surface expression toward the poles or equator over time. The presence of such a mass would indicate some driving force such as convection. Applied to Enceladus, this could explain why the tiger stripes have reorientated to their present location at the south pole.
While this may explain the tectonic activity at the south pole, the presence of active water ice plumes requires further explanation. If the tiger stripes are strike-slip faults like the San Andreas Fault on the Earth, then rapid enough motion back and forth along the faults could lead to uplift and shear heating. This friction leads to sublimation of water ice, the primary component of the crust of Enceladus, with some of that vapor being released with heat along the tiger stripes in the form of the spectacular plumes photographed by the Cassini spacecraft.
After developing this model, which Nimmo believes is a better explanation than other theories that have been put forward, such as a near-surface ocean or clathrate decomposition, Nimmo and his colleagues explored possible predictions. While a near-surface ocean does not provide the water vapor to geysers in his model, the model still suggests the presence of an ocean underneath a solid ice crust of at least 5 kilometers (3 miles). The model suggests that this ocean may be transitory, with significant freezing out or remelting depending on the tidal dynamics that change as Enceladus’ orbit around Saturn changes over time.
Furthermore, the orientation of the tiger stripes should change over time, and their fault motions should result in hotter or colder relative temperatures depending on how fast they are moving. In the paper, the team of researchers specify two portions of stripes they predict will have the highest relative tempatures. Some researchers believe fossil tiger stripes may be present elsewhere on Enceladus. Further observations of Enceladus may allow them to test this prediction and confirm or deny the presence of fossil tiger stripes.
Nimmo acknowledged that at present this model of Enceladus is simplistic with several questions still outstanding. Why, for example, does heating appear only at the south pole and not also at the north pole, as predicted by some models of subsurface tidal heating? What prevents the ocean, if it exists, from freezing out completely? Enceladus remains a mystery and will continue to be an important study target for planetary scientists still marveling at a frigid but incredibly active outer solar system.