Rice University logo


The mysterious world of Enceladus

Color-coded Cassini image of dust plumes from the south pole of Enceladus.

NASA’s Cassini spacecraft has been exploring the mini solar system surrounding the planet Saturn since its arrival in July 2004. The Saturn system includes the familiar bright rings, a multitude of moons, and Saturn’s magnetosphere, the vastly larger but invisible region of space that is controlled by Saturn’s magnetic field.

The magnetosphere is populated and dynamically driven by hot plasma (ionized gas) deriving from the satellites and ring particles that reside within it. One of those satellites, Enceladus, appears to be geologically active, and hence a dominant source not only of solid ring material but also of magnetospheric plasma.

The first figure (courtesy of the Cassini imaging science team) shows a color-coded image of icy dust particles emanating from the south pole region of Enceladus, already recognized as a “geothermal hot spot” based on other Cassini measurements during its close encounter with Enceladus in July 2005. During that same encounter, the Cassini Plasma Spectrometer (CAPS) found dramatic deviations from the otherwise smooth rotational flow of magnetospheric plasma in a surrounding region about 30 times larger than Enceladus.

The second figure shows the observed plasma velocities (in red) compared to a theoretical model of the plasma flow (in blue) developed recently at Rice. The data-model comparison suggests that Enceladus expels nearly a ton/second of water vapor into the surrounding space, exceeding previous estimates, and making Enceladus the engine that powers the much larger magnetosphere of Saturn.

This work is reported in a paper by Duane Pontius and Tom Hill submitted to the Journal of Geophysical Research. Pontius (Rice Ph.D., 1987) is a professor at Birmingham Southern College who spent his Fall 2005 sabbatical at Rice collaborating with Hill (Rice Ph.D., 1973), now a professor in the Rice Physics and Astronomy Department and a co-investigator for the CAPS experiment. The principal investigator for CAPS is David Young (Rice Ph.D., 1970) of the Southwest Research Institute in San Antonio.


Plasma flow observed by CAPS (red arrows) compared with theoretical flow model of Pontius and Hill (green contours).