Enceladus: Cold Moon With a Hot Spot

Saturn’s tiny moon Enceladus is a cold and icy place. But somehow, there’s enough heat being generated on Enceladus’ south pole to eject plumes of ice and vapor high above the moon. These plumes are extremely intriguing to the Cassini mission scientists and they want to know more about this hot spot on a very cold moon. In fact, Enceladus has become a major priority for study by the Cassini team and they are anticipating learning more about the moon in an upcoming fly-by.

The temperature at Enceladus’ south pole is about -220 degrees Celsius, but the hot spot is at least 100 degrees warmer. The leading model for the cause of the plumes on Enceladus is that the moon’s tides cause its crust to ratchet, or rub back and forth, in a set of faults near the south pole. The forces between Enceladus, the big planet Saturn and another moon, Dione cause what’s called dynamical resonance, and Enceladus is continually squeezed under this gravity field. This process creates a small hot spot, in relative terms, for an icy satellite.

Cassini has actually flown through the plumes, giving scientists a glimpse of the plume’s make-up.

“The plume particles are like smoke, ice smoke,”said William B. McKinnon, professor of earth and planetary sciences at Washington University in St. Louis. “If you were standing on Enceladus’ surface you wouldn’t even be able to see the plumes. The particles are just larger than the wavelength of light, about one-thousandths of a millimeter. Most icy bodies of this size are geologically inert, but this is a clear indication of geological activity. Cassini has found active venting of water vapor. This leads to scientifically intriguing speculations and questions.”

The scientists are pondering if Enceladus has active ice volcanism, and if so, is it due to ice sublimating, like a comet, or due to a different mechanism, like boiling water as in Old Faithful at Yellowstone. Even though there may be water on the moon, McKinnon doesn’t believe there is the possibility of life on Enceladus. This is because measurements made from Earth don’t indicate there is enough sodium present in the plumes to warrant the “life” question.

“The emerging view is that there’s not obvious evidence for a subterranean ocean in contact with rock, no boiling or venting,” said McKinnon.

The Cassini science team has made Enceladus a major priority and there will be seven additional close fly-bys of the moon by the spacecraft through mid-2010 (provided the mission is extended to that period.) The next fly-by will be on March 8, 2008 and Cassini will approach Enceladus at an incredibly close 25 km in altitude at the low latitudes and fly over the south pole at 580 km altitude. The spacecraft will actually fly through the plumes and should be able to take high-phase images of the plumes, map the temperatures of that region, search for any activity at other latitudes as well as image other interesting features on Enceladus, such as “tiger-stripe”-like fissures found near the south pole.

“We still can’t say how truly ‘hot’ the hot spots are,” said McKinnon. “We’ll probably learn this in March.”

Original News Source: Washington University Press Release