July 11, 2006 | David F. Coppedge

Saturn E-Ring Oxygen Bubble Blown by Enceladus

From a distance, the little moon Enceladus at Saturn looks for all the world like a leaking water balloon.  The Cassini Mission just released a new photo of Enceladus that fits that description well.  The plumes are faintly visible emanating from the south pole of the 300-mile-across moon as it orbits beyond the rings.  A second photo was released July 7; both were taken about 2.4 million miles away from Enceladus, and a third on July 19 with Rhea in the background.
    Two years ago, Cassini scientists were puzzled by a surge of oxygen detected in the E-ring as the spacecraft closed in on the planet (see 07/02/2004).  At first they thought some moonlets had collided in the extended, ephemeral ring of micron-sized particles.  It now turns out, according to another Cassini press release June 29, that Enceladus fits the bill as the source.  “We were able to measure the shape of the cloud, estimate the amount of water it contained and the rate it would be destroyed and produce oxygen,” said Larry Esposito, leader of the ultraviolet instrument team.  The little moon puts out a million tons of water.

The mystery of the atomic oxygen was solved.  At the same time, its source, the diminutive Enceladus revealed itself to be completely different than the cold, dead icy moon it should have been.  Small as it is, it has an internal heat source and is geologically active.  Its geysers throw out enough water vapor and ice to maintain the moon’s atmosphere, feed the vast E ring, and decompose into clouds of oxygen like the one first spotted by Cassini on its way to Saturn.

A third Cassini press release from July 5 said that the E-ring has structure.  There’s an arc of bright material racing around the ring (movie), and two distinct bands of material (see Space Science Institute press release).  Scientists are not sure what is causing these orbital dynamics, but hope to get a closer look at Enceladus on March 12, 2008 when the spacecraft flies within 100 miles of its surface.  There are five distant flybys (25,000 to 70,000 miles) before then.
    Cassini was launched on October 15, 1997 and arrived at Saturn seven years later, on July 1, 2004.  Enceladus is sure to be a target of the extended mission that begins July 2008 for three more years, through 2011 – perhaps even longer if all systems continue to function.

Enceladus is one of the biggest surprises of the entire highly successful, surprise-laden Cassini mission.  Scientists expected Enceladus would be a highlight, and it sure has fulfilled that prophecy.  What is heating up this little moon?  They have no answer at this time.  The usual suspects (radioactive heating, tidal flexing) fall short by a factor of 10.
    More observations will be needed to constrain the variability of the jets.  If the early 2004 output was an unusually large outburst, how often do similar bursts occur?  Pictures at each encounter suggest a continual ejection of a substantial mass of material.  Could Enceladus really have been spouting this much ice for billions of years?  How could a small moon, smaller than other Saturnian moons with no signs of activity, have maintained an internal heat source since its origin up to 2006?  Planetary scientists are attempting to come up with explanations after the fact, but finding a small moon jetting water out of one pole was surely not what their theories – or imaginations – predicted.

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