July 2, 2004 | David F. Coppedge

Cassini Watches Explosion in Saturn’s E Ring

Something strange happened in the E ring of Saturn last January.  The incident is forcing scientists to conclude the ring cannot be very old.
    The E ring is the broad, extended ring that extends from Mimas to Rhea (click here for diagram), over three times as broad as the main ring system but much more diffuse.  It reaches its maximum density at the orbit of Enceladus.  This fact leads Cassini scientists to anticipate finding ice geysers on the moon when Cassini flies by it at close range next March.
    Dr. Don Shemansky reported at a news conference today that the Cassini UVIS instrument (ultraviolet imaging spectrograph) measured a surge in atomic oxygen coming from the E ring.  Measurements beginning in December showed a sudden rise in oxygen in late January that dissipated by April.  Apparently a collision in the ring ionized water molecules among the icy particles.  Electrons in the plasma sheet around Enceladus quickly recombine with these ions, forming neutral atoms which are swept into the vacuum, eating away the ring.  With evident surprise, he told the press that this one incident resulted in a mass loss equal to the total mass of micron-sized particles in the entire E ring.  Extrapolating backward, assuming this event was not atypical, he calculated an upper limit of 100 million years for the lifetime of the ring.
    Dr. Larry Esposito said later in private conversation that the incident may have been caused by two large bodies colliding within the ring.  He estimates such an event could occur once every 4 to 10 years; if so, it was a fortuitous circumstance for Cassini, at the start of its 4 year tour, to witness the event.  Whatever happened, it was abrupt, severe, and short-lived.
    In the press conference Q&A, Dr. Shemansky, noting the bland expressions in the audience, said he must have understated the “spectacular nature” of this discovery.  So he repeated it, with emphasis: the mass quantity lost in the event was equal to the total mass of all the micron-size particles in the entire E ring – in just four months.
    A press release and image can be found at the Cassini website.
Update 07/11/2006: the source of the oxygen is the moon Enceladus, which is erupting water out of its south pole.  See the 07/11/2006 and 11/28/2005 reports.

This discovery adds to others that have led most ring scientists to conclude that planetary rings must be young.  A hundred million years sounds like a long time, but is a mere blink of an eye compared to the assumed age of the solar system (about one fiftieth of 4.5 billion years).  Notice that they did not say the ring is 100 million years old, but that it could not be older than that.  To believe it is much younger is reasonable in light of the evidence; it would seem a stretch to imagine an E ring many orders of magnitude thicker millions of years ago.  Today the E ring is tenuous and mostly composed of fine particles.  A much more massive E ring would seem to correspond to an epoch with more impactors flying around, resulting in exponentially faster erosion.
    This age limit is not a problem for those who believe the solar system was created recently.  It is only a difficulty, an anomaly, a surprise, a puzzle for those locked into a mindset that the earth, and life, evolved slowly over vast periods of time.

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Categories: Solar System

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