June 14, 2004 | David F. Coppedge

Phoebe Shows Her Dark, Icy Face

Planetary scientists are reveling in the sharp new pictures of Phoebe taken last Friday by the Cassini Spacecraft.  Phoebe is the outermost moon of Saturn, an oddball since it revolves around Saturn in the “wrong” direction at high inclination.  Nine images have been released to the public so far (click here for the gallery).  The high-resolution images, like this one, are a thousand times better than what Voyager took 23 years ago.
    Phoebe is four times farther out than Iapetus, the next-outermost moon.  Some of the craters are so large, the impacts must have come close to smashing the body apart.  The big one is 62 miles across, almost half the diameter of the moon, and it has slopes leading 12 miles down to the crater floor.
    Phoebe appears to be an ice-rich body with a coating of dark material that may be up to 1600 feet thick.  Preliminary indications suggest it is similar to Kuiper-Belt Objects (KBO) like those seen beyond Neptune, but some scientists are not sure.  More will be known once the density and composition have been determined by infrared, ultraviolet and radio science observations which are still being processed.
Update 06/25/2004: Project scientists reported the density is 1.59, above that of water ice but below that of rock.  The surface material contains ferrous iron and, surprisingly, carbon dioxide mixed with other unknown material.  Impacts all seem to punch through a dark layer into lighter subsurface material.  Best guesses at this point is that Phoebe is indeed a captured Kuiper Belt Object similar to Pluto or Triton.  If so, it could be exuding volatiles from the surface or below, but so far, ultraviolet measurements have not detected any emissions.  It does not look like the other icy moons of Saturn, nor does it look like an asteroid.  Interdisciplinary scientist Torrence Johnson doubts that Phoebe is the source of the dark material on Iapetus.  For more information on these results released at a press conference at JPL June 23, see the Cassini media resources page.
    Now warmed up after its spectacularly successful flyby of Phoebe, the Cassini spacecraft is in excellent health as it accelerates toward its Saturn orbit insertion June 30 – July 1, a critical maneuver vital to the remainder of the mission.  It will be Cassini’s closest encounter with Saturn and its rings.  First images may be reported shortly after 5:00 a.m. PDT July 1.

This is just a foretaste of even greater and more spectacular encounters ahead.  Cassini, with its Huygens probe that will parachute to the Titan’s surface on January 14, is poised to be one of the great historical voyages of exploration.  Now, after nearly seven years in flight it is on the verge of reaching its destination: the planet Saturn, with its 31 moons, rings, magnetosphere and mysterious large moon Titan.  Watch for news of the orbit insertion on June 30 and its closest-ever flight over Saturn’s magnificent rings.

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Categories: Solar System

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