Closest Pass Yet
NASA's Cassini spacecraft capped off 2004 with an encounter of
Saturn's ying-yang moon Iapetus (eye-APP-eh-tuss) on New Year's
Eve.
It was Cassini's closest pass yet by one of Saturn’s
smaller icy satellites since its arrival around the ringed giant on
June 30th. The next close flyby of Iapetus is not until 2007.
Iapetus is a world of sharp contrasts. The leading hemisphere is
as dark as a freshly-tarred street, and the white, trailing
hemisphere resembles freshly-fallen snow.
Cassini flew by the two-toned moon at a distance of
approximately 76,700 miles Friday.
"I can think of no better way than this to wrap up what has been
a whirlwind year," said Robert T. Mitchell, program manager for the
Cassini mission at NASA's Jet Propulsion Laboratory, Pasadena, CA.
"The new year offers new opportunities, and 2005 will be the year
of the icy satellites."
In 2005 Cassini will have 13 targeted encounters with five of
Saturn's moons. "We have 43 close flybys of Titan still ahead of us
during the four-year tour. Next year, eight of our 13 close flybys
will be of Titan. We will also have a number of more distant flybys
of the icy satellites, and let's not forget Saturn and the rings
each time we come around," said Mitchell.
With a diameter of about 1,400 kilometers (890 miles), Iapetus
is Saturn's third largest moon. It was discovered by Jean-Dominique
Cassini in 1672. It was Cassini, for whom the Cassini-Huygens
mission is named, who correctly deduced that one side of Iapetus
was dark, while the other was white.
Scientists still do not agree on whether the dark material
originated from an outside source or was created from Iapetus' own
interior. One scenario for the outside deposit of material would
involve dark particles being ejected from Saturn’s little
moon Phoebe and drifting inward to coat Iapetus. The major problem
with this model is that the dark material on Iapetus is redder than
Phoebe, although the material could have undergone chemical changes
that made it redder after its expulsion from Phoebe. One
observation lending credence to the theory of an internal origin is
the concentration of material on crater floors, which implies that
something is filling in the craters. In one model proposed by
scientists, methane could erupt from the interior and then become
darkened by ultraviolet radiation.
Iapetus is odd in other respects. It is the only large Saturn
moon in a highly inclined orbit, one that takes it far above and
below the plane in which the rings and most of the moons orbit. It
is less dense than objects of similar brightness, which implies it
has a higher fraction of ice or possibly methane or ammonia in its
interior.
The last look at Iapetus was by NASA's Voyager 1 and 2
spacecraft in 1980 and 1981. The Cassini images will be the highest
resolution images yet of this mysterious moon.
The Iapetus flyby by Cassini follows the successful release of
the Huygens probe on December 24.
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