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"Cassini at Saturn: Rings, Atmosphere and Moons"
Prof. P.D. Nicholson
Department of Astronomy, Cornell University, USA
	   
When:  Thursday 31 May 2007 at 5:00pm
Where: 3rd Floor Seminar Room

Abstract:

Since Saturn orbit insertion on July 1, 2004, instruments on the
Cassini orbiter have been observing the planet, its rings, Titan and
several of the icy satellites. I will briefly review some of the
highlights of these observations, concentrating on optical and
near-infrared imaging. Saturn's main rings have been imaged and
scanned in unprecedented detail, revealing density waves and "wakes"
driven by nearby and embedded satellites, evidence for unresolved
100-meter-sized moonlets and subtle, large-scale compositional
variations.  Mosaics show the complex, kinky, multi-stranded structure
of the F ring, while images taken in the planet's shadow show two new,
extremely tenuous rings.  Saturn itself sports a complex,
ever-changing pattern of ring shadows, as well as several giant
thunderstorms, ultraviolet aurorae and a mysterious hexagonal feature
at the north pole.  Studies of the planet's radio emissions seem to
show that its rotation period has lengthened by about 5 min since
1981.

The surface composition of the large moon Titan remains enigmatic,
partly due to our very limited ability to see through its thick,
methane-rich atmosphere outside of a few narrow `windows'. However,
significant albedo variations exist in the near-IR which may
correspond to elevated regions dominated by water ice and lower areas
covered by dark, probably organic material.  Recent radar images have
revealed what appear to be "lakes" in the far north, while the
equatorial regions are dominated by immense dune fields and occasional
sinuous channels reminiscent of terrestrial rivers.  Although smaller
than Jupiter's Galilean satellites, Saturn's icy moons turn out to be
equally intriguing. Icy plumes erupt from Enceldus' south polar
region, feeding the tenuous E ring, while chaotically-tumbling
Hyperion has a surface texture similar to coral. Enigmatic Iapetus
shows evidence of ancient giant impacts, as well as more recent
geologic activity. Distant Phoebe, likely a captured Kuiper belt
object, is heavily-cratered with a much more chemically-diverse
surface than the inner satellites.

This work was supported by NASA under a contract with the
Cassini-Huygens Project.

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