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| New Knowledge |
Cassini's mission is a four-year, close-up study of the Saturnian system, including the planet's atmosphere and magnetic field, the rings and several moons. The mission represents a rare opportunity to gain significant insights into major scientific questions about the creation of the solar system and the conditions that led to life on Earth, in addition to a host of questions specific to the Saturn system. As the best-instrumented probe ever sent to another planet, Cassini will produce the most complete information about a planet system ever obtained.
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Chief among the scientific goals is a thorough characterization of the large moon, Titan, a world thought to resemble a frigid, primordial Earth. Like Earth, Titan has a nitrogen-rich atmosphere. Complex organic molecules make up the haze that clouds Titan's surface from view. These molecules must eventually fall to the surface in the same way that organic molecules fell from Earth's sky at the time life originated on our planet. For this reason, understanding the chemistry of Titan's atmosphere may be key to understanding the evolution of early life on Earth.
A large portion of Cassini's Titan studies will be accomplished by the European Space Agency's Huygens Probe, which will be released from the main Cassini orbiter to descend by parachute through Titan's opaque atmosphere. During this descent, instruments mounted on the probe will directly sample the atmosphere and determine its composition. The surface of Titan may be solid ice or rock, or it may hold shallow seas of liquid ethane or methane. In addition, it is possible that over billions of years, the rain of complex organic molecules has built up a deep, brownish-orange sludge on Titan's surface. The descending probe will gather data on this exotic landscape and could briefly return information, including images, directly from the surface.
In addition to the Huygens Probe, Cassini's Titan studies will be carried out by imaging radar, which passes signals through clouds or atmospheric haze and showers the surface of its target with a swath of radar pulses. Characteristics of the returned radar signals are processed to create detailed images of the terrain. Imaging radar has been used to great advantage in mapping cloud-covered regions of the Earth where other mapping instruments cannot "see" the surface, and was used on NASA's Magellan spacecraft to produce a global terrain map of cloud-shrouded Venus.
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A Cassini instrument designed by the Applied Physics Laboratory at Johns Hopkins University will allow the first-ever imaging of a planet's magnetic field. The Magnetosphere Imaging Instrument (MIMI ) will obtain images of the plasma and radiation surrounding Saturn and enveloping its moons, including Titan. MIMI will observe the glow of Titan's exosphere due to the bombardment by high-speed protons trapped in Saturn's magnetic field. This pioneering investigation will open a new observational window in the study of planetary magnetic fields, including Earth's. Despite the spacecraft's long travel time to Saturn, Cassini is likely to produce the first images of any magnetosphere in the solar system.
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Saturn's rings are another principal target for study by Cassini. Explorations by the Voyager 1 and 2 spacecraft showed the rings to be made up of thousands of individual rings. The rings themselves were found to be largely composed of ice particles ranging in size from sugar grains to small houses. Slight color variations indicate the rings include some rocky material. Even the short-term Voyager studies showed a wide range of unexplained phenomena in the rings, including various wave patterns, small and large gaps, clumping of material and small, so-called "moonlets" embedded in the rings. Long-term, close-up observations of the rings by Cassini will help resolve whether the rings are material left over from Saturn's original formation, or whether they are the remnants of one or more moons shattered by comet or meteor strikes. Applied to larger scale, disk-shaped systems, the detailed studies of Saturn's rings proposed for Cassini will provide important contributions to theories about the origin and evolution of the dust and gas from which the planets first formed. Additional studies of the Saturnian system as a microcosm may be applicable to examinations of even larger disk systems so common in the universe, including our own spiral galaxy, the Milky Way.
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The tilt of Saturn's ring plane changes as the planet orbits the Sun, and the changing angle of sunlight illuminating the rings dramatically alters their visibility. Cassini's arrival at Saturn is timed for optimum viewing of the rings, during a period when they will be well-illuminated by sunlight. Upon arrival at Saturn in 2004, the tilt of the ring plane and resulting illumination angle will allow Cassini's instruments an unsurpassed view of the ring disk. Orbiting Saturn, Cassini will be able to detect small moonlets inside the rings, determine the composition of the particles, determine the effects of magnetic field interaction with the rings and conduct intensive observations of ring dynamics over a four-year period.
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Among the icy satellites Cassini will explore is Enceladus, made entirely of water ice with few indications of impact craters on its surface. Cassini will determine if Enceladus has some internal heat source that melts the ice enough to erase impact craters. It will search Enceladus for small, geyser-like volcanoes; some scientists suspect such volcanoes may shoot ice particles into space, where they are captured by Saturn's gravity and made part of the outer E-ring. The moon Iapetus is another subject for detailed study because of its unique surface: half the moon is covered with a snow-bright substance, while the other half is covered with material as dark as asphalt and thought to be complex organic material. Cassini will help determine the surface composition of Iapetus, discover the nature of the dark material and determine whether it came from within the moon or was deposited from another source.
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