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When considering such a complex, collaborative effort as the Cassini-Huygens mission, one might ask the question, "Why build it in the first place?"

The answer comes even before the mission's conception. At the beginning, a basic set of science goals are illustrated in a detailed, comprehensive mission plan that scientists hope to achieve and bring to the public at large.

In this case, through Cassini we hope to gain a better understanding of the planet Saturn, its famous rings, its magnetosphere, its principal moon Titan and its other moons or "icy satellites."

Along with extensive preparation, planning and tracking throughout the mission, science objectives are divided into two parts: The goals that occur on the way to the ringed planet, and then those that will occur after the mission's arrival at the Saturnian system.

The Cassini spacecraft is the first to explore the Saturn system of rings and moons from orbit. Cassini entered orbit on Jun. 30, 2004 and immediately began sending back intriguing images and data. The European Space Agency's Huygens Probe dove into Titan's thick atmosphere in January 2005. The sophisticated instruments on both spacecraft are providing scientists with vital data and the best views ever of this mysterious, vast region of our solar system.

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Current Top Article

Cassini Data Show Ice and Rock Mixture Inside Titan

Thursday, March 11, 2010

Titan's Cool and Sluggish Interior
Hi-Res and Full Caption

Credit: NASA/JPL
By precisely tracking NASA's Cassini spacecraft on its low swoops over Saturn's moon Titan, scientists have determined the distribution of materials in the moon's interior. The subtle gravitational tugs they measured suggest the interior has been too cold and sluggish to split completely into separate layers of ice and rock.

The finding, to be published in the March 12 issue of the journal Science, shows how Titan evolved in a different fashion from inner planets such as Earth, or icy moons such as Jupiter's Ganymede, whose interiors have split into distinctive layers.

"These results are fundamental to understanding the history of moons of the outer solar system," said Cassini Project Scientist Bob Pappalardo, commenting on his colleagues' research. Pappalardo is with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We can now better understand Titan's place among the range of icy satellites in our solar system."

Scientists have known that Titan, Saturn's largest moon, is about half ice and half rock, but they needed the gravity data to figure out how the materials were distributed. It turns out Titan's interior is a sorbet of ice studded with rocks that probably never heated up beyond a relatively lukewarm temperature. Only in the outermost 500 kilometers (300 miles) is Titan's ice devoid of any rock, while ice and rock are mixed to various extents at greater depth.

"To avoid separating the ice and the rock, you must avoid heating the ice too much," said David J. Stevenson, one of the paper's co-authors and a professor of planetary science at the California Institute of Technology in Pasadena. "This means that Titan was built rather slowly for a moon, in perhaps around a million years or so, back soon after the formation of the solar system."

This incomplete separation of ice and rock makes Titan less like Jupiter's moon Ganymede, where ice and rock have fully separated, and perhaps more like another Jovian moon, Callisto, which is believed to have a mixed ice and rock interior. Though the moons are all about the same size, they clearly have diverse histories.

The Cassini measurements help construct a gravity map, which may help explain why Titan has a stunted topography, since interior ice must be warm enough to flow slowly in response to the weight of heavy geologic structures, such as mountains.

Creating the gravity map required tracking minute changes in Cassini's speed along a line of sight from Earth to the spacecraft as it flew four close flybys of Titan between February 2006 and July 2008. The spacecraft took paths between about 1,300 to 1,900 kilometers (800 to 1,200 miles) above Titan.

"The ripples of Titan's gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the ground antennas of the Deep Space Network within 5 thousandths of a millimeter per second [0.2 thousandths of an inch per second] even as the spacecraft was over a billion kilometers [more than 600 million miles] away," said Luciano Iess, a Cassini radio science team member at Sapienza University of Rome in Italy, and the paper's lead author. "It was a tricky experiment."

The results don't speak to whether Titan has an ocean beneath the surface, but scientists say this hypothesis is very plausible and they intend to keep investigating. Detecting tides induced by Saturn, a goal of the radio science team, would provide the clearest evidence for such a hidden water layer.

A Cassini interdisciplinary investigator, Jonathan Lunine, said of his colleagues' findings, "Additional flybys may tell us whether the crust is thick or thin today." Lunine is with the University of Rome, Tor Vergata, Italy, and the University of Arizona, Tucson. "With that information we may have a better understanding of how methane, the ephemeral working fluid of Titan's rivers, lakes and clouds, has been resupplied over geologic time. Like the history of water on Earth, this is fundamental to a deep picture of the nature of Titan through time."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of Caltech, manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL. Cassini's radio science subsystem has been jointly developed by NASA and the Italian Space Agency (ASI).

Source: NASA/JPL

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Recent Articles
Scientists Explain Puzzling Lake Asymmetry on Titan

Dec. 01, 2009
Researchers at the California Institute of Technology, NASA's Jet Propulsion Laboratory, and other institutions suggest that the eccentricity of Saturn's orbit around the sun may be responsible for the unusually uneven distribution of lakes over the northern and southern polar regions of the planet's largest moon, Titan. A paper describing the theory appears in the Nov. 29 advance online edition of Nature Geoscience.
Cassini Captures Ghostly Dance of Saturn's Northern Lights

Nov. 25, 2009
Cassini has spotted the tallest known “northern lights” in the solar system, flickering in shape and brightness high above the ringed planet. The new video reveals changes in Saturn’s aurora every few minutes, in high resolution, with three dimensions. The images show a previously unseen vertical profile to the auroras, which ripple in the video like tall curtains. These curtains reach more than 1,200 kilometers (750 miles) above the edge of the planet’s northern hemisphere.
Cassini's Big Sky: The View from the Center of Our Solar System

Nov. 19, 2009
When NASA's Cassini spacecraft began orbiting Saturn five years ago, a dozen highly-tuned science instruments set to work surveying, sniffing, analyzing and scrutinizing the Saturnian system.
But Cassini recently revealed new data that appeared to overturn the decades-old belief that our solar system resembled a comet in shape as it moves through the interstellar medium (the matter between stars in our corner of the Milky Way galaxy).
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Saturn / The Jewel of the Solar System

Saturn filled Cassini's narrow angle camera as the spacecraft approached in March 2004.

With its stunning rings and dozens of moons, Saturn is an intriguing planet for many reasons. The giant planet has a huge magnetosphere and a stormy atmosphere with winds clocked at about 1,800 kilometers per hour (1,118 mph) near the equator. These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in its atmosphere.

Like Jupiter, Uranus and Neptune, Saturn is a gas giant. It is made mostly of hydrogen and helium.

Saturn's rings in true color.



'Jewel of the Solar System'

Saturn's beautiful rings are what set it apart from the other planets in our solar system. It is the most extensive and complex ring system in our solar system, extending hundreds of thousands of miles from the planet. Made up of billions of particles of ice and rock - ranging in size from grains of sugar to houses - the rings orbit Saturn at varying speeds.

There are hundreds of individual rings, believed to be made of pieces of shattered moons, comets and asteroids. Each of the billions of rings particles orbits the planet on its own path.

For centuries, Saturn and its rings puzzled observers. Italian astronomer Galileo Galilei, the first to use a telescope to explore space, couldn't understand why Saturn looked so different in the night sky at different times. We now know this is caused by a shifting in our view of the ring plane. The rings are virtually invisible when they are edge-on to Earth. The rings seem to reappear months later as our angle of view changes.

Despite tremendous advances in observations in the 400 years since Galileo began studying Saturn's rings, many questions remain about the composition and structure of the rings.

Titan's thick, hazy atmosphere.
 
Iapetus' light and dark terrain.



Many Moons

Saturn's 34 known moons are equally mysterious, especially Titan. Bigger than the planets Mercury and Pluto, Titan is of particular interest to scientists because it is one of the few moons in our solar system with its own atmosphere. The moon is cloaked in a thick, smog-like haze that scientists believe may be very similar to Earth's before life began more than 3.8 billion years ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth as well.

In addition to Titan, Saturn has many smaller icy satellites. Some, like Pan, Atlas, Prometheus, and Pandora, are "shepherd moons" that herd Saturn's orbiting particles into distinct ring. Some moons produce twisting and wave patterns in the rings.

One moon, Enceladus, is one of the shiniest objects in the solar system. It's about as wide as Arizona and covered with water ice that reflects sunlight like freshly fallen snow. And strange Iapetus has one side black as asphalt and the other as bright as snow. All of Saturn's moons are unique and intriguing science targets.

Taking a Closer Look

Four NASA spacecraft have been sent to explore Saturn. Pioneer 11 was first to fly past Saturn in 1979. Voyager 1 flew past a year later, followed by its twin, Voyager 2, in 1981.

The Cassini spacecraft is the first to explore the Saturn system of rings and moons from orbit. Cassini entered orbit on Jun. 30, 2004 and immediately began sending back intriguing images and data. The European Space Agency's Huygens Probe is set to dive into Titan's thick atmosphere in January 2005. The sophisticated instruments on both spacecraft are providing scientists with vital data and the best views ever of this mysterious, vast region of our solar system.

Cassini-Huygens is an international collaboration between three space agencies. Seventeen nations contributed to building the spacecraft. The Cassini orbiter was built and managed by NASA's Jet Propulsion Laboratory. The Huygens probe was built by the European Space Agency. The Italian Space agency provided Cassini's high-gain communication antenna. More than 250 scientists worldwide are studying the data streaming back from Saturn on a daily basis.

Source / Credit: NASA / JPL

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