Volume: 03, Issue: 09 05/04/2005 
During its very close flyby on March 9, 2005, the Cassini spacecraft captured this false-color view of Saturn's moon Enceladus, which shows the wide variety of this icy moon's geology. Image courtesy NASA/JPL/Space Science Institute.
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These three views of Titan from the Cassini spacecraft illustrate how different the same place can look in different wavelengths of light. Image courtesy NASA/JPL/Space Science Institute.
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During its closest flyby of Titan on April 16, the Cassini spacecraft found the outer layer of the thick, hazy atmosphere is brimming with complex hydrocarbons. Image courtesy  NASA/JPL/University of Michigan.
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With this false-color view, Cassini presents the closest look yet at Saturn's small moon Epimetheus (epp-ee-MEE-thee-uss). Image courtesy NASA/JPL/Space Science Institute.
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Other Articles in This Issue:
NASA Delays Shuttle Launch Window
Florida Welcomes Mars Reconnaissance Orbiter
Energy of Earth Is Out of Balance
Send Your Students to Camp

Cassini Targets Unique Saturnian Moons

The Cassini spacecraft has been hard at work in the past few months, exploring the unique features of Saturnís many moons. In recent months, Enceladus, Titan, and Epimetheus have been particularly intriguing targets for the spacecraftís watchful eyes.

As of late, the small moon Enceladus has been the source of several fascinating finds. Enceladus measures 310 miles in diameter and reflects nearly 100 percent of the light that hits its ice- covered surface. It orbits Saturn at a distance of approximately 147,500 miles, about two-thirds the distance from Earth to the Moon.

Enceladus is rapidly becoming a very interesting target for Cassini -- so much so that scientists and engineers are planning to revise the altitude of the next flyby on July 14, 2005 to get a closer look. The flyby was to be at an altitude of 620 miles, but the mission team now plans to lower that altitude to about 109 miles. This will be Cassini's lowest-altitude flyby of any object during its estimated four-year tour. An additional encounter is scheduled for March 12, 2008.

Cassiniís most recent discovery on Enceladus was the existence of dust particles around the moon. These particles might indicate the existence of a dust cloud around Enceladus, or they may have originated from Saturn's outermost ring, the E-ring. According to Dr. Thanasis Economou, lead researcher on Cassiniís high rate detector, scientists are taking measurements around the E-ring to distinguish the point at which the dust flux is originating.

Dust particles had been evidenced on two of Cassiniís earlier flybys of the moon. On Feb. 17, Cassini encountered Enceladus at an altitude of 725 miles. On that date, the cosmic dust analyzer with its high rate detector recorded thousands of particle hits during a 38-minute period.

Cassini executed another flyby of Enceladus on March 9 at an altitude of 310 miles, and Economou and the high rate detector team found a ďstream of dust particles." The largest particles detected measure no more than the diameter of a human hair -- too small to pose any danger to Cassini.

Scientists have speculated that Enceladus is the source of Saturn's E ring, the planet's widest, stretching roughly 188,000 miles. It's possible, scientists say, that tidal interactions with Mimas, another moon of Saturn, have heated Enceladus' interior, causing water volcanism.

Another of Cassini's instruments, the magnetometer, recently discovered water ions that could be part of a very thin atmosphere around Enceladus. Since it is a relatively small moon, the amount of gravity it exerts is not enough to hold an atmosphere for long; a strong, continuous source is therefore required to maintain the atmosphere.

The cosmic dust analyzer provides direct observations of small ice or dust particles in the Saturn system in order to investigate their physical, chemical, and dynamical properties. With further analysis, the cosmic dust analyzer might be able to determine whether the particles are made of ice or dust.

But Enceladus is not the only Saturn moon captivating researchers: Titan, the planetís largest moon, has provided several important discoveries. During Cassiniís closest flyby of Titan on April 16, the spacecraft came within 638 miles of the moon's surface and found that the outer layer of the thick, hazy atmosphere is brimming with a variety of complex hydrocarbons. Scientists believe Titan's atmosphere may be a laboratory for studying the organic chemistry that preceded life and provided the building blocks for life on Earth.

Cassini's ion and neutral mass spectrometer detects charged and neutral particles in the atmosphere, providing scientists with valuable information from which to infer the structure, dynamics, and history of Titan's atmosphere. Complex mixtures of hydrocarbons and carbon- nitrogen compounds were seen throughout the range of masses measured by the Cassini ion and neutral mass spectrometer instrument.

"We are beginning to appreciate the role of the upper atmosphere in the complex carbon cycle that occurs on Titan," said Dr. Hunter Waite, principal investigator of the Cassini ion and neutral mass spectrometer and professor at the University of Michigan, Ann Arbor. "Ultimately, this information from the Saturn system will help us determine the origins of organic matter within the entire solar system."

Hydrocarbons containing as many as seven carbon atoms were observed, as well as nitrogen- containing hydrocarbons (nitriles). Titan's atmosphere is composed primarily of nitrogen, followed by methane, the simplest hydrocarbon. The nitrogen and methane are expected to form complex hydrocarbons in a process induced by sunlight or energetic particles from Saturn's magnetosphere. It is surprising, however, to find the plethora of complex hydrocarbon molecules in the upper reaches of the atmosphere. Titan is very cold, and complex hydrocarbons would be expected to condense and rain down to the surface.

Interstellar clouds produce abundant quantities of organics, which are best viewed as the dust and grains incorporated in comets. This material may have been the source of early organic compounds on Earth from which life formed. Atmospheres of planets and their satellites in the outer solar system, while containing methane and molecular nitrogen, are largely devoid of oxygen. In this non-oxidizing environment under the action of ultraviolet light from the Sun or energetic particle radiation, these atmospheres can also produce large quantities of organics. This same process is a possible pathway for formation of complex hydrocarbons on early Earth.

This was Cassini's sixth flyby of Titan, but its exploration has just begun. Thirty-nine more flybys of this remote world are currently planned. The next Titan flyby is Aug. 22.

Besides Enceladus and Titan, Cassini has also made recent discoveries concerning Epimetheus (epp-ee-MEE-thee-uss), another small moon. Cassini recently captured the closest-ever image of the moon, which closely resembles swiss cheese in appearance.

Epimetheus is irregularly shaped and dotted with large, soft-edged craters. These craters indicate a surface that is several billion years old. The moon shares an orbit with another of Saturn's small moons, Janus. The two dance in a planetary tango as they move in almost identical orbits, exchanging orbits every four years rather than colliding. Both play a role in creating intricate waves in Saturnís rings, and both have densities significantly lower than that of solid ice, suggesting they may be "rubble piles" held together by gravity.

At 72 miles across, Epimetheus is slightly smaller than Janus, which is 113 miles wide. Spectra of Epimetheus from the Cassini visual infrared mapping spectrometer indicate that the moon is composed mostly of water ice.

For images and information about the Cassini mission, please visit the following websites:

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