July 2, 2008 -- Four years ago, the Cassini spacecraft arrived at Saturn to answer questions about how planets and other bodies have come to exist. Accompanied by a large and eclectic family of moons and encircled by an intricate collection of rings, Saturn, located 1 billion miles from Earth, is like a mini solar system unto itself.
Cassini's $3.3-billion primary mission ended this week, having successfully completed its key science objectives. In the process, however, it raised even more puzzles, prompting NASA to extend operations for at least another two years.
"What we have seen over the course of our travels has informed, moved and amazed us," said planetary scientist Carolyn Porco, who heads Cassini's imaging team.
Porco's team has analyzed more than 150,000 pictures relayed from the spacecraft, which also is equipped with cloud-penetrating radar, sensors to detect and measure magnetic fields and electrically charged particles, chemical-analyzing spectrometers and other instruments.
Cassini's Ringside Seat
Cassini also dispatched a free-flier probe named Huygens to study the large moon Titan, the most Earth-like world yet discovered and the only other known body with liquids on its surface. Rather than the water that comprises Earth's oceans, lakes, rivers and streams, Titan's liquid appears to be predominately methane. Scientists don't know from where it comes.
"If you just replace the water with liquid methane, the erosion, the weather systems are very much like Earth cycles," Cassini program manager Robert Mitchell, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in an interview with Discovery News.
Assessing Titan was one of Cassini's major accomplishments, though the work is far from complete.
"Everybody knows from Mars how easy it is to get a wrong impression of the place," said Candice Hansen-Koharcheck, a member of both the Cassini and Mars Reconnaissance Orbiter science teams at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We need to map at least 30 to 50 percent to really know what's going on."
Scientists had expected to find a global ocean on Titan and were a bit surprised to find instead chains of lakes and a few seas. "It's unlikely there's life there," Hansen-Koharcheck added.
Moons, Ice Geysers and Other Curiosities
Totally unexpected was Cassini's prized find: ice-spewing geysers on Titan's sister moon Enceladus.
"The lakes on Titan were pretty fabulous, but we had predictions that they were there. Enceladus was one of those 'oh-my-god!' moments," Hansen-Koharcheck said. "That's kind of why I'm addicted to this stuff. I like these surprises, where you go 'Wow, my little brain would have never thought of that.' "
The Cassini team plans to hone in on Enceladus during the extended mission, with seven flybys scheduled over the next two years. Scientists would love to discover what is powering Enceladus' geysers. Among the contenders: radioactive isotopes and tidal forces.
"There's not a good story yet," Hansen-Koharcheck said. "It's one of those science questions that people love to work on because it's hard, it's a challenge."
Scientists also want to use Cassini to explore if another moon Dione has similar, but much less powerful, geyser-like activity and also if the moon Rhea does indeed have rings.
Delayed Retirement
Cassini's extended mission occurs as Saturn shifts in position so that the sun passes through equinox in August 2009.
The alignment creates a window of opportunity for detailed studies of Saturn's rings with sunlight passing directly through the plane of the rings. Eventually the ring shadows will slip past the equator and fall into the southern hemisphere as the sun moves northward, relative to Saturn.
The perspective will be similar to the view of Voyager as flew by Saturn 29 years ago.
Scientists plan to use Cassini to assess ring structure and composition and to test a new theory that the rings are continually reforming from old material.
On Saturn itself, scientists want more information about a storm that has been sitting on the planet's south pole for the last 50 years.
"The phenomenon is not understood," Mitchell said.
Already, scientists are looking ahead to a Cassini follow-on mission after the two-year extension and an eventual plunge from orbit to avoid contaminating any possible indigenous life that may exist on a moon or elsewhere in the Saturn system.
One idea is to fly Cassini through a gap between Saturn and its innermost ring, a risky maneuver that could be its final contribution to science.
Already, Cassini is laying the groundwork for a successor probe. NASA is considering destinations for a new outer planet mission and while Jupiter's moon Europa, which has an underground ocean, remains a top contender, the Cassini team is building a case for Titan, where, as Mitchell put it, "The things of interest are sitting on the surface."
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