— Saturn’s rings and moons turned out wilder than any scientist could have imagined, but unknowns remain as the Cassini spacecraft concludes its primary mission and embarks on a new one.
“One of the greatest surprises about Cassini’s science results is that some of the most extreme predictions have turned out to be correct,” said Bob Pappalardo, a geologist at NASA’s Jet Propulsion Laboratory in , who recently signed on as Cassini project scientist.
Findings from the four-year primary mission include liquid lakes on Saturn’s moon Titan, icy plumes spouting from the moon Enceladus, and gigantic storms that make Saturn seem like Jupiter.
Pappalardo and other scientists look forward to watching as Saturn serves up even more surprises – and perhaps some answers as well – during the extended, two-year Cassini tour that began June 30.
Saturn’s rings appear less like orderly lanes of traffic and more like highways from hell, with moonlets the size of football stadiums plowing through the frozen debris or pulling off streams of the stuff with their gravitational pull. “Seeing the gravitational effects between moons and ring particles is pretty revolutionary in understanding how the rings work,” Pappalardo said.
The discovery of the moonlets suggested that Saturn’s rings are the remnants of a moon shattered by collision with a comet or asteroid.
Still, Saturn’s iconic rings almost took a backseat to the wacky weather that dominates the gas planet. A hurricane-like storm appeared at the planet’s south pole, while Cassini also confirmed a bizarre hexagon shape circling the north pole that still puzzles scientists.
Cassini similarly witnessed several electrical storms, including a long-lived one that first appeared in late 2007. That monster storm produced lightning 10,000 times more powerful than any seen on Earth, over an area spanning thousands of kilometers.
Scientists find equal fascination in what lies beyond the ringed planet. Scientists now know that Saturn harbors more than 60 larger moons; when Cassini launched in 1997 the number of known moons was about 18.
Cassini dropped the European Space Agency’s Huygens probe into Titan’s clouds in early 2005 and discovered Earth-like weather and environmental effects, but combined with completely different chemical building blocks. Methane and ethane rain down instead of water to form lakes and rivers. A hidden ocean also apparently lurks beneath Titan’s crust.
In March 2008 Cassini flew through an icy geyser emanating from the moon Enceladus and found organic molecules such as carbon dioxide and methane. That suggests the moon may contain conditions for life, although scientists have yet to confirm liquid water.
Pappalardo described Enceladus as “a tiny moon that’s incredibly active” despite its relatively small size, equal to that of . Saturn’s gravitational influence flexes the moon and produces tidal heating that may allow for a liquid ocean beneath the surface of Enceladus, although scientists still debate this.
“Seems like an ocean should freeze up pretty quickly,” Pappalardo said, adding that tidal heating and impurities within the ocean might keep it liquid. “It’s a great mystery, and it’s teaching us about how icy satellites work.”
The extended Cassini tour will include more visits to Titan and Enceladus, and not only because Cassini uses Titan for gravitational boosts during flybys. Both moons seem to suggest the possibility of life in the outer solar system, along with Jupiter’s moon Europa.
Sunset to see
Scientists also have their eyes set on the upcoming Saturn equinox in August 2009, when the sun “sets” on the ring and passes through the ring plane. That event allows for an unprecedented view of the overall ring.
“We can look for subtle warping of the ring, thermal effects and shadow effects,” Pappalardo said. “It may not be a perfectly flat disk.”
Cassini’s extended mission also could resolve a longstanding mystery and pin down the exact length of Saturn’s day. The measurement of Saturn’s rotation is based on the rotation of the planet’s magnetic field, but that magnetic rotation rate has changed between the time of the Voyager encounters with Saturn in the 1980s and Cassini’s arrival in 2004.
Pappalardo compared Cassini to a ship floating in Saturn’s magnetic field ocean – a metaphor that also could apply to the ongoing Cassini mission as it continues to plumb Saturn’s secrets. “Slowly over time, you cross the ocean at different times and days and seasons … Eventually you start to get a picture of what goes on.”