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 Pasadena, Calif., 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 Saturn serving up even more surprises
during the extended, two-year Cassini tour that began June 30 — and perhaps
some answers as well.
Mysteries
remain
The rings of
Saturn got a true close-up when Cassini arrived. The 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 told SPACE.com.
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.
Electric
world
Cassini similarly
witnessed several electrical storms, including long-lived one that first
appeared in late 2007. That monster
storm produced lightning 10,000 times more powerful than any seen on Earth,
spanning thousands of miles.
Scientists
find equal fascination in what lies beyond the ringed planet. Saturn now
harbors more than 60 larger moons, compared with just 18 known moons when
Cassini launched in 1997.
The
spacecraft dropped its Huygens probe into Titan's clouds and discovered
Earth-like weather and environmental effects there in early 2005, 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 the moon's crust.
Incredible
moon
More
recently, Cassini flew in March 2008 through an icy geyser emanating from the
moon Enceladus and sniffed
out 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 Arizona. 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 about pinning 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 could also 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,"
Pappalardo said. "Eventually you start to get a picture of what goes on."
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