Exploring Saturn
- From: "Lance" <lachenicht@xxxxxxxxxx>
- Date: 5 Jul 2005 04:25:11 -0700
New York Times
July 5, 2005
On Saturn, a Spacecraft Is Finding New Worlds
By JOHN NOBLE WILFORD
Of all the planets in the Sun's family, the most spectacular seen from
afar is Saturn, a sphere of ethereal pastels encircled by shimmering
rings of ice. Even up close and under repeated scrutiny by the Cassini
spacecraft for a full year now, Saturn does not disappoint. The new
familiarity becomes the ringed planet and its host of outlying moons of
all sizes and aspects, and excites the mission's attending scientists.
"The mission is going fabulously well, everything we had hoped for and
more," said Dr. Carolyn C. Porco of the Space Science Institute in
Boulder, Colo., the leader of the Cassini imaging team.
The first spacecraft to orbit Saturn, Cassini arrived there a year ago,
on June 30, with plans for at least a four-year tour of the Saturnian
environs. Scientists are already talking up the benefits of an extended
mission, if the craft remains healthy and the National Aeronautics and
Space Administration will foot the bill.
In the first year alone, Cassini threaded the rings for the closest
observations ever of the spreading disk of glistening water ice and
recently climbed into a higher orbit looking down on the rings. From
there the spacecraft sent radio signals penetrating the ring system for
the most detailed look ever at the size, distribution and density of
the icy material. Several similar observations will be made over the
summer.
Other instruments detected lightning and swirling storms on Saturn
itself, and auroras at both poles. They picked up signals from a new
radiation belt in a surprising place, between the inner edge of the
rings and atmosphere of Saturn, the solar system's second largest
planet. They discovered a four-mile-wide moon - scientists call it a
moonlet - that sweeps clear a gap in the rings and makes waves in the
surrounding ring material.
Photographic and radar surveys of Saturn's largest moon, Titan, show a
varied landscape with ridges of ice gravel and boulders, a possible
volcano spewing ice and liquid methane and dark drainage channels
leading to what appears to be a shoreline of a dry lake bed, though no
signs yet of the seas of liquid methane scientists had expected to
find.
But last week NASA announced that Cassini had photographed a dark
feature on Titan that may be a lake of liquid hydrocarbons 145 miles
long and 45 miles wide, about the size of Lake Ontario.
Dr. Alfred McEwen, a member of the imaging team from the University of
Arizona, said, "This is definitely the best candidate we've seen so far
for a liquid hydrocarbon lake on Titan." And Dr. Elizabeth Turtle,
another team member from Arizona, added, "Its perimeter is intriguingly
reminiscent of shorelines of lakes on Earth that are smoothed by water
erosion and deposition."
Dr. Linda Spilker, deputy project scientist at the Jet Propulsion
Laboratory in Pasadena, Calif., said, "The biggest, most exciting
highlight of the mission has been the probe of Titan, seeing this hazy
world for the very first time and landing on the surface."
Titan is a planet-size satellite, larger than Mercury or Pluto and the
only one in the solar system with a substantial atmosphere, primarily
nitrogen with about 3 percent methane. Ever since two Voyager
spacecraft flew by Saturn, in 1980 and 1981, scientists have speculated
on the source of the atmospheric methane and the rich soup of complex
hydrocarbons that envelop Titan in dense smog. A favored model
predicted that the frigid moon (minus 290 degrees Fahrenheit) had a
global ocean of liquid methane.
Riding piggyback on Cassini was the small craft Huygens, developed by
the European Space Agency and the Italian Space Agency to break away
and descend through Titan's atmosphere and parachute to the mysterious
surface. The successful landing occurred on Jan. 14.
Huygens transmitted 350 pictures during descent and the short time it
operated on the surface, and even in the hazy atmosphere the images
were relatively clear and revealing of dark patches of ice presumably
mixed with tarlike hydrocarbons. Dr. Martin G. Tomasko of the
University of Arizona, the principal scientist for the Huygens camera
system, likened the operation to "taking pictures of an asphalt parking
lot at dusk."
The scientists were elated, and confounded. Dr. Laurence A. Soderblom,
a planetary geologist with the United States Geological Survey, said,
"Titan turned out to be unlike anything that I expected."
Dr. Porco, the imaging team leader, had a somewhat different reaction.
"Titan has turned out to be both alien and familiar," she said. "Alien
enough to be thrilling, but familiar enough to give us a prayer of
working out what is going on on the surface."
Liquid methane appears to be the water of Titan. When it rains there,
it rains methane. Flowing methane seems to carve out the drainage
channels and create deltas at the shore. There may even be methane
springs.
When Huygens touched down on the surface, its warmth released a
detectable increase of methane in the immediate atmosphere. The lake
bed at the landing site, though dry now, may be underlain with a
reservoir of liquid methane. Perhaps in other seasons, Dr. Soderblom
suggested, methane rain and methane rivers fill up this lake bed. At
present, he noted, clouds over the southern polar region indicate a
rainy season there.
What the scientists are not finding in the pictures and radar maps is
any evidence of the expected global methane ocean. "We are having to
rethink what's going on," Dr. Spilker said.
Solving the puzzle is crucial to understanding Titan. The presence of
atmospheric methane and a complex hydrocarbon chemistry, combined with
ample nitrogen, intrigues scientists because these conditions (except
for the much lower temperatures) most likely resemble those on Earth
just before life emerged. No one thinks any life exists on Titan, but
it could serve as a laboratory for studying prebiotic Earth.
A slushy mixture of water ice, methane and hydrocarbons appears to have
eroded and recoded Titan's surface periodically. Cassini's radar
surveys show the landscape to be relatively flat and unscarred by
craters. "That's telling us Titan's surface is young relative to the
solar system," Dr. Spilker said.
So where is all the methane coming from to replenish the atmosphere and
possibly resurface Titan?
On a recent close flyby of Titan, Cassini's infrared imaging system
detected a circular feature about 20 miles wide that scientists have
tentatively interpreted as an icy volcano, a cryovolcano. A central
dark region looked like the bowl-shaped caldera of a volcano. Extending
outward were what appeared to be two "flow patterns" where water ice
and methane from an erupting volcano may have spread across the land.
Researchers said the flow patterns were similar to physical features
left by molten lava issuing from volcanoes on Earth and Venus. Voyager
II observed ice geysers spouting from a volcano on Triton, Neptune's
largest satellite. Such eruptions on Titan could be caused by heat
generated by tidal forces flexing the moon as it moves in an elliptical
orbit, drawing closer to Saturn and its powerful gravity and then
receding to a greater distance.
After these findings were reported in the June 9 issue of the journal
Nature, Dr. Bonnie Buratti of the Jet Propulsion Labaratory, a member
of the infrared mapping team, said: "We all thought volcanoes had to
exist on Titan, and now we've found the most convincing evidence to
date. This is exactly what we've been looking for."
Dr. Louise Prockter, a planetary scientist at Johns Hopkins University,
reserved judgment on the interpretation. Writing in Nature, she
cautioned, "The images are not of sufficient resolution to provide
details below a few hundred meters, and the feature may well turn out
to be an impact crater."
Nevertheless, Dr. Prockter wrote, "With 40 more planned close flybys of
Titan, as well as of several other Saturnian moons, we are only at the
beginning of this fantastic journey."
Cassini is scheduled to pass close to Titan again on Aug. 22 and
conduct radar observations of regions, including the site of the
supposed icy volcano. Scientists hope the new data will clarify the
nature of the object.
The next few months should also bring even more dazzling and revealing
pictures of Saturn's rings. Each time Cassini flies by Saturn - usually
on every other orbit of the planet - the large moon's gravity changes
the spacecraft's course. In this way, flight controllers at the Jet
Propulsion Laboratory are gradually shifting Cassini's orbit to reach
higher above Saturn's equatorial plane occupied by the disk of rings.
Eventually, Dr. Porco said, the plan is to have Cassini "looking down
on the rings almost like a bull's-eye."
Previous observations have determined that seven main rings and many
separate ringlets make up the disk circling Saturn. From edge to edge,
the ring system stretches farther than the distance between Earth and
its moon. Although until now their thickness has not been reliably
measured, the rings are estimated to be extremely thin and filled with
mostly water ice ranging from particles less than two inches thick to
frozen objects tens of yards wide, like frozen boulders.
The Voyager spacecraft discovered small moons that scientists say act
as shepherds, their gravity keeping the ring material from straying
from well-defined courses. They also found the gravity of another moon
clearing out the wide Encke Gap in one of the major rings. Cassini, Dr.
Porco said, has now shown that "the interactions between moons and
rings are way more complex than we thought."
Recent Cassini photographs showed a tiny moon orbiting the Keeler Gap
in the ring system. Its presence left a telling pattern on the edge of
the adjacent ring: a scalloped border shaped by the moon's gravity.
Looking closer, scientists detected the rippling effects of nearby
moons, many as yet unseen, running deep in the interiors of the largest
bands of ring material. Forty such "density waves" were observed in the
A ring, one of the broadest.
Dr. Torrence V. Johnson, a planetary scientist at the Jet Propulsion
Laboratory, said the gravitational effects of moons on the rings
produce "something that looks an awful lot like waves in the ocean." He
thinks these dynamic forces could give scientists clues to one of
Saturn's enduring mysteries, the origin and history of the rings.
Dr. Porco has her eye on an even bigger scientific question.
"Some of the most illuminating dynamical systems we might hope to study
with Cassini are those involving moons embedded in gaps," Dr. Porco
said in a statement in May after the discovery of the moonlet that
makes waves. "By examining how such a body interacts with its companion
ring material, we can learn something about how the planets in our
solar system might have formed out of the nebula of material that
surrounded the Sun long ago."
.
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