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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."
a.. Copyright 2005 The New York Times Company
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--------------------------------------------------------------------------------
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."
a.. Copyright 2005 The New York Times Company
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