March 09, 2005 —
Smoggy Titan, a moon of Saturn almost the size of Mars, has kept its
secrets well hidden beneath a soupy mix of
Rocks about the size of a fist and smaller pebbles appear to
be eroded, perhaps by flows of liquid methane, in this first-ever photo
of Titan’s surface.
But four close encounters by the Cassini spacecraft have changed all of
In an unprecedented first look into the moon’s mucky atmosphere, USC’s
Donald Shemansky, professor of aerospace and mechanical engineering in
the Viterbi School of Engineering and a co-investigator on the Cassini
Ultraviolet Imaging Spectrograph (UVIS) team, has begun to measure the
temperature structure and composition of Titan’s thick cloud
cover. Titan’s haze extends hundreds of miles above its surface
and is so thick that it blocks sunlight from reaching the moon’s
Shemansky says Titan’s atmosphere is made up of roughly 95 percent
nitrogen and 3 percent methane, but the air also contains a variety of
carbon-hydrogen compounds, called "hydrocarbons," which create thick
lower layers of atmospheric haze. That rich organic soup covers Titan
in a gooey mud of methane, ethane, acetylene and propane — what on
Earth would be considered one colossal oil refinery. If oxygen
were present, Titan's air would ignite with the stroke of a match.
New ultraviolet data from Shemansky’s instrument — the first
close-range data ever gathered of Titan’s opaque atmosphere — reveals a
moon that is locked in a deep freeze of primordial ingredients that
formed with Titan billions of years ago. The UV imagers are able
to determine the composition, distribution, aerosol content and
temperatures of Titan’s atmosphere from the stratosphere, 10 miles up,
to the thermosphere, about 180 miles up, at which point the UV
instrument’s transmission is blocked by haze. Shemansky and his
imaging team will publish the results of their look at Titan’s
atmosphere in a June issue of Science magazine.
It’s just the beginning of Cassini’s flights past the giant moon — with
40 more Titan encounters to go — but the scientific payoff this early
in the mission has been tremendous, Shemansky says.
An Oddball Moon
Titan, the second largest moon in the solar system, is something of an
oddball. Its surface pressure is about 50 percent greater than
the atmospheric pressure at sea level on Earth.
Like Earth, its atmosphere is predominately nitrogen. Much of it
probably came from ammonia, a compound of nitrogen and hydrogen that is
quite common in the outer solar system, Shemansky says. Ammonia is
easily broken into hydrogen and
Cassini saw fewer clouds over Titan’s south pole on its third
flyby in February 2005 than it did in previous flybys. This mosaic of
images was taken from a distance o fabout140,000 - 150,000 miles from
nitrogen atoms by the Sun’s ultraviolet
radiation. But because Titan’s gravity is too weak to retain hydrogen
atoms, these atoms escape into space and leave the more massive
nitrogen atoms behind.
The second most abundant gas on Titan is methane, which is the
principal component of the “natural gas” used on Earth as fuel. The gas
constitutes about 5 percent of the atmosphere near Titan’s surface,
according to the latest data from Shemansky’s team. As it mixes with
other molecules at higher altitudes, it separates into novel
subspecies, called “dissociation products,” which interact with
nitrogen atoms to form “nitriles,” such as hydrogen cyanide. These
radical hydrocarbons polymerize and form the aerosol haze that is seen
at altitudes up to and above 600 kilometers (372 miles).
At 372 miles above the surface all of the chemical changes in
the atmosphere seem to cease, Shemansky reports. Solar extreme
ultraviolet (EUV) radiation does not reach any farther down or mix to
produce radical hydrocarbons. Organic soot drifting down from the upper
atmosphere will eventually accumulate in a thick bottom layer of haze.
Scientists thought the Cassini-Huygens probe, which landed with a
“splat” on Titan’s surface Jan. 14, would break through that
Titan’s upper atmosphere consists of a surprising number of layers of haze extending
hundreds of kilometers above the surface.
about 72 kilometers (45 miles) above the surface, but they were
surprised. The probe did not break through the haze until it was
32 kilometers (20 miles) above ground. Then it encountered
methane clouds at about 16 kilometers (10 miles) above the
Below that, the probe found a dark, rocky landscape washed clean by methane showers
and soaked in pools of standing liquid.
Ingredients for Life
Putting all the data together, Titan appears to have all of the organic
molecules that were present in the chain of chemistry that led to life
on Earth. But there are differences that precluded life from taking
hold on a moon so far away from the Sun.
“Titan’s too cold and it can’t support liquid water,” says Shemansky,
who analyzes UV spectra in his second-floor USC office in Rapp Engineering
Laboratory. “Sunlight doesn’t reach Titan’s surface, so daytime
would be about as bright as a moonlit night on Earth. The surface is
never warmed as it is on Earth; ground temperatures average –175
degrees Celsius (-283 degrees Fahrenheit).”
Lifeless as it may be, Titan’s intricate chemistry produces methane
showers. On Earth, methane exists as a gas, but in very cold
climates, methane can exist as a liquid or be frozen into ice. That
explains Titan’s tar-like permafrost and possible oceans of liquid
Radar images taken by the 705-pound Huygens probe as it parachuted
through Titan’s atmosphere revealed icy highlands “washed bare by
full disk view of Titan was taken at a distance of about 621,000 miles,
as Cassini prepared for its first close encounter on Oct. 24, 2004,
with the giant moon.
methane rain.” The rain could be carving channels and emptying
into vast floodplains, where the methane gradually seeps back into the
ground, European Space Agency scientists say. Examining data from
the probe, they also reported seeing “cryovolcanic” features, most
likely produced by eruptions of melted water mixed with ammonia.
Like many of his colleagues, Shemansky
remains guarded about the interpretations and processes that have
shaped Titan’s surface. It’s been 25 years since the Voyager 1
spacecraft first attempted to peer into Titan’s opaque womb of
clouds. Shemansky knows it will take many more years to
understand the thermal structure of Titan’s atmosphere, which
influences conditions on its surface. Right now, Titan’s
atmosphere seems to be unlike anything found on Earth.
No Warming Effect on Titan
“Different atmospheric constituents (species) cause
the radiative cooling process on Titan and haze prevents solar
radiation from reaching the surface, so clearly, we are not dealing
with an Earth-like environment,” he says. “Heating of the air on Earth
is controlled at the surface by convective transfer, as the air comes
in contact with the ground and radiates back up. That gives us a
warming effect that isn’t present on Titan.”
Another key difference lies in the “mesopause,” a
level of Earth’s upper atmosphere at an altitude of about 90 kilometers
(56 miles), above which temperatures rise as altitude increases,
instead of falling as they do in the lower atmosphere. On Earth,
heating occurs in this layer from oxygen interactions with solar
radiation. On Titan, the heating is controlled mainly by molecular
nitrogen interactions with UV light.
mosaic of images taken by the Huygens probe as it parachuted through
Titan's atmosphere reveals what looks like a shoreline.
“The differences are striking,” Shemansky says. “The
temperature in Titan’s mesopause is 114 Kelvin (-254 degrees
Fahrenheit), almost the same as we find in Earth’s mesopause, but at a
much higher altitude of 615 kilometers (381 miles) above Titan’s
surface. The warming is occurring much higher up.”
Mysteries also surround the source of Titan’s
nitrogen. No one really knows where it came from. Did molecular
nitrogen accumulate as Titan formed or is it a byproduct of the ammonia
that formed with Titan? Perhaps it came from comets.
Shemansky says it is too soon to tell. “I
don’t think we really know yet what is going on in Titan’s atmosphere,
or on the surface, for that matter,” he admits.
Not The Only Mystery
Titan’s alien atmosphere isn’t the only riddle keeping scientists busy.
As the spacecraft flew through Saturn’s sparkling necklace of icy rings
and entered orbit last July, the planet let loose with a massive
eruption of atomic oxygen way out in the outer rings. UV data indicated
that about 275 million pounds (125 million kilograms) of oxygen was
abruptly released in a short period of time.
“That was our first surprise in the ultraviolet,” says Shemansky, who
continues to analyze the data along with Janet Hallett, a postdoctoral
aerospace research associate. The huge oxygen burst may be an
indication that Saturn’s wispy E ring is eroding so fast that it could
disappear within 100 million years if not replenished.
“We aren’t sure yet whether this was a transient event or part of a
routine recycling process in Saturn’s magnetosphere,” Shemansky says.
“Right now scientists are speculating that the oxygen eruption may have
been caused by a collision of ice particles from the planet’s distant E
ring with material in one of the main ring systems, A, B or C. Or
it could have been a meteorite collision or an eruption of icy slush on
Enceladus, a moon that orbits in the E ring.”
The observations suggest that Saturn’s placid appearance from Earth is
anything but that. On the contrary, Saturn commands a dynamic
world of complex, braided ice rings, cannibalistic moons, 1,100
mile-per-hour planetary winds and electrifying auroral displays high in
the night skies.
The planet, its moons and highly structured rings live inside a huge
cavity in the solar wind created by the planet’s strong magnetic
field. The magnetosphere is a bubble of particles including
electrons, various species of ions, neutral atoms and molecules,
several populations of very energetic charged particles like those
found in Earth’s Van Allen Belts, and charged dust grains. These
ionized (electrically charged) gases are called plasmas. However,
unlike Jupiter’s magnetosphere, Shemansky says Saturn’s magnetic cocoon
is smaller and filled primarily with neutral gas rather than
dazzling ring system, shown in this view from beneath the ring plane,
is highly structured, with gaps, gravitational resonances and wave
“Saturn’s magnetosphere is turning out to be very different from
Jupiter’s,” he says. “It’s dominated by neutral gas and
water-rich ingredients produced by its rings, as icy moon debris
collides, or by the more energetic collisions of incoming
meteorites. It doesn’t have nearly as many charged particles, and
many of them are absorbed by the rings, so the plasma processes we are
observing are entirely different.”
Photos from NASA/JPL/Space Science Institute