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.

brownish-orange haze.  But four close encounters by the Cassini spacecraft have changed all of that.    

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 surface.  

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 Titan.

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.

haze at 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 surface.    

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 methane. 

Radar images taken by the 705-pound Huygens probe as it parachuted through Titan’s atmosphere revealed icy highlands “washed bare by

This 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.

A 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.  

Donald Shemansky

“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 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 ions.