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About Saturn & Its Moons

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Titan - Atmosphere

Titan is the only moon in the solar system with a thick atmosphere. Because Titan is less massive than Earth, its gravity doesn’t hold onto its gaseous envelope as tightly, so the atmosphere extends to an altitude 10 times higher than Earth's – nearly 600 kilometers (370 miles) into space. In 1980 the Voyager 1 spacecraft showed that the air pressure at the surface is one-and-a-half times that of Earth at sea level and four times the density, but the surface temperature is extremely cold, at -290 degrees Fahrenheit (-179 degrees Celsius).

Shifting Northern Hazes
Shifting Northern Hazes
At Saturn's orbit, more than nine times farther from the sun than Earth, the solar illumination is weak, and beneath Titan's smoggy skies it is even weaker. An observer on Titan's surface would experience daytime as dim as deep twilight on Earth.

Chemical Processing Factory

Titan's atmosphere is mostly nitrogen (about 95 percent) and methane (about 5 percent), with small amounts of other carbon-rich compounds. High in the atmosphere, methane and nitrogen molecules are split apart by the sun's ultraviolet light and high-energy particles accelerated by Saturn's magnetic field, and the products of this splitting recombine to form a variety of organic molecules. (Organic molecules contain carbon and hydrogen, and often include nitrogen, oxygen and other elements important to life on Earth.)

In fact, Titan's rich mixture of organic chemicals is one of the qualities that make it such an interesting place to study, since conditions on our own world might have been similar (although much warmer) early in Earth's history, before the appearance of life. Studying Titan could provide insight into how Earth's atmosphere has evolved through the ages.

Some of the compounds produced by the splitting and recycling of methane and nitrogen in the upper atmosphere create a kind of smog – a thick, orange-colored haze that obscures the moon's surface from view. And some of the heavy, carbon-rich compounds also fall to the surface. Some of these hydrocarbons go on to form grains that make up the "sand" of vast dune fields on Titan's surface.

Comparing Earth and Titan
Comparing Earth and Titan
At higher altitudes the haze has multiple layers and a complex structure. In many Cassini images the moon is seen to be enveloped by a thin, high-altitude haze layer which forms a sort of hood over the north polar region.

One of Titan's great mysteries is the source of its methane, which makes all of this complex chemistry possible. Since sunlight breaks down methane in the atmosphere, there must be a source that replenishes what is lost. Researchers suspect methane could be belched into Titan's atmosphere by cryovolcanism, or volcanoes with water as lava, and Cassini continues to look for such activity.

Alien Weather

At such frigid temperatures (-290 degrees Fahrenheit or -179 degrees Celsius at the surface), water is frozen hard as rock, but methane, which is the main component in natural gas, is able to be liquid and solid. Methane, instead of water, forms Titan's clouds, rivers and lakes. Cassini RADAR Team member Dr. Ralph Lorenz has determined that, with Titan's low gravity and dense atmosphere, methane raindrops could grow twice as large as Earth's raindrops, and they would fall more slowly, drifting down like snowflakes. Scientists think it rains perhaps only every few decades, but when it rains on Titan, it really pours –- scouring the high altitude terrain, like the mountainous region of Xanadu, and washing the hydrocarbon grains down to lower altitudes where they collect in the great dune fields. Most of the rain falls on the poles, where it pools in myriad ponds and vast lakes.

As on Earth, the climate is driven mostly by changes in the amount of sunlight that come with the seasons, although the seasons on Titan (and Saturn) are about seven Earth years long. Large fields of puffy clouds have been observed near the south polar region by Cassini and the Hubble Space Telescope. These clouds were plentiful near the time of that hemisphere's summer solstice, which occurred not long before Cassini arrived in the Saturn system, but then became scarce as autumn approached. In the intervening years Cassini has also witnessed streak-like clouds at middle latitudes in both the northern and southern hemispheres, as well as a giant cloud of ethane that formed over the northern polar region.

Spring comes to the northern hemisphere during the Cassini Equinox Mission, and it will be a top priority for scientists to watch for new developments in the atmosphere there. They will also try to measure qualities like how dense the atmospheric layers are, determine exactly what is the mix of chemical compounds in the different layers, and learn how fast the winds blow at and near the moon’s surface. In addition, scientists will be looking to see if there are any changes in the ponds and lakes on the surface.