Saturn's Moons 
Titan: Smog World
Surrounded by a thick atmosphere that's topped by orange smog, and with a landscape that in some ways resembles Earth, Titan is one of the solar system's most intriguing moons.
Titan's frigid atmosphere is about 60 percent denser than Earth's at the surface, and clouds float across the sky. A global haze of organic molecules tops the atmosphere, so it was almost impossible to study the surface until the Cassini spacecraft, which carries instruments that can peer through the haze, arrived at Saturn in 2004.
|Discovery||1655, Christiaan Huygens||1789, William Herschel||1671, Jean Dominique Cassini||1789, William Herschel|
|Distance from Saturn||745,000 miles
1.2 million km
|2.2 million miles
3.6 million km
|Orbital Period||16 days||1.4 days||79.3 days||22 hours, 37 minutes|
Scientists had expected to find large seas or oceans of liquid methane, but Cassini found that there may be only a few small bodies of liquid. The Huygens probe, which parachuted through Titan's atmosphere in early 2005, photographed a landscape that, in some ways, is similar to Earth's, with possible drainage channels carved into hillsides. Huygens' instruments also found a thin layer of ice coating the surface at its landing site.
Titan's surface consists of rock and ices. Volcanoes may belch methane into the atmosphere, contributing to the moon's smog layer.
Tracking Titan's Temperature
As the Huygens probe parachuted through the atmosphere of Saturn's moon Titan in January 2005, it took the first direct measurements of temperatures, wind speeds, and the atmosphere's composition. Yet it sampled only one small region on one day. To fully understand Titan's complex atmosphere, scientists must study the entire moon, and repeat their observations to see how it changes over time.
In support of that goal, McDonald Observatory astronomer Laurence Trafton and his colleagues monitored Titan around the time of the Huygens landing from the Infrared Telescope Facility atop Hawaii's Mauna Kea. Their observations looked at the abundance of an organic molecule that could help yield clues to the atmosphere of the early Earth, which Titan's present-day atmosphere may resemble. The observations also can serve as a comparison to those made by Huygens, helping scientists calibrate the probe's discoveries.
Trafton was using a Texas-built spectrograph, TEXES, to study the molecule cyanoacetylene. This organic molecule is of particular interest because it contains nitrogen, which is one of the ingredients of amino acids, which are some of the "building blocks" of life. The early Earth probably contained similar molecules, so studying them on Titan could yield new information about the atmospheric conditions that led to life on Earth. The molecule also provides a look at temperatures in Titan's upper atmosphere across the entire moon. The Texas researchers found that the temperatures were a little lower than expected. But their observations, which covered primarily the southern hemisphere of Titan, were similar to those of the Voyager 2 spacecraft, which studied Titan's northern hemisphere when it flew past Saturn more than two decades ago.
Their data will form part of the overall picture of Titan that is emerging from Huygens and the Cassini orbiter, with support from astronomers at McDonald and other observatories on Earth.
Enceladus, a small ball of ice and rock, is the most reflective surface in the solar system: it reflects 99 percent of the visible sunlight that strikes it back into space. Cassini discovered that Enceladus has a tenuous atmosphere, which is resupplied by water vapor and water ice leaking from parallel cracks in the crust near its south pole (shown in blue in this color-enhanced Cassini image, right).
Most of this material falls back onto the surface as clean ice, which is highly reflective. In essence, Enceladus re-polishes its surface with material from its interior. Because of the moon's weak gravity, however, much of the material escapes Enceladus and supplies fresh ice to Saturn's outermost ring.
One side of Iapetus is as bright as snow, while the other is almost as dark as charcoal (left). The leading hemisphere may pick up particles of dark material that are blasted off the surface of another moon by impacts with meteorites, or the material may be ice that bubbled up from below the surface and was darkened by exposure to radiation. A ridge up to eight miles (13 km) high circles the equator, making Iapetus look a bit like a walnut. There is no clear explanation for what created the ridge.
Mimas is nicknamed the "Death Star Moon" because a circular crater about one-third of the moon's diameter makes it resemble the big battle stations from the "Star Wars" movies. The crater (top center, above) formed when an asteroid or comet slammed into Mimas. The impact was so powerful that it almost blasted the moon to bits. Had the collision been just a little more powerful, it would have pulverized Mimas, forming a new ring around Saturn. Instead, Mimas appears to sweep out a wide gap in the rings, as some of the ring particles march in step with the Death Star Moon.
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