Titan (moon of Saturn)

Discovered by: Christiaan Huygens, 1655
Distance from Saturn: 1,220,000 km
Radius: 2,580 km
Mass: 1.35 x 1023kg
Mean density: 1880 kg/m3
Mean temperature at solid surface: 94 K (-178°C)
Atmospheric pressure: 1.5 bars
Major atmospheric constituents: nitrogen, methane

Saturn's moon Titan was long thought to be the largest satellite in the Solar System, however, recent observations have shown that Titan has a very thick, opaque atmosphere which hides its solid surface. Due to this extensive atmosphere, the surface of Titan cannot be seen at all with visible light. However some surface details are visible in the infrared.

Titan's atmosphere has a surface pressure that is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. These organics are formed as methane, which dominates Titan's upper atmosphere and is destroyed by sunlight.

There are probably two layers of clouds at about 200 and 300 km above the surface. Complex chemicals in small quantities are responsible for the orange color as seen from space.

Observations have revealed that Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. There is some speculation that its interior may still be hot. At the surface, Titan's temperature is about 94 K (-178°C). At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere.

It seems likely that the ethane clouds would produce a rain of liquid ethane onto the surface perhaps producing an "ocean" of ethane (or an ethane/methane mixture) up to 1000 meters deep. Recent ground-based radar observations have cast this into doubt, however.

Recent observations with the HST show remarkable near infrared views of Titan's surface. Voyager's camera couldn't see through Titan's atmosphere but in the near infrared the haze becomes more transparent, and HST's pictures suggest that a huge bright "continent" exists on the hemisphere of Titan that faces forward in its orbit. These Hubble results don't prove that liquid "seas" exist, however; only that Titan has large bright and dark regions on its surface. The landing site for the Huygens probe has been chosen in part by examining these images. It will be just "offshore" of the largest "continent" at 18.1 degrees North, 208.7 degrees longitude.