Titan (moon)

Titan

Discovery
Discovered by	Christian Huygens
Discovered in	1655
Orbital characteristics
Semimajor axis	1,221,850 km
Eccentricity	0.0292
Orbital period	15d 22h 41m
Inclination	0.33°
Is a satellite of	Saturn
Physical characteristics
Mean radius	2575.5 km
Surface area	83×106km2
Mass	1.345×1023 kg
Mean density	1.88 g/cm3
Surface gravity	1.35 m/s2
Rotation period	15d 22h 41m (synchronous)
Axial tilt	1.942°
Albedo	0.21
Surface temp	min mean max K 94 K K
Atmospheric pressure	160 kPa

Titan is Saturn's biggest moon. It is larger than either of the planets Mercury or Pluto and is the second largest natural satellite in the solar system after Ganymede. (It was originally thought to be slightly larger than Ganymede, but recent observations have shown that its thick atmosphere caused an overestimation of its diameter). Titan was discovered on March 25, 1655 by the Dutch astronomer Christian Huygens, making it one of the first non-terrestrial moons to be observed.

Table of contents

1 Physical characteristics 1.1 Atmosphere 2 Exploration of Titan 3 Titan in fiction 4 See also 5 External links

Physical characteristics

Titan is similar in bulk properties to Ganymede, Callisto, Triton and (probably) Pluto. 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. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.

Atmosphere

Titan is the only known moon with a fully developed atmosphere that consists of more than just trace gases. The presence of a significant atmosphere was first discovered by Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of a an atmospheric partial pressure of methane of order 100 millibars. Since that time, observations from Voyager program space probes have shown that, in fact, Titan's atmosphere is denser than Earth's, with a surface pressure more than one and a half times that of our planet and supports an opaque cloud layer that obscures Titan's surface features. It is thought that Titan may posses bodies of liquid ethane. Recent radar measurements from Earth suggest that there is no large-scale ocean of ethane on Titan, but it may still be present in smaller lakes.

The atmosphere is 94% nitrogen with significant traces of various hydrocarbons making up much of the remainder (including methane, ethane, diacetylene, methylacetylene, cyanoacetylene, acetylene, propane, along with carbon dioxide, cyanogen, hydrogen cyanide, and helium). These hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog, and Titan's surface may be coated in a tar-like layer of organic precipitate called tholin. Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere, directly exposing it to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.

At the surface, Titan's temperature is about 94 K. At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.

Exploration of Titan

Titan was examined by both Voyager 1 and Voyager 2, with Voyager 1's course being diverted specifically to make a closer pass of Titan. Unfortunately Voyager 1 did not possess any instruments that could penetrate Titan's haze, which had not been known about up until that point in time.

The Cassini-Huygens Mission is scheduled to reach Saturn on July 1, 2004 and will map Titan's surface using radar; it will also drop a probe named Huygens into Titan's atmosphere where detailed measurements will be taken during its descent on January 14, 2005. The Huygens probe may even survive impact with Titan's surface for long enough to send back data on the conditions there.

Titan in fiction

In Arthur C. Clarke's Imperial Earth Titan is home to a human colony with a population of 250.000 and provides an important role in the Solar System's economics; Titan's atmosphere supplies the hydrogen needed to support interplanetary travel.

See also

• Saturn
• List of Saturn's moons

• The Darian Calendar for Titan