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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.
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 possess 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— the only nitrogen-rich atmosphere in the
solar system aside from our own— 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.
Surface Features
At present, maps of Titan's surface are vague and imprecise, owing to the obscuring atmosphere. However, a large, highly
reflective area about the size of Australia has been identified in infra-red images, and is already
detectable in visible light images from the Cassini
spacecraft. This region has been unofficially named Xanadu; it is not certain what kind of terrain it represents. There is a similarly-sized dark area
on the other side of the moon, which some have speculated may be a methane sea.
Titan's surface features will be better understood in late 2004, when Cassini will make several close flybys of the moon.
See list of geological features on Titan.
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 release a probe named Huygens on
December 25 2004 which will then dive 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 novel 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.
- In Stephen Baxter's novel Titan, a NASA mission to Titan must struggle to survive after a disastrous
landing.
- In the BBC television show Red Dwarf, the character Lister illegally imports a cat from Titan that goes on to found an
intelligent species.
- In the television show Starhunter, Titan features prominently as the former home of the character Dante, and is the site
of a large colony.
External links
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