is the largest of Saturn's satellites. It is the second
largest satellite in the solar system, and the only one
known to have a dense atmosphere.
may be the most interesting body, from a terrestrial perspective,
in the solar system. For almost two decades, space scientists
have searched for clues to the primeval Earth. The chemistry
in Titan's atmosphere may be similar to what occurred in
Earth's atmosphere several billion years ago.
of its thick, opaque atmosphere, astronomers believed Titan
was the largest satellite in the solar system. Their measurements
were necessarily limited to the cloud tops. Voyager 1's
close approach and diametric radio occultation show Titan's
surface diameter is only 5,150 kilometers (3,200 miles)
- - slightly smaller than Ganymede, Jupiter's largest satellite.
Both are larger than Mercury. Titan's density appears to
be about twice that of water ice; it may be composed of
nearly equal amounts of rock and ice.
Titan's thick haze layer
(Click on the image for a
surface cannot be seen in any Voyager photos; it is hidden
by a dense, photochemical haze whose main layer is about
300 kilometers (200 miles) above Titan's surface. Several
distinct, detached haze layers can be seen above the opaque
haze layer. The haze layers merge with the main layer over
the north pole of Titan, forming what scientists first thought
was a dark hood. The hood was found, under the better viewing
conditions of Voyager 2, to be a dark ring around the pole.
The southernhemisphere is slightly brighter than the northern,
possibly the result of seasonal effects. When the Voyagers
flew past, the season on Titan was the equivalent of mid-April
and early May on Earth, or early spring in the northern
hemisphere and early fall in the south.
pressure near Titan's surface is about 1.6 bars, 60 percent
greater than Earth's. The atmosphere is mostly nitrogen,
also the major constituent of Earth's atmosphere.
surface temperature appears to be about 95 Kelvins (-289
degrees Fahrenheit), only 4 Kelvins above the triple-point
temperature of methane. Methane, however, appears to be
below its saturation pressure near Titan's surface; rivers
and lakes of methane probably don't exist, in spite of the
tantalizing analogy to water on Earth. On the other hand,
scientists believe lakes of ethane exist, and methane is
probably dissolved in the ethane. Titan's methane, through
continuing photochemistry, is converted to ethane, acetylene,
ethylene, and (when combined with nitrogen) hydrogen cyanide.
The last is an especially important molecule; it is a building
block of amino acids. Titan's low temperature undoubtedly
inhibits more complex organic chemistry.
has no intrinsic magnetic field; therefore it has no electrically
conducting and convecting liquid core. Its interaction with
Saturn's magnetosphere creates a magnetic wake behind Titan.
The big satellite also serves as a source for both neutral
and charged hydrogen atoms in Saturn's magnetosphere.