Titan is the only known moon with a significant atmosphere.
Its atmosphere is the only nitrogen-rich dense atmosphere in the Solar System
aside from Earth's. Observations of its atmosphere made in 2004 by Cassini
suggest that Titan is a "super rotator", like Venus, with an
atmosphere that rotates much faster than its surface. Observations from the
Voyager space probes have shown that Titan's atmosphere is denser than Earth's,
with a surface pressure about 1.45 times that of Earth's.
Titan's atmosphere is
about 1.19 times as massive as Earth's overall, or about 7.3 times more massive
on a per surface area basis. It supports opaque haze layers that block most
visible light from the Sun and other sources and renders Titan's surface
features obscure Titan's lower gravity means that its atmosphere is far more
extended than Earth's. The atmosphere of Titan is opaque at many wavelengths and
a complete reflectance spectrum of the surface is impossible to acquire from
orbit. It was not until the arrival of the Cassini–Huygens spacecraft in 2004
that the first direct images of Titan's surface were obtained.
Titan's atmospheric composition in the stratosphere is 98.4%
nitrogen with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen
(0.1–0.2%).There are trace amounts of other hydrocarbons, such as ethane,
diacetylene, methylacetylene, acetylene and propane, and of other gases, such
as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen,
argon and helium. The 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. Titan spends 95% of its time
within Saturn's magnetosphere,
Energy from the Sun should have converted all traces of
methane in Titan's atmosphere into more complex hydrocarbons within 50 million
years a short time compared to the age of the Solar System. This suggests that
methane must be somehow replenished by a reservoir on or within Titan itself.
The ultimate origin of the methane in Titan's atmosphere may be its interior,
released via eruptions from cryovolcanoes.
On April 3, 2013, NASA reported that complex organic
chemicals could arise on Titan based on studies simulating the atmosphere of
Titan. On June 6, 2013, scientists at the IAA-CSIC reported the detection of
polycyclic aromatic hydrocarbons in the upper atmosphere of Titan.
On September 30, 2013, propene was detected in the
atmosphere of Titan by NASA's Cassini spacecraft, using its composite infrared
spectrometer This is the first time propene has been found on any moon or
planet other than Earth and is the first chemical found by the CIRS. The
detection of propene fills a mysterious gap in observations that date back to
NASA's Voyager 1 spacecraft's first close flyby of Titan in 1980, during which
it was discovered that many of the gases that make up Titan's hazy brown
colored haze were hydrocarbons, theoretically formed via the recombination of
radicals formed by the ultraviolet photolysisof methane, the second-most common
gas in Titan's atmosphere. Voyager 1 also discovered propane, the heaviest
member of the three-carbon family, and propyne, the lightest member of that
family, but did not detect propene.
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