linguistic:paraphrasing, quality:corruption, structural:expansion
Atmosphere

Titan is the only moon in the Solar System with an atmosphere denser than Earth's, with a surface pressure of 1.448 atm, and it is one of only two moons whose atmospheres are able to support clouds, hazes, and weather—the other being Neptune's moon Triton. The presence of a significant atmosphere was first suspected by Catalan astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903. Due to the extensive, hazy atmosphere, Titan was once thought to be the largest moon in the Solar System until the Voyager missions revealed that Ganymede is slightly larger. The haze also shrouded Titan's surface from view, so direct images of its surface could not be taken until the Cassini–Huygens mission in 2004.

The primary constituents of Titan's atmosphere are nitrogen, methane, and hydrogen. The precise atmospheric composition varies depending on altitude and latitude due to methane cycling between a gas and a liquid in Titan's lower atmosphere — the methane cycle. Nitrogen is the most abundant gas, with a concentration of around 98.6% in the stratosphere that decreases to 95.1% in the troposphere. Direct observations by the Huygens probe determined that methane concentrations are highest near the surface, with a concentration of 4.92% that remains relatively constant up to 8 kilometres (5.0 mi) above the surface. Methane concentrations then gradually decrease with increasing altitude, down to a concentration of 1.41% in the stratosphere. Methane also increases in concentration near Titan's winter pole, probably due to evaporation from the surface in high-latitude regions. Hydrogen is the third-most abundant gas, with a concentration of around 0.1%. There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, and propane, and 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.

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 replenished by a reservoir on or within Titan itself. The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes. On April 3, 2013, NASA reported that complex organic chemicals, collectively called tholins, likely 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 (CIRS). 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 planetary flyby of Titan in 1980, during which it was discovered that many of the gases that make up Titan's brown haze were hydrocarbons, theoretically formed via the recombination of radicals created by the Sun's ultraviolet photolysis of methane.