Atmosféra Titanu

Mlhové vrstvy Titanovy atmosféry ve skutečných barvách

Atmosféra Titanu je atmosféra Saturnova měsíce Titanu. Je to plně vyvinutá atmosféra, která obsahuje více než jen zbytkové plyny. Její tloušťka je odhadována na 200 až 800 km. Atmosféra Titanu je na většině vlnových délek neprůhledná a znemožňuje tak z dálky pozorovat odrazové spektrum povrchu měsíce. Zamlženost atmosféry bylo i příčinou chyb v odhadu jeho průměru.

Přítomnost významnější atmosféry poprvé tušil španělský astronom Josep Comas Solà, který pozoroval okrajové ztemnění Titanu v roce 1903 a potvrdil ji Gerard Kuiper toku 1944, kdy pomocí spektroskopie odhadl parciální tlak metanu na povrchu měsíce na řádově 10 kPa. Z pozorování sond Voyager 1 a 2 vyplynulo, že Titanova atmosféra je hustší než zemská s tlakem na povrchu 1,5 vyšším. To vysvětluje její neprůhledné vrsty, které blokují většinu viditlného světla ze Slunce a dalších zdrojů a vytvářejí dojem obskurních objektů na povrchu Titanu. Atmosféra je ta hustá a gravitace měsíce tak nízká, že lidé by v ní mohli létat za pomoci křídel na svých rukou. Sonda Huygens během svého sestupu k povrchu měsíce nemohla detekovat směr ke Slunci. Snímky, které pořídila z povrchu pak její operátoři přirovnávali k „fotografovaní asfaltového parkoviště za soumraku“.

The atmosphere is 98.4% nitrogen—the only dense, nitrogen-rich atmosphere in the solar system aside from the Earth's—with the remaining 1.6% composed of methane and trace amounts of other gases such as hydrocarbons (including ethane, diacetylene, methylacetylene, acetylene, propane), cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium. The orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates. 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 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. In November 2007, scientists uncovered evidence of negative ions with roughly 10 000 times the mass of hydrogen in Titan's ionosphere, which are believed to fall into the lower regions to form the orange haze which obscures Titan's surface. Their structure is not currently known, but they are believed to be tholins, and may form the basis for the formation of more complex molecules, such as polycyclic aromatic hydrocarbons.

Energy from the Sun should have converted all traces of methane in Titan's atmosphere into hydrocarbons within 50 million years; a relatively 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. That Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, since comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon. Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes. A possible biological origin for the methane has not been discounted (see below).

There is also a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east. Observations by Cassini of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.

Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural ELF waves (see above) on Titan is unclear as there does not appear to be extensive lightning activity.

V tomto článku byl použit překlad textu z článku Titan (moon) na anglické Wikipedii.