TI­TAN

The sec­ond Earth

All About Space - - Strange Moons -

Mass: 1.3 x 1023kg (2.9 x 1023lbs) Di­am­e­ter: 5,150km (3,200 miles) Par­ent planet: Saturn Dis­cov­ered: 1655, Chris­ti­aan Huy­gens

Saturn’s largest moon, Ti­tan is unique in the So­lar Sys­tem as the only satel­lite with a sub­stan­tial at­mos­phere of its own – a dis­cov­ery that frus­trated NASA sci­en­tists when im­ages from the Voy­ager probes re­vealed only a hazy orange ball. The Cassini or­biter was fit­ted with in­frared and radar in­stru­ments that pierced the opaque at­mos­phere, re­veal­ing a soft­ened land­scape of rivers and lakes that is un­like any other world in the So­lar Sys­tem ex­cept for Earth. De­spite be­ing larger than Mercury, Ti­tan can only hold onto its thick at­mos­phere be­cause of the deep cold. Found some 1.4 bil­lion kilo­me­tres (0.9 bil­lion miles) from the Sun, the moon’s av­er­age sur­face tem­per­a­ture is a freez­ing -179 de­grees Cel­sius (-290 de­grees Fahren­heit).

Ti­tan’s at­mos­phere is dom­i­nated by the in­ert gas ni­tro­gen – also the ma­jor com­po­nent of Earth’s air – but it gets its dis­tinc­tive colour, opaque haze and clouds from a rel­a­tively small pro­por­tion of meth­ane. Amaz­ingly, con­di­tions on Ti­tan are just right for meth­ane to shift be­tween its gaseous, liq­uid and solid forms, gen­er­at­ing a ‘meth­ane cy­cle’ rather sim­i­lar to the water cy­cle that shapes Earth’s cli­mate. In cold con­di­tions meth­ane freezes onto the sur­face as frost and ice. In mod­er­ate tem­per­a­tures it con­denses into liq­uid droplets and falls as rain that erodes and soft­ens the land­scape be­fore ac­cu­mu­lat­ing in lakes, while in warmer re­gions it evap­o­rates and re­turns to the at­mos­phere.

Ti­tan ex­pe­ri­ences chang­ing sea­sons very sim­i­lar to those on our planet, though its year is 29.5 Earth years. Tem­per­a­tures at the win­ter pole seem to favour rain­fall, so the lakes mi­grate from one pole to the other over each Ti­ta­nian year. With all this ac­tiv­ity, Ti­tan is an in­trigu­ing tar­get in the search for ex­trater­res­trial life, though most bi­ol­o­gists find it hard to en­vi­sion or­gan­isms that could ex­ist in such harsh and chem­i­cally lim­ited con­di­tions, and most agree that Ti­tan’s wa­tery in­ner neigh­bour Ence­ladus of­fers more promis­ing prospects for life.

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