Titan - Saturn's moon with hydrocarbon lakes under a dense atmosphere
Ti­tan - Sat­urn's moon with hy­dro­car­bon lakes un­der a dense at­mo­sphere
Credit: NASA/JPL/University of Arizona

Titan - Saturn's moon with hydrocarbon lakes under a dense atmosphere

At 5150 kilo­me­tres across, Ti­tan is the sec­ond-largest moon in the So­lar Sys­tem and one of the most mys­te­ri­ous. An at­mo­sphere sur­rounds many plan­ets. Ti­tan, how­ev­er, is the on­ly moon in the So­lar Sys­tem with a sig­nif­i­cant gaseous en­ve­lope. The at­mo­sphere is a brown­ish-or­ange and so dense that the moon’s sur­face can­not be seen at vis­i­ble light wave­lengths. On­ly by us­ing what are known as 'at­mo­spher­ic win­dows' – nar­row wave­length bands in the near and mid­dle in­frared is anal­y­sis of the sur­face pos­si­ble. This tech­nique is used by the in­frared spec­trom­e­ter VIMS (Vis­i­ble and In­frared Map­ping Spec­trom­e­ter) on the Cassi­ni space probe, which has been analysing the Sat­ur­ni­an sys­tem since Ju­ly 2004. Be­cause of the dis­tance of Ti­tan from the Sun, its sur­face tem­per­a­ture is about mi­nus 180 de­grees Cel­sius.

The pic­ture shows a su­per­po­si­tion of VIMS im­ages of Ti­tan in three dif­fer­ent in­frared wave­lengths: 1.3 mi­crons (thou­sandths of a mil­lime­tre, blue),) 2 mi­crons (green) and 5 mi­crons (red). The cir­cu­lar struc­ture in the mid­dle is prob­a­bly an old­er im­pact basin. Ti­tan's equa­to­ri­al lat­i­tudes are most like­ly dry ar­eas, with­out ex­ten­sive 'wa­ter'. The nu­mer­ous liq­uid bod­ies in the north­ern hemi­sphere, one of which is de­scribed in this web ar­ti­cle, the Krak­en Mare, are prob­a­bly part of an ac­tive flu­id cir­cu­la­tion. These lakes are fed by liq­uid hy­dro­car­bons that a drainage sys­tem car­ries out of the sur­round­ing val­leys. The drainage sys­tem, in turn, is fed by methane and ethane pre­cip­i­ta­tion. Many sci­en­tists sus­pect that the ni­tro­gen at­mo­sphere of Ti­tan ex­hibits strong sim­i­lar­i­ties with the prim­i­tive at­mo­sphere of the Earth.

First discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan is the largest of the 53 moons of Saturn we know to exist. With a diameter of 5150 kilometres, Titan is the second largest moon in the entire Solar System second only to Jupiter’s moon Ganymede with a diameter of 5262 kilometres.

Titan is the only moon in the whole Solar System to have an atmosphere, making it a particularly interesting and intriguing research prospect for scientists. Because this atmosphere has an aerosol layer (a layer of tiny suspended particles of hydrocarbons) at a height of 200 to 300 metres and a high proportion of methane in its nitrogen atmosphere, it is impenetrable to telescopes and cameras. Together with distant Pluto, Titan is the last major body in the Solar System about whose surface we know virtually nothing. And we’re talking about a surface area the same size as Africa, Asia and Europe put together.

To researchers, this Saturnian moon is like a trip back in time to our planet’s past that could give them a glimpse of a primitive Earth. Titan and the Earth are the only bodies in our Solar System whose atmospheres are composed principally of nitrogen; in the case of Titan, the proportion of nitrogen is ten times greater than that of our own atmosphere here on Earth. Scientists believe that the atmosphere of Titan might be similar to Earth’s early atmosphere. By exploring Titan, they hope to find clues as to how the 'primitive' Earth could have developed into a planet on which life could form.

A cold, dark world

Titan is approximately 1500 million kilometres away from the Sun. This huge distance from the Sun, combined with its nebulous atmosphere, mean that a person walking about on Titan would have to make do with roughly one thousandth of the daylight we have on Earth. For this reason, solar radiation does little to warm up Titan: the average surface temperature is minus 179 degrees Celsius. It is possible that Titan could have preserved many of the chemical components that preceded the formation of life on Earth by deep-freezing them. Perhaps this Saturnian moon has a 'hydrological' cycle like we do on Earth with clouds, rain, rivers and oceans, but with ethane and methane taking the place of water. Researchers are faced with a whole range of questions. To come closer to answering them, the Cassini orbiter will perform 45 flybys of Titan at distances of as little as 950 km. One high point of the Cassini-Huygens mission was the landing of the Huygens probe on the mysterious moon on 14 January 2005.

  • Elke Heinemann
    Ger­man Aerospace Cen­ter (DLR)
    Pub­lic Af­fairs and Com­mu­ni­ca­tions
    Telephone: +49 2203 601-2867
    Fax: +49 2203 601-3249

  • Ulrich Köhler
    Pub­lic re­la­tions co­or­di­na­tor
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Rutherfordstraße 2
    12489 Berlin

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