Ce n'est pas du tout mon domaine de compétence professionnel, mais je trouve cela intéressant +++
Et le fait que ces étendues liquides connaissent le même type de "cycle" que l'H2O sur Terre soulève vraiment le débat d'une vie basée sur un autre "schéma" que le notre ayant évolué dans cet environnement.
Ci-dessous l'article originel qui a été publié dans
Nature ce W.E :
Nature 454, 587-589 (31 July 2008) | doi:10.1038/454587a; Published online 30 July 2008
Organic lakes on Titan
François Raulin1
While orbiting Saturn, the Cassini spacecraft has spotted lakes containing ethane on Titan, the planet's largest moon. Titan is so far the only planetary object other than Earth that is known to have liquid bodies on its surface.
Titan is the largest satellite of the giant planet Saturn. Its diameter of 5,150 kilometres makes it bigger than Mercury and only 25% smaller than Mars. It is the second-largest satellite in the Solar System after Jupiter's Ganymede based on diameter alone, and would claim first place if its dense atmosphere, which extends more than 1,000 kilometres above its surface, were included. Fly-bys of Titan by the Voyager spacecraft in the early 1980s helped determine the main composition, temperature and pressure of this atmosphere. Although much colder than Earth's (around 90–94 kelvin at the surface), and lacking molecular oxygen (O2), it shows many similarities to the atmosphere of our planet. It consists mainly of molecular nitrogen (N2), with a surface pressure of 1.5 bar and, like Earth, it has a structure comprising a troposphere and a stratosphere. On page 607 of this issue, Brown et al.1 confirm the existence of a further similarity between Earth and Titan — the presence of open bodies of liquid on its surface.
The idea that liquid bodies exist on Titan's surface is not new. After the discovery that Titan's atmosphere contains a substantial amount of methane, the question of its origin and evolution was seen as a key to understanding Titan's mysteries. A combination of light from the Sun and high-energy electrons from Saturn's magnetosphere would have destroyed all methane in the atmosphere within a few tens of millions of years, implying that the methane was being replenished by a source on, or in, Titan. The main volatile organic product of the light-induced breakdown of methane is ethane, and it was proposed that an ocean consisting of liquid ethane and methane with dissolved N2 covered Titan2.
The Voyager probes were unable to get a peek at Titan's surface, as it was masked by hazy layers in the atmosphere. But with the Cassini-Huygens mission, this veil has finally been lifted. Several instruments on the Cassini spacecraft have been observing the moon in regions of the electromagnetic spectrum in which the atmosphere is transparent.
These observations quickly ruled out the possibility of a global ocean3, but radar data strongly supported the possibility of smaller seas and lakes, mainly in the colder northern regions4. Furthermore, camera images taken in mid-2005 showed a dark surface feature near the south pole as big as North America's Lake Ontario, speculatively named Ontario Lacus. Brown et al.1 have now identified the characteristic spectrum of liquid ethane in infrared spectra of Ontario Lacus.
The Cassini-Huygens mission has demonstrated the existence of a complex cycle of methane in Titan's geochemistry similar to the water cycle on Earth (Fig. 1). Large stores of methane seem to exist within Titan in the form of methane hydrates (clathrates) trapped during the formation of the satellite5. Methane may also be produced through the reaction of water with igneous rocks under high pressures, which would also form hydrogen gas. Ethane, formed by photodissociation of methane in the atmosphere, accumulates on Titan's surface, replenishing the surface lakes — which are therefore one possible ethane reservoir, as are haze particles in the atmosphere into which ethane can be sequestrated6.
Figure 1: Titan's methane/ethane cycle.
Methane (CH4) is released into the atmosphere from Titan's interior stores through volcanic action, and evaporates from the lakes of methane and ethane (C2H6) identified by the Cassini spacecraft on the satellite's surface1. Chemical reactions in the atmosphere convert it to ethane; complex organic aerosols consisting of carbon, hydrogen and nitrogen; and hydrogen gas (H2), which escapes into space. Ethane and methane partly condense, forming clouds and hazes that precipitate, replenishing the lakes and bearing many organic species in solution.
High resolution image and legend (202K)
Desolé, l'image est protégée.
Titan's lakes are probably a liquid ethane–methane mixture together with dissolved nitrogen, as previously proposed for the speculative oceans2. Also dissolved in them will be a variety of solutes, mainly organic compounds produced in the atmosphere and rained down in aerosol particles consisting of a nucleus of macromolecular materials coated with volatile hydrocarbons and nitriles7. Such solutes will be much more concentrated in the lakes than in the atmosphere8. Despite the low temperatures, the action of high-energy cosmic rays reaching the satellite's surface may produce additional organic compounds in this exotic chemical reactor.
It has been suggested that cold liquids such as those found in the lakes of Titan could contain life9, but they also have less speculative interests for astrobiologists10. Although composed of a low-temperature, nonpolar solvent very different from water, they provide an analogue to Earth's oceans and are a potential chemical reactor. It is to be hoped that these organic lakes are made a priority target for future exploration missions, such as the Titan/Saturn System Mission (TSSM), a joint venture between NASA and the European Space Agency currently undergoing feasibility studies
