05/06/2013
Scientists have confirmed the presence of PAHs - Polycyclic Aromatic Hydrocarbons - in the upper atmosphere of Saturn's largest moon, Titan
The study, based on data from the VIMS instrument on the Cassini orbiter, provides an explanation of the origin of the aerosol particles found in the lowest haze layer that blankets Titan's surface. The PAHs, which form high up in the atmosphere later grow into larger aggregates that drift down - much like snow flakes - and eventually give rise to aerosols.
Of all the bodies in the Solar System, Saturn's largest moon, Titan, has the atmosphere most resembling that of Earth. Like that of our planet, Titan's atmosphere is largely composed of molecular nitrogen (N2); unlike Earth's atmosphere, however, Titan's contains only small traces of oxygen and water. Another molecule, methane (CH4), plays a similar role to that of water in Earth's atmosphere, and makes up about 2 per cent of Titan's atmosphere. Scientists have speculated that the atmosphere of this moon may resemble that of our planet in its early days, before primitive living organisms enriched it with oxygen via photosynthesis.
When sunlight or highly energetic particles from Saturn's magnetosphere hit the layers of Titan's atmosphere above 1000 km, the nitrogen and methane molecules there are broken up. This results in the formation of massive positive ions and electrons, which trigger a chain of chemical reactions, producing a variety of hydrocarbons - a wide range of which have been detected in Titan's atmosphere. These reactions eventually lead to the production of carbon-based aerosols, large aggregates of atoms and molecules that are found in the lower layers of the haze that enshrouds Titan, well below 500 km.
Aerosols in this lower haze have been studied using data from the descent of the Huygens probe, which reached the surface in 2005, but their origin remained unclear. A new study of Titan's upper atmosphere might have solved the puzzle with the detection of Polycyclic Aromatic Hydrocarbons (PAHs), which are large carbon-based molecules that form from the aggregation of smaller hydrocarbons. The detected PAHs appear to be the precursors to aerosols, triggering the first reactions that cause these large, solid particles to sink, like snow flakes, into Titan's lower atmosphere. The study is based on data collected with the Visible and Infrared Mapping Spectrometer (VIMS) on board the Cassini mission.
Riferimenti: Per informazioni: Bianca Maria Dinelli - Isac-Cnr - Via Gobetti, 101 - 40129 Bologna - email: BM.Dinelli@isac.cnr.it - tel.: +39 051 6398055 - fax: +39 051 6398132
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