Cassini sees precursors to aerosol ‘snow‘ on Titan  

A new study has proved the presence of Pahs (polycyclic aromatic hydrocarbons) in the upper atmosphere of Titan, the Saturn's largest moon. They form high up in the atmosphere, later grow into larger aggregates that drift down, giving rise eventually to aerosols. The research, carried out by different scientific institutions, including the Italian Cnr-Institute of Atmospheric Sciences and Climate (Isac) and Inaf, is published in 'The Astrophysical Journal' and it based on data collected with the Visible and Infrared Mapping Spectrometer (Vims) on board the Cassini-Huygens mission, the international project between Nasa, Esa and Italy's Asi space agency.

The detected Pahs trigger the first reactions that cause the large, solid particles that sink, like snow flakes, into Titan's lower atmosphere. "We can finally confirm that Pahs play a major role in the production of Titan's lower haze, and that the chemical reactions leading to the formation of the haze start high up in the atmosphere", comments Manuel López-Puertas from the Instituto de Astrofísica de Andalucía (Csic) in Granada, Spain and first author of the paper. "This finding is surprising: we had long suspected that Pahs and aerosols were linked in Titan's atmosphere, but didn't expect we could prove this with current instruments", he adds.

The scientists have been studying the emission from various molecules in Titan's atmosphere when they stumbled upon a peculiar feature in the data. One of the characteristic lines in the spectrum, due to emission by methane, had a slightly anomalous shape, and the scientists suspected it was hiding something. "We subtracted from the observed spectra the signal caused by methane, which is very strong because this molecule is quite abundant in Titan's atmosphere. And that's when we found out that it was covering up something else", explains co-author Bianca Maria Dinelli from the Cnr-Isac in Bologna, Italy. "We found an additional emission feature. But we had no idea what it was", she adds.

Painstaking investigation followed to identify the chemical species responsible for this emission. The additional signal was found only during daytime, so it clearly had something to do with solar irradiation. "The central wavelength of this signal - about 3.28 microns - is the typical one of the emission from aromatic compounds - hydrocarbon molecules in which the carbon atoms are bound in ring-like structures", clarifies Dinelli.

The researchers tested whether the unidentified emission could be produced by benzene, the simplest aromatic compound consisting of one ring only, which had been earlier detected in Titan's atmosphere. After they ruled benzene out, the scientists tried to reproduce the observed emission with Pahs, which are more complex aromatic molecules containing several rings. And they were successful: the data can be explained as emission by a mixture of many different Pahs, which contain an average of 34 carbon atoms and about 10 rings each. "Although less abundant than benzene, Pahs are very efficient in absorbing ultraviolet radiation from the Sun, redistributing the energy within the molecule and finally emitting it at infrared wavelengths", explains co-author Alberto Adriani from the Istituto di Astrofisica e Planetologia Spaziali (Inaf) in Rome, Italy. Adriani is part of the Cassini-Vims co-investigator team based at Inaf that collected and processed the data.

"It is not only this 'solar pumping', but also the peculiar characteristics of Pahs that cause these molecules to radiate so profusely, after having absorbed ultraviolet photons from the Sun,  even in the rarefied environment of Titan's upper atmosphere, where the collisions between molecules are not very frequent", comments López-Puertas.

"The direct detection of Pahs in Titan's atmosphere is an important step in understanding the role of carbon compounds in another body in the solar system", says Nicolas Altobelli, Cassini-Huygens project scientist at Esa. "In the future, we plan to study how these compounds behave with the seasons in the data from Cassini: aerosols in the lower haze are known to undergo seasonal variations, so finding a similar trend in the Pahs would be a further proof of their close connection".

Fonte: Bianca Maria Dinelli, Istituto di scienze dell'atmosfera e del clima, Bologna, tel. 051/6398055, email BM.Dinelli@isac.cnr.it

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