June 5, 2013
Cassini Spacecraft Observes Aerosol ‘Snow’ On Saturn Moon Titan
Lee Rannals for redOrbit.com — Your Universe OnlinePAHs) in the upper atmosphere of Saturn's largest moon. This study provides an explanation of the origin of the aerosol particles found in the lowest haze layer that blankets Titan's surface.
Titan has an atmosphere most resembling that of Earth, compared to other Saturn moons. The large moon's atmosphere is mostly composed of molecular nitrogen and contains only small traces of oxygen and water. Methane in Titan's atmosphere plays a similar role to that of water in Earth's atmosphere. Scientists believe the atmosphere of this moon may resemble that of our own planet during its early days, before living organisms.
When sunlight or highly energetic particles from Saturn's magnetosphere hit the layers of Titan's atmosphere, the nitrogen and methane molecules are broken up, forming massive positive ions and electrons. 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.
Aerosols in this lower haze have been studied using data from the descent of the Huygens probe, but their origin remained unclear. The new study of Titan's upper atmosphere may have solved this puzzle with the detection of PAHs. This detection appears to be the precursor to aerosols, triggering the first reactions that cause these large, solid particles to 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," said Manuel LÃ³pez-Puertas from the Instituto de AstrofÃsica de AndalucÃa (CSIC) in Granada, Spain. "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."
The scientists had been studying the emission from various molecules in Titan's atmosphere when they stumbled upon a striking feature in the data. One of the characteristic lines in the spectrum had a slightly anomalous shape, and the scientists agreed 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 Istituto di Scienze dell'Atmosfera e del Clima (CNR) in Bologna, Italy. "We found an additional emission feature. But we had no idea what it was," she adds.
They identified the chemical species responsible for this emission, as well as another signal only during daytime.
"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," explains Dinelli.
The scientists tested whether the emission could be produced by benzene, which is the simplest aromatic compound consisting of one ring only. However, they saw low abundances of benzene, making it not sufficient enough to explain the emission. After ruling this out, the scientists tried reproducing the observed emission with PAHs and found success.
"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.
LÃ³pez-Puertas said it is also the peculiar characteristics of PAHs that cause these molecules to radiate so profusely. The PAHs are a product of photoionization of smaller molecules in the upper atmosphere of Titan. Models show how PAHs can coagulate and form large aggregates due to their greater weight.
"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."