Secrets of Titan’s smog

16 May 2007

Researchers have identified molecules in the atmosphere of one of Saturn’s moons that are responsible for its smog-like haze.

The findings, published in the 11 May 2007 issue of ‘Science’, were gathered using the Cassini spacecraft, cooperative mission of NASA, the European Space Agency and the Italian Space Agency.

The atmosphere of Titan is of great interest as it is the only one in the solar system remotely like that of Earth, containing a nitrogen-rich mix of gases.

Data collected using the Cassini plasma spectrometer and the UK-led sensor on it, the electron spectrometer, have revealed an unexpected cloud of very heavy ions 1000km above Titan’s surface.

These ions are complex organic molecules formed from methane and nitrogen when exposed to intense sunlight, such as polycyclic aromatic hydrocarbons (PAH’s) and similar compounds containing nitrogen. They gradually form more and more complex molecules – reaching masses of 8,000 times that of a single hydrogen atom.

These molecules sink towards Titan’s surface, forming a group of compounds named ‘tholins’. Tholins were first observed in a 1953 experiment that demonstrated that organic molecules could be formed from inorganic precursors. As such, they may provide the building blocks from which life forms.

Co-author of the paper Dr Andrew Coates (UCL Mullard Space Science Laboratory), said: “It’s humbling to think that, with our instrument at Titan, we may be seeing processes which were at work in Earth’s early atmosphere and which eventually led to life on Earth. It turns out that Titan’s atmosphere is an organic chemical factory on a grand scale. To see such heavy negative ions was a big surprise for us, and is a key finding linking processes in Titan’s atmosphere to the surface of Titan itself – and perhaps to dark, PAH-related deposits on Saturn’s other moons.“

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Image: Titan’s murky atmosphere (credit: NASA/JPL/Space Science Institute)


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