A study published this week using data from NASA's Cassini mission describes in more detail than ever before how aerosols in the highest part of the atmosphere are kick-started on Saturn's moon, Titan. Scientists want to understand aerosol formation on Titan because it could help shed light on fundamental processes underlying the formation of life, including the early Earth. Understanding the chemistry of such processes also could predict the behavior of smoggy aerosol layers on Earth.
"Titan has the most complex chemistry of any body in the solar system," said Roger Yelle, a professor in the University of Arizona’s Lunar and Planetary Laboratory who co-authored the study. "Cassini discovered large molecules and aerosols high up in Titan’s atmosphere. We have long thought there is a continuum between the two, and with this study, we are able to show that."
According to the new paper, published this week in the Proceedings of the National Academy of Sciences, Titan's trademark reddish-brown smog appears to begin with solar radiation on molecules of nitrogen and methane in the ionosphere – the uppermost layer of the moon’s atmosphere – which creates a soup of negative and positive ions.
"Measurements by Cassini showed large concentrations of aerosol in the ionosphere, where UV sunlight is being absorbed," Yelle said. "In this paper, we report that this is because the sun’s radiation creates charged particles, which interact faster than non-charged particles, and therefore accelerate the chemical reactions."
Collisions among the organic molecules and the ions help the molecules grow into bigger and more complex aerosols. Lower down in the atmosphere, these aerosols bump into each other and coagulate, and at the same time interact with other, neutral particles. Eventually, they form the heart of the physical processes that rain hydrocarbons on Titan's surface and form lakes, channels and dunes.
"This research is relevant if you’re interested in the origin of life, and even though we don’t think there is life on Titan, we believe the same or similar processes could have been possible early on here on Earth," said Yelle, who has been involved in the Cassini mission for more than 25 years. "But since they are no longer happening here, we can’t study them."
The paper was led by Panayotis Lavvas, a Cassini participating scientist who completed a postdoctoral fellowship in Yelle’s lab at the UA and is now based at the University of Reims, Champagne-Ardenne, France. The team analyzed data from three Cassini instruments – the plasma spectrometer, the ion and neutral mass spectrometer, and the radio and plasma wave science experiment, and was able to determine those processes quantitatively.
Previous work done in Yelle’s lab by former graduate student Sarah Horst revealed that basic building blocks of what are thought to be pre-biological molecules are are being made in Titan’s atmosphere.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL is a division of Caltech.