Sunlight scattering through the periphery of Titan's atmosphere forms a ring of color.

A detailed analysis of data from NASA's Cassini spacecraft has directly linked the density of the ionosphere – a region in the upper atmosphere that is dominated by electrically charged particles – at Saturn's moon Titan to the 11-year boom-and-bust cycle of activity at our sun. The confirmation was possible because of Cassini's long-term stay in the Saturn system, which began in 2004, and a Swiss-Army-knife-like suite of many instruments.

At planets and solar system bodies with an atmosphere – such as our own Earth – X-rays and extreme ultraviolet radiation from the sun split atmospheric molecules in the upper atmosphere into electrons and ions. Scientists have known about the existence of an ionosphere at Titan since NASA's Voyager 1 spacecraft flew by the moon in November 1980 and have been studying it close up since Cassini arrived.

Some scientists have made predictions that the density of ions in that part of Titan's atmosphere should increase proportionally to solar radiation, but now actual observations by Cassini have been analyzed by an international team of scientists led by Niklas Edberg at the Swedish Institute of Space Physics in Uppsala, Sweden. They compared the amount of ionization at Titan early in the mission, when the sun was at a moderate-to-low level of activity, with data from recent flybys completed at a time of fairly high solar activity. They analyzed data from Cassini's radio and plasma wave science instrument, which features a Langmuir probe that monitors the density, temperature and bulk speed of plasma (electrons and ions).

In a recent paper, published in the Journal of Geophysical Research: Space Physics, the authors said the flux of extreme ultraviolet radiation from the sun had a definite correlation to the amount of ionization in Titan’s ionosphere.

At the beginning of Cassini's mission, solar activity was moderately low and the data did not show any large changes in Titan's ionosphere. But between 2010 and 2012, when solar activity picked up rapidly, Edberg and colleagues found that the electron density in the ionosphere's densest region increased by 15 to 30 percent, compared to the previous average and despite the fact that the Saturn system was moving away from the sun at that time.

For more information about the study, visit: http://sci.esa.int/jump.cfm?oid=53074 .

The Cassini–Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C.

Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.