A team of researchers led by Dr Andrew Coates of University College London, UK, has found that the interactions between the atmosphere of Saturn’s moon Titan and the solar magnetic field and radiation create a wind of hydrocarbon and nitrile molecules being blown away from its polar regions into space. This is very similar to the wind observed coming from the polar regions of Earth.
This image is a composite of several images taken during two separate Titan flybys on October 9 and October 25, 2004. Credit: NASA / JPL / University of Arizona.
“Titan’s atmosphere is made up mainly of nitrogen and methane, with 50 percent higher pressure at its surface than on Earth,” said Dr Coates, first author on the study published in the journal Geophysical Research Letters.
“Data from the CAPS plasma spectrometer on NASA’s Cassini spacecraft proved a few years ago that the top of Titan’s atmosphere is losing about 7 tons of hydrocarbons and nitriles every day, but didn’t explain why this was happening. Our new study provides evidence for why this is happening.”
Dr Coates’ team found that this atmospheric loss is driven by a polar wind powered by an interaction between sunlight, the solar magnetic field and the molecules present in the upper atmosphere.
“Although Titan is ten times further from the Sun than Earth is, its upper atmosphere is still bathed in light. When the light hits molecules in Titan’s ionosphere, it ejects negatively charged electrons out of the hydrocarbon and nitrile molecules, leaving a positively charged particle behind,” Dr Coates said.
“These electrons, known as photoelectrons, have a very specific energy of 24.1 electronvolts, which means they can be traced by the CAPS instrument, and easily distinguished from other electrons, as they propagate through the surrounding magnetic field.”
Unlike our planet, Titan has no magnetic field of its own, but is surrounded by Saturn’s rapidly rotating magnetic field, which drapes forming a comet-like tail around the moon.
In 23 flybys which passed through the moon’s ionosphere or its magnetic tail, CAPS detected measurable quantities of these photoelectrons up to 6.8 Titan radii away from the moon, because they can easily travel along the magnetic field lines.
The scientists found that these photoelectrons, spread throughout Titan’s ionosphere and the tail, set up an electrical field.
The electrical field, in turn, is strong enough to pull the positively charged hydrocarbon and nitrile particles from the atmosphere throughout the sunlit portion of the atmosphere, setting up the widespread ‘polar wind’ that planetary researchers have observed there.
This phenomenon has only been observed on Earth before, in the polar regions where Earth’s magnetic field is open.
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Andrew J. Coates et al. A new upper limit to the field-aligned potential near Titan. Geophysical Research Letters, published online June 18, 2015; doi: 10.1002/2015GL064474
