Saturn's moon Titan is known for having an atmosphere similar to the Earth's, and even more similar to what we think Earth's was like billions of years ago. An explanation has now been found for an unexpected similarity: winds of hydrocarbons blown into space from the poles.
"Titan's atmosphere is made up mainly of nitrogen and methane, with 50% higher pressure at its surface than on Earth," said Professor Andrew Coates of University College London. Using the Cassini Plasma Spectrometer (CAPS), Coates observed that Titan's poles are spitting out a mix of hydrocarbons and nitriles, organic compounds with a triple-bonded carbon and nitrogen functional group.
"Data from CAPS proved a few years ago that the top of Titan's atmosphere is losing about seven tonnes of hydrocarbons and nitriles every day, but didn't explain why this was happening,” Coates said.
In Geophysical Research Letters, Coates explains that these chemicals are being driven into space by sunlight and a magnetic field interacting with Titan's upper atmosphere, which contains an array of organic molecules.
"Although Titan is ten times further from the Sun than Earth is, its upper atmosphere is still bathed in light," said Coates. "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. 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."
CAPS found photoelectrons following the magnetic field lines for distances of up to 6.8 times Titan's radius. The negatively charged field from these electrons pulls the positively charged molecules out of the sunlit side of the atmosphere, creating the “polar wind” CAPS observed.
“We have been working up to this [explanation] since first noticing ion escape via Titan's tail a few years ago and estimating the rate in 2012,” Coates told IFLS. “Now, we have enough statistics from several Titan encounters in the ionosphere and the tail – and can see the 'fingerprint' of photoelectrons via the peak we see.”
The Earth produces a similar polar wind, made up of an array of ionized atoms and molecules. It was first discovered by researchers trying to explain how the helium produced by radioactive decay within the Earth was escaping the atmosphere, but more complex molecules have also been found. Similar such winds are thought to exist on Mars and Venus, with Coates presenting evidence for the latter earlier this year.
However, the finding is remarkable because Titan has no self-produced magnetic field, but is surrounded by Saturn's. Coates pointed out that this is the first time that such a wind has been seen, or even suspected, on an unmagnetized body. The paper also notes that other neutral particles, coming from geysers on Enceladus, experience the same ionizing effect and have also been seen following the magnetic field lines.
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