21 November 2017
Titan’s winter polar vortex imaged by the Cassini Spacecraft’s ISS camera. The vortex is now in deep winter and can only be seen because the polar clouds within the vortex extend high above Titan’s surface into the sunlight. The vortex was extremely cold from 2012-2015 giving rise to unusual nitrile ice clouds.
(Credit: NASA/JPL-Caltech/Space Science Institute/Jason Major)
A new international study, led by the UK, has shown that unexpected temperature changes on Saturn’s largest moon, Titan, are caused by its unique atmospheric chemistry.
The study, which was funded in part by the Science and Technology Facilities Council, investigated why Titan’s polar atmosphere suddenly dropped in temperature recently. This behaviour was contrary to all model predictions and quite different to all other terrestrial planets in our Solar System.
Usually, the high altitude polar atmosphere in a planet’s winter hemisphere is warm because of sinking air being compressed and heated – but Titan’s atmospheric polar vortex seems to be extremely cold instead.
Before it took its final dive this year, the Cassini spacecraft obtained a series of observations of Titan’s polar atmosphere, which showed that the expected polar hot-spot did begin to develop in 2009, but this soon turned cold in 2012, with temperatures as low as 120 K being observed until late 2015.
Only in the most recent 2016 and 2017 observations has the hot-spot returned.
Lead author Dr Nick Teanby from the University of Bristol’s School of Earth Sciences, said: “For the Earth, Venus, and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace gas CO2 and because CO2 has a long atmospheric lifetime it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation.
“However, on Titan, exotic photochemical reactions in the atmosphere produce hydrocarbons such as ethane and acetylene, and nitriles including hydrogen cyanide and cyanoacetylene, which provide the bulk of the cooling.”
The research team showed that the presence of these gases in Titan’s atmosphere is enough to cause the temperature to plunge.
Dr Teanby added: “This effect is so far unique in the Solar system and is only possible because of Titan’s exotic atmospheric chemistry.
“A similar effect could also be occurring in many exoplanet atmospheres having implications for cloud formation and atmospheric dynamics.”
More information available on the Bristol University website.