Autumn sets in rapidly on Saturn's giant moon

November 28, 2012

Thanks to NASA's Cassini spacecraft which has been orbiting Saturn since 2004, scientists have been able to observe for the first time ever the seasonal atmospheric circulation direction change on Titan - an event which only happens once every 15 years and is never observable from Earth. Their findings are published today in Nature.

Titan, while technically only a moon, is bigger than the planet Mercury, and is often considered a planet in its own right. It is the only known moon to have a significant atmosphere and is one of only four terrestrial atmospheres in our Solar System (the other three being Earth, Venus, and Mars). As Titan's rotation axis is tilted by a similar amount to that of the Earth, it experiences seasons in a similar way, but at a much more relaxed pace as Saturn takes 29.5 Earth years to orbit the Sun.

Dr Nick Teanby of the University of Bristol and colleagues used infrared spectra measured by Cassini's Composite InfraRed Spectrometer (CIRS) instrument to determine atmospheric temperature and global distributions of chemical tracers. This allowed them to map out the seasonal changes in great detail.

The team observed an enormous increase in Titan's exotic trace gases over the south pole within a relatively short time. These trace gases are produced high in Titan's atmosphere, where sunlight and highly energetic particles break down nitrogen and methane and recombine to form a vast array of more complex molecules like benzene and hydrogen cyanide.

Co-author, Dr Remco de Kok of SRON Netherlands Institute for Space Research said: "We were waiting for signs that the trace gas abundances would change together with the new season, but we did not expect such a large and rapid change: some gas concentrations increased more than a thousand times within only a few months. Also surprising was that this was happening at altitudes above 450 km, much higher than initially anticipated."

At these high altitudes the atmosphere goes around the planet much faster than the rotation of Titan's solid surface and can have horizontal wind speeds around the planet of up to 200m/s (450mph). Vertical winds caused by the seasonally varying hemisphere to hemisphere atmospheric circulation are much slower at a rather sluggish few millimetres per second and are hard to measure using conventional means.

Lead author, Dr Nick Teanby of Bristol's School of Earth Sciences said: "Using measurements of temperature and chemical tracers by Cassini, we were able to observe changes in the subtle vertical winds and reveal the pole-to-pole circulation cell. For the first time ever, we observed the circulation cell direction reverse over the south pole around the time of the 2009 southern autumnal equinox. The resulting distribution of gases shows that the circulation must extend much further than previously thought to 600 km or even higher. This calls into question our current understanding of how Titan's atmosphere works and suggests new avenues to explore."

"Our results provide a powerful new constraint for atmospheric models of Titan. Titan provides a natural laboratory for an Earth-like atmosphere in the cold outer solar system. So, these results could eventually lead to a more complete understanding of atmospheric processes on Earth, other Solar System planets, and the many exoplanetary systems now being discovered."

In the coming years Cassini will continue to observe how the seasons develop. Dr Conor Nixon at NASA Goddard Space Flight Centre said: "These results are giving us the first detailed look at changes occurring in Titan's atmosphere around the time of equinox, a season which has not been viewed up close by a spacecraft previously. This shows the really great science that is coming out of the Cassini extended mission phases since 2008, and we look forward to seeing the further changes that will occur over the next five years until the end of mission in 2017."

'Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan' by Nicholas A. Teanby, Patrick G. J. Irwin, Conor A. Nixon, Remco de Kok, Sandrine Vinatier, Athena Coustenis, Elliot Sefton-Nash, Simon B. Calcutt & F. Michael Flasar in Nature

University of Bristol

Related Solar System Articles from Brightsurf:

Ultraviolet shines light on origins of the solar system
In the search to discover the origins of our solar system, an international team of researchers, including planetary scientist and cosmochemist James Lyons of Arizona State University, has compared the composition of the sun to the composition of the most ancient materials that formed in our solar system: refractory inclusions in unmetamorphosed meteorites.

Second alignment plane of solar system discovered
A study of comet motions indicates that the Solar System has a second alignment plane.

Pressure runs high at edge of solar system
Out at the boundary of our solar system, pressure runs high.

What a dying star's ashes tell us about the birth of our solar system
A UA-led team of researchers discovered a dust grain forged in a stellar explosion before our solar system was born.

What scientists found after sifting through dust in the solar system
Two recent studies report discoveries of dust rings in the inner solar system: a dust ring at Mercury's orbit, and a group of never-before-detected asteroids co-orbiting with Venus, supplying the dust in Venus' orbit.

Discovered: The most-distant solar system object ever observed
A team of astronomers has discovered the most-distant body ever observed in our solar system.

Discovery of the first body in the Solar System with an extrasolar origin
Asteroid 2015 BZ509 is the very first object in the Solar System shown to have an extrasolar origin.

First interstellar immigrant discovered in the solar system
A new study has discovered the first known permanent immigrant to our solar system.

A star disturbed the comets of the solar system in prehistory
About 70,000 years ago, when the human species was already on Earth, a small reddish star approached our solar system and gravitationally disturbed comets and asteroids.

Scientists detect comets outside our solar system
Scientists from MIT and other institutions, working closely with amateur astronomers, have spotted the dusty tails of six exocomets -- comets outside our solar system -- orbiting a faint star 800 light years from Earth.

Read More: Solar System News and Solar System Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to