"Indian summer" on Titan
The changing of the seasons appears to be lagging on Saturn’s largest moon. A phenomenon causing much intrigue amongst the scientists.
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Clouds in the southern hemisphere of Titan, signs of a summer which is lasting much longer than predicted. Infrared image taken on 26 March 2007.
Credit: NASA/JPL/University of Arizona/University of Nantes |
«The seasons aren’t what they used to be!» - a cry that illustrates just how attached we are to regular seasons on our own planet. It is a complaint often heard when there is a sharp cold snap in Spring or if it is unusually warm in autumn... It is comforting to know that Earth is not the only body in the solar system with a complex climactic mechanism which sometimes defies the experts’ calculations. Titan, Saturn’s largest moon, currently appears to be experiencing an autumn which is warmer and more humid than predicted.
An extended Summer
The only known moon to possess an atmosphere, Titan has been regularly observed by the Cassini probe which has been orbiting around Saturn’s system since July 2004. This joint unmanned mission involving NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI) has already brought back some remarkable scientific results and superb photographs (see the Enjoy Space portfolio). It has provided us with a better understanding of Titan, shrouded in thick mist in its upper atmosphere, concealing a world of ice, rock and methane (a hydrocarbon) rain at an average temperature of -180 °C. The Cassini probe radar has even detected the presence of lakes of liquid methane at its poles which fill when it rains. The enigmatic moon experiences seasons, just like the Earth, but over much longer periods. As scientist Jean-Pierre Lebreton points out, «a season lasts approximately 7 years on Titan» (see his interview in the Find out more section at the end of the article).
Titan’s surface is constantly shrouded by mist in its upper atmosphere. However, certain instruments on the Cassini probe, like its radar are capable of «seeing» through it. Photo on the right: liquid methane lakes at the poles.
Credit: NASA/JPL/Space Science Institute |
Saturn and Titan are currently in their equinox phase which means that the Sun is more or less over their equators, lighting up their poles equally. It is a time of seasonal change, as on Earth*. According to the theoretical models, summer in Titan’s southern hemisphere should have already shown obvious signs of ending such as the clearing of its clouds and a drop in temperatures (summers on Titan are «hot» and humid) as far back as 2005!
However the Cassini probe is still detecting clouds at average southern latitudes near the equator as highlighted in the NASA press release of 3 June 2008. An extended summer which reminds us of the North America Indian summer on Earth when near summery weather appears to last into the period of the first autumn frosts. Obviously, an Indian summer on Titan differs from its terrestrial version in that it is neither dry nor temperate but «hot» (a few degrees above the average -180 °C) and humid.
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Artist’s illustration of the Cassini probe approaching Saturn in July 2004.
Credit: NASA/JPL |
Equinox Mission
Scientists are closely studying the lag in seasonal change observed on images as far back as 2007. To investigate it in more detail they will be utilising Cassini’s multiple flights over Titan starting on 6 and 22 June when the probe is set to pass at an altitude of approximately 950 km at the nearest point. It is necessary to point out that since July 2008 the unmanned explorer has been on an extended mission called Equinox (chosen to emphasis the season that the Saturn system is currently experiencing). As part of an ambitious observation programme, Enceladus, another moon, is set to be the object of a series of flights. Its South Pole features geysers of ice particles which could come from pockets of underground liquid water. As soon as we talk about liquid water the possibility of life (albeit in a very simple form) automatically springs to mind. Cassini will fly over the moon once more passing through the spray from the geysers in order to analyse its composition more accurately. The manœuvre is scheduled for 2 November of this year and its minimum flight altitude will be just 99 km!
The geysers on Enceladus spew out ice particles which could come from underground pockets of liquid water.
Credit: NASA/JPL/Space Science Institute |
The Equinox mission is scheduled to last until September 2010. However, if the Cassini probe remains «fit for purpose» it is hard to imagine that the only device ever to be placed in orbit around Saturn will be decommissioned.
(*) On Earth, the March and September equinoxes mark the arrival of spring and autumn respectively, at least in the Northern hemisphere (the contrary is true in the southern hemisphere).
Cassini mission: NASA - ESA - ASI websites
NASA/JPL 3 June press release
Flight over Titan on 6 June
List of up-and-coming flights over Titan and Enceladus
Published on 8 June 2009
