Titanic weather: Methane recycled

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  • Published: Jan 5, 2012
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Titanic weather: Methane recycled

Mapping Titan's hydrocarbon weather

Titan, the planet Saturn's largest and perhaps its most intriguing moon, has weather patterns that are akin to those found on earth but rather than being underpinned by a water cycle, it is methane that circulates across the orb. A new study published in Nature corroborates infrared mapping to reveal methane lakes in the moon's polar regions, "dry" low latitudes with fluvial features, and occasional rainstorms, as well as tropospheric clouds found so far mainly in mid-southern latitudes and the polar regions.

The Cassini-Huygens space probe finally confirmed that Titan has lakes of methane and ethane in 1987, although astronomers had suspected their existence for quite some time before that. Indeed, the Voyager 1 and 2 missions first hinted at liquid hydrocarbons lapping at these distant shores. Data from the Hubble Space Telescope in 1995 corroborated this earlier evidence.

Super models

Models have attempted to explain the evidence to provide an underpinning for the methane cycle that apparently exists on Titan and is analogous to the terrestrial water cycle on which we depend here on Earth. Now, Tapio Schneider, Sonja Graves, and Mike Brown of the California Institute of Technology, in Pasadena and colleague Emily Schaller of NASA Dryden Aircraft Operations Facility, at the National Suborbital Education and Research Center, in Palmdale, California have created simulations of the methane cycle on Titan. They suggest that their model, which supports the Cassini infrared mapping data, may help to explain the formation of features such as lakes, clouds and rivers of methane. It might also predict future changes on the surface of this moon.

Schneider and colleagues use general circulation models to perform simulations of Titan's methane cycle. In their model, the formation of lakes in polar regions, they suggest, is due to the accumulation of cold-trapped methane. This occurs preferentially in the northern hemisphere where the "rainy" season is much longer than in the south. In low-latitude regions, rare but intense storms occur around the equinoxes. This causes sufficient precipitation to carve river-like features out of the surface rock, the evidence suggests.

Flexible models

The researchers explain how their model is based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory (http://fms.gfdl.noaa.gov). "It is similar to standard models for Earth's atmosphere, but with Titan's radius, planetary rotation rate, material properties, and seasonally varying insolation, and with a methane cycle instead of a water cycle," the team says.

For the period of 2001 onwards, their model indicates a lack of clouds in the northern hemisphere, which is consistent with ground-based telescopic observations of Titan and data from the Cassini Visual and Infrared Mapping Spectrometer.

The team's simulation of cloud distribution indicates that clouds form primarily in middle and high latitudes of the summer hemisphere, which until recently has been the southern hemisphere. From their model, Schneider and colleagues predict that as the Titanic northern summer begins, clouds begin to form around the northern polar regions and lake levels rise. " We predict that in the northern polar region, prominent clouds will form within about two (Earth) years and lake levels will rise over the next fifteen years," the team says. Future observations of Titan will confirm or reject these predictions and so confirm the description of the methane cycle offered by the team's simulations.

 


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 Credit: Nature/Schneider et al -  Annual cycle of zonal-mean climate statistics in Titan GCM
Modelling the methane cycle on Titan for years to come

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