Wind blows west on Titan

GALE-FORCE winds on Titan, Saturn's largest moon, blow from west to east, researchers have now discovered says a report in Geophysical Research Letters. Knowing which way the winds blow should help international space agency scientists in their plans to land a probe on the icy moon in 2004.

Using the 3 m-wide Infrared Telescope Facility on Mauna Kea, Hawaii, Theodor Kostiuk of the NASA Goddard Space Flight Centre in Greenbelt, Maryland, and his colleagues found that the winds some 200 kilometres above Titan's surface were blowing at tornado speeds of about 750 km/hr.

Not knowing the direction of the winds complicated plans by NASA and the European Space Agency (ESA) to explore Saturn, its rings and its 17 moons, including Titan. Launched in 1997, ESA's 350- kilogram Huygens probe will parachute in from NASA's 2-tonne Cassini orbiter.

The probe will beam data about Titan's clouds, atmosphere and surface to Earth via the Cassini spacecraft orbiting above. Knowing the wind direction will help NASA and ESA to predict the probe's path as it descends through Titan's thick, smoggy atmosphere. They can then point Cassini's antenna more precisely towards the probe as it lands.

Huygens' batteries will last only 153 minutes, and so it is important to maximise the contact between the orbiter and probe to gain as much information as possible from Titan's surface. But these measurements of Titan's stratospheric wind are at only one height.

It "tells us nothing about what's going on below, where Huygens will spend most of its time", warns Michael Bird of the University of Bonn in Germany, leader of an experiment the probe will carry out to investigate wind on Titan's surface. To measure Titan's wind, Kostiuk and his team used a technology analogous to that used by police to detect speeding drivers. A police officer sends out a signal of a specific frequency; this frequency changes as it bounces back from a moving car, and the change is dependent on the car's speed.

The telescope and other equipment detected infrared light emitted by gaseous ethane in Titan's upper atmosphere. The speed and direction of the moving ethane molecules altered the frequency of the infrared light. Thus, by comparing the frequencies from the eastern and western edges of Titan, the researchers calculated which way, and how fast, the wind was blowing.

Now they plan to confirm their results using a larger, 8-metre telescope that will cut down on the noise from space. Titan, the second- largest moon in the Solar System, and about 40 per cent of the size of Earth, interests astrophysicists because its conditions probably resemble those of Earth before the appearance of life.

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