Titan winds pummelled Huygens

Huygens had a rough ride during its descent to Titan

Scientists have successfully measured the wind speeds that pummelled Huygens during its bumpy descent through the atmosphere of Titan.

Researchers had feared the information was lost because one of Cassini's receivers was not switched on.

But a network of terrestrial radio telescopes has managed to salvage the data, to the delight of the team.

It reveals wind speeds are weak near the surface of Titan, becoming stronger with altitude, up to 120m/s.

"Our team has now taken a significant first step recovering the data needed to fulfil our original scientific goal, an accurate profile of Titan's winds along the descent trajectory of Huygens," said scientist Michael Bird, of the University of Bonn, Germany.

Doppler effect

The preliminary estimates of wind speeds on Titan were obtained by measuring the frequency of radio signals from Huygens, recorded during the probe's descent on 14 January 2005.

Winds in the atmosphere affected the horizontal speed of the probe's descent and produced a change in the frequency of the signal received by the radio telescopes on Earth.

This "Doppler" phenomenon is similar to the change in siren pitch heard near a passing police car.

This is a stupendous example of the effectiveness of truly global scientific co-operation Jean-Pierre Lebreton, ESA

The ground based Doppler measurements were carried out and processed jointly by scientists from Nasa's Jet Propulsion Laboratory and the Joint Institute for Very Long Baseline Interferometry (VLBI) in Europe.

Their data shows that winds are flowing in the direction of Titan's rotation (from west to east) at nearly all altitudes.

The maximum speed of roughly 120m/s (430 km/h) was measured at an altitude of about 120km.

The winds are weakest near the surface and increase slowly with altitude up to around 60km.

Rough ride

This gradual pattern does not continue at altitudes above 60km, however, where large variations in the Doppler measurements were observed.

Scientists believe that these variations may arise from significant vertical wind shear.

Huygens had a rough ride in this region, which was already known from the data recorded on board the probe.

The very successful signal detection on Earth provided a surprising turnabout for the Cassini-Huygens Doppler Wind Experiment (DWE), whose data could not be recorded on the Cassini spacecraft due to a commanding error needed to configure the receiver.

"This is a stupendous example of the effectiveness of truly global scientific co-operation," said Jean-Pierre Lebreton, ESA Huygens Project Scientist.

"By combining the Doppler and VLBI data we will eventually obtain an extremely accurate three-dimensional record of the motion of Huygens during its mission at Titan."

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