In a remarkable technical coup, astronomers working from Earth have measured the winds of Saturn's moon, Titan.
They listened in to a faint radio signal emitted by the European Space Agency's Huygens probe as it descended through Titan's atmosphere on 14 January. Analysing tiny shifts in the signal's frequency betrayed the probe's motion.
The Cassini orbiter should have picked up this signal for analysis, but a missing command meant that one of its receivers was not switched on. The wind experiment would therefore have been a total failure but for the huge technical advances in the sensitivity of radio telescopes since the mission was launched in 1997.
Huygens was more than a billion kilometres away, and its transmitters had roughly the power of a mobile phone. But the signal came through clearly to several radio telescopes on Earth, including Green Bank in West Virginia, US, and the Parkes dish in Australia.
The clarity allowed astronomers to monitor the frequency of the 2 gigahertz signal to within about one hertz. That is good enough to detect a doppler shift caused by motion of less than 20 centimetres per second, about the speed of a hurrying tortoise.
The measurements taken begin 150 kilometres above Titan's surface, where Huygens was blown eastwards at more than 400 kilometres per hour. That roughly fits with earlier measurements of the winds at 200 kilometres altitude, made over the past few years using Earth-based telescopes.
At ground level, the doppler shifts show gentle winds of a few metres per second, again roughly in line with expectations.
But in between there was something new: between 60 and 80 kilometres, Huygens was buffeted by rapidly fluctuating winds, which the team put down to vertical wind shear. "It's a bit of a surprise," says Michael Bird of the University of Bonn, Germany, who leads the doppler wind experiment. "It's something for the theoreticians to get to work on."
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18:53 05 September 2008
