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Planetary Geology
Listening to Titan

During the Huygens probe’s 147-minute descent toward Titan’s surface on Jan. 14, it carried onboard a microphone poised to record any sounds from Saturn’s largest moon, from a booming clap of thunder to the tiny splash of methane raindrops. But while Huygens was listening to Titan, 750 million miles away, scientists on Earth were listening to Huygens.

As the Huygens probe descended through Titan’s smoggy atmosphere, scientists on Earth were able to listen in on sounds from the moon’s surface. Formed when sunlight interacts with hydrocarbons, a 300-kilometer-thick layer of orange smog covers Titan’s surface, preventing light from escaping. Courtesy of ESA.


As part of an experiment to determine wind speeds on Titan, radio telescopes in the United States, China, Japan and Australia honed in on the radio signal emitted by Huygens. The plan was for Cassini (the spacecraft that carried Huygens to Titan and is still orbiting Saturn) to focus on detecting east-west winds, and the radio telescopes to focus on any north-south winds, but things didn’t go as planned.

The National Radio Astronomy Observatory in Green Bank, W.V., was the first to detect the probe’s successful deployment from Cassini. But after waiting 67 minutes for the transmission delay to pass, scientists were crestfallen when no data arrived from the orbiter. Fortunately, however, the radio astronomers who were eavesdropping on the Huygens’s signal intended for Cassini were able to reconstruct the lost data.

“I’ve never felt such exhilarating highs and dispiriting lows than those experienced when we first detected the signal from the Green Bank Telescope, indicating ‘all’s well,’ and then discovering that we had no signal at the operations center, indicating ‘all’s lost,’” said Michael Bird, the radio astronomer at the University of Bonn, Germany, who led the Doppler Wind Experiment, part of the larger European Space Agency/NASA Cassini-Huygens project, in a press release. “The truth, as we’ve now determined, lies somewhat closer to the former than the latter.”

As Huygens descended through Titan’s thick atmosphere, it also drifted in the wind. These subtle changes in position caused the frequency of its radio signal to shift slightly — producing longer wavelengths as it moved farther away from observers on Earth and shorter wavelengths as it moved closer, a phenomenon called Doppler shift. Scientists analyzing the Doppler shift from the weak signal were then able to reconstruct Huygens’ motion and then calculate the wind strength.

Researchers found that Titan’s winds move from west to east at all altitudes, in the same direction that the moon rotates. The highest wind speed, 270 miles per hour, was detected at an altitude of 75 miles, and the lowest was found near the surface where a light breeze blows. Wind speeds increased at altitudes up to 37 miles, where Huygens encountered some turbulence. Scientists interpreted the turbulence as vertical wind shear, but they are still investigating it.

Green Bank and the other radio telescopes are using the radio signal to reconstruct a 3-D record of Huygens’ position, orientation and motion throughout the Titan mission, despite how weak it is. “A typical cell phone on the moon would provide a considerably more powerful signal,” Bird says. Scientists are also still analyzing the acoustic data from Huygens’ microphone.

Listening to Titan’s atmosphere should provide crucial information about weather and storms that may hold clues to how life began on Earth. For example, lightning could supply electrical energy for certain organic chemical reactions. Titan’s atmosphere and surface are thought to closely resemble those of early Earth.

Sara Pratt
Geotimes contributing writer


To “hear” Titan, click here to download files from the ESA Web site, of sounds picked up by the acoustic sensor aboard Huygens.


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