The Huygens probe landed on Saturn's moon Titan in 2005, but it never encountered chilly seas of liquid methane as mission scientists had hoped — it landed in a mud field.
In spite of the disappointment, scientists have re-created a turbulent picture of Titan's atmosphere using data from sensors intended to measure oceanic properties. In addition to showing Huygens probably plunged through turbulent methane ice clouds, the research may aid in the design of balloon probes for future Titan missions.
"We knew Huygens had a bumpy ride down to Titan's surface. Now we can separate out 20 minutes of air turbulence — probably due to a cloud layer — from other effects such as cross winds or air buffeting," said Mark Leese, a Huygens project manager at The Open University in Britain.
The Huygens probe jettisoned off of the Cassini probe on Dec. 25, 2004 and reached Titan's surface in Jan. 14, 2005, deploying a parachute after entry into the moon's planet-like atmosphere to begin a 2.5-hour descent.
Huygens engineers did not intend to precisely measure the minor, chaotic changes in air known as turbulence, but Lorenz and his team gleaned such data by looking at information from two of the probe's sensors. One of the two, a "liquid density sensor," was designed to measure the properties of Titan's purported methane seas if the probe had floated on one.
"Although never designed with atmospheric measurements in mind, this device works as a weakly damped accelerometer," write the study's authors in an upcoming issue of the journal Planetary and Space Science. Accelerometers measure changes in speed at extremely minute scales. NASA, for example, uses one on NASA's space shuttle leading wing edges to detect vibrations caused by micrometeoroid impacts, among other phenomena.
Lead author Ralph Lorenz, a planetary scientist at the Johns Hopkins University in Baltimore, Md. Lorenz and his colleagues partnered the density sensor's data with that from a pendulum-like "tilt sensor" to create a play-by-play picture of Huygens' atmospheric dive.
"Such information may offer insights into the meteorological processes prevalent on Titan, a world believed to share many characteristics with the Earth," Lorenz said.
The researchers found that Huygens may have plunged into icy methane cloud layer — which scientists have proposed to create a chilly methane "rain" — about 12 to 19 miles (20 to 30 kilometers) above the moon's surface.
Although the turbulence findings may not give airline passengers on Earth relief from the nauseating buffets of wind, the information could be used to help design balloonlike probes.
"Future Titan exploration might use lighter-than-air vehicles, which would have to compensate for wind gusts in order to keep above targets of interest for sampling," the study's authors said.
Scientists imagine such balloons would be outfitted not only with cameras to detail out the moon's surface, but also spectrometers that could map mineral deposits on Titan. NASA, however, has neither approved nor scheduled such a mission.
Lorenz and his colleagues hope to use more Huygens data, including that from radio emissions, to create a more detailed reconstruction of Huygens' descent as well as Titan's nitrogen-thick atmosphere.