Ever spilled your drink on an airline because of
turbulence? Researchers on both sides of the
Atlantic are finding new ways to understand the violent
phenomenon, both in Earth's atmosphere and
in that of Saturn's moon Titan, aided by data gathered from
the Cassini-Huygens probe.
"Exactly the same physical processes are occurring on
Titan and Earth, although under
interestingly different conditions," said planetary
scientist Ralph Lorenz of Johns Hopkins' Applied
Physics Laboratory. "Scientists who look at the Earth
and those who study other planets working
together can gain new insights into both places."
Turbulence is an important process in our weather and
can be more than an inconvenience:
Hundreds of injuries have occurred on commercial flights
due to turbulence.
Giles Harrison, an atmospheric physicist in the
Department of Meteorology at the University of
Reading in the United Kingdom, devised an inexpensive way
of measuring turbulence effects using
weather balloons. The instrument package--a
radiosonde--measures the Earth's magnetic field to
detect turbulent motion. In a paper subtitled "A Compass
for a Radiosonde," magnetic changes
measured on the weather balloon were shown to arise from
motions within cloud, using Earth's stable
magnetic field for reference.
Lorenz found Harrison's results key to making sense of
data from the Huygens probe, which
descended by parachute through Titan's atmosphere in
January 2005. The Surface Science Package
on board Huygens included tilt sensors that measured
motions of the probe during its descent. These
tilt sensors act much like a drink in a glass, using a
small slug of liquid to measure tilt angle. But as the
probe plummeted at high speed on Titan, there was a lot of
buffeting, even though the air itself was
fairly still.
By knowing the particular signature of cloud-induced
turbulence in Harrison's Earth balloon
data, where nearby weather radar could document what was
causing the turbulence, Lorenz was able
to find this signal at Titan despite the buffeting during
the Huygens descent. "The Huygens tilt
history was just this long squiggly complex mess, but
seeing the fingerprint of cloud turbulence in
Harrison's work showed me what to look for," Lorenz
said.
Armed with that information, Lorenz found that a
20-minute period during Huygens' 2.5-hour
descent, at an altitude of around 12 miles, was affected
by this kind of in-cloud turbulence.
Having experimented with instrumentation on small
models, even Frisbees, to understand the
dynamics of aerospace vehicles like the probe, Lorenz was
familiar with the sensors Harrison used. His
analysis helped identify a turbulent cloud layer in Titan's
atmosphere, a significant result for the
investigation of Titan's meteorology. In the process, he
also found a way to improve Harrison's
magnetic sensor arrangement on the weather balloon, simply
by changing its orientation.
"We went to Titan to learn about that mysterious body
and its atmosphere," he said. "It's neat
that there are lessons from Titan that can be usefully
applied here on Earth."
Lorenz's analysis was recently published online in the
journal Planetary and Space Science, and
an exchange of ideas between Lorenz and Harrison appears in
the August issue of Journal of
Atmospheric and Oceanic Technology.
The Cassini-Huygens mission is a cooperative project
of NASA, the European Space Agency and
the Italian Space Agency. The Jet Propulsion Laboratory, a
division of the California Institute of
Technology, manages the mission for NASA. Additional
information about the mission is available
online at
saturn.jpl.nasa.gov.