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Posted by baalke on July 26, 2006, 11:03 am
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http://www.esa.int/esaSC/SEM23SVT0PE_index_0.html
Titan's pebbles 'seen' by Huygens radio
European Space Agency
25 July 2006
An unexpected radio reflection from the surface of Titan has allowed
ESA
scientists to deduce the average size of stones and pebbles close to
the
Huygens' landing site. The technique could be used on other lander
missions to analyse planetary surfaces for free.
When Huygens came to rest on the surface of Titan on 14 January 2005,
it
survived the impact and continued to transmit to the Cassini
mothership,
orbiting above. Part of that radio signal 'leaked' downwards and hit
the
surface of Titan before being reflected back up to Cassini. On its way
up, it interfered with the direct beam.
As Miguel P??rez-Ay??car, a member of the Huygens Team at ESA's
European
Space Research and Technology Centre (ESTEC) in The Netherlands, and
his
colleagues watched the signal coming back, they were initially puzzled
to see the power of the signal rising and falling in a repetitive
manner.
"Huygens had not been designed to necessarily survive impact so we had
never thought about what the signal would look like from the surface,"
says P??rez. After making a joke that aliens must be dragging the
craft
along the surface, P??rez and the team began work at once to
understand
the signal.
The clue was the repetitive oscillation of the power. It made
P??rez
think about the interaction of the direct signal with that reflecting
from the surface of Titan. As Cassini travelled away from the Huygens
landing site, the angle between it and Huygens changed. This altered
the
way in which the interference between the reflected and direct beams
was
detected, perhaps causing the variation in power.
He began running computer models and saw that not only could he
reproduce the received signal but also it was sensitive to the size of
pebbles on the surface of Titan.
Cassini collected data for 71 minutes after Huygens landed. After that
time, the spacecraft's motion took it below the horizon as seen from
Huygens' landing site. Until then, it soaked up radio signals that
encoded information about a swathe of Titan's surface from 1 metre to 2
kilometres to the west of the landed probe.
To accurately mirror the true signal, P??rez and his team
discovered that
the surface swathe must be relatively flat and covered mostly in stones
of around 5-10 centimetres in diameter.
This unique result complements the data taken by the Descent Imager and
Spectral Radiometer (DISR) instrument. When Huygens came to rest on the
surface of Titan, DISR was pointing due south. Its images show stones
and terrain in good agreement with the newly deduced western facing
radio data. "This is a real bonus to the mission. It requires no
special
equipment, just the usual communications subsystem," says P??rez.
Now that the scientists have understood the process using the
unexpected
Huygens data, the technique could be implemented on future lander
missions. "This experience can be inherited by any future lander," says
P??rez, "All that will be needed is a few refinements and it will
become
a powerful technique."
By subtly altering the properties of the radio beam for instance, the
radio transmitter and receiver can be optimised to help deduce the
chemical composition of the planetary surface.
Note to editors:
The results appear in the 25 July 2006 issue of the Journal of
Geophysical Research (Vol. 111. Doi: 10.1029/2005JE002613, 2006). The
paper, titled 'Bistatic Observations of Titan?s Surface with the
Huygens
probe radio signal', is by M P??rez-Ay??car, J.P.Lebreton,
N.Floury and
R.Prieto-Cerdeira (ESA-ESTEC, Noordwijk, The Netherlands), and
R.D.Lorenz (Univ.of Arizona, Tucson, Arizona, USA).
For more information:
Miguel P??rez-Ay??car, ESA-ESTEC, The Netherlands
Email: mperez @ rssd.esa.int
Jean-Pierre Lebreton, ESA Huygens Project Scientist
Email: jean-pierre.lebreton @ esa.int
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