Titan's mysterious radio wave (Forwarded)

ESA News
http://www.esa.int

1 June 2007

Titan's mysterious radio wave

Huygens scored a first in 2005 by measuring the electrical conductivity of
Titan's atmosphere. The results hint at a new way to investigate the
subsurface layers of Titan and could provide insight into whether or not
Titan has a subsurface ocean.

The Permittivity, Waves and Altimetry (PWA) sensor on the Huygens Atmosphere
Structure Instrument (HASI) detected an extremely low frequency (ELF) radio
wave during the descent. It was oscillating very slowly for a radio wave,
just 36 times a second, and increased slightly in frequency as the probe
reached lower altitudes.

If the PWA team confirms that the signal is a natural phenomenon and not an
artefact of the way the instrument worked, they will have discovered a
powerful new way to probe not just the atmosphere of Titan but its
subsurface as well.

The only other world on which ELF waves were detected before was Earth. They
are reflected by both the surface of the Earth and its ionosphere, the
rarefied region of the atmosphere where most particles are electrically
charged. This turns the atmosphere into a giant 'sound box' where certain
frequencies of ELF waves resonate and are reinforced, whilst other die away.

On Titan, however, the surface is a poor reflector because of its low
conductivity and so these waves penetrate the interior. "The wave could have
been reflected by the liquid-ice boundary of a subsurface ocean of water and
ammonia predicted by theoretical models," says Fernando Sims,
CETP/IPSL-CNRS, France, and a member of the PWA team.

If Sims is right, successful modelling of how ELF waves resonate on Titan
could lend support to the ocean's existence and tell scientists about the
depth at which it sits. Understanding the resonance however, is difficult.

Above about 100 kilometres altitude on Earth, the ionosphere provides the
upper reflecting layer of the resonating cavity. At Titan, PWA revealed that
things are more complicated. Apart from the ionosphere, at a much higher
altitude of about 1200 kilometres, Huygens found a layer of ionized
particles consisting of electrons, at 63 kilometres. "This does not match
any previous prediction for Titan," says Sims. To some extent, it splits
Titan's atmosphere into two resonating chambers.

With so much at stake, the PWA team are checking to make sure the detection
is real and not an artefact generated by the spacecraft. They have already
ruled out electrical interference from the instrument itself.

Two small arms, one on either side of Huygens, create an antenna and the
team's next goal is to investigate whether the arms could have oscillated
during the descent. Sims and colleagues are building a special chamber to
hold a replica of the instrument at the low temperature of Titan's
atmosphere, between 100-200 degrees Kelvin (about -173 to -73 C), in order
to check whether the antenna resonates at 36 hertz. If it does, it probably
means that the signal is an artefact. If it does not, confidence in the
signal's reality will increase and the investigation of the atmosphere and
subsurface can begin.

But perhaps the biggest mystery is what generated the ELF wave in the first
place. On Earth, they are initiated by lightning strikes that make electrons
in the atmosphere oscillate, releasing the ELF waves.

The PWA was designed to search for ELF waves on Titan while a microphone on
Huygens kept an ear out for thunder -- a sure sign of lightning. Cassini has
also been watching for lightning using its cameras.

However, Huygens suggests that there is no lightning, or very little. "If
there is lightning on Titan, it is significantly less than the amount of
lightning that Earth experiences," says Sims. So what generated Titan's ELF?
No one is quite sure yet. "It might be generated by an interaction with
Saturn's magnetosphere or related to Titan's intrinsic fields," suggests
Sims. "Titan is proving to be an intriguing environment."

One thing is certain: there is plenty to investigate. "The measurement of
atmospheric electricity is something really new and exciting," says
Jean-Pierre Lebreton, ESA Huygens Project Scientist. "We could send similar
instruments to study atmospheric electricity on other celestial bodies, in
particular Venus, Mars, and the giant planets," adds Sims.

The PWA team expect to release more definitive results when their
investigation is complete.

Notes for editors

This article is based on two papers that will appear in a special issue of
the Planetary and Space Science magazine dedicated to Huygens results:
'Electron conductivity and density profiles derived from the Mutual
Impendence Probe measurements performed during the descent of Huygens
through the atmosphere of Titan', by M. Hamelin et al., and 'A new numerical
model for the simulation of ELF wave propagation and the computation of
eigenmodes in the atmosphere of Titan: did Huygens observe any Schumann
Resonance?', by F. Sims et al.

Cassini-Huygens is a joint mission between NASA, ESA and the Italian Space
Agency (ASI).

For more information

Fernando Sims
CETP/IPSL-CNRS, France
Email: Fernando.Simoes @ cetp.ipsl.fr

Michel Hamelin
CETP/IPSL-CNRS, France
Email: Michel.Hamelin @ cetp.ipsl.fr

Jean-Pierre Lebreton, ESA Huygens Project Scientist
Email: Jean.Pierre.Lebreton @ esa.int

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