Dissecting the dirt on Titan (Forwarded)

ESA News
http://www.esa.int

1 June 2007

Dissecting the dirt on Titan

Planetary scientists are a step closer to understanding the composition of
the dust in Titan's atmosphere. A decade-long programme of laboratory
studies, aiming to reproduce Titan's unique dust, or 'aerosol' population
in specially constructed reactors, has proved invaluable.

Aerosols are small, solid particles that float in the air. On Earth, they
are often the result of pollutants in the atmosphere. On Titan, they occur
naturally and are abundant in the atmosphere, masking its surface.

To analyse these particles, Huygens heated the samples to 600 C, in order
to vaporize them into volatile fragments. This technique, known as
pyrolysis, was performed by the Aerosol Collector and Pyrolyzer (ACP)
experiment. The resultant gases were then passed on to the Gas
Chromatograph Mass Spectrometer (GCMS) for analysis.

The GCMS results effectively give scientists a list of chemical components
from which they can derive the chemical composition of these particles.
Scientists then have to work out what their precise chemical makeup is and
determine how they formed. In anticipation ten years ago, a team of French
scientists started making their own laboratory analogues, called
'tholins', for comparison.

Tholins are complex nitrogen-rich substances that form in the laboratory
when ultraviolet radiation or electrons react with simpler molecules such
as methane and ethane in a surrounding atmosphere of nitrogen. On Titan,
the methane and nitrogen-rich atmosphere makes their formation easy and
they drift to the surface where they continue to react with other atoms
and molecules.

Faced with creating such alien molecules, the French team designed a
special reaction chamber to simulate Titan's atmosphere and produce the
tholins for study. "We can generate over 200 chemical species," says
Patrice Coll, a team member at Laboratoire Interuniversitaire des Systèmes
Atmosphériques (LISA), Paris, "We do not yet know the detailed pathways
that build the chemicals, but we believe that are very similar to those on
Titan."

The aerosols govern what you can see on Titan. They create Titan's hazy
conditions, revealed by Huygens, and give the moon its dull orange glow.
If you could stand on the surface of Titan and magically tune your eyes to
infrared light, the haze and the clouds would seem to disappear and Saturn
would loom large in the night sky. This is because the aerosols are
largely invisible at infrared wavelengths. Change your eyes to
ultraviolet, however, and you would be plunged into darkness because, at
these wavelengths, the tholins behaves like a thick fog that absorbs all
ultraviolet radiation falling on it.

The team's work has already solved one mystery in the data from Huygens by
showing that the aerosols must contain ammonia-like structures, even if
there was little or no ammonia in the atmosphere from which the particle
was built. The team reached this conclusion by analysing the products of
the pyrolysis and the ratio of carbon isotopes in their laboratory
tholins.

Carbon, in common with many elements, can exist in a number of different
isotopes. Isotopes contain different numbers of electrically neutral
particles in the atomic nucleus and thus have different weights. An atom's
weight influences how easily it reacts with other atoms. Usually light
isotopes react faster and build up into molecules faster than their
heavier cousins. Mai-Julie Nguyen, a team member at LISA, analyzed the
carbon isotopic ratio in the laboratory tholins and, surprisingly, did not
find them to be enriched in the lighter isotope of carbon, in spite of
their chemical complexity.

The team then used these new results to interpret the ACP-GCMS data from
Huygens. They discovered that the material collected by ACP in Titan's
atmosphere releases ammonia when heated to 600 C. This gives essential
information on the elemental and molecular composition of Titan's
aerosols.

The task of analysing the data from GCMS continues. "This latest paper
shows that to correctly interpret these results, we must have clear
information about the complexity of the aerosols," says Francois Raulin,
Huygens Interdisciplinary Scientist, at Laboratoire de Physique et Chimie
de l'Environnement, Paris.

Note 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:
'Carbon isotopic enrichment in Titan's tholins? Implications for Titan's
aerosols', by M. Nguyen et al., and 'A technique to determine the mean
molecular mass of a planetary atmosphere using pressure and temperature
measurements made by an entry probe: demonstrating using Huygens data', by
P. Withers.

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

For more information

Mai-Julie Nguyen
LISA, Paris, France
Email: Nguyen @ lisa-paris12.fr

Paul Withers
Center for Space Physics, Boston Univ. (USA)
and
The Open University (UK)
Email: Withers @ bu.edu

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

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