This week, astrobiologists are discussing what ESA’s Huygens
spaceprobe might discover when it parachutes to the surface
of Saturn’s mysterious moon, Titan, in 2005. Titan possesses
a rich atmosphere of organic molecules, which Huygens will
analyse. Recently some scientists have begun to think that,
by redefining life, in broader terms, what we may find on
Titan may be life. If this is the case, it certainly will
not be life as we know it …

Titan is an astrobiologist’s dream laboratory. Its atmosphere
is composed of nitrogen and methane gas. Ultraviolet light
from the Sun can break the methane molecules apart, leading
to the formation of complex organic molecules by which
scientists mean molecules containing carbon. Carbon compounds
are the first step towards life, as we know it on Earth.
Life, itself, is based on extremely complicated carbon
molecules such as DNA. Some scientists believe the
composition of Titan’s atmosphere closely resembles that of
early Earth, before life began on our planet.

Huygens’s investigations may reveal how life began on Earth.
Jean-Pierre Lebreton, ESA’s Project Scientist for Huygens
says, “One of the key questions we hope to address is how
complex the organic molecules have grown in Titan’s
atmosphere.”

However, organic molecules are still a long way from life
itself. So, what defines life? What is the difference between
the living and the non-living? Scientists are still unsure.
No satisfactory definition has been found so far. Any attempt
to define life’s characteristics either excludes some types
of life or includes some inanimate objects. When looking for
an appropriate definition of life, there is one property all
scientists seem to agree on: all life needs energy to sustain
its metabolism. For example, plants use sunlight, while
animals extract energy from organic molecules in the food
they eat. This happens not only in these higher-level
organisms, but also in the simplest forms of life on Earth,
microbes. Microbes are single-cell organisms that capture
their life-energy from a dizzying array of inorganic
chemical reactions. Such chemical metabolisms are so
different from those in the animals and plants of Earth,
that astrobiologists now wonder if life could arise in any
place that can sustain a rich network of chemical reactions,
such as on Titan. Moreover, on Earth, microbes have adapted
to the extreme environmental conditions. Scientists
therefore now ask, “Could life arise on Titan?”

By all standards, Titan is an extreme and hostile environment
to life, as we know it. Any life on Titan would have to be
totally different from all Earthly forms. Lebreton says,
“The conditions on Titan are not adequate for the kind of
life we understand today. It is very cold and there is no
liquid water but we should be ready for surprises.”
Identifying life is tricky, especially when you are unsure
what to look for. Huygens’s geological and environmental
investigations, and Cassini’s mapping from orbit, might
record chemical anomalies or curious geological structures
that warrant further investigation as possible life
indicators.

Another chemically puzzling place is the planet Venus.
Similarly to Titan, Venus is a world that scientists would
traditionally call hostile to life, as we understand it.
However, there is something odd in its clouds. Venus’s
chemically laden atmosphere displays some curious phenomena,
such as the planet’s ability to absorb ultraviolet radiation.
Scientists cannot explain this. Some speculate that perhaps
microbes in the atmosphere are responsible. If ESA’s Venus
Express is given the final go-ahead later this year, it
might help solve the mystery.

For centuries, scientists have struggled to define life.
Space investigations present the best chance for
astrobiologists to find the missing link in our
understanding of what separates the living from the
non-living. When we know that, we will finally have
defined life here on Earth.

Note to editors

There are other European Space Agency missions with strong
emphasis on astrobiology coming soon. Rosetta will study
organic molecules on Comet Wirtanen, investigating how
comets might have seeded the early Earth with such compounds,
which possibly favoured the origin of life. Mars Express and
its lander, Beagle 2, will scour Mars for environments
likely to harbour past or present life.

Huygens

Huygens will be the first spaceprobe to land on a world in
the outer Solar System. In early 2005, it will land on the
surface of Titan, Saturn’s largest moon, and the only moon
in the Solar System to possess a thick atmosphere. The
Huygens data may offer clues about how life began on Earth.
Huygens is currently in space, hitching a ride on NASA’s
Cassini mission, which was launched by a Titan IVB/Centaur
rocket on 15 October 1997.

Venus Express

Venus Express is the latest mission to be added to ESA’s
Cosmic Vision 2020 Science Programme. It is scheduled for
launch in November 2005 and will be build around the design
of Mars Express, making it quicker and cheaper to develop.
It will study the Venusian atmosphere and surface in detail
and use radar to conduct the first investigation of the
planet’s subsurface layers. With Venus Express, Mars
Express, and BepiColombo, ESA is the only space agency in
the world with current plans to visit each planet in the
inner Solar System.

Astrobiologists are gathering this week at the Second
European Workshop on Astrobiology in Graz, Austria.

USEFUL LINKS FOR THIS STORY

* Life in extreme conditions
http://spdext.estec.esa.nl/content/doc/56/30550_.htm

* What is life?
http://spdext.estec.esa.nl/content/doc/57/30551_.htm

* More about Huygens
http://sci.esa.int/huygens/

* More about Rosetta
http://sci.esa.int/rosetta/

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=12&cid=12&oid=30548&
ooid=25335]
Cassini-Huygens approaching Saturn.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=12&cid=12&oid=30548&
ooid=13955]
Artist’s impression of Huygens descending through Titan’s.