May 1, 2003
Looking for Life of Any Shape or Form
How does life begin? How does it evolve?
On 25 April 25 1953, James D. Watson and Francis H. Crick published
an historic paper in Nature that would change the fate of modern science.
They proposed that DNA, the molecule of complex life forms, had the
shape of a double helix. Today, scientists from all areas are working
together to answer the ultimate question: can life (in any shape or form)
exist anywhere else in the Universe?
Astrobiology is a young interdisciplinary science that tries to find
answers to tough questions. Experts want to know how life begins and
evolve and if it can exist anywhere else outside the Earth. Life in the
Universe - as we know it - began with the synthesis of some key
elements: hydrogen, carbon, oxygen, nitrogen, sulphur, and phosphorus.
Carbon is the element that allows the formation of organic compounds
that form the base of more complex molecules. Nucleic acids or sugars,
constituting DNA and RNA, are examples of such complex molecules.
With telescopes like ISO, the European Space Agency Infrared Space
Observatory, studying molecules and organic compounds in space began
to pay off. Operating from 1995 to 1998, ISO performed nearly 30 000
observations. Scientists have discovered more than a hundred complex
organic molecules in space so far, some of them widespread in our Galaxy.
"Most of them", explains Alberto Salama, ISO Project Scientist,
"form close to where stars form and near to old dying stars. Violent
phenomena take place there, like high-velocity winds and high-energy
fluxes, so the chemistry of the gas changes greatly and many molecules
can form. Strong radiation from the stars destroys some molecules.
Others survive in the interstellar space and may be incorporated on dust grains".
Will Mars Express find traces of life?
Water is one of the basic (inorganic) molecules
needed for life to develop. How wet is space? "ISO found water basically
everywhere. It was in our Solar System, including the atmospheres of the
giant planets and Saturn's moon Titan, in the areas around stars,
in the cold interstellar medium, and in other galaxies. In an interstellar
gas cloud in Orion, we discovered enough water generated every day
to fill all of the Earth's oceans 60 times!", exclaims Salama.
Water and complex organic molecules are there, but just having them
in space is not enough for life to develop. DNA and RNA molecules,
for example, use a life-replication mechanism that is very safe, ensuring
the maintenance, stability, and diversity of its basic components.
These molecules have distinct components, a very complex structure.
It is unlikely you would found them just anywhere. Is it therefore also
unlikely that there is life anywhere else? Many astrobiologists are optimistic.
Playing with the same components of DNA and RNA, you can make
other molecules that can store genetic information, even with a different
structure. There are also other types of nucleic acids, called peptide-nucleic
acids (PNA), which have different backbones. They contain no sugars, no
phosphates, just amino-acid derivatives. Forming compounds of nucleic
acids might therefore be easier than we think.
Moreover, some nucleobases, like adenine and guanine, form easily and
are quite stable. Scientists do not rule out these complex molecules also
being in interstellar space. They have already been found on 4.6 thousand
million-year-old meteorites. Other sugar derivatives have also already
been discovered on meteorites.
Will Mars Express find traces of life?
Many ESA missions have a clear astrobiological
side. For example, Rosetta will land on a comet and Huygens on Saturn's
moon Titan. Their analyses will help us to better understand so-called
prebiotic chemical processes on those bodies. Mars Express, scheduled
for launch in June 2003, will be able to identify signs of water in liquid,
solid, or vapour form on Mars. Its lander, Beagle 2, will take a good
chemical and morphological look at its landing site, looking for water in
the soil, on rocks, and in the Martian atmosphere. It will investigate the
existence of carbonate minerals and organic residues to detect possible
signs of past or present life.
From 2005, Columbus, the European research module on the
International Space Station, will be using a new space biology tool, EXPOSE.
EXPOSE will examine whether, and to what extent, meteorites may offer
enough protection for life to continue after their long existence in space.
Scientists identified the underlying structure of life on Earth only a
relatively short time ago. Now they are taking the first steps to find the
chemicals of life far away in space and study them. Who or what will they discover?