At the dawn of the twenty-first century, the search for life beyond our planet is getting serious
and Europe is playing a full role. Over the next two years, the European Space Agency will be
sending two spacecraft (Rosetta and
Mars Express)
to search for clues to life’s origins elsewhere in our Solar System. A third ESA probe,
Huygens
is already on its way to Titan, a planetary-sized
laboratory for pre-biotic chemistry. ESA is also planning a series of spacecraft
(Eddington,
Gaia
and Darwin) to find planets orbiting other
stars and even to look for life’s signatures on other worlds that look much like our own. These
missions are providing scientists from all over Europe with unprecedented opportunities to unravel
the origins of life.

Life in the Solar System

We’re pretty sure that we’re the only intelligent beings lurking in our Solar System. There’s
plenty of excitement, though, about finding micro-organisms elsewhere in our neighbourhood.
Recent discoveries on Earth suggest that where there’s organic (carbon-based) chemistry, water
and an energy source, there’s life – no matter what the conditions. As these essentials are
commonplace in space, there’s a good chance that life is too – or so the argument goes.

Living organisms have been found alive and well in environments on Earth so apparently hostile
that the presence of life on other Solar System bodies seems quite feasible. Mars, the planet
that most closely resembles Earth, and Europa, one of Jupiter’s moons, both show evidence of
water, past or present, and so are the focus of plans to look for life elsewhere in the Solar
System. The possibility that Earth is not the only home for life, however, begs questions.

Did life arise independently on each body? If not, could it have been transferred from one to
another? If so, was the common origin a ‘seed’ planted, perhaps, during collisions with comets,
or interstellar dust? Crucial to answering these questions will be greater knowledge about the
structure and composition of comets and interstellar dust. Saturn’s moon, Titan, also shows
promise of revealing the conditions needed for basic organic chemistry to evolve into the chemistry
that eventually lead to life.

Life on Mars?

“If there’s life on Mars today, I reckon it’s a couple of kilometres underground living in melted
permafrost – or it could be alive and well in pockets of water under the north polar ice cap,”
says David Wynn-Williams, a microbiologist with the Antarctic Astrobiology Project at the
British Antarctic Survey, Cambridge, United Kingdom
and a member of the Beagle 2 (the Mars Express lander)
adjunct scientists group. His conclusion is based on similarities between habitats in Antarctica,
where microbial life exists in rocks and at the bottom of extremely salty ice-covered lakes,
and plausible Martian environments where primitive micro-organisms could be eking out a meagre living.

The surface of Mars, however, is too dry, cold and corrosive for any form of life to exist there
today. But that may not always have been the case. There’s plenty of evidence that the elixir
of life probably flowed copiously during the planet’s youth.
Early in its history, Mars, like Earth, may have been warm and wet. Indeed the conditions for
life may have arisen on Mars first, as the planet is smaller than Earth and would have cooled
down first to temperatures suitable for life.

If water did once flow and pool on Mars, where did it go? Did life evolve there? Does it still
hang on by a thread in some protected niche? “The only way of answering such questions is to
go to Mars and take a look,” says Agustin Chicarro, Mars Express project scientist at ESA.
“Mars Express, ESA’s mission to the Red
Planet due for launch in 2003, will be the first spacecraft equipped to search specifically for
underground aquifers – and Beagle 2, the small lander it will
carry, will search for signatures of past and present life,” he adds.

Europa – an ocean of liquid water?

Slightly smaller than our own Moon, Europa is one of Jupiter’s 16 known moons. Its surface is
covered entirely with water ice. Beneath could lie an ocean of liquid water – or so recent
space-based observations suggest.

If there’s water could there be life? There’s almost certainly carbon on Europa, deposited
perhaps by meteorites or crashing comets, or originating in the interior. However, the sunlight
penetrating the thick ice will be too weak to power the chemical processes needed for life.
So the answer probably depends on whether Europa’s interior is hot. If it is, then hot gases
and molten rock, issuing from vents in the ocean floor, could create a chemical cocktail in the
surrounding water just to the taste of some forms of life, as happens near such hydrothermal
vents on the Earth’s ocean floor.

Titan – a natural laboratory for prebiotic chemistry

Titan is one of Saturn’s 30 known moons. It is slightly less than half the volume of Mars. The chemistry
going on in its dense atmosphere, which consists largely of nitrogen, methane (a carbon source) and
hydrogen, is thought to be similar to the chemistry that went on in Earth’s early atmosphere before
life transformed it into the air we breathe today.

“When you study Titan, it’s a bit like going back in a time machine to Earth 4 billion years ago.
The atmosphere is a natural laboratory for studying prebiotic chemistry on early Earth – the chemistry
that led to life,” says
Helmut Lammer from the Space Research Institute
of the Austrian Academy of Sciences, Graz, Austria who
is associated with a co-investigator team on Huygens. “There’s a very low probability of finding
real life there (it’s far too cold and all the water is deep frozen on or below the surface), but
the chemistry in the atmosphere may be very similar to the chemistry that preceded life on Earth.”

When ESA’s Huygens probe enters Titan’s atmosphere
in early 2005, one of its main objectives will be to study this chemistry. The probe is hitching a
lift aboard NASA’s Cassini spacecraft, which will also study Titan from orbit around Saturn. The two
spacecraft are already on the final leg of their journey between Jupiter and Saturn.

“Methane in Titan’s atmosphere is continuously destroyed by ultraviolet light. To explain the
amount of the gas present in the atmosphere, we think there must be a large source either on or
under Titan’s surface. It could be in the form of lakes or oceans, or subsurface reservoirs,”
says Jean-Pierre Lebreton, ESA’s project scientist for Huygens.

“Of course, we may not find the precise transformation that turned complex organic compounds
into living things. But the better we understand the chemistry, the better our chance of working
out how it led to life,” says Francois Raulin, an astrobiologist from the University of Paris,
France and a co-investigator on Huygens.

Did comets bring water and the seeds for life?

Simple organic molecules containing carbon and nitrogen are the essential building blocks
of the complex organic chemistry found on Titan, on the early Earth or possibly on early
Mars. How did these simple organics come to be on these bodies in the first place? Were
they indigenous? Or were they deposited by impacting comets together with large amounts of water?

Scientific opinion is shifting towards the view that comets have played a significant role
in seeding the chemical building blocks of life. “An impacting comet has the power to destroy
life on Earth, but we now think that comets may have also helped to create life in the first
place,” says Gerhard Schwehm, ESA’s project scientist for the
Rosetta mission.

Comets are loose agglomerations of dust and ice that orbit the Sun in the outer reaches of
the Solar System. Unlike the planets, they are thought to have undergone little chemical
processing since their birth with the rest of the Solar System 4.5 billion years ago.
Preserved inside them may be unadulterated samples of the raw material out of which the
Solar System formed.

“We know that organic material is present in comets from remote-sensing observations from
Earth and spacecraft such as Giotto, which flew by comet Halley in 1986. But we haven’t been
able to tell how complex the organic molecules are,” says
Simon Green a Solar System scientist
from the Open University, Milton Keynes,
United Kingdom and co-investigator on ESA’s Rosetta spacecraft.
Finding out will be one of Rosetta’s tasks when it arrives at Comet Wirtanen in 2012. “The
Rosetta orbiter and lander will carry sophisticated payloads that will study the composition
of the dust and gas released from the comet’s nucleus and help to answer the question: did
comets bring water and organics to Earth?”, says Schwehm.

Dust and gas

The Solar System condensed from a cloud of molecules and dust, similar to the clouds of interstellar
dust and gas observed in our Galaxy today. Discoveries over the past ten years are revealing many
similarities between dust elsewhere in the Galaxy and the composition of comets.

“When we look at dust and gas in the interstellar medium, we see molecules that are found on comets
and on Earth. They are important components for building up more complex molecules,” says Pascale Ehrenfreund, an astrochemist from the
Leiden Observatory
in the Netherlands and a co-investigator on ESA’s Infrared
Space Observatory (ISO).
Of the 120 molecules so far detected in interstellar gas, about half are organic. “The largest
has 13 atoms (HC11N), but we think there are much larger
molecules out there,” says Ehrenfreund.

ISO helped detect many of these molecules. Most notable was its finding that
water is just about
everywhere in space. In 2007, ESA’s far infrared and submillimetre mission the
Herschel Space Observatory, will continue to
unravel the complexities of interstellar chemistry and further our
understanding of star formation.

“We know that our Sun formed in a similar way to other stars which are forming in front of our
eyes (or at least our telescopes) today,” says Göran Pilbratt, ESA’s project scientist for Herschel.
“What we don’t know is to what extent planets form with these stars – and the nature of these
planets. This is a problem that must be addressed by looking for and studying planets around
already-formed stars, and by trying to understand the process of star formation itself, which
is one of the areas where Herschel is expected to have a large impact.”

Life beyond our Solar System

The chance of finding life beyond our Solar System took a leap in 1995 when Michel Mayor and
Didier Queloz at the Geneva Observatory
discovered the first planet orbiting another star.
Since then, ground-based telescopes have detected no fewer than 74 planets orbiting 60 stars – and
the number is rising almost monthly.

Most of these planets, however, are large (at least the size of Jupiter) and orbit too close
to their stars for life’s comfort. Earth-like planets, capable of supporting life as we know
it, will be much smaller and orbit their stars within ‘habitable zones’, that is at such a
distance and in such a way that liquid water can exist on their surfaces most of the time.

“About 1000 stars are being searched for planets at the moment and it seems that about 5 per cent of
them have closely orbiting Jupiters. The reason we haven’t detected small planets is because
our methods are not sensitive enough,” says
Alan Penny, a member of the Darwin study team from
the Rutherford Appleton Laboratory, United Kingdom.
“If you think of the extra-solar planets as the
occupants of a zoo, we’ve only looked inside one cage so far – the one containing large
planets near to their star,” he adds.

Increases in the sensitivity of ground-based telescopes are expected to take the size of
detectable extra-solar planets down to one sixth of a Jupiter, or 50 Earths in the next
ten years. But “space is best”, says Penny and plans are afoot at ESA to launch three
spacecraft to look for Earth-like planets and the signs of life on them.

First will be Eddington
with a possible launch date of 2008, which will search 500 000
stars for orbiting Earth-like planets. Next will be
Gaia which will watch for stars
wobbling along their orbits because they are being tugged by planets. Finally,
Darwin
will employ a sophisticated technique called nulling interferometry to detect Earth-like
planets directly and determine the composition of their atmospheres through spectral analysis.

“If we see oxygen or ozone, we’ll know that something is pumping it out continuously,
as oxygen is very reactive and doesn’t stay around long. People have wracked their
brains to think of methods of getting a lot of oxygen into the atmosphere that have
nothing to do with life – and they can’t think of any. So in all probability, if
we see oxygen or ozone, it will mean life,” says Penny.

Darwin, however, is a complex and challenging mission technologically. “Recent studies
by industry have shown Darwin to be feasible within the next decade,” says Malcolm Fridlund,
Darwin study scientist at ESA. “Nevertheless, a very large amount of technological
preparatory work is needed which will be carried out in pre-cursor missions over the
next few years.”

Whether it’s looking for life close by in our Solar System or far away in
distant galaxies, ESA is forging forward with exciting and ambitious
exploration plans to enable European scientists and engineers come closer to
answering the question: “Is there life out there?”