For years the pantheon of characters dreamed up by
science fiction writers has both excited and alarmed us. Taking the myth out of
science fiction and into the realms of respectable science is the relatively
new interdisciplinary field of astrobiology.
Saturn's largest moon Titan is home to oceans of methane
which, unlike water on Earth, is not an ideal environment to sustain
life.
What is life?
If we are going to look for life, we need to be able to
define what it is that distinguishes living from non-living. Unfortunately for
us, life defies simple definition.
There is no neat sentence that sums up what life is, no
mathematical formula, no straightforward schematic. Instead we have resorted to
describing life, with lists of characteristics that living things have. These
familiar characteristics can be found in any biology text, and include cellular
organisation, ability for growth and reproduction, heredity, metabolism,
movement, and response to stimuli.
While all living organisms on Earth exhibit these
characteristics, vexingly, so do some non-living entities. Fire can be said to
metabolise, that is convert energy from one form to another, but fire is not
alive. Crystals can reproduce, but they are not alive. Viruses are seemingly
living when they take over the machinery of a host cell, but by themselves are
not alive.
Although there are difficulties with the way we answer this
most fundamental of questions, without some idea of what constitutes life, we
will find it very difficult to go and look for it. So, while the clumsy
definitions that we currently employ have a range of limitations, we do not
have a great deal of choice but to use them in our search to see if we are the
only creatures in the universe that exhibit this peculiar set of
characteristics.
Origins of life on Earth
In the search for life beyond Earth, it's also important
that we have some understanding of how and where life on Earth originated. As
we can be 100% certain that life has emerged once in the universe, discovering
the origins of life on this planet has the potential to tell us a great deal
about the occurrence of life on others.
There are a number of theories on how life began on Earth.
It may have cooked up in a primordial soup of increasingly complex compounds on
the Earth's surface 3.5 billion years ago. Alternatively, it could have originated
many miles underground in the exceedingly hot and chemically volatile regions
of the Earth's still forming crust. It may have even arrived from space, riding
in on one of the vast number of meteorites that impacted the surface of the
newly formed earth. We can not be sure.
The latter theory, widely known as panspermia, has for the
most part been widely disregarded. Recently, in light of findings such as the
discovery of amino acids in the Murchison meteorite, and evidence of
microfossils in a meteorite of Martian origin, the theory has undergone a
resurgence of popularity.
Primordial Soup?
The primordial soup theory, while still popular, is losing some support in favour of the idea that life may have evolved deep in the Earth's crust.
Evolution of life on the surface of the relatively young
Earth would have had a lot of obstacles to overcome, not least of which was
frequent bombardment by meteorites and radiation. Although the subterranean
environment would have provided shelter from bombardment, and allowed early
life a reasonably uninterrupted chance to establish, the extreme conditions present
there were thought to be too harsh for life to exist. Now, with the relatively
recent discovery of a totally new order of life, known as Archaea, this belief
is being reviewed.
Archaean microbes live in environments of extreme
temperature, pressure, salinity and pH. Broadly termed extremophiles, the
different groups have been given equally inventive names to describe their
particular habitat. Thermophiles live in temperatures of 50-80°C, while
hyperthermophiles have been found in the temperature range 80-115°C. On the
other end of the scale are the psychrophiles, which live at temperatures of
around -2°C. Halophiles live in very saline environments. Barophiles live in
high pressure environments (up to 110 Mpa). Acidophiles live in conditions
where pH ranges from 0.7-4, while alkalophiles can be found in pH ranges of
8-12.5.
The interest in these organisms, apart from the very novelty
of their existence, is that the inhospitable conditions in which they thrive
may be similar to what Earth was like 1 billion years or so after its
formation. The discovery of extremeophiles lends a great deal of support to the
theory that life may have emerged on Earth in the high pressure, high
temperature, chemically volatile depths of the planet, and only emerged once things
had settled down on the surface.
If this is the case, and life could have emerged in such
unfriendly conditions on Earth, why couldn't the same be said for other planets
that until now were thought not to be suitable for life?
What's the chance of life beyond Earth?
"If science fiction authors and Star Trek writers can envision life as we don't know it ... then surely the Universe is equally creative." — Michael Lemonick
It was recently estimated that there are 70 thousand million
million million observable stars in the universe, not to mention those that are
beyond our detection. Given this, it is my personal belief is that we are not
alone in the universe. There's no real science behind this belief, but to me
the size and numbers involved seem to indicate that there is more than a fair
chance that there is life, intelligent or otherwise, somewhere out there.
Otherwise, it would be an incredible waste of space.
There are, of course, many people who are more scientific in
their approach to determining the existence of life beyond earth than I am. One
such person is Frank Drake. Currently Chairman of the Board of the SETI
Institute, in 1961 he developed the now famous Drake equation, which for the
first time attempted to quantify the probability of detecting life (in this
case, intelligent life) beyond Earth.
The Drake equation basically states that the number of
civilizations we could detect will depend on the rate at which stars like our
sun form, then the fraction of these stars that form planets, then the number
of these planets that are hospitable to life, then the number of these planets
where life actually emerges, then the number of these planets were life evolves
to develop intelligence, then the fraction of these planets where interstellar
communication evolves and, finally, the time that communication is carried on
for before these intelligent civilizations die out or stop trying. More
succinctly, the equation looks like:
The Drake equation - N = R* Fp Ne Fl Fi Fc L
Where:
N the number of detectable civilizations
R* the rate at which Sun-like stars form
Fp the fraction of stars that form planets
Ne the number of planets per solar system
hospitable to life
Fl the fraction of planets where life
emerges
Fi the fraction of life bearing planets
where intelligence evolves
Fc the fraction of such planets where the
inhabitants develop interstellar communication
L the length of time such civilizations continue to
communicate before they end
Not only does the Drake equation convert the question of the
existence of extraterrestrial neighbours from one of metaphysics to hard
science, but it gives those looking for life beyond Earth a place to start.
What are we looking for?
It's accepted that life on Earth is highly unlikely to be
representative of all life in the universe, but we have to start somewhere.
The most basic requirement of life on Earth is the presence
of liquid water. Water is important to life because, in liquid form, it is an
excellent medium for carrying chemical and biological compounds. It is also
stable as a liquid over a wide temperature range, a temperature range that
(conveniently) accommodates a wide range of biological processes. In
identifying places where life may exist, astrobiologists are looking for signs
of water, particularly in liquid form.
Astrobiologists are also looking for the right cosmic
chemistry in their search for life. The presence of organic (carbon) compounds,
while not conclusive, could be suggestive of life. Atmospheric concentrations
of certain substances could also be indicative of living organisms. Oxygen and
methane, for example, are both found in our atmosphere, but are both highly
reactive molecules. Their individual presence suggests that molecules are being
constantly produced to replenish the numbers in the atmosphere, and the source
of this replenishment could be life.
Given that life did emerge and evolve on Earth, it seems a
logical step to look for Earth-like planets as potential hosts for
extraterrestrial life. These planets would be of a similar age and size to
Earth, and orbit a similar distance from sun-like stars — far enough away from
the star that any water present doesn't evaporate, but close enough that it
doesn't freeze.
If there are highly evolved life forms out there we may even
intercept signals from them. This search is the whole premise of the SETI
program - the Search for Extraterrestrial Intelligence. Rather than looking for
chemical and biological artefacts, SETI scientists are aiming to make contact
with ETI through radio astronomy.
Of course, finding all of these things does not mean that we
should not expect to find life forms (particularly evolved or higher life
forms) that are in any way similar to life as we know it. The Earth's biota is
the result of a set of unique conditions shaping the products of the natural
life giving processes — the laws of chance dictate that finding a planet whose
population has survived five great extinction events, not to mention geological,
meteorological , physical, chemical and biological conditions that ensued as a
result of each other, is exceedingly slim, and even if we did, the probability
of life beyond Earth following exactly the same evolutionary pathway is too
remote to contemplate.
Where are we looking?
On Earth
Although it may seem an odd place to look for our
extraterrestrial neighbours, there are a vast number of astrobiological
projects taking place here on Earth. Apart from being easier to access and a
whole lot cheaper to study than sites in deep space, the terrestrial laboratory
that is our planet provides an array of fascinating opportunities for
astrobiologists. Extremophile studies may help to unlock the origin of life on
Earth, and so offer insights into life beyond it. Animal communication studies
utilising information theory, which allows the complexity of a given signal to
be measured, will hopefully allow us to identify the long awaited signal from
space once it comes from random noise.
Other studies that are being undertaken involve examining
materials from space that we find here on Earth. Over 22,000 meteorites have
been discovered on Earth, including 28 of Martian origin. As mentioned earlier,
studies of these meteorites have broadened our ideas about the beginnings of
life, and about its distribution in the solar system and beyond.
These lines of enquiry are but a few of the many being
examined on Earth in the search for life beyond it. NASA's astrobiology site
gives details of many more.
In the Solar System
Mars
Mars has always been a favourite source of speculation when
it comes to extraterrestrial life. Its proximity means that it is also a target
for scientific expeditions. Since 1960 there have been 34 missions to Mars.
Of the successful ones (16 have failed), four have involved
landing spacecraft on the surface of Mars. In 1971 the first Martian landing
was accomplished by the Soviet Mars 3 mission. Although only broadcasting
information for 20 seconds, landing a craft on another planet was a huge
success. NASA followed with the successful deployment of two orbiter-lander
pairs in 1976 — Viking 1 and Viking 2. The landers conducted experiments
looking for signs of life, but found no conclusive proof at their landing
sites. Most recently, the Carl Sagan Memorial Station lander and Sojourner rover
of NASA's 1997 Pathfinder mission collected information suggesting that Mars
was at one time warm and wet — conditions suitable for life.
Mars is again the destination du jour with three separate
craft winging their way to the red planet. The European Space Agency (ESA)
launched its Mars Express mission in June 2003, with the primary objective
being the search for subsurface water. The Mars Express spacecraft is carrying
the Beagle 2 lander which will perform exobiological and geochemical research
after it lands on the Martian surface in December 2003. NASA's Mars Exploration
Rover program is also looking for signs of water, and has two separate rovers
on their way to Mars. Spirit, launched in June 2003, and Opportunity, launched
in July 2003 are set to arrive at their destination in January 2004.
In addition to the missions landing on the surface of the
red planet, there have been a number of orbiting spacecraft sent to try and
unlock some of its mystery. At present the Japanese spacecraft Nozomi is on its
way there. Although plagued with problems since its launch in 1998, it is hoped
that Nozomi will make it to Mars where it will study the upper Martian
atmosphere. A summary of all missions to Mars, past and present, is on the NASA
website.
Europa
Europa is one of the four large "Galilean
satellites" orbiting Jupiter. Although it is the smallest of these
satellites, Europa is still the sixth largest satellite in the solar system,
only slightly smaller than our own moon. Europa has a relatively smooth, icy surface
under which there is good evidence for the presence of liquid or semi-liquid
"oceans". As liquid water is one of the key signs of potential life
beyond Earth, Europa has caused a great deal of excitement in astrobiological
circles.
Pioneer 10 and 11, and Voyager spacecraft have flown by
Jupiter, but Galileo has given us the most information about Europa. Galileo
was launched in October 1989, and after arriving at Jupiter in July 1995, made
11 orbits of Jupiter and its moons over the two year period of its prime
mission. In addition, a probe was sent plummeting through the Jovian atmosphere
early in the mission, where it recorded 58 minutes of data before being
destroyed by the harsh conditions it encountered. In 1997 after the prime
mission was completed Galileo completed an additional 14 orbits, eight of which
were around Europa.
Titan
Titan is Saturn's largest moon, and it is believed that the
atmospheric composition (nitrogen, methane, ammonia and argon) and surface
conditions might be similar to those that we would have found on Earth when
life was first emerging.
Pioneer 11 made the first direct observations of Saturn in
1979, with the two Voyager spacecraft following in 1980-81. These spacecraft
took photographs of Titan (although the hazy atmosphere of the moon obscured
the surface) and obtained atmospheric pressure and composition readings.
The latest mission to head to Saturn is Cassini-Huygens, an
international collaboration between NASA and the ESA. Scheduled to reach Saturn
in the second half of 2004, the craft consists of the Cassini orbiter (NASA),
and the Huygens probe (ESA). On arrival, the Huygens probe will be deployed to
the surface of Titan, where it will relay information about what it finds to
the Cassini orbiter. This part of the mission is expected to last for four
hours. The Cassini orbiter will continue to orbit Saturn and its moons for
another four years.
Beyond the Solar System
Earth-like planets
Although more than 100 planets have been found orbiting
stars outside of our solar system, they have all been more
"Jupiter-like" than "Earth-like". At present, we do not
have sensitive enough equipment to detect the presence of relatively tiny
planets like Earth. A number of missions are being planned in an attempt to
overcome these limitations such as NASA's Terrestrial Planet Finder which it is
hoped will be implemented in 2006, and the ESA's Darwin mission, to be launched
in 2014.
Search for Extraterrestrial Intelligence (SETI)
Perhaps the most well known search for life beyond earth is
the Search for Extraterrestrial Intelligence. Projects under the SETI banner
are not just looking life beyond earth, but highly evolved, intelligent life.
The search is based on the premise that the intelligent
civilizations will be either deliberately or inadvertently transmitting signals
that we will be able to detect on earth. The largest program being undertaken
at present is Project Phoenix. Starting in 1995 at the Parkes Radio Telescope
in Australia, the program is now based at the world's largest single-dish radio
telescope at Arecibo in Puerto Rico. It involves the systematic scrutiny of
space in the vicinity of sun-like stars. To date approximately half of the
target stars have been investigated with no success. However, there are still
an awful lot of stars to go...
... so the search continues
The search for life beyond earth is potentially one of the
most exciting, illuminating and confronting pieces of science ever to be
undertaken. Its success will change the face of science and life as we know it
forever. The journey through space and time that this success could take us on
has profound implications, but none more so I suspect, than the realisation
that at the end of the day, there's no place like home. Maybe then we'll give
our own planet the care and attention it deserves.
