The truth is out there

Did bacteria from Mars kick-start life on Earth? No this isn't an X-Files storyline, but just one of the fascinating possibilities revealed by the science of astrobiology, says Becky Parker

Could there be life elsewhere in the Universe? Has there ever been? To date there has been no definitive proof either way, but that's not stopped me from standing outside on a clear night gazing up at the stars while quietly humming the theme from Close Encounters of the Third Kind and wondering if someone out there might be trying to make contact.

An expanding field of science, astrobiology seeks new ways of searching for life in our Solar System and beyond. It's a far cry from the sensation in 1967 when radio astronomer Jocelyn Bell Burnell discovered pulsars and they were thought to be cosmic signals from little green men, but some of the current work is amazing.

But how do you define life when searching for extraterrestrial evidence? In science fiction, beings from other universes can usually walk and talk. In fact they are often described quite rightly as super races - after all, any alien race must be super intelligent if it has developed the technology to travel close to the speed of light speed, cut through "wormholes" in space (see page 18) and so on in order to travel the vast distances necessary to make contact with us.

In real science, the key is to look for more primitive life forms, such as bacteria. Scientists have proved that life can survive in the most extreme environments, so could life from elsewhere in the universe have reached Earth before any other life here developed? And could that life perhaps have come from Mars? If so, that would mean that we are, in a sense, Martians!

But why is Mars a key planet for such speculation rather than others in the Universe? One reason is that it's our nearest neighbour - only about 78 million kilometres away when closest to the Earth. Another is because there was once water on Mars - an essential ingredient of life. There is also an amazing piece of evidence for earlier primitive life on Mars that is still under debate. Researchers studying a four-and-a-half thousand million-year-old Martian meteorite claim that 25 per cent of the magnetic material within it was produced by ancient bacteria. Adding weight to these claims is the fact that the lead researcher on the study, Kathie Thomas-Keprta, is an astrobiologist at NASA's Johnson Space Centre in Houston.

Magnetotactic bacteria (see page 18), found on Earth in aquatic habitats, arrange magnetite crystals in chains within their cells to make compasses that help them locate food and energy. The researchers used a biosignature - a physical andor chemical marker of life - to compare the crystals in the Mars meteorite with those produced from bacteria on Earth. They found striking similarities and there is evidence that Mars did have a magnetic field at the same time these magnetites were formed. This research is currently under scrutiny, yet the implications if confirmed would be enormous.

It is a major research goal to understand how life began on Earth. We know that our life system needs water in liquid form, an environment friendly enough to assemble complex organic molecules and energy sources to sustain metabolism. As it is so difficult to establish how the first organism came to life, why couldn't it have come in from space? Central to this theory is discovering the mechanism by which it could have made the journey to Earth. Could it have been delivered by meteorite?

Exciting research done at the University of Kent in Canterbury by Mark Burchell and his team suggests this may be possible. Using a two-stage light gas gun (see picture above), researchers fired bacteria-laden projectiles at simulated oceans in the form of agar plates. Certain bacteria survived these hypervelocity impacts and grew. The speed of impacts that they survived - around five kilometres per second - is typical of the escape velocity needed to break free from Mars's atmosphere. Although this is not quite as fast as the typical impact of a meteorite on Earth, it does establish that bacteria (the trial ones belonged to the genus Rhodococcus - they're red and rod-shaped) can survive being accelerated to high speeds to escape from a gravitational field and may survive a subsequent impact. If similar bacteria could have survived a meteoric impact with Earth, they could have been our predecessors as life forms.

So could we really be Martians evolved from bacteria that survived an impact with Earth around four-and-a-half thousand million years ago? It's a fantastic theory and opens up the possibility of "panspermia" - life migrating round the Universe - but exercise caution when looking up this term on a web search because you'll find a variety of sites dealing with subjects ranging from creationism to life having arrived on a spaceship.

It's impossible to say that this is how life started on Earth, because scientifically we still do not know how and when this happened. But it throws up questions such as is the Universe really teeming with life? For definitive proof of whether there has been life on Mars we need to send a probe there and perform tests. This is where the Beagle 2 mission will be extremely exciting and far reaching. This British-led effort to land on Mars as part of the European Space Agency's Mars Express Mission is due to be launched from Kazakhstan in June 2003. The project is a huge collaboration masterminded by a team of scientists at the Open University. Its crucial mass spectrometer package (see page 18) will analyse samples from beneath the soil, gathered by using a robotic arm to deploy a crawling mole. It will look for the chemical signatures of biological processes. The equipment is expected to be able to determine whether there was ever life on Mars. Beagle 2 should reach its destination by Christmas 2003 and begin its analysis a few weeks later.

The study of astrobiology extends far beyond the question of whether there has been life on Mars and whether it came to Earth. But how can we look to other planetary systems and speculate about whether life exists on them? NASA seems to be the key player in this emerging science, although it has only been running its astrobiology programme for five years. Its stated astrobiological goals are to:

* Understand habitable planets in the Universe.

* Explore for past or present habitable environments, prebiotic chemistry and life that might exist elsewhere in our Solar System.

* Understand how life originates from cosmic and planetary precursors.

* Understand the interactions between life on Earth and its planetary and Solar System environment.

* Understand how life in general evolves on the molecular, organismal and ecosystem levels.

* Understand the principles that govern changes in ecosystems on Earth and shape the future biosphere.

* Explore the physical and chemical limits to which life adapts as a guide for searching for life on other worlds.

* Determine how to recognise signatures of life on other worlds and on early Earth.

Scientists have identified more than 100 planets, but the trick if we are to search for life similar to our own is to find one that's suitably sited at a reasonable distance from its star. And that's just what happened earlier this year. On June 13, it was announced that "after 15 years of looking, a top planet-hunting team has finally found a distant planetary system that reminds them of home". Geoffrey Marcy, astronomy professor at the University of California, Berkeley, and astronomer Paul Butler of the Carnegie Institution of Washington, had found a Jupiter-like planet orbiting a Sun-like star at about the same distance that Jupiter orbits the Sun. The star was 55 Cancri in the constellation Cancer. The planet itself would not be a suitable site for life, but the "Doppler wobbling" (see page 18) observed suggests there could be another planet orbiting closer to the star. If the planet is found to be in the "habitable zone" - a narrow band round a star where it is neither too hot nor cold for water to exist as a liquid - perhaps it may support life.

The next task is to analyse the atmospheres of these types of planet. The NASA Hubble Space Telescope has contributed to this analysis already. Being located above the Earth's atmosphere, it has dramatically expanded our views of space and has detected sodium in the atmosphere of a planet orbiting a star. Although this doesn't signify the presence of life, it does point the way for more sophisticated missions to come, such as the Kepler Mission to be launched in 20067. This is part of NASA's new Discovery programme, which concentrates on lower cost, highly focused planetary science investigations. It will search for planetary systems, monitoring about 100,000 nearby stars, looking for the slight dimming that occurs when an orbiting planet blocks out some of the parent star's light. Over a four-year period it will examine a far greater area of space than would be possible with the Hubble and should be able to observe thousands of stars like our own Sun.

The equivalent mission run by the European Space Agency, called Darwin, is due for launch in 2015. It will look for planets and their atmospheres and try to discover whether they have the appropriate chemical composition for life. Commenting on the inherent difficulties of this kind of project, the Agency says:"Looking for planets around nearby stars is like trying to discern the feeble light from a candle next to a lighthouse from a distance of 1,000 kilometres."

Once planets suitable for further study have been established, NASA's Terrestrial Planet Finder (TPF) will then examine all their aspects, from formation to final characteristics. In addition to measuring the size, temperature and placement of Earth-sized and other planets, TPF will analyse the light they give out to establish the make-up of gases in their atmosphere. It will look for carbon dioxide, water vapour, ozone and methane, the presence of which would show that life could be supported.

To detect the faint light from distant planets, TPF will reduce the glare of their parent stars 100,000 times, taking pictures of planetary systems as far away as 50 light years. Using images 100 times more detailed than those of the Hubble, TPF will also make it possible for astronomers to study the black hole at the centre of the Milky Way and other exciting phenomena in the Universe. But don't hold your breath - it's not due for launch until 2012.

To get some idea of just how difficult it is to detect the signatures of life, consider the Galileo probe which was launched in 1989 to take a detailed look at Jupiter. Astronomers also used the mission to observe Earth from space and discovered that careful interpretation of the data received was needed even before they could reveal the presence of life on our planet!

The final strand of this search for extraterrestrial life takes us to SETI, the Search for Extraterrestrial Intelligence. This international collaboration, which has an institute in California and a department in the University of Western Sydney, seeks to listen into the Universe. Ever since the first experiment was carried out in 1960 by Professor Frank Drake at the Green Bank radio telescope facility in West Virginia, increasingly sophisticated equipment has been developed to try to establish whether we are alone. Now radio telescopes a 100 trillion times more sensitive are listening in. You can get involved by downloading the program from http:setiathome.ssl.berkeley.edu

As well as carrying out radio searches, SETI is also developing an optical programme by which it is seeking to detect pulsed and continuous wave laser beacon signals in the visible and infra red spectrums.

There is a massive effort on many fronts to establish whether beings exist beyond our world. Wouldn't it be incredible if examples were found from our own "tree of life", to use Darwin's description of life branching out from fundamental roots. But why should this be the only system that exists? Surely there must be completely different life forms developed around different chemical combinations? Quite how we would begin to search for these stretches the imagination beyond belief and would require breakthroughs in the fundamental laws of physics. Yet these are coming. For example, Paul Davies, visiting professor at Imperial College London and adjunct professor at the University of Queensland, has recently suggested that the speed of light may have slowed over time.

It seems that anything is possible, so hold tight for an exciting ride. This topic must surely continue to amaze, excite and baffle anyone who starts to wonder "are we alone?"

Becky Parker is a former head of science currently working as a science education consultant. She is also a member of the development team for the new AS, Perspective On Science

The SETI Institute's comprehensive website has published its plans for the future up to 2020, which you can order over the web.

http:www.seti.org

http:seti.uws.edu.au

For more details on Mars, visit: http:mars.jpl.nasa.gov

The United Kingdom Astrobiology Forum and Network's website lists the interests of all those working in areas embraced by the science and can be found at:

http:astrobiology.rl.ac.uk

The other main European country interested in Astrobiology is Spain with its Centro de Astrobiologia (CAB) - its web address is:

www.cabinta.espagehome_cab_e.htm

For more details on the Terrestrial Planet Finder visit NASA's Origins website at:

http:origins.jpl.nasa.gov

Other useful websites include:

the interactive "Four ways to find a planet"

http:planetquest.jpl.nasa.gov

www.beagle2.com

http:astrobiology.arc.nasa.gov

www.spaceref.com

www.astrobiology.com

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