Are we unique, or is there life elsewhere in the universe? Colin Pillinger considers Martian puzzles
No one knows whether there is life on other planets, but new information gleaned from Martian meteorites makes it worthwhile to have another look. It has certainly made it worth all the efforts my team has been making to obtain funds for the Beagle 2 project, which will fly to Mars in 2003 in an attempt to answer the question.
The subject has been good value for at least 200 years; the first time it appeared in our daily ration of news was in the 1780s. The earliest story was not, however, from a science correspondent. It arose in a legal report of a court case: the defence counsel, in a trial for attempted murder, decided to make the case that his client was insane - and therefore not responsible for his actions - because he believed that men lived on the Sun. (It was dangerous ground because at the time the king's favourite astronomer, Sir William Herschel, believed that both the Sun and the Moon supported life.) The lawyer was never required to prove his point - not because the defendant was obviously a very disturbed man (he starved himself to death while in prison), but because the judge had already ruled that the prosecution's case was flawed. There had been plenty of witnesses to events, the attacker was relieved of his weapon at the scene, and the intended victim had burns on her stays because the pistol was so close to her when it was fired. But in spite of all this, the learned judge nevertheless decreed that unless the ball the pistol had fired was produced, there was no evidence of intent to kill. After all, a man's life was at stake.
So it is with science: the more spectacular your claim, the better your evidence must be. Fortunately, science, unlike the law, is able to go on searching, almost indefinitely if necessary, for the bullet; we do not have to prove our case first time round. As far as life on Mars is concerned, we appear regularly to change our minds.
In 1976, the Viking mission, launched by the US space agency, Nasa, sent two spacecraft to the Red Planet with the best experiments it was possible to conceive to seek viable life forms. Very soon the results began to be beamed back. It appeared that something in the soil was utilising a radioactive material that had been sent on the mission as a possible nutrient source for any Martian organism that might be present. The mission also carried carbon dioxide with a radioactive carbon atom present, and some of this labelled gas was "fixed" by an unidentified agent. In addition, when the samples scooped up were moistened, copious amounts of gas, mostly oxygen, were evolved. At least two, if not all three, of these results could be taken as evidence of biological processing.
Editors of the day should have been commissioning their stories and writing their headlines, but something held them back. Another experiment, carried out by the spacecraft and designed to measure the abundance of organic material in the Martian soil, had failed to detect anything above a few simple molecules - and even these could be ascribed to cleaning procedures. The Viking scientists were faced with a dilemma. They had what looked like a biological function without the physical manifestation of its cause. Viking seemed to have life without a body. There was nothing for it but to opt for a not-proven verdict.
In fact, the scientists went a little further; they claimed that Mars was playing an enormous practical joke on them. Its surface environment showed signs of being incredibly oxidising so they explained away the results as chemistry mimicking biology.
Everybody fully expected that other space missions would rapidly follow Viking and that samples of Mars would soon be returned to Earth to demonstrate the truth or otherwise of the hypotheses formulated in 1976. But it was not to be. The not-proven verdict became not guilty; indeed, it became the conventional wisdom that Mars was too hostile an environment to be considered a habitat for life.
For some scientists investigating other lines of inquiry, however, the case was never closed. Enter Martian meteorites. As a direct result of the Viking mission, it has been shown that we already have rocks from Mars here on Earth, thrown by giant impacts on the Red Planet. Painstakingly (every new piece of information has been extensively challenged), it has been revealed in a number of ways that these rocks have witnessed the passage of the essential ingredient for life - water - at temperatures appropriate for biological activity to occur. Organic matter, a chemical relict of biology, was discovered accompanying carbonate, a mineral deposited by water. Carbonate-containing sediments on Earth are petroleum source rocks - not that this means oil on Mars - but oil is one of Earth's most obvious manifestations of a vigorous past biology.
The carbon isotopic characteristic of the organics and carbonate on Mars can be shown to be very different; isotopic differences between co-occurring organics and carbonates are taken as demonstrating that biological activity began on Earth 4 billion years ago - in other words, almost as soon as the planet became solid enough to support a biology of sorts. This alone suggests that life might be rather easy to get started, and not, as some would have it, some unique happenstance peculiar to Earth.
Then there was the case of the "Martian fossil" - what appeared to be a nanometre-sized segmented worm. Love it or hate it, the Martian fossil - which, real or not, remains extremely controversial - has prompted the search on Earth for the smallest features in the geological record. It has made researchers look hard for corroborating evidence such as the existence of biominerals - magnetite, for example.
So where are we now? Although much of what has been learnt about Mars from Martian meteorites can be definitely shown to have occurred on our neighbour in the solar system, a few important things cannot be proved. Most important, we cannot unequivocally demonstrate the provenance of the organic matter. This is why Beagle 2 is returning to Mars in 2003 armed with new experiments.
We are going to have a much more careful search for the bullet, to use the court analogy - the body that was so elusive for Viking . Beagle 2 carries a method based on combustion that detects every atom of carbon in all its forms. We are going to look for organic matter - chemical fossils - under the surface, particularly under a large boulder that has not been moved from the time it was deposited. We will also look inside rocks. The premise behind seeking out subsurface and interior locations is that organic matter would have a better chance of surviving if it were protected from an oxidising environment.
Beagle 2 's mass spectrometer will also be capable of searching for trace constituents of a biological nature in the atmosphere. The composition of Earth's atmosphere would be incredibly boring were it not for the products of biology. All manner of species, perhaps the best known being methane, are there only because biology continuously produces them. It would be the same on Mars. If we could show that the oxidising atmosphere contained unstable reduced molecules, such as methane, which is the end product of the simplest metabolic reaction, we could argue for a biological source. It might be 1,000km away or 1,000m down, but the important thing would be its existence.
About 80 per cent of the population would like to believe that life does exist on other planets. We do not like being on our own. In my opinion, it is incredibly arrogant to think that humankind is the pinnacle of evolution. While we probably will not have a definitive answer about planets beyond the solar system for many years to come, it is worth bearing in mind that in science the absence of evidence is not evidence of absence.
Colin Pillinger is professor of planetary sciences at the Open University and leader of the Beagle 2 project, a part of the European Space Agency's Mars Express mission.
The Big Questions in Science is published on October 3 by Jonathan Cape, £15.99.