The Beagle 2 lander, to be carried on ESA's Mars Express, is equipped with a suite of instruments designed to look for evidence of life on Mars. Credit: ESA

Of all missions sent to Mars only one, the Viking 26 years ago, has dared to search for life. Its only conclusive result was that finding proof of extraterrestrial life proved to be much harder than expected. Second attempts never followed. Until now. ESA's Mars Express, the next mission to the Red Planet and the first European one, has an ambitious goal. To be launched in 2003, Mars Express will be the first spacecraft after Viking to search for direct and indirect evidence for past or present life on Mars. This time, scientists are equipped with more knowledge and insight in how to detect Martian life. The chances of success look very good.

The expectations regarding life on Mars have changed substantially since the Viking missions. Today's scientists are considering several alternatives:

1. Martian life exists, but the lifeforms are so small you can barely see them and they probably live underground;

2. Martian life is not only small but also dead and extinct by now, so the search is for fossils and not for living organisms; and

3. there is no life on Mars now and there never has been.

Each of the two Viking landers, launched in 1976, carried three biological experiments. All of them searched for microbes or microorganisms, or their 'signature', in soil samples. All three experiments, based on different concepts, quickly produced positive results. The thrill died down as scientists soon realised that a non-biological process could easily explain most of the results. Surprisingly, the non-biological process that had tricked scientists had not been anticipated by anyone prior to the launch.

ESA's Mars Express will arrive at Mars in December 2003 and will follow a strategy quite different from that of the Viking. It consists of an orbiter plus a lander, called Beagle 2, "as an homage to the ship on which Charles Darwin found the inspiration to write his theory of evolution," says Agustin Chicarro, ESA Project Scientist for Mars Express, also pointing out that "indeed this mission could be as revolutionary as Darwin's ideas because it is the first one after the Viking to search for life."

A key difference between Mars Express and the Vikings is that now scientists are aware that they should also look for past, fossilised life. A few biological experiments are not enough. Mars Express's scientists will combine many different types of test findings, for example, to help discard contradictory results.

Some of the evidence will be indirect, mostly focused on the search for water. The Mars Express orbiter will have seven instruments on-board, apart from the lander Beagle 2. One of these instruments will image the entire planet in full color, in 3D, at a resolution of about 10 metres. Another will map the mineral composition of the surface with great accuracy. "These data will be key to determine how much water there was in the past, and from that you can estimate how much water there is left," says Chicarro.

A third instrument on-board the Mars Express orbiter will search for water below the surface, to measure the thickness of the layer of ice or permafrost, that is, a thick subsurface layer of soil that has a temperature below 0 deg C all year round. Other studies will determine the amount of water in the atmosphere and the water cycle: how the water is deposited in the poles and how it evaporates depending on the seasons.

The search for direct evidence of past or present biological activity will be the task of the lander, Beagle 2. Once deployed, in an area that was probably flooded in the past, Beagle 2 will unfold its robotic arm where most of the instruments are located. Beagle 2 carries several instruments, among them a gas analysis package that will determine whether carbonate minerals on Mars, if they exist, have been involved in biological processes. If there are certain gases on Mars, such as methane, that scientists believe can only be produced by organisms living either on the surface or below it, Beagle's 'nose' will detect them.

The feeble Martian atmosphere cannot prevent ultraviolet radiation from the Sun killing potential life. For this reason, it is important to get samples from places below the surface, under large boulders, and within the interiors of rocks. Beagle 2 will collect samples with a mole able to crawl short distances across the surface, about 1 centimetre every six seconds, and to dig down to 1.5 metres deep. If the digging proves to be hard, a grinder will help access the rocks' protected interior. With all these available tools, Mars Express will be the best mission ever to discover life on Mars. There can be no place for life to hide from it.