Martian territory

Picture the scene: a desolate, alien-looking plain, cowering beneath a blistering sun. In the middle of the plain a robotic vehicle, about the size of a Mini Cooper, is trundling along at a slow walking pace, as it has been ever since the sun came up a few hours ago. It stops at a scarred, wind-scuffed rock. Lights flash, cameras whirr. Then it moves on.

Picture the scene: a desolate, alien-looking plain, cowering beneath a blistering sun. In the middle of the plain a robotic vehicle, about the size of a Mini Cooper, is trundling along at a slow walking pace, as it has been ever since the sun came up a few hours ago. It stops at a scarred, wind-scuffed rock. Lights flash, cameras whirr. Then it moves on.

Cut to Mission Control, a darkened room in Pittsburgh. It's late evening. A dozen excited scientists crowd round monitors showing the wind-scuffed rock. The atmosphere is tense as they examine the image, pointing at spectral data scrolling down the side of the screen. Finally, a cheer goes up. It's official. They've found it - life on Earth!

Earth? Yes, the alien plain is actually in the Atacama Desert of Chile, probably the dryest place on Earth. You would be forgiven for thinking it was the surface of another planet - like Mars. But this is precisely the point: the scientists and engineers behind the robotic rover are testing the technology they hope will one day be used on a world like Mars.

"If you can't detect life in the most inhospitable place on Earth, what chance will you have on Mars?" says Alan Waggoner, an Atacama team member and the director of the Molecular Biosensor and Imaging Center at the Carnegie Mellon University in Pittsburgh. Waggoner and his colleagues are one of several teams developing schemes to detect life on other worlds, with the backing of Nasa's Astep (Astrobiology Science and Technology for Exploring Planets) programme.

The project is headed by the Carnegie Mellon Robotics Institute for the technology part, and by the Nasa Ames Research Center/Seti Institute. It involves experts in robotics, plus geologists, chemists, physicists and, of course, biologists.

If you're going to look for life on another planet, you need a good definition of life. No such definition exists, although most can agree on life's central characteristics - the ability to reproduce, move around, compete for resources, pass information from generation to generation, and so on.

Waggoner and his colleagues look for four of the chemical "building blocks" of life - DNA, the genetic material; proteins, the scaffolding and chemical engines of cells; lipids, the molecules of cell walls; and carbohydrates, the fuel of life.

Waggoner is an expert in fluorescent biosensors - molecules that can be attached to target biological molecules and which, when illuminated, "fluoresce", or give out characteristic flashes of light. They are usually used to show the chemical pathways in cells. "But it's also an ideal technology for detecting life on another planet," Waggoner says.

The rover, christened Zoe, uses four fluorescent dyes, which attach to DNA, proteins, lipids and carbohydrates respectively. When it stops at an interesting location, these are sprayed on the ground beneath and allowed to soak in for a few minutes. When the ground is lit up with artificial light from Zoe, the dyes that have found their targets fluoresce.

The difficulty is that the solar-powered rover has to "rove" in bright daylight, making it hard to detect the fluorescence. The solution is to illuminate the ground with an intense but ultra-short pulse of light, and to open the shutter of the rover's camera for the same interval. The sunlight picked up in so short a time - about 10 microseconds - is easily overwhelmed by the fluorescent light.

So far, the rover has been used in the foggy, coastal region of the Atacama, where hardy lichen can be seen on rocks, and in the apparently lifeless desert inland. What has got the team excited is that, even in the inland desert, they have picked up the unmistakable signal of life with their DNA and protein "probes". Waggoner says: "I suspect that life is so incredibly hardy that there is virtually nowhere on the surface of the Earth where it is absent."

To assess the accuracy of their detectors, some of Waggoner's colleagues have set up camp in the Atacama. They scoop up samples of the soil the rover has examined and send them to Pittsburgh. The same team is spraying the fluorescent dyes manually - just two dyes at the moment - on the ground beneath the rover. But, in future experiments, the rover will do this itself.

Every day, from about 3pm into the early hours, the Pittsburgh team examine the data, argue over their interpretation and decide the rover's schedule for the next day. The schedule is then sent to the Atacama team, who program the rover. The science team is led by Nathalie Cabrol, a planetary scientist at Ames Research Center at Moffett Field, California.

So, how long before we see this kind of technology used in an actual mission to a planet? Waggoner believes it could be within 10 years. He points out, however, that the kind of technology the Carnegie team is developing may need to be integrated with other technology if it is to track down the location of life. "This is the key problem. It's not simply identifying life in a sample - picking out the needle in the haystack," he says. "First, you have to find the haystack."

What Waggoner means is that conditions on other worlds are so extreme that any life may be extremely well hidden. On Mars, there is very little atmosphere, the temperature on a hot summer's day barely reaches freezing, and the surface is raked by savage ultraviolet and particle radiation from the Sun. "To survive, Martian life may have buried itself deep in the soil," Waggoner says. "Winkling it out may be very difficult."

It is a similar story in other locations in the solar system, such as Jupiter's moon, Europa, which is also a possible site for life. On Europa, any living things may be swimming about in a giant moon-wide ocean beneath a kilometres-thick ice sheet.

Though the prospect of finding life elsewhere in the solar system was once considered remote, this is not the scientific view today. This is because life has proved so tenacious on Earth, surviving deep down in the solid rock of the planet's crust, in boiling hot volcanic vents on the sea floor, and even in the cores of nuclear reactors. "Mars was much warmer and had water in its early days," Waggoner says. "If life got started on the planet, it is entirely possible it has clung on until today."

So he has high hopes. "All I can say is that if there's life anywhere, and a sample can be got hold of, our technology could probably detect it."

Marcus Chown is the author of 'The Universe Next Door' (Headline)