The H.M.S. Beagle's captain, Robert FitzRoy, conceived the idea of taking a naturalist only in the summer of 1831, making Darwin - the "scientific package" aboard the original Beagle - something of an afterthought.

Beagle 2 was also something of an afterthought, says lead scientist Colin Pillinger.

"Mars Express was originally going to be a rescue mission. It was going to relaunch instruments that had been lost on the Russian Mars '96 mission," Pillinger explains. But the discoveries related to signs of life in Martian meteorites and the 1996 scientific revelation that ALH84001 might contain a fossil sparked a new idea, Pillinger says. "I suggested to ESA that if they were going to have a mission to Mars that they really needed to have a lander and address some of these new issues that had arisen 20 years after Viking had said "We don't think there's any evidence of life on Mars."

Other researchers in France and Finland had conceived a "net lander" strategy: many small landers making measurements on Mars. But with no plans for one lander, let alone many, Pillinger says, Mars Express had neither space nor a mass allocation for numerous net landers, no matter how small. "So it came down to a competition as to who could propose a [single] lander for a 60 kilogram limit. And Beagle is the only one that was a viable possibility," he says.

Because of the severe mass restrictions, Beagle 2 has no propulsion system. Instead it relies on parachutes and a slowly deflating balloon in a controlled crash landing. And it cannot move once it lands. Instead it relies on a robotic arm studded with scientific instruments, like digits on the end of a living limb. Scientists call the instrument package the paw.

Beagle 2, once on the surface, deploys four solar panels and the robotic arm. The paw includes a subsurface sampler called a mole, which taps itself into the ground - preferably under a large boulder, Pillinger says - a millimeter at a time. To sample unweathered rock, the paw also includes a grinder - developed conceived by a dentist - a microscope, spectrometers and other instruments.

But the part of the package that excites Everett K. Gibson, geochemist, adjunct scientist on the Beagle 2 project and a senior scientist at NASA's Johnson Space Center in Houston, is the gas-analysis instrument. "The real beauty of this is to be able to sample on the surface, beneath the surface, make fresh surfaces of the rocks to get samples and then to take those and send them into the gas-analysis package, where we can begin to get a handle on the nature of the biogenic elements that might be present."

The gas-analysis experiment is a miniaturized version of the lab equipment developed by Pillinger to analyze Martian meteorite samples on Earth. It slowly heats a sample in the presence of oxygen, then analyzes the gases driven off. At each step in the heating cycle, different chemicals burn. During the process, the instrument can detect carbonates produced by water percolating through cracks in rocks and organic matter - the chemical signs of life.

Because the instrument analyzes gases, it can also analyze the Martian atmosphere. Gibson explains that the Beagle instrumentation eclipses that of Viking "I'm really pleased that we'll have information in early '04 about the nature of the light elements, in particular carbon and the biogenic elements, from an in situ package that is well, well advanced beyond anything that Viking could do in 1976."

Unlike Viking, Beagle 2 can garner isotopic information about individual carbon atoms. Carbon atoms come in two stable forms, called isotopes: carbon-12 and carbon-13. The only difference between the two is the number of neutrons it contains. Carbon 12-and carbon-13 are mixed together in atmospheres. As biological processes build organic molecules, they use more carbon-12 than -13. This distinctive carbon-12:-13 ratio becomes a detectable signature both of living organisms and their leavings. It is this signature that Beagle-2 will look for.

But if the same instrumentation has found evidence of life in Martian meteorites, why take them to Mars? Because, Pilinger argues, however enticing, the evidence from meteorites is not complete.

"You can prove that the meteorites come from Mars and that the carbonates were formed on Mars and that Martian water trickled through the rocks. And you see organic matter there. But what you cannot prove is that the organic matter is indigenous. You can't prove it's Martian. We just have one more step to make in this puzzle, we think, of going back to Mars and seeing whether the organic matter we've seen in the meteorites is in fact in Martian rocks and whether it meets the same criteria that we've recognized in the meteorites."

Plans for the various parts of Beagle 2 are taking shape. But the very genius of the Beagle 2 design - complete integration, with no separate boxes containing separate experiments - poses a significant problem, that of cleaning and sterilizing the whole spacecraft to remove all microbial contamination. "This is a tricky issue for Beagle," Pillinger says. "You have to sterilize everything which is part of the spacecraft and you also have to clean parts of the spacecraft as well so that you don't take dead bodies any more than you take live ones. The rules on planetary protection are quite extreme in that you have to meet an internationally agreed protocol. This is something that is exercising us quite a lot at the moment." The team is currently building a facility for cleaning and decontaminating.

Gibson, meanwhile, is already thinking past Beagle 2. "I would love to see sons of Beagle scattered throughout the whole surface of Mars," he says. "Any spacecraft that's going to Mars, that's going into orbit about the planet, should have a probe of the Beagle type." Because sons of Beagle would be cheap, mission planners could risk sending some into rugged terrain, which also might have a higher probability of having harbored life. "If we can send a multitude of these vehicles onto the surface in some of these high-risk areas, we have a good chance of getting some really interesting data on the nature of potential living systems that might have been on the planet in the past," Gibson says.

Pillinger, who trained as a chemist, has found astrobiology a great stimulus to learn many other fields. He has dabbled in biology, physics, astronomy and earth sciences and added a bit of plain old invention in his search for the evidence of life on other planets, he says.

"It takes you back to the days of Victorian scientists, when you were allowed to be interested in what fascinated you."

Related Links
Mars Express
The Beagle 2 Landing Site
NASA Astrobiology Institute
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