麻豆传媒

The creation of a human outpost on Mars is still some way off, but that hasn鈥檛 stopped us planning the garden. At Kennedy Space Center on April 15, President Barack Obama announced the intention to send humans to Mars by the mid-2030s. If all goes to plan, NASA will kick off an era of space exploration not seen since the Apollo moon programme in the 1960s.

If getting to Mars is a big step, ultimately staying there will be a giant leap. But already, experiments conducted in space and in simulated Martian conditions on Earth have yielded micro-organisms that will eventually help turn Martian rocks into soil, generate oxygen for us to breathe, purify water and recycle waste. Think of these microbial colonies as the first gardens on Mars.

at the Planetary and Space Sciences Research Institute of the Open University in Milton Keynes, UK, is part of a team that subjects terrestrial organisms to extreme stresses. The group has conducted experiments on the International Space Station (ISS) and as part of the Biopan VI mission run by the European Space Agency (ESA).

Biopan was a capsule loaded with 鈥� among others 鈥� samples of rocks from the sea cliffs at Beer on the south-west coast of England. It was carried by a Soyuz rocket into low Earth orbit in 2007. Once there, the capsule exposed its contents to the vacuum of space. The Beer rocks are home to a broad spectrum of microbes, including photosynthesising cyanobacteria. 鈥淲e thought it would be fun to send Beer into space,鈥� says Olsson-Francis.

By the time the Biopan samples were recovered, they had endured 10 days of space exposure, though the sun鈥檚 lethal UV radiation had mostly been filtered out. The team discovered a cyanobacterium new to science that had survived the ride (Applied and Environmental Microbiology, ). The experiments were not designed with space gardens in mind, but their findings are relevant. The high levels of UV radiation on Mars essentially sterilise anything left on the surface, so space gardens would need the protection of greenhouses (see diagram). 鈥淭he experiments demonstrated that we can use low Earth orbit to select for resistant organisms that could potentially be used in space applications,鈥� says Olsson-Francis.

Rocks placed into orbit also provide support for the lithopanspermia hypothesis 鈥� the idea that living cells can be transported through space in meteorites. Meteor impacts onto a life-bearing planet may throw rocky debris containing living micro-organisms into space, and if those organisms can survive the vacuum of space then they can start to grow again if they ever find themselves landing on a barren planet with otherwise suitable conditions.

The organisms in the planned Martian gardens will, of course, be transported by spacecraft. Nevertheless, astronauts will need them to be tough enough to survive in a compact, dormant state 鈥� as seeds and spores, for instance 鈥� and able to tolerate equipment failure and exposure to unshielded extremes.

Cyanobacteria photosynthesise, so they are good candidates for use in long-term space missions or on crewed outposts. That鈥檚 because colonists will need organisms that are 鈥減rimary producers鈥�, which are able to directly use the energy of sunlight to grow. For this reason, cyanobacteria are part of the self-contained recycling system being designed by ESA. The MELiSSA loop, or Micro-Ecological Life Support System Alternative, recycles human waste into water, oxygen and nutrients that astronauts and Martian colonists can use. Cyanobacteria are used to make the protein-rich food supplement spirulina, which ESA has named as one of its nine essential crops to grow on Mars.

As non-terrestrial locations for gardens go, Mars isn鈥檛 too bad. On the Martian equator in midsummer, the temperature can reach 20 掳C, and the atmosphere is 95 per cent carbon dioxide, which is fabulous news for photosynthesisers. But there鈥檚 one fundamental garden ingredient missing: soil.

The first Mars gardener could just slop fertiliser onto pulverised Martian rocks; experiments using rocks from Antarctica have shown that plants can grow in this way. In the longer term, though, we need a way of turning the volcanic basalt that makes up most of the Martian surface into a plant-supporting structure.

Basalt-eating bacteria could be the answer, suggests Paul Wilkinson, also at the Open University. He is hunting inside Icelandic basalts for organisms capable of surviving on a diet of Martian rocks. 鈥淭he [terrestrial] rocks are positively teeming with microbial life,鈥� he says. 鈥淚f these microbes were on Mars and capable of surviving and growing, they would go some way to conditioning the rock for higher-order plant life.鈥�

鈥淚f these microbes were on Mars, they could help to condition the rock for higher-order plant life鈥�

Wilkinson, who presented his results in April at the Astrobiology Society of Britain meeting at Royal Holloway, University of London, used genetic profiling to identify some of the organisms in the Icelandic rocks. He also managed to grow them in media rich in heavy metals such as copper, nickel, zinc and chromium. This suggests space gardeners will be able to call on bacteria tolerant of the iron prevalent in the rust-hued rocks of the Red Planet.

The real kings of rock processing, though, are the lichens, says Wilkinson. Happily, rock-colonising lichens were also among the organisms that survived exposure to space in the Biopan experiment. What鈥檚 more, a team led by Jean-Pierre de Vera at the Institute of Planetary Research in Berlin, Germany, has subjected lichens to conditions similar to those found on some parts of Mars 鈥� but without the lethal levels of UV radiation. The lichens were able to photosynthesise (Astrobiology, ).

None of this is to say that astrobiologists are seriously talking about terraforming Mars, not in the immediate future at least. But, piece by piece, the elements of a starter kit for the first colonists are coming together. Housed in a pressurised greenhouse and populated by vats of micro-organisms that will supply oxygen and food, and transform Martian rocks into fertile soil, the first alien garden is not as far off as it appears.