麻豆传媒

WHEN the Lunar and Planetary Sciences conference first met back in 1969 it was a historic occasion. It was the first time that astrobiologists had got together to discuss the rock samples the Apollo astronauts had brought back from the Moon.

Thirty years on, a new generation of researchers gathered in Houston to plan how they would rerun their own version of the great lunar adventure鈥攖his time with rocks transported billions of miles back from Mars. Now that dream may never happen.

Unearthing the Red Planet鈥檚 secrets could, for the moment, prove impossible for two reasons. The first is simple: there just isn鈥檛 enough money to fund such an audacious research programme. The second is more technical. The latest research suggests that getting people to Mars could be a lot more dangerous than anyone envisaged.

After possible evidence for life on early Mars was discovered in the meteorite ALH84001 in 1996, NASA planned a series of orbiter and lander missions. Its ambitious centrepiece was to be the Mars Sample Return Mission鈥攁 fast-track programme scheduled for 2005 that would bring Martian soil and rocks back to Earth for analysis. But the launch date for this complex mission began to slip鈥攆irst to 2011 and then 2014.

When NASA recently asked for a new batch of proposals for sample return missions, all the options came up with a figure of around $2 billion. Michael Meyer, one of NASA鈥檚 senior astrobiologists, told the conference that 鈥渟ample return has moved beyond the budget horizon鈥�.

Many scientists are now quietly questioning the entire Mars programme. Superficially, NASA鈥檚 approach is 鈥渇ollow the water鈥�, finding and analysing water and ice on the Red Planet should be a quick route to identifying extraterrestrial life. But some prominent scientists who spoke to 麻豆传媒 say the current rationale is closer to 鈥渇ollow the money鈥�. These critics wonder whether NASA is secretly worried about bankrolling an expensive sampling mission to Mars. The huge bill will be hard to justify if the mission fails.

Astrobiologists are also concerned that public enthusiasm, and the taxpayers鈥� commitment, will wane after any successful sampling mission, whether or not they turn up firm signs of life. Either life is found and the mystery of 鈥渁re we alone?鈥� will be solved once and for all, or confidence in our ability to search for life will ebb away.

Against this gloomy backdrop, the conference heard even more disconcerting news. Biomedical experts have long insisted that the greatest obstacle to sending people to Mars is protecting them from the high levels of radiation they would be exposed to on the journey. The latest data from Mars Odyssey suggests that they may be right.

The Mars Odyssey team presented the results from the Martian Radiation Environment Experiment (MARIE), which started collecting data about the levels of radiation Odyssey encountered on its outbound trip. In August, four months after MARIE was switched on, it suddenly stopped transmitting. NASA engineers could not make contact again until this month.

They have now analysed the data that MARIE managed to collect and found that Odyssey was exposed to a severe burst of radiation from the Sun on its way to Mars. While the radiation hazard on the surface of Mars is about the same as on board the International Space Station鈥攚hich researchers describe as 鈥渄ifficult but manageable鈥濃�攖he radiation Odyssey experienced on its journey to Mars was more than twice as severe. And it was bombarded by three times as many heavy ions as the ISS.

With little hope of retrieving samples from Mars in the near future, researchers are now focusing on the next best thing鈥擬artian meteorites. To date, 20 meteorites from the planet have been found. But investigators are stepping up the hunt for new rocks in the hope that this will help to understand the geological processes that have shaped the planet, and possibly find further clues in the search for life there.

Already a new Martian meteorite made of basalt has turned up in Morocco. But nothing could compare to the long-running debate on the celebrated ALH84001. In 1996, scientists from the Johnson Space Center (JSC) in Houston claimed they had possible evidence of microscopic life on the rock, unearthed in the Allan Hills of Antarctica. Since their announcement, well over $10 million has been invested in research on this single rock.

ALH84001 contains organic minerals called carbonates, as well as tiny magnetite crystals that resemble fossilised bacteria. Much of the recent research has focused on trying to show that the crystals could have formed through non-biological processes, or that the rock could have been contaminated after it landed on Earth.

Andrea Koziol at the University of Dayton, Ohio and Adrian Brearley of the University of New Mexico have taken carbonates like those in ALH84001 and subjected them to a short blast of heat, to try to simulate the impact that would have formed the meteorite in the first place. The heat broke down the carbonates, they told the conference, forming randomly oriented magnetite grains 10 to 20 nanometres across, roughly similar to the grains seen in ALH84001.

But JSC team member Kathie Thomas-Keprta maintains that the grains were probably formed by a range of processes, one of which took place inside the cells of Martian bacteria. She estimates that roughly 25 per cent of the ALH84001 grains are biological in origin. David McKay, also of JSC, pointed out that nobody had been able to recreate the Martian grains exactly.

It seems increasingly unlikely that ALH84001 will ever provide a definitive answer to whether life existed on Mars. The question now is whether another Martian meteorite might reveal further clues, or whether a mission to bring back samples is the only way to solve the mystery.

So some scientists are now calling for an open competition to judge proposals for a Mars sample return mission. That could be the only way to deliver a cheap, affordable mission that could put the search for life on Mars back on track.