雅思阅读练习：火星

　　Missions to Mars: a rocky road to the Red Planet

　　Missions to Mars may have stalled, but the search for signs of life continues – by analysing the 'DNA' of Martian meteorites, writes Roger Highfield.

　　Are we alone in the cosmos? For centuries, that question has been purely speculative. But in recent years scientists have gathered evidence of alien life on Mars that is as tantalising as it is inconclusive.

　　We thought we might have a definitive answer in 2003, when Britain's ￡50 million Beagle 2 probe was scheduled to touch down on the Red Planet, carrying an instrument that could have detected traces of living things. But we never heard from the little probe again.

　　The loss was a massive disappointment to the professor behind the mission, Colin Pillinger of the Open University. During the late Nineties, I had seen him doggedly enlist support for the project from fellow space scientists, the government and even the likes of Blur and the artist Damien Hirst.

　　The European Space Agency promised Prof Pillinger that there would be a follow-up programme, with a mission as soon as 2007. That date slipped back again and again. The Mars mission is now scheduled for 2018, when a joint mission with Nasa is due to send two rovers to search for life. Towards the end of this year, Nasa will launch the Mars Science Laboratory mission, which will set down a rover called Curiosity that will study whether conditions have ever been favourable for microbial life.

　　There is, however, another way to answer this giant question. In 1989, Prof Pillinger's team found organic material, typical of that left by the remains of living things on Earth, in a meteorite called EETA79001. This is one of a relatively small number of rocks – fewer than 100 – that chemical analysis reveals must have been blasted off the surface of the Red Planet by an asteroid impact and then subsequently fallen to Earth.

　　The Open University team stopped short of saying they had discovered life on Mars – but, in 1996, Everett Gibson and his colleagues at Nasa announced that they believed that they had discovered a fossil no bigger than a nanometre in another meteorite, known as ALH84001, which had fallen to Earth roughly 13,000 years ago. Other researchers, studying the data collected by America's Viking landers, which touched down in 1976, concluded that life signs had been detected then, too.

　　Sceptics – and there are many – remain convinced that inorganic (non-living) processes could have produced the same data and features that have been interpreted by some as signs of microbial life. But how can we even tell these rocks came from Mars?

　　Well, a few days ago, I found myself back at the Open University, to test another Martian meteorite, which we will offer as a prize to readers of New Scientist in the next issue. I bought it from Luc Labenne, a well-known collector based in France. It was a piece of a rock that crashed into the desert in Algeria, hence the designation NWA2975 ("North-West Africa").

　　One measure of its rarity is its astonishing value – one 102g sample of the same rock is on sale for $100,000 (our prize is 1.7g). To ensure that it was genuine, I enlisted the help of Prof Pillinger's colleagues. Andy Tindle studied a slide of NWA2975 provided by Ted Bunch of Northern Arizona University, a member of the team who originally described the meteorite in 2005. This revealed a mixture of rounded desert sand grains and various minerals of the kind found on Mars, such as pyroxene, which contains manganese and iron in a ratio typical of the Red Planet.

　　To make absolutely sure, Richard Greenwood and Jenny Gibson removed around ten-thousandths of a gram for further analysis. Using an instrument called a mass spectrometer (think of it as an atomic weighing machine), they studied the relative abundance in the meteorite's silicate minerals of three isotopes of oxygen – oxygen-16, oxygen-17 and oxygen-18. They were released for analysis with the help of a laser and a powerful reagent.

　　Because the relative abundance of these isotopes varies throughout the solar system, it is possible to use them like a DNA test in order to identify whether a meteorite comes from the Moon, an asteroid or Mars. In this case, they found a slight excess in the abundance of oxygen-17 and oxygen-18 compared with rocks from Earth, just as we would expect from a Martian rock.

　　What this tells us is that we don't have to go to Mars to get all kinds of insights into the Red Planet. We can reveal a lot simply by studying its meteorites to reveal data from the composition of the atmosphere to the presence of water. And, of course, these meteorites offer us a welcome opportunity to search for life signs, as we wait for the

　　next mission to land on the planet's dusty, pink surface.

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