STANDING in the picturesque English town of Canterbury, you would never believe that so many lives are being sacrificed here in crash tests of unimaginable violence. Hundreds of thousands of unsuspecting victims are accelerated to more than 18 000 kilometres per hour and slammed into walls of granite, steel or glass. Only a traumatised few survive.
Luckily, only bacteria perish in the crash tests, and they do not die in vain. By splatting into targets at those unearthly speeds, they are shedding light on the origins of life on Earth. Did our earliest ancestors emerge in warm puddles on the youthful Earth, or did they arrive with a spectacular thump in a comet or meteor that fell from the sky? The hapless bugs may also warn scientists whether they should prepare to meet living aliens for the first time in 2006, when a spacecraft returns to Earth carrying a smidgeon of comet dust.
Since the 1950s, when Stanley Miller of the University of Chicago reconstructed the Earth鈥檚 early atmosphere in a jar, the received wisdom has been that life began here on Earth. With the help of a huge electric discharge-something like a primordial lightning bolt-Miller cooked up some amino acids, the building blocks of life, in his jar of gases.
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But the case was never clinched. An alternative view called panspermia, championed by the British astronomer Fred Hoyle, is that life originated not on Earth, but in space. Simple life forms may have formed in interstellar clouds, then hurtled around solar systems on comets and meteors, sometimes fertilising the worlds where their cosmic ferries crash-landed. The idea proved too way-out for some, especially with the advent of interplanetary spacecraft. 鈥淚t languished because when people finally got some spacecraft to other planets, they were obviously dead,鈥� says Mark Burchell, a planetary scientist in the physics lab at the University of Kent in Canterbury.
Life in space
Views have changed a little in the past decade. In 1994, astronomers saw the telltale spectral fingerprint of the amino acid glycine shining out from an interstellar cloud in the constellation Sagittarius. This year, scientists in India reported that their computer simulations suggest the DNA base adenine also forms in these clouds (快猫短视频, 22 January, p 4). And by mixing up artificial interstellar dust and gas and simulating the cold, vacuous conditions of space, scientists at NASA鈥檚 Ames Research Center in California have spawned not just essential biological molecules, but also tiny vesicles that look like empty cells (快猫短视频, 12 September 1998, p 30).
It鈥檚 also clear now that some meteorites found on Earth made their way here after being ejected from other planets. And then there鈥檚 the Martian meteorite found in Antarctica that scientists sensationally claimed in 1996 might contain tiny fossil bacteria. Even though the claims have floundered since then, says Burchell, the meteorite saga has given panspermia a boost. 鈥淧sychologically, things have changed,鈥� he says. 鈥淭he finding has made people think and, crucially, people from different disciplines have started looking into the problem.鈥�
The case for panspermia hangs on how hardy primitive life forms such as bacteria really are. For instance, could they really survive the gruelling extremes of temperature and radiation in space? There are hints that they might. A colony of the common soil bacterium Bacillus subtilis, left unprotected in orbit on a spacecraft for nearly six years, was alive when the craft was retrieved. And Streptococcus mitus stowaways on the Surveyor 3 spacecraft, which was left behind on the Moon in 1967, were easy to revive when they were rescued by astronauts and taken back to Earth three years later.
So far, so good. But microbes would face a much bigger challenge when it comes to plunging into a planetary surface on board a comet or meteor. Meteors smack into the Earth at tens of thousands of kilometres per hour. So Burchell teamed up with research students in space sciences and staff at the University鈥檚 biosciences laboratory to see if bacteria could survive a similar impact in a crash test.
Bugs under pressure
As their guinea pig, the team chose Rhodococcus rhodochrous, which can live at the huge pressures found in deep-sea vents 5.5 kilometres below the ocean surface. These bugs are perfectly harmless to people, and helpfully appear red and easy to spot when they鈥檙e cultured using standard techniques. The researchers loaded porous ceramic projectiles up to a few millimetres in diameter with roughly 100 000 Rhodococcus bacteria each, by soaking the bullets in a solution of the bugs. They also kept some clean bullets, sterilised in alcohol, to use as controls.
The firing squad then prepared to accelerate the bacteria using a two-stage gas gun, about 5 metres long. When a shotgun cartridge detonates at one end, it drives a piston down a barrel to compress a reservoir of hydrogen. This ruptures a metal disc and explodes outwards, a process that fires the projectile into a target at the other end of the gun at speeds of around 18 000 kilometres per hour.
Before a comet or meteor from space ploughed into the Earth鈥檚 surface, it would first be slowed down by the atmosphere. So the researchers wanted a target material that the bugs could pass through before being stopped completely. They chose aerogel, an air-filled translucent solid made from silica. With a density just a thousandth that of glass, it would slow the projectile to a halt in a track around 1 centimetre long-not quite the effect of the entire atmosphere, but it鈥檚 a start. 鈥淭he aerogel is a crude simulation of the planetary capture mechanism,鈥� says Burchell. Out of interest, the team also used other targets of steel, glass, chalk and granite, carrying out 20 crash tests in total.
To find out if any traumatised bacteria endured their plunge into aerogel, the team injected the punctured aerogel with a fluorescent dye that would make any living bacteria glow. Result: no signs of life. In a second attempt to find living bacteria, they broke up the aerogel target and powdered it, then tried to culture any surviving bugs from the powder in a Petri dish. Again, living Rhodococcus were nowhere to be seen. Similarly, all the bacteria were shown to have met their maker as they slammed into glass or steel.
But to Burchell鈥檚 surprise, some bacteria survived their ordeal when they were fired into chalk or granite. When the scientists gathered up the debris around the collision sites and placed it on warm Petri dishes, Rhodococcus growth appeared after several days. No bacteria grew from the debris of control impacts in which bullets hit chalk or granite targets without a bacterial cargo.
Why collisions with chalk and granite ended happily, while softer landings in aerogel did not, is baffling. 鈥淚 was certainly very surprised-I didn鈥檛 expect that result,鈥� says Burchell. One possibility is friction. Since the other solids would have stopped the projectile more or less immediately, they would not have been heated up by friction. With the aerogel, though, the bugs barge through-heating up as they go-and the friction perhaps boils them alive. Alternatively, the bugs may have survived, but they may have been entombed inside the bullet as the melted aerogel formed a glass film around it.
Burchell plans to test these options. And NASA 鈥減lanetary protection鈥� scientists, who are charged with safeguarding the Earth from infectious alien microbes, are keen to know what he finds. In February last year, NASA launched a spacecraft called STARDUST which will rendezvous with Comet Wild 2 in January 2004. A lump of aerogel on the craft will soak up dust from the comet, then return the extraterrestrial material to Earth in 2006. Serendipitously, the speed at which the dust from Wild 2 will hit the aerogel is exactly the same as in Burchell鈥檚 Earth-based crash tests.
鈥淚f we were ever to succeed in capturing bacteria in aerogel, the Americans would start to be concerned,鈥� says Burchell. 鈥淭heir view is that a mission to a comet is not a life sciences mission. But if we demonstrate that STARDUST can bring bacteria back, they鈥檒l then have to think about how likely that is, and arrange a handling procedure accordingly.鈥�
Burchell is also running tests to see whether bacteria can handle being blasted off the surface of a planet in the debris thrown up by a comet impact. 鈥淚f we can demonstrate that this is possible, suddenly this field would be taken much more seriously,鈥� he says. He admits that showing bugs are capable of jumping from planet to planet doesn鈥檛 prove that they do. But if one day we find signs of life on a comet, the case will strengthen. 鈥淚t would really raise the question: are we all extraterrestrials?鈥�