Sydney - Nov 04, 2002
In 1996, the world was stunned by one of the greatest scientific controversies of the past decade. A team of scientists in the USA had announced evidence suggesting that a meteorite from Mars bore fossilised evidence of microorganisms. To many observers, it seemed that the first real traces of alien life had been uncovered. But the results were quickly called into question, and debate on the issue is still unresolved.
The controversy also drew attention to the concept that meteorites could serve as natural spaceships, carrying life between the planets. If bacteria could survive the stresses of being dislodged from the planet, crossing space for eons and hitting the surface of another world, then life on Earth or Mars could have originally been imported from elsewehere.
Studying life in space has always been difficult. We lack any proof that it exists, and we're not even sure about the origins of life on our own planet. But dozens of scientists around the world are carefully exploring niche areas of research that are bringing us closer to understanding the place of life in the universe.
Dr Mark Burchell from the University of Kent at Canterbury, England, is one scientist exploring the idea of life hitchhiking around the galaxy on meteorites. He is one of several researchers performing experiments that turn bacteria into microscopic crash test dummies. Small pellets containing bacteria are loaded into gas guns, then fired at high velocities into different materials such as rock and aerogel, the soft, porous material being used to gently catch cometary particles on the Stardust mission. The ejecta blown out from these impacts, as well as the impact zones themselves, are then tested for any surviving bacteria. Amazingly, these tiny lifeforms have demonstrated an ability to survive collisions at five kilometres per second! As Dr Burchell notes in his published research, impacts from space on Earth and Mars are typically several times faster than his laboratory simulations, but the sheer hardiness of bacteria in the laboratory tests suggests that survivability at higher speeds could be possible.
Dr Burchell discussed his research by email with SpaceDaily correspondent Dr Morris Jones.
Q: Your research suggests that bacteria can survive high impact speeds. To what extent can you propose "safety limits" for physical shocks on bacteria?
A: Our research is conducted as a function of impact speed with the target, but we can convert these results into the peak shock pressure experienced during the impact. We find that bacteria can survive impact pressures of some 50 GigaPascals. So we hypothesise that bacteria could survive an impact that produces a similar shock pressure.
Q: You use tiny ceramic pellets as "bullets" to carry the bacteria in your tests. Would the properties of a meteorite have any influence on the survivability of an impact?
A: We used ceramic material because it is porous and we could infuse bacteria into it through soaking. It is possible that a degree of porosity might aid survival by affecting the propagation of shock waves through the material during the impact.
Q: Ceramic material is essentially rocky in its physical structure. How do you feel about bacterial transfer in icy bodies such as comets?
A: Comets are often held to be icy, but they are also known to contain a lot of silicate materials (rock). Descriptions of comets range from "dirty snowballs" to "frosty rubble piles". Ice will be less useful as a shield against solar ultraviolet radiation than rock, which could sterilise the material during its time in space. But if you add some silicates into the material then the shielding improves.
The real, question, though, is how would bacteria get into a comet in the first place?
Q: Some members of the scientific community have proposed the idea of naked bacteria or spores wafting through space unprotected. What do you think of this?
A: There's the issue of shielding against solar radiation, cosmic rays and other problems. Space is a harsh place. I find it very hard to consider this without seeing large obstacles.
Q: Your tests have involved Rhodococcus bacteria. You selected this strain because of its hardiness. Have you tested anything else? What could influence the survivability of different bacteria?
A: We have looked at Bacillus Subtilis and spores. So far we have no systematic study which looks at changes in survivability as a function of the structure of the organism.
Q: Is there any connection between the ability of bacteria to survive hypershocks and its potential survivability in space?
A: We compartmentalised the problem. First we needed to show that the shock of an impact is not fatal. We've done that. Then we started using organisms that are standard in "survival in space" tests, such as Bacillus Subtilis. We are now using this strain in our tests.
Q: How would the properties of an impact area affect the survivability and reproduction of microorganisms after their arrival?
A: I could write a whole book on this single question! You need to release the lifeform after arrival. It then needs to find itself in a congenial environment. There must be no predators or conditions that could kill it or frustrate its growth. Landing in an ocean would be very different to a desert, and an impact at a highly oblique angle would be different to a vertical drop.
Q: Aerogel sounds like the most impact-friendly target you could choose, but your aerogel impact tests failed to generate any evidence of bacterial transfer. Do you have any theories about this? Is a "hard knock" on a surface like rock necessary to dislodge bacteria?
A: We are still working on this. I don't think we have done enough experiments yet to say that "no positive result" so far means that it doesn't happen.
Q: What other sorts of experiments are being done in this field?
A: Other researchers are testing for survival by passing a peak shock wave through a sample. Some are using centrifuges. Others are interested in survival in space. The field is growing.
Q: Do you have any comments to make on the 1996 Martian meteorite controversy?
A: No. Leave it to the experts in mineralogy and meanwhile go direct to Mars!
Morris Jones is a Sydney-based correspondent. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.
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Astronomers Find Life On Earth
Boston - Nov 01, 2002
Now that the discovery of extrasolar planets, or planets around distant stars, has become relatively routine, scientists are now tackling the next step: finding life-bearing worlds. To do this, observers must know what signs to look for in the feeble light from these faraway planets.
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