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Out of this world

2013; Wiley; Volume: 11; Issue: 7 Linguagem: Inglês

10.1890/1540-9295-11.7.396

1540-9309

Adrian Burton,

Space Science and Extraterrestrial Life

You may be surprised to learn that in 1973, Francis Crick – he of the double helix – published a paper with Leslie Orgel of the Salk Institute (San Diego, CA), proposing that life on Earth may have been the outcome of “directed panspermia”: that is “the result of infection by microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship” (Crick FHC and Orgel LE. 1973. Icarus 19: 341–46). Earlier, in 1960, Thomas Gold, a professor of astrophysics at Cornell University (Ithaca, NY), proposed the rather less enchanting idea that we might in fact all be the result of “accidental panspermia”, the evolutionary offspring of microbes left over in an extraterrestrial visitor's trash bags (Gold T. 1960. Air Force and Space Digest May: 65). Beware, however, lest you too readily write these ideas off as the stuff of crackpots (Nobel Prizes notwithstanding) and cast them onto your own pile of garbage, for NASA and the world's other space agencies take their implications very seriously. NASA's Office of Planetary Protection is not the place you go to find out how to live a “greener” lifestyle; its job is one of stewardship, ensuring that our space missions do not seed Earth life unintentionally on other planets. Missions are categorized depending on where a probe is going. A Category I mission is one that targets a celestial body with no direct interest regarding the origin of life; for these missions, no protection requirements are imposed. Category IV missions, however, such as those involving rovers on Mars, require all kinds of contamination analyses, sterilization procedures, and the installment of bioshields. Clearly there are very weighty ethical and exoecological reasons why we should not contaminate the worlds we visit, but there are practical ones too. Contamination might ruin our chances of understanding any secrets to the origin of life those bodies might hold, and we certainly don't want to spend billions going to Saturn's moon Enceladus just so our instruments can detect the E coli that snuck a ride. Curiosity's wheels on Mars. Let's hope they're clean. But what about that ride? Unmanned space probes are not designed to carry any life-forms safely into space, so wouldn't exposure to the void's dehydrating vacuum, temperatures near absolute zero, and deadly radiation kill any stowaways? Some research suggests it might not. Several missions have involved hanging biological specimens outside a spacecraft, and the list of organisms that can take that kind of punishment is growing. For example, Bacillus subtilis spores are quite capable of surviving exposure to space – for years if well shielded from radiation. The lichens Rhizocarpon geographicum, Xanthoria elegans, and Aspicilia fruticulosa are also reported to withstand exposure (apparently with little or no protection), seeds of the plants Arabidopsis thaliana and Nicotiana tabacum have survived 18 months outside the International Space Station, and our probes could even carry tardinauts; yes, tardigrades (also known as water bears) have successfully endured being dangled in space for 10 days. “All these organisms survive much better if they are shielded from solar radiation by say a little soil or rock”, says Ingemar Jönsson (Kristianstad University, Kristianstad, Sweden), the man who blasted the water bears into orbit. “Although we don't know how long they might eventually survive, it's plausible that they could endure a long space journey, especially if they were protected by the probe's structure. So we could unwittingly carry Earth life to other planets, and if that's the case, we might concede that we could bring extraterrestrial life back on the return journey.” That's where Category V comes in. Category V procedures are designed to protect our planet from contamination by life-et from contamination by life-forms hidden in samples collected on other worlds and then brought home on a returning spacecraft. In fact, the very spacecraft itself would be subject to strict containment and sterilization measures. The lengths to which NASA now goes to protect other worlds are highlighted by the end-of-life burn-up of the Jupiter probe Galileo. Despite the intensely sterilizing radiation fields around Jupiter, it was still feared that some stowaway might survive, and that if the abandoned probe crashed into Europa or Ganymede (two of Jupiter's moons) it might contaminate these candidate havens of extraterrestrial life. The Juno probe, which will reach Jupiter in 2016, will also eventually be burned up in the Jovian atmosphere. For me, it's still too much to believe that any life-form, even if trapped in rocks ejected into space from one planet by a collision with another, could survive crossing space for years on end, live through a fiery entry into another planet's atmosphere, and then upon arrival discover that conditions for growth are just peachy. And, as far as I know, there are no probes of any kind sent from other worlds to visit ours, nor any aliens who leave their trash behind. It's a reality, however, that we are already taking steps to not be those aliens ourselves.