Should we seed the galaxy with life?

Franco Brambilla/Airstudio

40 | NewScientist | 5 February 2011

Go forth and multiply If there is no life on other planets, let’s send it there. Stephen Battersby reports

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ARTH’S first interstellar expedition seems to be a disaster. During the long journey most of the passengers die from radiation sickness. When at last the spacecraft arrives, it crash-lands on the surface of a bleak and barren world. The capsule splits open and the alien air finishes off many of the remaining explorers. Over the ensuing days, some of the few survivors succumb to the extreme temperatures, while others die after drinking from pools of acid. But one stalwart survives. Soon there is even better news: our explorer divides into two clones. Earth life reproduces for the first time under the light of an alien star. Its offspring mutate and begin to adapt to their new home, eventually spreading across the planet and evolving into new forms of life. That’s one small step for a bug, one giant leap for bugkind. Why would we want to replace Captain Kirk with a bacterium? Because the dream of humans travelling to other stars, while not impossible, may yet turn out to be unfeasible. If we can’t go in person, then instead we could recruit our single-celled cousins as astronauts. “We are at a point now where we almost have the ability to send micro-organisms to other worlds,” says Michael Mautner of Virginia Commonwealth University in Richmond. “We can generate

a vast amount of life in the universe. It would give our own existence purpose.” The idea that simple life forms could be carried from planet to planet, known as panspermia, is an old one. Ever since the 19th century, scientists have been debating whether life could survive the long journeys between star systems. Mautner thinks the process should not be left to chance. “I started to become interested in the 1970s, at the height of the cold war and the nuclear arms race, when there were questions about whether we were going to survive,” he says. “What if Earth has the only life? Earth will be destroyed eventually, then all life is gone. For me that’s a very empty and meaningless universe.” The answer, he concluded, is that we should become the agents of panspermia. He is not alone in advocating directed panspermia, as this idea is known. “Expanding the richness of life in the universe is what we ought to be doing,” says Chris McKay, an astrobiologist at the NASA Ames Research Center in Moffett Field, California. Mautner outlined his ideas for spreading Earth life across the galaxy in a recent paper (Journal of Cosmology, vol 5, p 982). He envisages sending out colony ships filled with microbes and pulled by solar sails. The first solar-sailing craft was launched by Japan’s space agency last year, and by Mautner’s calculations such craft could reach speeds of up to 150 kilometres a second by swooping close to the sun before unfurling their sails. Where should we send the first microvoyagers? The obvious target is a young, temperate rocky planet similar to Earth, the kind of planet we may soon start to find thanks to NASA’s Kepler mission, launched in 2009. A seeding mission could aim to put a spacecraft in orbit within the habitable zone around the host star, from where it could disperse millions of seed capsules, some of which should end up on the target planet. But this would not be easy. Such a distant stellar target would need precise targeting, and more critically, the craft would have to slow down to enter orbit around the target star. It could decelerate by using its solar sail to catch the light of the star, but it is not clear whether this would be possible without an active guidance system, which would have to remain in working order for tens of thousands of years. “I would like to stay away from any farfuture technologies if possible,” says Mautner. In that case, a softer target might be a disc of gas and dust around a young star, such > 5 February 2011 | NewScientist | 41

as Beta Pictoris, 63 light years away. Here the tactics of the swarm come in: “If you send billions of small vehicles, hopefully some will arrive,” says Mautner. Each vessel could hold 100,000 freeze-dried bacteria in a capsule just 40 micrometres across, towed behind a sail less than 4 millimetres across. When these seed pods arrive, drag from the gas in the disc would slow them down. As comets and rocky bodies form in the disc, says Mautner, some seed pods will become incorporated and eventually a few should end up on the surfaces of planets. The journey will take a long, long time. Even at a speed of 150 kilometres per second, the trip to Beta Pictoris would take more than 120,000 years. Can any living organism survive such an epic voyage in space? “That is the biggest open question,” says Mautner. The toughest passengers may be freezedried bacteria, which are often stored for long periods in laboratories. Some bacteria can dry themselves out and produce a hardy dormant form called an endospore. There are

controversial claims of endospores being revived after being locked in amber for 40 million years, or after being trapped in salt crystals in a cave in New Mexico for 250 million years. Even if some bacteria really can snooze for a quarter of a billion years, though, they are far less likely to survive in space than in a cave.

Dead on arrival The big danger is cosmic rays – energetic protons and other charged particles that can smash up DNA. We are shielded from most cosmic rays by Earth’s atmosphere and the solar wind, but in interstellar space the microbe passengers of a small seed capsule would face the radiation unprotected. We know that they could cope for a few years, at least. Bacteria have survived for more than 18 months outside the International Space Station. Much longer-term exposure would be more challenging, but might not be terminal, says Lewis Dartnell of University

The right stuff If we want to seed distant planets with life, what should we send? The first challenge is to survive the journey, which makes Deinococcus radiodurans – aka Conan the Bacterium – a tempting choice. It is not only extraordinarily radiationresistant, but can also survive extreme cold and dehydration. However, D. radiodurans cannot form long-lasting endospores and it needs oxygen and organic compounds, which won’t be available on a barren new world. What might be available is methane, hydrogen or sulphide compounds, food to microbes like the ones found around deep-sea vents. “As long as they find an ocean, they could probably find a living,” says Lewis Dartnell of University College London. Other obvious candidates include photosynthetic organisms such as cyanobacteria, which could make their own food and produce oxygen into the bargain, although it might take billions of years for oxygen to reach the levels that animals need. Fertile ground should not be hard to find. Michael Mautner of Virginia

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Commonwealth University in Richmond has grown algae and even small asparagus plants in pulverised chunks of meteorites (Icarus, vol 158, p 72). “The material is as productive as agricultural soil here on Earth,” he says. Mautner is not planning to sow the galaxy with asparagus, but he does speculate about sending more advanced organisms. It may have taken almost 2 billion years for complex cells to evolve from bacteria, and sending single-celled algae, for example, might bypass an evolutionary bottleneck and speed up the appearance of multicellular organisms. Complex cells would be far less likely to survive an epic trip, though. To maximise the chances of a few microbes surviving, Mautner suggests sending a diverse mix of simple cells, including extremophiles that thrive in high or low temperatures, fierce acidity and so on. Better still, genetic engineers could create superbugs able to survive extended space travel, exploit many different energy sources and live in a wide range of environments.

College London, who studies the potential for microbes to survive on Mars. “The numbers might work out if you can send enough microbial voyagers in each capsule. The vast majority would die on the way from radiation, but a tiny fraction would survive.” After a million years with negligible shielding, he calculates, about one in a million freeze-dried bacteria would remain alive. At the solar-sail speeds envisaged by Mautner, a million years is long enough to travel 500 light years. Then again, maybe it does not matter if the bugs are dead on arrival. Last year, Paul Wesson of the Herzberg Institute of Astrophysics in Canada suggested that even the shattered corpses of microbes, just fragments of DNA and other biomolecules, could help life to emerge. He called the idea “necropanspermia”. Alternatively, shielding a few metres thick on the spacecraft would cut out the bulk of cosmic ray damage. Another solution might be to revive the passengers from time to time so they can repair any DNA damage, before suspending their animation again. These options would require much larger spacecraft, though, which would spoil one of Mautner’s aims – to make directed panspermia relatively cheap. After all, a project that may not bear fruit for billions of years, and whose success or failure may never be known, seems unlikely to attract vast funds. The cost of Mautner’s lower-tech approach depends on a lot of factors. How many capsules must land on a young planet, say, to achieve a fair chance of some bug becoming established? Mautner guesses a hundred, although McKay feels that is optimistic. “The chances of any particular organism growing or any particular capsule falling on fertile ground is vanishingly small,” says McKay. “The good thing is that it’s easy to make billions of them.” Billions, perhaps many billions, will be needed. Even the closest planetary systems are tiny targets, and most capsules will miss altogether. They are also moving targets, and we will need ultraprecise measurements of their motions before an unguided mission could succeed. That should be possible with space-based telescope arrays within a few decades, Mautner says. Marc Millis of the Tau Zero Foundation, which promotes research into interstellar travel, is sceptical. “It’s hard to hit interstellar targets, and it is much harder to hit targets with passive sails than with a vehicle that can correct its course as it goes along.”

l. allen (harvard-smithsonian cfa) & d. padgett (ssc-caltech)/jpl-caltech/nasa

”The only thing that is a source of value is life. Humans should expand its richness and diversity”

Aim becomes less of a problem in one of Mautner’s grander plans. He hopes to seed entire star-forming regions holding dozens of new stars, such as the Rho Ophiuchi cloud, about 500 light years away. That is a big target, no problem to hit. On the downside, such large-scale carpet-bombing would probably need millions of times as many seed capsules as a single planet or planet-forming accretion disc. And once there, most of the intrepid bugs might have wait millions of years, all the while exposed to the hard rain of cosmic radiation, before anything solid forms. If fleets of simple spacecraft can’t do the job, a more high-tech approach will be needed. Sails propelled not by sunlight but by huge lasers in Earth orbit could theoretically reach speeds of thousands of kilometres per second, slashing travel time and radiation exposure, and they could probably be aimed more precisely than sun-catching sails. Advanced robotics could even guide microbial passengers to the most promising havens on new worlds. While the challenges are huge, there is no doubt that it will be easier to send bacteria than people. They are only very distant cousins of ours, but as far as Mautner is concerned, kin is kin. “Life is one big family, and the purpose of life is to propagate,” he says. “If we manage to seed life on a few

hundred planets, we can start many chains of evolution. Hopefully some will evolve into intelligent beings.” McKay agrees. “When we look around the universe we see a lot of different things, but the thing that is most interesting, the only thing that is a source of value, is life,” he says. “I like the argument that humans should seek to expand the richness and diversity of life.”

Complete annihilation There is a risk that we would be doing the opposite, however. The presence of Earth colonists might prevent new forms of life evolving from scratch. Worse still, the colonists might kill off native life forms. Consider the opposite situation. “How would we react if another civilisation sent to Earth a directed panspermia package containing alien microbes, and it affected the Earth’s biosphere in a negative way?” asks astrobiologist and writer Barry DiGregorio, affiliated with Cardiff University in the UK. If we cannot be sure that microbes won’t harm existing life, then we shouldn’t send them, he says. “The only reason I can think of to try it, as a last resort, is if the Earth was facing complete annihilation by an impending solar event, asteroid or comet catastrophe.” Others are less worried. “My feeling is that

Billions of seed capsules could be sent to clouds where new star systems are forming

any natives, adapted to their environment, would be better equipped and so outcompete the new arrivals,” says Dartnell, “but that might not always be the case.” Proposed space telescopes such as NASA’s Terrestrial Planet Finder could check for signs of life on other worlds before capsules are sent. They would not be able to detect the early stages of life, Dartnell says, but they should reveal where a biosphere is well established. If these searches do not find any such signs, it will be evidence that life does not readily get started and needs our helping hand. If, on the other hand, life is found to be plentiful, there would be no need for directed panspermia. A galaxy teeming with aliens might be a sign that life evolves readily, or spreads rapidly between star systems by natural panspermia, or both. Or maybe, as Carl Sagan suggested in 1966, another civilisation had this idea billions of years ago and successfully spread their seed throughout the galaxy. Was our ancestor the lone survivor of a tiny starship that crash-landed on a bleak and barren planet far from home? n Stephen Battersby is a consultant for New Scientist based in London 5 February 2011 | NewScientist | 43