| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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skywatcher Panspermia: Seeds from Space Eons after the Big Bang, a meteoroid speeds through the frigid blackness of space, a colony of microbes living deep within its icy core. Once part of a distant planet, this pitted, house-sized chunk of interstellar rock has been hurtling through the universe for millions of years. Now snared by the gravity of the Milky Way, it’s headed directly for a blue planet marbled with wispy white clouds. The plunge into the planet’s atmosphere sets the rock aflame, melting its surface into a white-hot skin of molten slag. With a crash that reverberates across the hemisphere, the meteorite collides with the planet, scattering fragments across hundreds of square miles of the virgin Earth. Four billion years later, the descendants of these microbes rule the planet. They call themselves humans. This scenario may seem unlikely. After all, mid-twentieth-century experiments demonstrated that chemical conditions found on the early Earth could have produced the building blocks of life. But today, a fleet of astrobiologists are investigating whether life on Earth could have been spurred by life from space. Outer space is looking increasingly hospitable to life, and microorganisms may actually be tough enough to survive long, dangerous trips between planets. The idea of “panspermia,” Greek for “seeds everywhere,” is hardly new. Since the 1970s, British astronomers Sir Fred Hoyle and Chandra Wickramasinghe have championed the view that life originated in deep space and was transported around the universe in comets, asteroids, and cosmic dust. Interstellar material floating through space contains Earthlike freeze-dried bacteria, they say. At the time, few working scientists accepted their claims, maintaining that chemical analysis of these materials also fits various nonliving carbon compounds. But widespread skepticism over the notion of interstellar panspermia hasn’t quelled interest in what’s beginning to look much more likely—interplanetary panspermia, the transfer of life between celestial bodies within our solar system. “I wouldn’t rule out interstellar panspermia; it’s not impossible,” says Christopher Chyba of the Search for Extraterrestrial Intelligence Institute (SETI). “But it would be vastly, vastly more difficult than interplanetary panspermia, because the timescales for which organisms would need to survive are so much longer and the probabilities are so much more daunting.” Conditions in space are far harsher to life than the Earth’s surface. Microorganisms in transit would face ionizing radiation from cosmic rays, ultraviolet (UV) radiation from the sun, and space’s vacuum—all known to severely damage DNA. In addition, any life would have to withstand both the extreme cold of space and the fiery plunge through Earth’s atmosphere. And microbial voyagers would require astonishing longevity to travel the mind-boggling distances between hospitable worlds. But discoveries of Earthly microbes known as “extremo-philes” make it clear that life just might be tough enough to survive those austere conditions. Life has been found in the crushing depths of the ocean’s hydrothermal vents, in scalding sulfurous cauldrons, deep inside Antarctic ice, and within rock more than a mile down in the dark bowels of the Earth. Gases trapped inside some meteorites found on Earth have been found to match the chemical composition of Mars’ atmosphere, while the minerals and textures of other meteorites are identical to rocks taken from the moon. Scientists think these geological aliens were blasted off their planets of origin and into space by the impacts of city-sized comets or asteroids. Such impacts were especially frequent when the universe was young. Space debris crashing into our planet violently enough to vaporize today’s oceans punctuated the stormy first half-billion years of Earth’s history, known as the Age of Bombardment. In the four billion years since then, impacts have trickled off, as most of the matter left over from the solar system’s formation has either been accreted into planets or thrown beyond the pull of the sun. But even today, thousands of meteorites fall to Earth’s surface each year, including an estimated half ton of material from Mars. Scientists long assumed a large asteroid’s impact would melt or vaporize all rocks it hurled into space. Then planetary scientist H. Jay Melosh of the University of Arizona bucked conventional wisdom by arguing in the 1990s that impacts would shock-heat most rocks but would leave a small percentage totally unharmed, implying that all rocks might not be sterilized. Melosh and colleagues have also run computer simulations that suggest some microbes can withstand being blasted off a planet’s surface. Scientists are now finding hard data to support Melosh’s ideas. Last fall, a team demonstrated that the interior of ALH-84001, a meteorite from Mars, had not been heated much at all. When a rock gets hot enough, magnetic particles within the heated area reorient with the local magnetic field and freeze into that position when the rock cools. The magnetic field of the meteorite’s outer five millimeters was strongly aligned in this way, whereas the inner portion of this mango-sized rock was more jumbled, indicating the interior had remained cool. Researchers Benjamin Weiss and Joseph Kirschvink of the California Institute of Technology remagnetized a slice of the meteorite by heating it to only 40 C, showing that the rock’s interior had not even warmed to that modest level during its fall to Earth. Weiss calls his team’s finding “the first direct experimental evidence that you can get a rock from the surface of Mars to the surface of Earth without heat-sterilizing it.” ALH84001 had already played a leading role in the debate over whether meteorites might transport life. It was within this hunk of rock that NASA scientists in 1996 declared that they had discovered multiple signs of past life. They described tiny shapes in ALH84001 (see “Message in a Meteor,” Pacific Discovery, Winter 1997) resembling fossilized bacteria; microscopic mineral residues similar to those produced by terrestrial microbes; and organic compounds matching some byproducts of Earthly bacteria. At the time, critics argued that inorganic processes could have formed each of these features. And the purported bacteria were much smaller than any found on Earth. But, since then, surveys have located microbes just as tiny on Earth. And this last winter, the NASA team strengthened their claim with the discovery of long chains of magnetic crystals in the meteorite, which they say could only have formed inside a living organism. Even if microbes can book a seat on a meteorite, they still need to survive the ride through space. Evidence from as far back as 1969 suggests that common Earth bacteria are capable of such an otherworldly feat; when Apollo astronauts retrieved the robot Surveyor 3 spacecraft after three years on the moon’s surface, NASA discovered viable Streptococci bacteria embedded in its camera. Most scientists believe the organisms survived the trip through space, though some contend that the camera was contaminated after returning to Earth. A number of labs testing how well microbes can survive the rigors of space are now discovering the little creatures would make impressive astronauts. In the 1980s, Gerda Horneck and other scientists at the German Aerospace Center exposed spores of the bacterium Bacillus subtilis to outer space for six years on NASA’s Long Duration Exposure Facility. Spores are the dormant form some bacteria assume to resist desiccation; the microbes wrap themselves in a shell of protective proteins. Horneck’s group found that 80 percent of the spores survived when shielded from radiation by material the thickness of a few cell layers, as they would be if embedded within a meteorite. During the 1990s the European Space Agency (ESA) expanded the list of incidental space travelers to include salt-loving microbes and dried lettuce seeds. Attached to the outside of a satellite in orbit, the voyagers were subjected to temperatures nearing -20 C, mutagenic cosmic rays, and the microgravity and vacuum of space. About 80 percent of the bacteria survived the vacuum and 10 to 20 percent lived through ultraviolet exposure and vacuum conditions combined. The lettuce seeds endured best of all probably because their seed coats afforded protection against UV radiation. In 2005, ESA will expose more microbes and organic materials to space from the International Space Station to examine how factors such as UV-filtering carotenoid pigments might affect their survival. As impressive as these feats are, a year and a half is nothing compared to how long a bacterium on Earth can survive in suspended animation. Last fall, researchers revived bacteria from a 250-million-year-old deposit deep in a New Mexico salt mine. Other microbial Methuselahs have been resurrected from 30-million-year-old amber and 3-million-year-old permafrost. Such longevity has major implications for panspermia; 250 million years is plenty of time for a microbe to travel from one star system to another, let alone between planets. Most martian meteorites may take several million years to reach Earth, but a tiny percentage on direct trajectories could make the trip in less than a year. However long the voyage takes, it would end in an unspeakably violent crash on a planet’s surface. Some researchers are trying to test just how well life can survive such crash landings. Jennifer Blank of the University of California at Berkeley is looking beyond meteorites and studying comets. Essentially dusty ice balls, comets seem to carry a wide array of organic molecules. To some, this suggests that comets deliver molecules to planets where they might help create or nurture primordial life. Braver speculators might say that life itself could have originated in comets’ watery interiors. To simulate the effects of a comet crash landing, Blank’s team shoots a dime-sized metal pellet containing a sample of amino acids down a “shock chamber” ten meters long and into a wall. Blank can simulate different impact magnitudes by varying the projectile size and the amount of gunpowder, generating forces up to 200,000 times the air pressure on the Earth’s surface, and temperatures well past the boiling point of water. Up to 70 percent of the amino acids withstand this abuse intact, a rate that defied Blank’s intuition. “I figured if you heat it up to 600 degrees, everything would be fried,” she says. But extremely high pressures can counteract the effects of extremely high temperatures, just as it takes more heat to boil water at lower altitudes with higher pressure. And not only did most of the molecules survive, the heat actually helped some of them form new compounds—chains of amino acids two and three links long—suggesting that comets might indeed be plausible couriers of life’s building blocks. In the future, researchers may implant microbes into artificial meteorites and examine their fate during reentry. Inspired by this flood of new information supporting the likelihood of panspermia, Curt Mileikowsky of Sweden’s Royal Institute of Technology synthesized the knowledge to date. He gathered a team of experts to review all the relevant studies and to brainstorm about panspermia. Published last summer in the journal Icarus, the team’s landmark report concluded that “if microbes ever existed on Mars, viable transfer to Earth is not only possible but also highly probable, due to microbes’ impressive resistance to the dangers of space transfer and to the dense traffic of billions of martian meteorites which have fallen on Earth since the dawn of our planetary system.” The paper adds that Earth-to-Mars transfer is possible, “but at a much lower frequency,” because Mars’ thinner atmosphere allows more meteoroids out and Earth’s stronger gravity draws more in. While Mileikowsky’s team determined that transfer from one star system to another is highly improbable, but not impossible, their work illustrates that, at the very least, Earth and Mars have already traded many thousands of meteorites potentially capable of carrying life. If life can indeed jump from planet to planet, what does this say about its origins? Scientists long thought life’s beginnings on Earth had been well explained. In the 1950s, Stanley Miller and Harold Urey showed that electrifying a mix of gases mimicking the Earth’s early atmosphere produced an abundance of organic molecules and amino acids, the building blocks of life. Scientists now believe Earth’s early atmosphere differed from the conditions in Miller and Urey’s experiment in ways that make production of organic molecules much less likely. New origination studies have expanded outside of Earth’s atmosphere, and the evidence is encouraging. Just this winter, scientists simulating an interstellar ice cloud produced complex chemicals and membrane structures that may resemble the materials of the first cells. The researchers, Jason P. Dworkin, Scott A. Sandford, and Louis J. Allamandola from NASA’s Ames Research Center, and David W. Deamer from the University of California at Santa Cruz, say such material raining down onto a planet could jumpstart the development of life there. So while theories about life’s emergence on Earth have grown murkier, its arrival from elsewhere has gone from being a vague unfounded notion to a respectable hypothesis. If organisms were found elsewhere in space, we would want to know whether they shared ancestry with Earth life or if they evolved independently. Alien life made up of molecules such as DNA, RNA, and proteins that are critical to Earth organisms would strongly suggest a common origin, since there’s little reason to expect that such complicated molecules would evolve the same way on two planets. More habitats congenial to life beyond Mars are emerging every year. Astronomers believe the frozen surface of Jupiter’s moon Europa hides a liquid ocean that could be hospitable to life. But since Jupiter’s gravity would snatch most meteoroids blasted off Europa, the chances of one making it to Earth is slim. “That makes Europa interesting,” says SETI’s Chyba, “because it means if there’s life on Europa, it’s probably of a separate origin [than Earth’s] and would be a very different type of life.” Other recent evidence suggests that Ganymede, another of Jupiter’s many moons, may also hold a watery subsurface ocean. Though panspermia may not address the question of how life originated, the concept helps alleviate one source of uneasiness for many scientists: the sudden appearance of life on Earth right on the heels of the Age of Bombardment. The oldest known fossils appear only 100 to 300 million years after bombardment slowed, and they suggest the first cyanobacteria were every bit as complex as those existing today. Perhaps a seed from space was all a life-friendly planet needed. Jay Withgott is a freelance science writer based in San Francisco. |
Spring 2001
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