Microbiologists Rocco Mancinelli and Lynn Rothschild have a thing for salt. Jagged hunks of it crowd the shelves of the couple's offices at the NASA Ames Research Center in Mountain View, California. Their favorite pieces are laced with translucent reds and greens that look like algae in a neglected pool. These crystals harbor colonies of hardy, salt-loving microbes called halophiles, a class of bacteria that can thrive in very nasty settings. So impressive are the survival skills of these single-celled organisms that Mancinelli and Rothschild suspect the microbes might be able to survive long journeys through the vacuum and radiation of space. And that possibility, in turn, could help explain how life began on Earth.
Mancinelli opens a drawer and points to scores of small, neatly arranged quartz disks. They were used to test whether the halophiles could survive a two-week flight on a satellite. "We dried about 10 million cells onto the surface of each quartz disk. When we analyzed them after the flight, we found that 10 percent to 75 percent had survived."
How? Rothschild says cells that thrive in a salty habitat evolve to endure long dry spells. That stress, along with the sun's relentless radiation, forces them to develop ways to mend nicks in their DNA. Their rich pigments may also provide protection.
"We don't have an answer yet for whether life could withstand space travel," muses Mancinelli. "But if it can, I wouldn't be surprised if a halophilic organism is the first extraterrestrial we find."
Photographs: Left to Right, Courtesy of NASA/Lunar and Planetary Institute; Courtesy of Wayne Nicholson/University of Arizona
Mancinelli and Rothschild belong to a cadre of researchers who are reviving an old idea that seems straight out of science fiction: Organisms might have hopped from planet to planet, spreading life far beyond their birthplace. The scenario is simple. When our solar system was young, comets and asteroids crashed into planets and moons, which blasted surface rocks back out into space (a few such impacts still happen today). If the space-bound rocks harbored lifeforms, they might migrate to other planets. Recent lab tests suggest that bacteria can withstand the shocks of such blasts. And decent-sized rocks could shield the ejected cells from radiation in space. What's more, some studies suggest that sheltered microbes can survive tens or hundreds of millions of years of dormancy, plenty of time to drift to a new home. Add it all up and you've got a case that life could have drifted to Earth from someplace like Mars.
The idea of life vagabonding through the cosmos has been around for millennia, but scientists first considered it seriously in the mid-19th century. In 1871, British physicist William Thomson Kelvin told his colleagues in Edinburgh: "We must regard it as probable in the highest degree that there are countless seed-bearing meteoritic stones moving about through space. If at the present instant no life existed upon this earth, one such stone falling upon it might . . . lead to its becoming covered with vegetation."
Three decades later, Swedish chemist and Nobel laureate Svante Arrhenius agreed, but he took issue with part of Kelvin's scenario. The fiery trauma of a meteoroid ejected from a planet or out of the solar system, he argued, would incinerate any cells it harbored. Instead of hitching rides within rocks, Arrhenius said, life could travel unaided. In 1903, he proposed that spores of plants and germs might drift through space propelled by the gentle pressure of starlight. He called this idea panspermia (from the Greek for "seeds everywhere").
When astronomers later grasped the true distances between stars and the vast size of the Milky Way, panspermia fell out of favor. Chemists and biologists devised other credible theories for how life might have arisen on Earth, such as the "warm little pond" of gentle chemical reactions envisioned by Charles Darwin.
Now panspermia is gaining credence again, but with more caveats. Planetary geologist Jeffrey Moore of the NASA Ames Research Center says that if panspermia simply means exchanges of life among bodies in our solar system, Kelvin's "seed-bearing meteoritic stones" could be spot on. "Panspermia redefined is perceived as reasonable by virtually everybody," Moore explains. "Say you have several places in the solar system where organisms could multiply. Once one gets it, all the planets and moons with suitable environments come down with life. It's the day-care effect. They infect each other." The inner solar system, he adds, with its friendly temperatures and hard surfaces, is the most likely place for such exchanges.
Still, migrating microbes face significant obstacles. Until recently, no researchers had evaluated every stage of the scenario. Then a Swedish scientist rounded up a team to do just that.
Photographs: Left to Right, NASA/SPL/Photo Researchers; Courtesy of NASA/Lunar and Planetary Institute.
Curt Mileikowsky, 78, works at the Royal Institute of Technology in Stockholm. A nuclear physicist by training, he is the former chief engineer for development of nuclear power reactors in his country. For 12 years he served as chief executive officer of Saab Scania, known for its high-performance planes, missiles, and rockets. His company also helped develop the Ariane rocket for the European Space Agency. "All of this got me very interested in space," he says.
Mileikowsky's academic career also included work with medical devices, such as neutron-beam guns for cancer treatment. That expertise in the study of radiation, combined with his knowledge of projectiles, perfectly prepared him to study life whizzing through space.
"This project isn't for money, or for planetary protection, or for some NASA program," he explains. "We just thought we needed an answer." So Mileikowsky pulled together a team of 10 scientists, each with expertise on a key panspermia question. They soon found that panspermia seems viable only within our own solar system. One hitch in the old theory, he explains, was that interstellar nomads would face lethal radiation from cosmic rays, which strike far more frequently beyond the sun's magnetic shield. Even more important, Mileikowsky's team has calculated the probability of ejected planetary material reaching Earth from elsewhere in the Milky Way or from another galaxy. "It is one in a billion," says Mileikowsky. Given those odds, the probability is virtually nil that even one ejecta from the galaxy with still-viable microorganisms on board could have arrived on Earth during its first 500 million years. So Mileikowsky concludes, "Our ancestor cell must have been created within our own planetary system or in a nearby sister system born at the same time." The question then becomes: Where?
In the young solar system, interstellar dust coalesced into clumpy particles, rocks, small bodies, and eventually, planets. Asteroids and comets blasted into these bodies for hundreds of millions of years. Among that chaos lay the possibility of life drifting not only to Earth but from Earth to neighboring planets and moons. For decades the problem with an extraterrestrial transfer of life was thought to be that everything would disintegrate in a shock-induced poof when an asteroid hit another planet.
In the 1980s, new evidence turned up. Analysis of trace gases within meteorites found on Earth revealed that some had originated on Mars or on our moon. "That changed everything," says Jay Melosh, an astronomer at the University of Arizona. "Suddenly, interplanetary transfer was feasible." It turns out that a high-speed impact on a planet's surface doesn't pulverize all the rock on the ground below. Instead, some rocks at the edge of the impact get lofted into space at tremendous speeds and remain intact.
But were enough rocks launched to make arrivals on the young Earth likely? Mileikowsky called upon Brett Gladman, an astronomer at the Observatoire de la Côte d'Azur in France, to tackle that question. Gladman's specialty is studying how the trajectories of small bodies in orbit are altered by the tugs of nearby planets. A count of lunar craters provided a rough estimate of how hard and how often Mars and Earth were hit in the period of early bombardment. Then, by modeling rates and speeds of launch, Gladman created computer simulations that track the loopy orbits rocks take after they're ejected from planets.
"It's surprisingly easy to get material from Mars to Earth," says Gladman. "If you launch stuff off Mars, there aren't a lot of other places to go." He found that up to 5 percent of the rocks launched from Mars land on Earth within 10 million years. Many arrive much sooner— some within a few years.
Photograph Courtesy of Stewart Pankartz
Mileikowsky's team then deduced that 50 billion Martian rocks landed on Earth during the first 500 million years of the solar system. Of those, about 20,000 rocks struck Earth within a decade. And throughout the subsequent 4 billion years, as many as 5 billion more Martian meteoroids journeyed our way. If life ever existed on Mars, it's quite possible that it contaminated Earth repeatedly.
The reverse trip, from Earth to Mars, has less traffic. First, it's harder to launch rocks off the more massive, atmosphere-enshrouded Earth. Second, Mars is a smaller target than Earth. The team estimates that the flow of rocks from Earth to Mars would be about one fiftieth the flow from Mars to Earth. Still, given the amount of traffic from Mars, that's a lot of boulders. The odds are slim, though, that rocks from Mars or Earth could strike planets or moons in the outer solar system. "Jupiter is a very efficient garbageman," says Gladman. "It gets hold of rocks, pumps their orbits, and chucks them out of the solar system in about 100,000 years."
The team's work established that a transfer of rocks could occur easily and often between planets in the inner solar system. The next question: Could microbes aboard survive ejection and impact? To escape a planet's gravity, a rock must accelerate from zero to at least 11,500 miles per hour in a thousandth-of-a-second jerk so intense it would liquefy a human. But when Jay Melosh and his colleague Rachel Mastrapa loaded bacteria into bullet casings and shot them into cold plastic modeling clay, they found that most bacteria survived. Mileikowsky, too, has tested this idea by firing cannon shells stuffed with pebbles holding hundreds of millions of ordinary bacteria. Again, most of the cells lived.
Another anticipated hurdle would be the intense heat at launch from one planet and the heat at impact on another. Yet last year a team led by graduate student Benjamin Weiss of the California Institute of Technology found that the inside of a Martian meteorite (ALH84001, made famous by researchers who believe that it contains clues of ancient life) never grew hotter than a summer day in Palm Springs. The team figured this out by analyzing faint traces of a magnetic field preserved within the meteorite. When researchers heated a small slice of it to 104 degrees Fahrenheit, the rock's magnetic signature— imprinted during its early days on Mars— vanished. That meant the meteorite's interior had never exceeded that temperature, not even during its odyssey to Earth. "It was actually quite amazing," says Weiss's supervisor at Caltech, geobiologist Joseph Kirschvink.
Microbial havens could, therefore, survive the trip between planets. "The only question is the lifetime of the bacteria," says Mileikowsky. "It is the aspect that must be tested more than anything else."
A few experiments show that bacteria can persist in space for at least a few years. Microbiologist Gerda Horneck of DLR, the German space agency, found that out when she sent organisms into a six-year orbit on a NASA satellite in the 1980s. The star performer was Bacillus subtilis. When deprived of nutrients, these bacteria form spores, hardened nuggets that protect each cell's vital components. Horneck found that although ultraviolet radiation killed all the spores in a top layer, the dead spores formed a protective shield for those beneath. Many survived the vacuum, cold, and lack of water, including about 30 percent of those embedded in salt.
Two years ago, Rocco Mancinelli followed up by sending his salt-loving microbes into space for two weeks on BioPan, a European satellite. Mancinelli showed that halophiles also survive, but they don't make spores. His result may mean that many ordinary, non-spore-forming microbes could travel within meteoroids. Horneck and Mancinelli acknowledge that short satellite flights can't compare with the millions of years required for most interplanetary crossings, or even the decades to millennia required for fast transfers between Earth and Mars.
Photograph Courtesy of Russell Vreeland/Nature
Survival is theoretically possible, though, because microbes dormant for millions of years on Earth have been reawakened. In 1995, microbiologist Raœl Cano and others at California Polytechnic University isolated a living spore from the gut of a bee preserved in amber. The spore had lain in suspended animation for 25 million to 40 million years. In a more spectacular experiment last year, geomicrobiologist Russell Vreeland of West Chester University in Pennsylvania and his team extracted and revived bacteria from a pocket of fluid within a 250-million-year-old salt crystal.
Some hail Vreeland's study as evidence that bacteria may be virtually immortal under certain conditions. Others suspect that the pockets of fluid— or the bacteria within— somehow migrated into the crystal far more recently than 250 million years ago. Vreeland counters that his team selected only unaltered crystals and fluid pockets. "There's no known way that a bit of fluid could enter a crystal after it formed," he says. "But we'll never get 100 percent acceptance. You probably wouldn't get that if you brought back a brontosaurus."
For David Des Marais, a NASA Ames microbial ecologist, the protective properties of salt as a packing material are obvious: "It's just a good way to get organisms from one planet to another." Des Marais is particularly interested in biofilms, thin mats of bacteria that can withstand the oxygen-poor atmospheres and environmental stresses that would have characterized newborn planetary worlds. The oldest fossilized evidence of life on Earth is found in biofilms called stromatolites, layers of blue-green algae that lived along salty shorelines.
Des Marais selects a rock from a bottom drawer in his lab and runs his finger along the cone-shaped layers that the microbes formed. "They were alive 3.458 billion years ago," he says with a smile. Depending on the tides or the weather, these slimes dried out for days to weeks at a time until the next water came along. "A biofilm community can get as dry as a board," Des Marais says. "It looks like tree bark. But when you add water, an hour later it's happily photosynthesizing." If any of those got launched into space by an early impact on Earth, Des Marais theorizes, they might well have been perfect interplanetary colonizers.
Knowing that some microbes easily hopscotched from planet to planet doesn't necessarily bring us any closer to pinpointing the fountainhead of life. But if future missions to Mars uncover microbes there, past or present, scientists may be able to isolate their biological molecules. Should that material contain something completely different, such as amino acids with a right-handed molecular twist, mirror images of the left-handed kind in all proteins on Earth, it would provide the first evidence that life also arose somewhere else. But if the micro-Martians share precisely the same language of DNA and proteins as cells on Earth, the question will remain open. That discovery, while startling, would merely show that Mars and Earth shared life-forms. The origin could have occurred on either planet.
In the meantime Mileikowsky is content with one profound implication of his team's research. "If life could exist on both planets, then it could have survived on one planet if a comet or asteroid had annihilated life on the other," he says. Then, a return transfer of ejected rocks could recolonize the sterilized planet once conditions there settled back down. "There was a refuge," Mileikowsky theorized. "Our solar system had a spare planet for life." To a former engineer, a spare part makes a lot of sense.
The eminent British astronomer Fred Hoyle and his former student astrophysicist Chandra Wickramasinghe of the Cardiff Centre for Astrobiology in Wales promote a far-reaching— and, to most scientists, far-fetched— view of panspermia. They believe that microbes migrate within comets and their dusty remnants. They also claim that the spectra— the signature of how something reflects light— of interstellar dust particles (right) reveal the "degradation products of bacterial life." Last fall, Wickramasinghe claimed that a shower of Leonid meteors held hints of heated bacteria. The likeliest explanation, he said, was that the meteor incinerated a layer of comet-strewn bacteria.
Photograph Courtesy of NASA/Lunar and Planetary Institute
Mainstream astrobiologists scoff at such ideas. No evidence supports the notion that comets harbor watery, microbial havens. Nor are there distinctive signs of bacterial life in the heavens. "That's wild speculation," says Peter Jenniskens, a meteor specialist at the NASA Ames Research Center. For example, much organic matter— living or nonliving— with chemical bonds between carbon and hydrogen atoms creates nearly the same spectrum, notes Jenniskens. "That spectral feature is not very discriminating," he adds. "It's also similar to the spectrum of good Sicilian wine." — R.I.