Aliens Among Us

The fireball bearing down on the little town of Tata, in southwestern Morocco, in July 2011 was like nothing the locals had ever seen. There was one sonic boom, then another as a yellow slash of fire cut across the sky. The yellow turned to a landscape-illuminating green, the fireball split in two, and a hail of smoldering rocks crashed to the ground across the surrounding valley. With that, the planet’s latest invasion from Mars was over.

Scientists quickly pounced on the incoming ordnance, dubbed the Tissint meteorite after the type of rock it was made of. They wanted to know its chemistry and mineralogy–which proved it came from Mars–and they wanted to know one more important thing: whether it was carrying passengers. It’s a question space scientists have begun asking a lot.

Life, as far as we can prove, exists only on Earth. There is our modest planet circling our modest star, and then there is the unimaginable hugeness beyond. Yet in that whole, great cosmic sweep, we’re the only little koi pond in which anything is stirring. That, at least, has been the limit of our science. But that limit is changing fast.

The cosmos, as scientists now know, is awash in the stuff of biology. Water molecules drift everywhere in interstellar space. Hydrogen, carbon, methane, amino acids–the entire organic-chemistry set–swirl through star systems and dust planets and moons. In 2009, NASA’s Stardust mission found the amino acid glycine in the comet Wild 2. In 2003, radio telescopes spotted glycine in regions of star formation within the Milky Way. And meteors that landed on Earth have been found to contain amino acids, nucleobases–which help form DNA and RNA–and even sugars.

That raises a tantalizing question: If the building blocks of life can rain down anywhere, why not life itself–at least in the form of bacteria? Such an improbable idea–dubbed panspermia–has been chattered about by scientists since the 19th century. But back then, there wasn’t much knowledge of what the cosmic ingredients of life would be or how to detect them even if they could be identified. That’s all changed. A welter of new studies in the past few years have shed light on the panspermia idea–and, in the process, have changed our very sense of our place in the cosmos. Never mind the old image of life on Earth existing in a sort of terrestrial bell jar, sealed off from the rest of the universe. Our planet–indeed all planets–may be more like a great meadow, open to whatever spores or seedlings blow by.

“I think there’s definitely a role meteorites have to play in at least getting prebiological materials to planets,” says Chris Herd, a meteorite expert at the University of Alberta, who has studied the Tissint rocks. “A lot has to go right for an actual microorganism to go from planet to planet. But in some cases, they just might survive the trip.” If they made that trip to the ancient Earth, we may not merely have encountered aliens; we may be the aliens.

Martian Misfire

The search for life in rocks from space has not always been smooth. On Aug. 6, 1996, NASA stunned the world with a midday press conference announcing that a meteorite from Mars, prosaically known as ALH84001, contained evidence of what appeared to be fossilized bacteria.

LIFE ON MARS, the headlines screamed–including one in TIME–and that was exactly the conclusion the researchers had tentatively reached. “It’s an unbelievable day,” said then NASA administrator Daniel Goldin. “It took my breath away.” President Clinton, campaigning for re-election, took a break to weigh in too. “If this discovery is confirmed,” he said in a White House statement, “it will surely be one of the most stunning insights into our universe that science has ever uncovered.”

Stunning, yes, but that confirmation never came. Further study of 84001 failed to rule out inorganic processes for the seemingly biological clues it contained, and while the rock continues to spark debate, no one disputes that the evidence was not the slam dunk it seemed to be.

In the years since, the research has proceeded apace, even if the press releases have been more measured, and the case for panspermia is being convincingly rebuilt. Last year, Herd and his co-authors published a paper in the journal Science showing not just how biological material could get to Earth but also how it could survive a long trip in space.

The study focused on what’s known as the Tagish meteorite, after the frozen lake in British Columbia on which it smashed itself to fragments on Jan. 18, 2000. Within days of the impact, scientists collected the debris–making no direct hand contact with it in order to prevent biological contamination–and put it in cold storage. When Herd and his colleagues got hold of four of the fragments and cracked them open, they found that the debris very much warranted such caretaking.

Distributed throughout the rock were not just the organics that had been seen before but also organics in different stages of sophistication, with simpler molecules giving way to complex ones and more complex ones still–a bit like finding caterpillars, cocoons and butterflies all in the same little nest. The rock, it seemed, had been acting as a sort of free-flying incubator, with traces of water trapped in its matrix combining with heat from radioactive elements to keep things warm and effectively pulsing.

“These asteroids form in space, you dump in organic molecules, a little water ice and a little heat, and then they just start to stew,” says Herd. That slow cooking went on for millions of years until the heat and water eventually were exhausted and the process shut down.

This doesn’t have to mean that similar rocks landing on Earth billions of years ago were the start of all terrestrial life–or even that they contributed to biological processes already under way. And yet the organics in the Tagish meteorite have a curiously familiar feature. Amino acids come in one of two varieties: left-handed and right-handed, defined by an asymmetrical structure that points either one way or the other. All earthly life uses the left-handed kind–a puzzle since right-handed amino acids should work just as well–and the Tagish amino acids are left-handed too. Somehow, that southpaw bias got started on Earth. Herd’s findings at least suggest that the influence could have come from beyond.

Cosmic Stowaways

It’s easy enough to imagine how a meteor that accreted in space and spent its life flying could eventually find its way into the gravity field of a planet if it came too close. Harder to figure is what it takes to get biologically contaminated material from the surface of one planet to another. Something, after all, has to launch the stuff in the first place. Typically that something is a meteor strike, which hurls debris into space, where it slowly drifts from one world to the next. Earth and Mars have exchanged material this way for billions of years, though more in the early days of the solar system, when the cosmic bombardment was greater.

The kind of life that can get started on the warm, wet surface of a planet, contaminate its rocks and hitch a ride to the world next door is a lot more complex than the mere prebiology that can get cooked up in space. Most of those organisms–probably the single-celled kind like those the ALH84001 scientists thought they found–couldn’t live through the shock heating that occurs when debris is blasted into space, but the ones deep within the rock might. Surviving the hundreds of thousands or millions of years it would take to travel from world to world would not be impossible. Earthly bacteria that live in extreme environments may go dormant or even freeze-dry until conditions improve and they stir to life again.

In June, investigators from the University of Colorado at Boulder studied bacteria found in the Atacama region of South America, where rain almost never falls and temperatures go from 13F (-11C) at night to 133F (56C) the next day. Microbes nonetheless thrive there, sucking energy from traces of carbon monoxide in the air and extracting moisture from exceedingly rare snowfalls. The rest of the time they hibernate. There’s no reason an adaptation that nifty should be confined to earthly life.

Whatever biology is flitting about out there would not even have to be limited to traveling from planet to planet; it could also hop from solar system to solar system. This idea, known as lithopanspermia, was long considered impossible. Not only would the transit times between solar systems be prohibitively long for even the hardiest bacteria–on the order of 1.5 billion years–but the speed a space rock needs to travel to escape the gravity of its home solar system is too great for it to be captured by another. In September, however, a team of researchers from Princeton University, the University of Arizona and the Centro de Astrobiologa in Spain figured out a neat solution that sidesteps these problems.

Most lithopanspermia models assumed that the only way a rock could escape a solar system was if it passed too close to a large body like Jupiter and was gravitationally ejected at a speed of about 18,000 m.p.h. (29,000 km/h). But the investigators in the recent study used a computer to model a slow-boat escape known as weak transfer, in which a rock gradually drifts out through a solar system until it’s so far from its parent sun that the slightest flutter in its trajectory could tip it into interstellar space.

“At this point,” says Princeton astrophysicist Edward Belbruno, one of the co-authors, “mere randomness determines whether it gets out or not.” And never mind the extreme distances to the nearest solar systems. About 4.5 billion years ago, the infant sun was part of a tight grouping of nascent stars known as the local cluster. The herd dispersed after less than 300 million years, but a weak-transfer rock that escaped within that window could have reached the next solar system in about a million years. “Trillions of rocks could escape a solar system,” says Belbruno. “Over the course of 300 million years, about 3 billion might have struck Earth.”

It’s impossible to know if even one of those 3 billion would have harbored biological material, especially so early in the history of the local stars. But if the new studies say anything, it’s that it’s equally impossible to continue to see the Earth and its organisms as somehow separate from the rest of the cosmos. The building blocks of biology are everywhere; life, it seems increasingly likely, could be too.