Astrobiology: The Search for Life in the Universe, by David Pacchioli
Once it was asked in whispers, or with winks. The timid among us, though undeniably curious, feared raised eyebrows. Jokes about little green men. Who could take such a question seriously, yank it from the misty realms of science fiction and drop it under the searchlight of science? Well, our national space agency, for one. What's more, NASA seems pretty confident these days about the answer: Astrobiology, as defined on an official agency Web site, is "the study of the living universe."
James Kasting is a bit more guarded. Astrobiology is the search for life in the universe, the Penn State professor of geosciences and meteorology told a keen audience at the first talk in last January's Frontiers of Science lecture series. Although the term itself may be recent, "This is not a new field," Kasting said. He got his first taste of it as an undergraduate, reading Intelligent Life in the Universe, a 1966 book by Russian astronomer I.S. Shklovskii and a young American named Carl Sagan, who later wrote, "We have every reason to believe that there are many water-rich worlds something like our own." Kasting was hooked.
In recent decades, Kasting acknowledged, the field has known a bit of a slump. It fell out of favor after the 1976 Viking mission to Mars. "Viking was very successful," he explained. "We learned a lot—but we didn't find life. The perception was that all that money was wasted."
Today, astrobiology is back. The reports, over the last five years, of some 30 planets spotted outside our Solar System —the first of these by Penn State astronomer Alexander Wolszczan —have made all those potential watery Earths that Sagan speculated about less hypothetical.
A great stir, too, has been caused by the discovery, in a melon-sized meteorite plucked from the ice of Antarctica, of a fossil-like remnant that, according to Kasting, looks a lot like Earthly bacteria—"except smaller by a factor of ten." Martian microbes? Opinions vary. The possibility was strong enough, however, to warrant a press conference at which President Clinton said, "If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered."
There have been other, quieter, advances. We know now, for instance, that organic, i.e., carbon-based, molecules— crucial to any sort of life we can imagine—are virtually everywhere in the universe. And that, here on Earth, living organisms thrive in what once seemed the unlikeliest of places, from hot springs to frozen lakes—even far below the planet's crust.
In 1998, NASA announced formation of an Astrobiology Institute, a partnership formed for study of "the origin, distribution, evolution, and future of life in the universe." Penn State is one of 11 lead members. No surprise, then, that last winter's annual Frontiers of Science series, organized by the Eberly College of Science and sponsored by the pharmaceutical company Pfizer, Inc., took astrobiology as its topic. On six straight Saturday mornings, the large lecture hall in the Thomas Building at University Park was filled to overflowing with people eager to hear talks by three planetary scientists, two molecular biologists, and a geologist. Astrobiology, these listeners learned, is no loopy fringe pursuit; it is coordinated, systematic, and broadly interdisciplinary. And it involves a lot more than just outer space.
Life in the Extreme
How hardy is life on Earth? Imagine a globe cased in ice: A cap a kilometer thick over land and sea, frozen solid for ten million years. The most recent Ice Age, during which Cro-Magnon's teeth chattered and great hunks of North America and Europe were covered by glaciers, was a tropical honeymoon in comparison.
Now imagine life beneath all that ice. Not a lot of life, mind you— almost everything with a pulse is turned into a Popsicle. But in a few hidden niches, those hot springs under the ocean, say, the hardiest specimens—bacteria, archaea—survive. And, in the long run, life prospers. For when things eventually thaw, they do so in such a way that they accelerate the process of evolution as it has not been accelerated before or since.
Such a scenario, said Paul Hoffman, a professor of geology at Harvard University, is not at all far-fetched—nor is the idea new. Rather, he said, he wanted to share in his lecture "a variety of new evidence supporting an old theory."
In 1964, Hoffman told us, British geologist Brian Harland found glacial deposits present in the ancient rock strata of every continent, even near the Equator and at sea level—evidence, Harland claimed, of the advance of great ice sheets over much of the Earth some 600 million years ago. "Harland proposed a series of extreme Ice Ages, and suggested that the amelioration of climate following these Ice Ages might have had something to do with the great burst in biological evolution that became known as the Cambrian explosion."
Doubts were voiced. With continental drift, Harland admitted, he couldn't be sure where the land masses had been when glaciers covered them. But the real problem was that he had no good explanation for how an ice-covered Earth could have happened. How could it get so cold? "In the absence of a theory," Hoffman said, "no one believed him."
Ironically, Hoffman added, there was a contemporary theory that fit Harland's evidence. A physicist named Mikhail Budyko, at the Leningrad Geophysical Observatory, had worked through a series of calculations based on the global energy balance: the fundamental principle that the heat Earth absorbs must always equal what it gives off. "This balance includes the planetary albedo, the energy reflected back to space," the amount of which is determined largely by surface cover. Dark cover, such as trees and other vegetation, absorbs energy, while a light-colored surface—snow and ice—reflects it away.
Budyko was most interested in something called the ice-albedo feedback. (Maybe it was those long winters in Leningrad.) The ice-albedo feedback, Hoffman explained, says that for any drop in global temperature, you get an increase in surface snow and ice, which means that in turn more heat is reflected away, insuring that things will get still cooler.
What Budyko determined was what Hoffman called "an underlying instability" in the ice-albedo feedback. In short, if temperatures ever went low enough to allow that ice cover to creep to within 30 degrees of the Equator—Houston, Texas, say —"the feedback would be so strong you'd get a runaway effect. It would be unstoppable. The Earth would quickly freeze over."
Budyko didn't think a snowball Earth had ever actually happened, Hoffman said. If it had, he thought, life would have been completely wiped out. Then too, Budyko thought a snowball Earth, once in place, would be permanent: What could generate the enormous heat it would take to undo such a hammerlock? (In 1992, Penn State geoscientists Jim Kasting and Ken Caldeira estimated that such a reversal would require raising atmospheric CO2 to 350 times its present level.)
Since Budyko's day, however, "a couple of things have happened," Hoffman noted. One is the discovery of living organisms in those deep-sea vents, creatures not dependent on sunlight. "We're not certain that these organisms could have survived—ocean chemistry would change in a snowball Earth—but it raises the possibility." A parallel discovery, he added, was of frozen lakes in places like Victoria Land, East Antarctica, where despite mean annual temperatures in the range of Ã±20 degrees C (Ã±4 degrees F), "things never completely freeze. And the water under the ice is teeming with life.
The other thing Budyko didn't know about," Hoffman said, "was plate tectonics. Plate tectonics drives the carbon cycle, which allows Earth to be a habitable planet."
Earth's crust is made up of a dozen great plates, like ill-fitting puzzle pieces, that float atop the hot molten rock below. The bumping and grinding of these plates shapes Earth's geography, raising mountains, occasioning earthquakes, breaching and redistributing continents. Pressures that build up at the heated core beneath all this activity are released via volcanoes, which belch out CO2.
In the normal course of events, Hoffman related, "Rainwater washes this CO2 out of the atmosphere as dilute carbonic acid, which falls on silicate rocks. This weathering produces alkalinity, which is washed by rivers into oceans and winds up as carbonate sediment on the sea floor." This limestone deposit is drawn by churning and settling down to the core, where it is reheated to liquid and gas, and eventually spewed back up volcanically into the atmosphere, renewing the cycle.
A snowball Earth, however, would screw up the carbon cycle something awful. "The oceans are frozen. The air is very dry. There is no source of atmospheric moisture, no way to scrub CO2." Meanwhile, "plate tectonics is continuing. CO2 is being emitted, but there's no way of getting rid of it. CO2 builds up and up, drives temperatures higher and higher—the escape mechanism is inevitable. And boy, what an escape." After about four million years, things warm to the point that dark ponds of open water appear at the equator. This sudden switch in albedo at low latitudes then kicks off wholesale melting, and from there, "Deglaciation is extremely violent. The ice will disappear in a few hundred years—much faster than you can get rid of the excess CO2."
That thick blanket of gas means an extreme greenhouse period: "Surface temperatures at the tropics over 40 degrees C (104 degrees F), super-hurricanes, torrents of carbonic-acid rain." And—with no ice and the maximum surface area of rock exposed—powerful carbonate weathering. This combination eventually resets the atmospheric chemistry to pre-Snowball levels.
A "freeze-fry" scenario, Hoffman called the whole process. And it fits nicely, he added, with the existing rock record. "Glacial deposits world wide are capped by carbonate sediments. This has long been a puzzle—why are warm-weather rocks sitting on top of glacial rocks? But with all this alkalinity being delivered in conditions of rapid warming, massive deposition of inorganic limestone is exactly what you would predict.
It seems pretty likely, given the evidence, that a Snowball Earth did take place, somewhere between 600 and 700 million years ago. And that likelihood brings us back to the Cambrian explosion.
The extreme environmental conditions post-Snowball, Hoffman suggested, may have ramped up the rates of evolution. "The crash in population size accompanying a global glaciation," he has written, "would be followed by millions of years of comparative genetic isolation in high-stress environments," conditions "favoring the emergence of new life forms." Whether this speed-up would create new branches on the tree of life (as the molecular data would determine) as well as new body types within existing branches (as fossil evidence may show) is not clear. But changes in molecular sequence, Hoffman noted, will always show up earlier than changes visible in the fossil record. Whichever type of explosion the Cambrian was, it seems reasonable to speculate that a string of freeze-fry events could have triggered it.
And how does all this relate to astrobiology?
"We're finding there are still many things to be discovered about the history of this planet," Hoffman concluded, "which shed light on the probability of finding life elsewhere. If life's expansion here depends on an event like a Snowball Earth, that's another thing that makes the persistence and evolution of life on this planet extremely remarkable."
Life as We Know It
In 1997, Charles Fisher, professor of biology at Penn State, discovered this remarkable creature (also shown on the cover of this special report) living on mounds of methane ice under half a mile of ocean on the floor of the Gulf of Mexico. The flat, pink worms, one or two inches in length, use their appendages like oars to move around the surface of the ice as they graze for the bacteria also living there. The new worm species, Hesiocaeca methanicola, may have some influence on the formation of natural gas deposits on the sea floor and, if so, on how we go about mining gas as a source of energy. It has already helped redefine "life as we know it." The bacteria the ice worms eat, and the methane both species grow on, could provide clues about early life on this and other planets.
Fisher came upon the worms by accident while collecting tubeworms near hydrocarbon seeps at the sea floor. Before the discovery, methane ice had been of most interest to geologists and energy companies, not biologists. The area where the ice worms live is under extremely high pressure and, at seven degrees C, very low temperatures. Adds Fisher, "The ice worm community is in itself a new ecosystem. We found an animal living in an environment that we never thought of as a habitat for animals." The ice was formed when methane gas rose up from deposits deep beneath the sea floor. Ancient bacteria that may have lived beneath the Earth's crust, feeding on this gas, migrated with it, eventually settling on the ice.
The ice worms, which are not ancient animals but are related to the common red mud worms we see after a rain, would have come along later. But the mere fact that they can survive such a harsh environment shows the long-term adaptive capabilities some animal species possess. Says Fisher, "The animals we study live in some very extreme, very strange environments and they adapt to it using special physiology, special anatomy, and special behavior."
To Bruce Jakosky, life's demonstrated ability to weather almost anything Earth can dish out makes a strong argument that life probably does exist elsewhere in the universe. One likely spot, he suggested, is an old favorite: Mars.
Given the fertility of our collective imaginings about the red planet over the years, Jakosky, professor of geology at the University of Colorado at Boulder and a member of the Laboratory for Atmospheric and Space Physics there, wisely began his talk with a few ground rules. His first slide was a cover from the tabloid Weekly World News, with a prominent photo of a shiny silver saucer hovering above a line of trees. "This," he said with deadpan aplomb, "is what I'm not going to talk about."
Mars, Jakosky went on to acknowledge, is a stone that's already been turned. Twenty-four years ago, two Viking landers touched down on the planet's surface, dug some soil samples, and headed home. Subsequent analysis turned up no trace of organic molecules, the bare-minimum evidence that would have pointed toward life. The search for extraterrestrials was dealt a stinging setback. But recent findings here on Earth, Jakosky said, warrant taking a second look. "Over the last couple of decades, our understanding of terrestrial life has evolved dramatically.
First of all, we know now that life originated quickly." Earth's early history, he explained, was exceedingly violent, with frequent catastrophic bombardments by asteroids not letting up until about four billion years ago. "Not until then could life have gained a foothold." Yet carbon-dating evidence shows that life was already firmly established by 3.8 billion years ago. "Life sprang up almost overnight once the right conditions were present," Jakosky concluded. "To me, this suggests that anywhere these same conditions exist, the odds are good that life could be—and probably is."
Second, he said, "We've found out that life on Earth is incredibly robust and capable," existing not only in surface hot springs and around thermal vents but deep within the planet's interior. "Twenty years ago we didn't know about life below the surface. Today we think that half of Earth's biomass exists there, inside rocks. We were missing half of the life on Earth!"
In short, "Life doesn't require much for its support," Jakosky said. The basic necessities are only three: a liquid medium, an energy source, and the presence of a few choice elements. Here on Earth that means water, sunlight, and an atmosphere shot through with carbon, hydrogen, nitrogen, and oxygen. "Of these elements," Jakosky said, "carbon is probably the most important," not just because of its abundance—it exists all over the universe—but also because of its versatility. "Carbon combines with oxygen to form a gas—carbon dioxide—that can be dissolved in water, so it's transportable. It can precipitate out and be stored as limestone when it's not needed. People ask, Ã«Does life have to be carbon-based? What about silicon?' But carbon is so much more capable."
Does Mars meet the three basic criteria? From this distance, it's difficult to say. But "we can learn a lot," Jakosky said, "by looking at pictures." Present-day Mars is much colder than Earth, too cold to sustain liquid water on its surface. But photographs depicting what looks like erosion of crater rims and other features suggest that abundant water has been present there even very recently. Other photos show networks of branching lines that look like river tributaries; still others, broad channels up to 100 kilometers wide. "That's an hour's drive here on Earth. That much water couldn't have come from just rainfall; there must have been some catastrophic release." Yet tracked to their sources, these channels reveal nothing. "It looks like water burst forth from beneath the crust," Jakosky said. "Almost certainly there is still water down there."
What about an energy source? Granted, the sun is too far off to power Earth-style photosynthesis, but geochemical energy—from volcanoes, and even from mineral weathering—is a viable alternative, Jakosky suggested. He showed a picture of Olympus Mons, a volcanic Martian peak that is twice as tall as Earth's Everest, with a summit area 100 kilometers across. "With volcanism and liquid water," he said, "there's a possibility of hydrothermal vents, like the ones we see at Yellowstone."
As for those life-building elements—carbon, hydrogen, oxygen, and nitrogen—they are all present in the Martian atmosphere. According to the recent Pathfinder mission, magnesium, iron, aluminum, and phosphate —all potential role-players, as well—are components of Martian rocks. "So life could have originated on Mars," Jakosky said. "That doesn't mean that it did, or that it's there now. But it's reason enough to look."
Oh, and there's one more reason: whatever it is that's embedded in the small set of Martian meteorites that have been recovered over the last 20 years. From a pocket Jakosky produced a sliver of dark mineral cased in clear plastic, and held it aloft. "This is part of one of about 15 rocks that have been picked up on the Antarctic ice sheets," he said, "where if you find a rock, the only place it can have come from is out of the sky. These rocks are young, volcanic, which means they came from a planet with recent geologic activity: Earth, Venus, or Mars." Gases trapped within the samples show that they're unearthly: there's not enough oxygen present for them to have been trucked down from New Zealand, say. More positively, the levels of argon, xenon, and krypton are identical to what is present in the Martian atmosphere—"and nowhere else," Jakosky said. "If these rocks didn't come from Mars, we don't know anything about the solar system."
In 1996 NASA created a splash by reporting that one of the Martian meteorites, known as ALH84001 (for its discovery in the Allan Hills region of Victoria Land, in 1984), contains some rather interesting tidbits. Lodged within limestone deposits formed in cracks in the rock were tiny tube-shaped structures that just might be fossilized life-forms. Make that extremely tiny: The largest of them is less than 1/100th the width of a human hair. "Nano-fossil-like structures," NASA has called them. "They look like terrestrial bacteria, except they're a thousand times smaller" in volume, Jakosky said. Apparently they formed, whatever they are, the same way fossils occur in limestone on Earth. But could they really be remnants of life?
"We don't have enough data to tell," Jakosky said. Researchers at Johnson Space Center, he noted, have also identified organic molecules in ALH84001 and some of the other fragments: polycyclic aromatic hydrocarbons, to be precise. "These could be precursors of life, but they are also typical of decay products from the earthly combustion of fossil fuels." They could be simple contamination, in other words. Again, "We will only find out by getting more samples from the Martian surface and bringing them back to study."
The chief difference between now and the Viking mission days, Jakosky said, is that, "We know better what to look for now. Twenty years ago, we didn't know to look for hydrothermal vents." He and his colleagues at NASA also have a better idea of where to look: "In river channels and canyons, places where there has been liquid water." Or at crater rims, some of which appear from photographs to be rimed with ice.
"It's possible that we won't find any evidence of life," Jakosky said. "But that would also be an important result. It would lead us to question again what we have learned about life's origin here on Earth."
An Ocean in Space
For a long time," said Chris Chyba, the last Frontiers of Science speaker, "the difficulty with looking for life on other planets was finding water." The concept of the "habitable zone," developed by Stephen Dole of the Rand Corporation and Michael Hart of NASA's Goddard Space Center and further elaborated by Penn State's Kasting, along with Ray Reynolds of NASA Ames and Dan Whitmore of the University of Southwest Louisiana, put this dilemma in black and white. Of the planets in our Solar System, Earth, Kasting and his colleagues calculated, is the only one close enough to the Sun to be warm enough for liquid water, yet not so close that the water boils away. Actually, Mars is in the ballpark too, except that present-day Mars has too little atmosphere to retain the necessary heat—at the surface. But what about down below?
Recent research has heightened interest in "worlds that may be rich in liquid water below the surface," said Chyba, associate professor of geological and environmental sciences at Stanford University and director of the Center for the Study of Life in the Universe at the SETI Institute. Mars is one such world. Another, in some ways even more tantalizing, is Europa.
Fourth largest of the 16 known satellites of Jupiter, Europa is a chunk of rock and metal about as big as Earth's moon, sheathed in ice. Voyager photographs taken 20 years ago show a smooth surface scored heavily with cracks, like a favorite skating pond in late winter. The absence of craters, Chyba said, shows that unlike our moon, Europa is geologically active. "Its surface is being reworked every 10, or 20, or 30 million years," by new material churned up from below.
The reason for this activity, he said, is the strong tidal pull exerted by Europa's giant parent, which causes bulging and shrinking of the satellite's crust as Europa moves through its orbit. All that movement creates friction—and heat. "And we can calculate how much," Chyba said. Doing so, he added, "enabled one of the few important successful predictions in the history of planetary science": that Io, Jupiter's closest satellite, was so heated by friction it would be "the most volcanically active world in the Solar System. And it in fact is—Voyager has taken pictures of its volcanoes erupting."
The pull on Europa, farther out, is less than that on Io. But there's still enough friction to heat Europa's core substantially —enough to melt away most of its icy layer from the inside, Chyba said. So, although the surface, which has no atmosphere, remains a rock-solid Ã±170 degrees C (Ã±274 degrees F), beneath Europa's ice in all probability lies a vast body of water.
The evidence of resurfacing seems to corroborate this, Chyba said, with smooth areas suggesting water flowing out from the interior only to be quickly re-frozen, like the contents of a bucket spilled across a frigid sidewalk. The wealth of cracks, he added, "seem to be related to stretching ice as it rides up on top of an ocean deforming underneath." But in a way the most compelling argument for an ice-bound sea is the magnetometer data.
Jupiter has the strongest magnetic field of any planet in the Solar System. That field sweeps past Europa every ten hours, as the giant planet spins on its axis. "If there were a conductor on Europa—salty water, for example—the changing magnetic field would set up a current in that conductor," Chyba explained, and that current would create Europa's own magnetic field. Such a field has now been measured—its strength consistent with an ocean 100 kilometers deep with a salt content about equal to that of the ocean on Earth.
"It's hard to avoid the conclusion that there's a salty conducting ocean on Europa," Chyba concluded. "But we're not completely certain. And we would like to be, because if there is a second ocean in the Solar System, we're going to go back and have a program of exploration on Europa that rivals the Mars program. I would go so far as to say that if there is an ocean on Europa, it is the most exobiologically interesting place in the Solar System. That is to say, there might be life there."
What do we mean by life? That's the first thing that needs to be agreed on. "There have been many attempted definitions—thermodynamic, metabolic, biochemical—but all of them seem to either leave something out that we know is life, or let something in that we know isn't," Chyba said. "So we have to fall back on a simpler idea, that of life Ã«as we know it,'" made of liquid water, organic molecules, and an energy source. On Europa, "there is almost certainly liquid water present. There are hints that there are organic molecules present." What about an energy source?
"It's hard to say anything at all about this," Chyba admitted. "You can't have photosynthesis. Light couldn't penetrate that surface ice." Might there be hydrothermal vents at the bottom of that ocean? "We have no idea."
A look at life on Earth, he continued, shows that higher life forms—eukaryotes—require something beyond the three basics: they need oxygen, too, to help metabolize energy. "Even tubeworms and clams at hydrothermal vents need oxygen; it's produced at the surface and finds its way down. If not for photosynthesis these organisms would die." Oxygen, whether in Europa's atmosphere or in its ice-covered ocean, is likely to be scarce, Chyba said. So, "as much as I would like to see giant squid swimming in Europa's ocean, we probably have to content ourselves with one-celled organisms analogous to bacteria or archaea."
On the other hand, he noted, there are some creatures on Earth that get along fine with no oxygen at all. Methanogens, for example, are a class of bacteria that digest hydrogen and carbon dioxide to produce methane. "And they probably get that hydrogen from rocks. If Earth froze over tomorrow and became a world that looked like Europa, we would probably continue to have an ecosystem living underground for billions of years."
Conceivable, too, are energy sources on Europa that we simply don't know about: Chyba offered a suggestion based in photochemistry. Jupiter's strong magnetic field, he said, acts like a particle accelerator, shooting charged particles—radiation— into Europa's ice. "We know from Galileo's observations that there are carbon dioxide molecules mixed in with that ice. Once you irradiate carbon-dioxide-bearing ice, you make simple organic molecules, like formaldehyde. And you can make oxidants from the ice itself. These molecules are frozen together, and at melt-through events they could get mixed into the ocean." Using Earth analogies, Chyba said, "We can estimate that Europa's ocean, in this way, could support a bacterial ecosystem." Not a very robust one—"only about 1/10,000 as dense as that in Earth's ocean"—but, hey, it's a start.
The only way to know whether such an ecosystem is out there," he said, "is to go look." That's the rationale behind NASA's Europa Orbiter, planned for launch in 2006. The Orbiter's primary objectives, which Chyba helped to draft,