by Ben Bova
Clouds of water vapor have been found deep in interstellar space, as well. NASA’s Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998, has found enormous amounts of water in deep space. The bright swirling complex of stars and interstellar gas that composes the Orion Nebula, a favorite site for stargazers, produces enough water to fill all the Earth’s oceans every twenty-four minutes!
But liquid water is another matter. The chemistry of life takes place in liquid water here on Earth, not in ice or steam. Earth is the only world we know of that has vast oceans of liquid water covering its surface, although some of Jupiter’s ice-covered moons may well contain oceans of liquid water beneath their ice sheaths.
However, as we will see in Chapter 9, the first steps in the creation of life may well have happened in the icy bodies of comets. Such comets bombarded the Earth early in our planet’s history, bringing much water to our world. They may well have also brought the earliest forms of life, microscopic spores that arose in the carbon-laced ice that forms the bodies of the comets.
ENERGY
The third need of life is energy. Life is a chemical process that consumes energy and converts it into living tissue. On Earth, the energy for life falls out of the sky: sunshine. Our intricate web of life on Earth, from bacteria to whales, depends on sunlight. Single-celled bacteria and gigantic sequoia trees use the complex molecule chlorophyll to create carbohydrate foodstuffs out of water, air, inorganic nutrients in the soil, and sunlight in the process called photosynthesis. The food chain for our kind of life is based on photosynthetic chlorophyllic organisms and cannot exist without sunlight. Therefore, the search for life on other worlds was focused on the surfaces of those worlds where sunlight could provide the energy for life.
That was a mistake. Life is much more versatile and much tougher than anyone imagined.
For example, consider what happened on the airless, waterless plain of the Moon’s Oceanus Procellarum (Ocean of Storms) during the Apollo 12 mission in 1969.
THE “MOON BUGS”
No one expected to find life on the Moon. The Apollo program was launched for Cold War political reasons, not for the sake of science. Of the twelve astronauts to reach the Moon, only one was by profession a scientist: geologist Harrison Schmitt of Apollo 17. Many Earthbound scientists bitterly criticized the $20 billion Apollo program for this lack of scientific emphasis, even though the program provided a tremendous wealth of new information about the nature and origin of the Moon, as well as many other scientific discoveries.
But no biology. After all, the Moon was well-known to be lifeless. Yet the astronauts found terrestrial life on the Moon!
Apollo 12 astronauts Alan Bean and Pete Conrad landed on the Moon’s Oceanus Procellarum (Ocean of Storms)5 in November 1969 close to the unmanned Surveyor 3 probe, which had touched down on the Moon in 1967. As planned, they detached the probe’s camera and returned it to Earth with them. NASA scientists were stunned to find that a small colony of Streptococcus mitus, a harmless bacterium that lives in the human nose, mouth, and throat, had “stowed away” aboard the camera’s foam insulation. Some of the bacteria had survived thirty-one months on the Moon without air or water, subjected to hard radiation and temperatures that varied from a high of 132°C in sunshine to a low of -151°C during the long lunar night.
Astronaut Conrad said, “I always thought that the most significant thing we ever found on the whole damned Moon was that little bacteria who came back and lived . . .”
LDEF
The hitchhiking lunar bacteria did not remain in the astrobiology equivalent of the Guinness Book of Records for very long, however.
On April 7, 1984, NASA astronauts deployed the Long Duration Exposure Facility from the space shuttle Challenger. The LDEF was supposed to remain in orbit for ten months to determine how exposure to the vacuum and hard radiation of space would affect the fifty-seven samples of various materials it carried. Unfortunately, the NASA program schedule slipped, and then the Challenger blew up on January 28, 1986, killing all seven astronauts aboard. All shuttle flights were suspended for more than a year, and NASA’s recovery of the samples was delayed until January 9, 1990, when LDEF was recovered by astronauts aboard the space shuttle Columbia.
Biologists found that samples of the bacterium Bacteria subtilis were still alive after more than six years without air or water. Ultraviolet radiation had killed the outermost layer of the bacteria colony, which then formed an opaque shell that kept the rest of the bacteria protected from the hard UV.
Life is much hardier than anyone had previously thought it could be.
4
The Extreme Road to Astrobiology
The creatures that inhabit these environments are called “extremophiles” because they are living without oxygen or light, and at temperatures that often exceed the boiling point of water.
—David W. Wolfe
Tales from the Underground: A Natural History of Subterranean Life
WHILE SPACECRAFT WERE LOOKING in vain for life on the other planets of our solar system, here on Earth oceanographers made a discovery in 1977 that was almost as startling as finding extraterrestrials.
Down at the cold and dark bottom of the ocean, far too deep for sunlight to penetrate, oceanographers in deep-diving submersibles such as the Woods Hole Oceanographic Institution’s Alvin were intently mapping the sea bottom. They were not biologists. After all, the frigid, sunless sea bottom was “known” to be just as inhospitable to life as the desolate surface of the Moon.
What the oceanographers were interested in was the jagged fissure that splits the Atlantic Ocean in a north-south direction from Iceland to Antarctica: the Mid-Atlantic Ridge. It is a saw-toothed 60,000-kilometer-long rift in the planet’s crust, an open wound that exposes the red-hot magma deep below, a searing glimpse into the pits of hell.
Vents of superheated water boil up from the ocean floor along the Mid-Atlantic Ridge, water that is more than three times hotter than its normal boiling point of 100°C. The pressure at that depth is so great that the water cannot boil, despite its blistering temperature. These “black smokers,” as the oceanographers dubbed them, are rich in sulfur compounds. The superheated water jetting up into the cold, dark sea bottom water carries dissolved salts of sulfur, magnesium, calcium, and other minerals that slowly, over time, build tall, slender spires along the ocean floor.
Similar fields of thermal vents have been found in the western Pacific and, most recently, in the Indian Ocean about 1,600 kilometers east of Madagascar. In many cases, over the years of their ceaseless eruption they have built up chimneys of rock as tall as apartment buildings. The biggest of these, dubbed the Lost City, was discovered in 2001. It is a spectacular assembly of hydrothermal vents, with spires up to 60 meters high, taller than a twenty-story building. It is estimated to be some 1.5 million years old.
Life exists there.
Clustered around those sunless sea-bottom jets of water that are often 350°C or even hotter, whole colonies of strange creatures live in utter darkness, organisms as weird and wonderful as any alien life-form could be: crabs and fish of exotic new types; shellfish such as clams and mussels; herds of transparent sea cucumbers that scuttle away from the researchers’ submersibles; giant tube worms, some of them 4 meters long, that have neither mouth nor anus.
How could this be? When biologists started to examine these weird new life-forms, they were greatly puzzled. More than 2 kilometers below the ocean’s surface, without sunlight, how can an entire ecological web of life exist? Life needs sunlight. Chlorophyllic plants are the base for the entire food chain—up on the surface of the sunlit world. Down in the darkness of the ocean floor, an entirely different food chain exists. A different ecology, as strange as any extraterrestrial world, clusters around the black smoker hydrothermal vents.
More than a decade earlier in 1965, scientists at Yellowstone National Park had discovered a bacterium thriving in the steaming hot springs at temperatures of 80°C, nearly th
e boiling point of water. (Comfortable room temperature for we fragile humans is about 21°C.) They duly named the little heat-lover Thermus aquaticus. But the life-forms that clustered around the ocean-bottom vents were living under pressures a thousand times greater than sea level, at temperatures more than three times hotter than what T. aquaticus lived in, and had no access to sunlight.
Yet they did have access to energy: the heat welling up through those vents from Earth’s molten interior. The superheated water is also rich in sulfur compounds. As oceanographer Cindy Lee Van Dover puts it:
Vent water is enriched in reduced chemical compounds, especially hydrogen sulfide. A sample bottle of vent water opened in the laboratory can clear a room in seconds as the ripe odor of rotten eggs escapes.
While we humans may not appreciate hydrogen sulfide, there are species of microorganisms that love the stuff. Using the heat energy welling up from deep inside the Earth, these bacteria break down the hydrogen sulfide and produce organic compounds from it, much the way that photosynthetic plants break down water and carbon dioxide (using solar energy) to produce carbohydrates. These chemosynthetic microbes are the foundation of the ocean-floor food chain, just as green photosynthetic plants are the foundation of our sunlit food chain here on the planet’s surface.
The chemosynthetic bacteria live inside the tube worms; the two species are symbiotes: They need each other to survive. The tube worms have a form of gill that looks like a bright red plume; they keep their plumes in the boundary area where the hot vent water and the surrounding cold ocean water mix. The plume takes in the sulfides, oxygen, and carbon dioxide from the vent water and supplies them, through the tube worm’s blood, to the bacteria living in the worm’s gut. The bacteria break down the sulfide compounds and produce organic nutrients that the tube worm lives upon—symbiosis at the bottom of the ocean.
Shellfish eat the tube worms. Crabs and benthic (deep sea) fish eat the shellfish. This is a viable ecological cycle without sunlight.
SLiMEs AND THE DEEP, HOT BIOSPHERE
The discovery of strange new life-forms on the ocean floor hardly affected the scientists who were seeking extraterrestrial life. They were looking at the stars, not at “black smokers” down at the bottom of the sea.
Enter Thomas Gold.
Tommy Gold is both a renowned astronomer/cosmologist and a maverick. Born in England, he has lived in the United States most of his life and holds a professorship at Cornell University. He was one of the trio of British cosmologists who proposed the controversial Steady State theory to explain the origin of the universe, in 1948.6
Gold has often said, “I would rather be wrong than dull.” Over the years he has proposed ideas in fields as diverse as lunar topography and paleontology. In 1992, he offered his concept of a deep, hot biosphere to a skeptical world.
He was thinking about oil drilling and energy shortages when the idea struck him that there must be abundant supplies of natural gas—methane—deep belowground. Methane is a common compound; plenty of it has been observed in the atmospheres of the gas giant planets. It must have been a component of the Earth when our planet first formed, Gold reasoned; lots of it should still be present, trapped deep underground. Drill deep enough and you will find methane. We don’t have to worry about running out of petroleum or about OPEC oil embargos; there is plenty of natural gas beneath our feet, deep down.
Hardly anyone believed him. A deep-drilling experiment was started in Sweden, but it was abandoned after going down several kilometers without striking any methane. Gold grumbled that they simply didn’t go deep enough.
Thinking about conditions deep underground led Gold to speculate that there are probably microbial forms of life living below, surviving at pressures and temperatures far beyond anything we on the surface are familiar with. Most biologists scoffed; another wild theory—from an astronomer, no less.
Yet other lines of investigation were leading toward the same conclusion. In the early 1980s, scientists from the Environmental Protection Agency were searching for microbes that might help clean up buried toxic wastes, a process called bioremediation. They knew that bacteria found in topsoils were capable of ingesting toxic chemicals and turning them into harmless by-products. Since they were primarily interested in removing harmful toxic wastes from underground supplies of drinking water, however, they wanted to find bioremediating microbes that lived at the depths where drinking water aquifers exist.
The EPA effort was led by James McNabb and John Wilson.Their studies discovered many different types of microbes living deep underground; some of them are now used regularly for bioremediation purposes.
Then the Department of Energy became interested in finding microbes that might bioremediate buried nuclear wastes. Geologist Frank J. Webber established a long-term Subsurface Science Program aimed at going deeper not only to look for bioremediation possibilities, but to study the deep-dwelling species themselves.
They found plenty. In 1987, near a nuclear facility in South Carolina, they discovered bacteria living nearly 500 meters underground. Soon other sites revealed microbes dwelling nearly 3 kilometers deep, in heat more than three times normal room temperature. A microbe dubbed Pyrolobus fumarii thrives at 106°C, and in a laboratory furnace it has even withstood 121°C for an hour. Its growth slows when it is exposed to temperatures below 100°C, and at about 90.5°C (194.9°F) it begins to “freeze” to death.
These microbes live in complete darkness, at pressures of more than 14 tons per square centimeter, thousands of times higher than we face on the surface. They have been called thermophiles (heat-lovers) because they thrive at high temperatures. They use the heat of the Earth’s interior as their energy source, rather than sunlight, and utilize the water that percolates through the ground. They are anaerobic: They do not require oxygen, as we do. Indeed, oxygen kills them.
Then there are the lithotrophs, literally “rock eaters.” They eat solid rock. In 1995, researchers from the Pacific Northwest Laboratory found microbes that metabolize rock nearly a kilometer below the Columbia River basin in Washington state. They dubbed them SLiMEs, which stands for Subsurface Lithotropic Microbial Ecosystems.
In 2002, a team of researchers from the U.S. Geological Survey and the University of Massachusetts, Amherst, announced their discovery of microbes living deep underground in volcanic terrain in Idaho that “eat” hydrogen percolating out of the hot rock and “exhale” methane. Similar microbes, they feel, could exist belowground on Mars.
Researchers drilling into the crust at the bottom of the northeast Pacific Ocean have found evidence of a diverse ecology of microbes that devour sulfur compounds and exhale ammonia.
ALL SORTS OF “ ’PHILES”
Biologists began to realize that Gold was right: There are all sorts of microbes living in environments that had previously been thought to be too hot, too cold, too acidic, too salty for life to exist.
There are thermophiles (heat-lovers) such as Sulfolobus and Thermoplasma that live in hot springs and geysers at temperatures up to 110°C.
There are psychrophiles (cold-lovers) that exist in ice and briny sea water within a fraction of a degree of the freezing point of water. Psychrophiles have been found at the ocean’s bottom, in samples drilled out of Antarctic ice, even within the rocks in the cold dry deserts of Antarctica. Microbes have been found in samples of icy brine from Lake Vida in Antarctica, living in salt concentrations seven times higher than sea water and at temperatures of -10°C (because of its high salt content, the water does not freeze even at that extremely low temperature).
There are acidophiles living in the ocean-bottom hydrothermal chimneys and in the smoking vents of active volcanoes at pH values of 0 to 0.7, hundreds of times more acidic than battery acid (see Appendix 4). These microbes ingest sulfur and convert it to sulfuric acid. Alkalophiles, living at pH values up to 12.5 (a hundred times more alkaline than lye-based soap), have been found in Lake Nakuru in Africa, which is called “the soda lake” because of its
extremely high alkalinity. Halophiles (salt-lovers) have also been found in such lakes, thriving in salt concentrations as high as 37.5 percent (ocean water averages about 3 percent salinity).
What to make of all these strange microbes in their harsh environments? Biologists began to speak of extremophiles.
EXTREMOPHILES
Thermophillic rock-eating SLiMEs. Tube worms living next to 350°C water at the bottom of the ocean in symbiosis with sulfur-digesting bacteria. Anaerobic microbes that exist in hot springs at temperatures that would boil water twice over here on the surface. Acidophiles. Alkalophiles. Halophiles. All these strange creatures have been lumped together into a rough category called extremophiles: Organisms that live under extremes of temperature, pressure, salinity, pH, or other conditions.
Then there is Deinococcus radiodurans, a pinkish little fellow whose name means “the strange berry that can withstand radiation.” Biologists have nicknamed D. radiodurans “Conan the Bacterium.” It is indeed the Arnold Schwarzenegger of the microbial world. Found in elephant dung, Antarctic granite, and the highly radioactive waste water of nuclear power plants, D. radiodurans can survive radiation doses of 1.5 million rads (500 to 1,000 rads is lethal to humans). Its DNA is shattered into hundreds of fragments by the radiation, yet within a few hours it knits its DNA together, good as new. An extremophile’s extremophile.
The extremophiles have taught astrobiologists an enormous lesson: Until this past decade, the astronomers, biologists, and other investigators who had been seeking life on other worlds were all victims of a false assumption. They tacitly assumed that they should search for life on the sunlit surface of worlds where liquid water might exist. The extremophiles have shown that life is much tougher, much more ubiquitous, than anyone had previously imagined.
