by Ben Bova
As yet, however, no firm evidence for extraterrestrial life has been found. No Martians, not even a bacterium.
But that will undoubtedly change.
2
Time and the Tide of Opinion
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune.
—William Shakespeare
Julius Caesar
THERE IS A TIDE in the status of scientific ideas, also.
A hundred years ago the general population, sparked by popular books about the planet Mars, readily accepted the idea that other worlds might be inhabited by intelligent extraterrestrials. Yet most professional astronomers disdained such “outlandish” ideas, and biologists hardly even considered the possibility.
In the early decades of the twentieth century, the popularity of searching for extraterrestrial life—on Mars or anywhere else—was definitely at an ebb in the astronomical community. Planetary astronomy in general was not a “hot” subject, and the possibility of alien life was derided—when it was considered at all.
Yet popular interest in Mars as a possible abode for not merely life, but intelligent life continued unabated. One of the most popular novels about Mars ever written, Ray Bradbury’s hauntingly poetic The Martian Chronicles, was published in 1950 and immediately gained a readership far beyond the usual audience for science fiction books.
Toward mid-century the tide of scientific interest in the possibilities of extraterrestrial life began to shift. As is often the case in science, new instruments gave astronomers new capabilities, and new capabilities led to new ideas.
RADIO TELESCOPES AND SETI
Radio astronomy began by accident.
In 1931, Karl Jansky (1905–1950), a young engineer working at the Bell Telephone Laboratories in New Jersey, was assigned the task of tracking down some of the sources of electrical interference—“static”—that often troubled long-distance radio communications.
To find where the interference was coming from, Jansky put together a radio receiver and an antenna that could be swiveled to point in various directions. He originally thought that the “static” was coming from electrical disturbances high in the Earth’s atmosphere, a region between 80 and 400 kilometers up that is called the ionosphere, because many of the atoms at that altitude are electrically charged (ionized). The ionosphere is the region where auroras light up the northern and southern skies.
By 1932 Jansky found that there was indeed a source of static coming from the sky, but it did not seem to be from anywhere in the atmosphere. The source rose in the morning, crossed the sky from east to west, and set at dusk. Jansky thought the source was the Sun.
He soon noticed, however, that the interference source rose above the horizon and set each evening a few minutes before the Sun. It crossed the sky in twenty-three hours and fifty-six minutes.
Jansky was no astronomer, but he started to teach himself the rudiments of astronomy. He soon learned that the stars go across the sky in twenty-three hours, fifty-six minutes. The source of radio interference was coming from the stars themselves.
Jansky had inadvertently built the first radio telescope. Stars and interstellar gas clouds emit radio energy as well as visible light. In fact, they also emit infrared, ultraviolet, X-ray, and gamma-ray “light,” although most of those wavelengths are blocked by our atmosphere. Some radio wavelengths penetrate to the ground, and this is what Jansky’s primitive antenna was receiving.
World War II fostered giant leaps forward in the technology of electronics, and after the war radio astronomy became an important new field of astronomical research. Astronomers were soon building large antennas, most of them dish-shaped, to study the radio emissions that stars and interstellar gas clouds give off naturally.
By 1959 radio astronomy was a vigorous and growing field, making important contributions to our understanding of the heavens. In that year, two highly respected physicists from Cornell University, Giuseppe Cocconi and Philip Morrison, proposed using radio telescopes to search for intelligent signals from alien civilizations. Their landmark paper, “Searching for Interstellar Communications,” was published in the British journal Nature in September 1959. It ended with these words:
The presence of interstellar signals is entirely consistent with all we now know, and if signals are present the means of detecting them is now at hand. Few will deny the profound importance, practical and philosophical, which the detection of interstellar communications would have. We therefore feel that a discriminating search for signals deserves a considerable effort. The probability of success is difficult to estimate, but if we never search the chance of success is zero.
Within a year, Frank Drake, then a newly graduated scientist at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, “bootlegged” a few hours on the observatory’s twenty-six-meter “dish” to begin the radio search for intelligent signals. As others joined the search, the effort became known as SETI—the Search for ExtraTerrestrial Intelligence. Pioneer Drake now heads the SETI Institute in California.
SPACECRAFT AND SAGAN
Meanwhile, another tool was becoming available to astronomers: rockets that could carry sensors above the Earth’s cloudy, turbulent atmosphere for unobstructed views of the heavens. Rocket-launched spacecraft could even fly to other planets for close-up examinations.
A leading figure in the use of spacecraft for seeking signs of life was Carl Sagan (1934–1996). Ostensibly a professor at Cornell, Sagan was actually—and more importantly—a man who could stir the public with his vision of the excitement of scientific exploration. Sometimes he could stir politicians, too, and he was instrumental in “selling” to the White House and Congress programs such as the Mariner and Viking missions to Mars and the Voyager flybys of the outer planets.
Sagan was the author of many popular books and, above all, a superb teacher. He knew how to make science understandable and even thrilling to the general public. For example, when the Mariner 4 mission to Mars sent back photos of a barren, crater-scarred planet, bereft of canals or any signs of life, most astronomers and media pundits proclaimed that Mars was a “dead planet.” Not Sagan. He ingeniously used photos of Earth taken by orbiting weather satellites to show that they didn’t detect any signs of intelligent life on our planet either. Since Mariner 4 and the weather satellites used cameras of similar sensitivity, Sagan’s point was that cameras that cannot detect London or New York should not be expected to find Martians.
As the tide of enthusiasm for studies of extraterrestrial life rose to a new high in the early 1960s, NASA created an office of Exobiology to coordinate the various programs seeking alien life. Not every scientist was pleased. Harvard biologist George Gaylord Simpson (1902–1984) commented dryly that this was the first time a scientific discipline had been started before any evidence of its subject matter had been found. For most academic scientists, the search for extraterrestrial life was still slightly less than respectable.
But with energetic pushing from Sagan and a host of NASA scientists and engineers less well-known to the public, the United States sent a pair of Viking spacecraft to Mars in 1976. Each of the two Viking craft included a lander specifically designed to look for life on the surface of Mars.
They didn’t find any.
EBB TIDE FOR EXOBIOLOGY
The general public, and the politicians in Washington, were hugely disappointed by Viking’s apparent failure. The tide began to turn once more against the search for life on other worlds.
By the early 1990s, radio astronomers had been using their giant antennas to search for signals from intelligent civilizations among the stars for more than thirty years. No such signals had been found. Although this effort had hardly begun to scratch the surface of the task of locating extraterrestrials, as far as the general public was concerned, if alien civilizations existed, the ETs were not signaling us. Not by radio, at least.
NASA had sent spacecraft probes to eve
ry planet in the solar system, except for distant Pluto. Most of them were “flyby” missions, such as the Pioneer and Voyager spacecraft that briefly visited the outer worlds of Jupiter, Saturn, Uranus, and Neptune. The Russians had landed several craft on the planet Venus, and in the American bicentennial year of 1976, NASA landed its two Viking vehicles on the rust-red surface of Mars.
No evidence of life was found by any of these spacecraft. No Martians. Not even a bacterium. Although scientists were learning more about the nature of the planets in our solar system than they had been able to glean in all the centuries preceding, to the public—and Congress—the space missions had failed to find life, and that meant they were failures.
VIKING AND MARS
The Viking landers were an especially bitter disappointment. They were outfitted with automated biochemistry experiments to search for life in the Martian soil.4 While the equipment produced results that were totally unexpected (see Chapter 13), those results did not provide evidence for Martian life. Indeed, another instrument designed to measure organic chemicals in the Martian soil found none at all. The Viking landers seemed to be telling the unhappy scientists on Earth that Mars was barren.
Prospects for finding life elsewhere in the solar system looked equally bleak by 1990.
“MAGNIFICENT DESOLATION”
Earth’s Moon was known to be a barren ball of rock, airless and waterless. No life was expected to be found there. In the words of Apollo 11 astronaut Buzz Aldrin, the Moon was a place of “magnificent desolation.” Robotic probes and, later, six teams of Apollo astronauts confirmed those gloomy expectations. The rock samples returned from the Moon by the astronauts were completely anhydrous: not a molecule of water was found in them.
Liquid water is a requirement for all life on Earth. While no one expected to find liquid water on the Moon, a few maverick enthusiasts contended that water ice might exist in “cold traps,” areas that are always shadowed from the Sun’s rays. They reasoned that comets must have crashed on the Moon since time immemorial. Comets are mostly water ice. If some of them crashed in spots that are always shaded from sunlight and thus kept well below freezing, their ice may still exist on the Moon.
In 1994, a small probe launched by the U.S. Department of Defense, Clementine, detected evidence of water ice in permanently shaded craters at the lunar south pole. Three years later, NASA’s more sophisticated Lunar Prospector found evidence for ice at both the north and south poles of the Moon. Still, the lack of liquid water makes the Moon an unlikely abode for life, although the presence of ice means that a supply of water might be made available for human explorers and settlers.
THE OTHER PLANETS
As we will see in subsequent chapters, no firm evidence of life has been detected by any of the probes sent to the other planets of our solar system. Venus is a hellhole of a world, the victim of a runaway greenhouse effect, its atmosphere a thick stew of choking carbon dioxide; the clouds that perpetually cover our “sister world” are composed of sulfuric acid.
The closest planet to the Sun, Mercury, is as airless and waterless as the Moon and much hotter.
Jupiter, Saturn, Uranus, and Neptune are completely unlike the rocky inner worlds. They are called “gas giants” because they are composed primarily of elements that are gaseous on Earth, such as hydrogen, helium, methane, and ammonia. On those giant worlds, however, these gases are compressed into liquids and even solids.
The larger moons of Jupiter and Saturn might tell a different story, however. We will look in detail at the possibilities for life existing on them in Chapter 15.
Pluto is so far from the Sun that no one considers it a likely abode for life. “Springtime” on Pluto sees the temperature rise to about -250°C.
THE “GIGGLE FACTOR”
With all these pessimistic results dogging the search for life, some politicians began to use the “giggle factor” to denigrate the search for extraterrestrial life. Senator William Proxmire of Wisconsin was particularly adept at poking fun at scientists and condemning NASA for spending taxpayers’ money searching for “little green men from Mars.” Eventually, Congress ordered NASA to stop all funding for SETI, making it increasingly difficult to get funds for new spacecraft probes of the planets.
Ironically, these political body blows were coming at a time when totally new ideas were starting to dawn in the field then known as exobiology. New discoveries were about to revolutionize old concepts. But these new discoveries did not come from astronomers or space scientists.
They came, for the most part, from biologists and geologists.
3
The Three Requirements of Life
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 . . .
—Pete Conrad
Apollo 12 Astronaut
ALTHOUGH BY 1990 not a shred of evidence for life had been found anywhere beyond the Earth, the biologists who interested themselves in the possibilities of extraterrestrial life had deduced that life needs three fundamental ingredients:
A building-block molecule
A medium in which chemical reactions can take place
Energy
THE BUILDING BLOCK
All life on Earth is built around long-chain carbon molecules. While other elements can form molecules of two or three atoms, such as molecular oxygen (O2) or water (H2O), the carbon atom has the almost unique capability of linking together with other carbon atoms to form long chains that chemists call organic molecules. Carbon also links easily with atoms of other elements, such as oxygen, hydrogen, nitrogen, etc. You and I are made of long-chain carbon-based molecules, some of them containing thousands of atoms. So is every living creature on Earth.
Carbon is versatile. Carbon atoms can join together to form ladders, rings, tubes, spirals, and springs, as well as chains of different lengths. Carbon-based molecules can absorb light or emit light by fluorescence; they can be endothermic (heat-absorbing) or exothermic (heat-releasing). Carbon atoms can even come together in congregations of sixty or more to form buckyballs, spheres that remind chemists of the geodesic domes designed by Buckminster Fuller. Materials made of spherical C60 molecules have been dubbed buckminsterfullerene.
No other element performs the way carbon does. While it may be possible for atoms of silicon to form long-chain molecules under certain circumstances, carbon atoms link into long chains under a much wider set of conditions. Organic chemistry, the chemistry of life, is all about carbon-chain molecules. Since carbon is also one of the most abundant elements in the universe (see Appendix 3), biologists and astronomers interested in extraterrestrial life have concluded that carbon-based life should be the most common form everywhere, and carbon-based life is what we should look for.
They reason that they understand (imperfectly, they admit) carbon-based life-forms. We know next to nothing about possible life-forms based on other molecular building blocks, which makes it extremely difficult to determine what we should look for in the way of possible non-carbon-based life on other worlds. Besides, what we do know about alternatives to carbon is not encouraging.
For example, silicon can form long chains, particularly at temperatures so high they would destroy carbon-chain molecules. But while silicon is the seventh most abundant element in the universe, carbon is almost ten times more plentiful. Despite its abundance, silicon is almost always found in combination with oxygen, as silicon dioxide or sili-cates: in meteorites, in comets, in interstellar clouds, even on Earth. In fact, silicon-oxygen compounds are the basis for rock-forming min-erals. Silanes—compounds of silicon and hydrogen—burst into flame spontaneously in air and are decomposed by water. No evidence has been found anywhere of a silicon version of hydrocarbons, long-chain molecules of silicon and hydrogen.
In oxygen-breathing, carbon-based organisms (such as us), carbon and oxygen combine to form carbon dioxide, which is gaseous and easily exhaled. Photosynthetic organisms take in the ga
seous CO2 as their primary feedstock. A silicon-based, oxygen-breathing creature, however, would form silicon dioxide as the waste product of its respiration. Silicon dioxide is a solid, even at high temperatures; its crystals would be much more difficult for an organism to get rid of than gaseous carbon dioxide.
Carbon is abundant, and we know that it works. Astrobiologists are looking for carbon-based life.
THE MEDIUM
The second requirement for life is a medium, or solvent, in which the building-block molecules can undertake chemical interactions. Water seems perfect for the job. Almost any chemical element found in living organisms dissolves easily in water, so that a huge variety of chemical reactions can take place in water. Water is really amazing stuff. You can see this in the everyday world. In our household, I am the dishwasher, and I’m constantly impressed with the way water can dissolve almost anything given time. The hardest-stuck grease or baked-on sugar can literally be floated away if you leave the pan in water overnight.
Water also has another virtually unique attribute: Its solid form (ice) is less dense than its liquid form. Water ice floats on liquid water. For most other substances, the solid form is invariably denser than the liquid and sinks in it. Not water. Large bodies of water do not freeze solid down to the bottom; their surfaces freeze, but the water beneath the ice coating remains liquid, protected from freezing temperatures by the cap of ice floating on the surface. Thus life, which may have begun in the water, was not frozen out of existence even during the many bitter, long Ice Ages that have periodically enveloped the Earth. Water remained liquid, despite the frigid temperatures of the air.