by Ben Bova
Ground-based and orbital telescopes could detect asteroids (or comets, for that matter) that are on a collision course with Earth months or even years before there is any danger. Spacecraft could be sent to the intruder with propulsion equipment that can change the asteroid’s or comet’s trajectory enough to make it miss Earth. Some have suggested using nuclear bombs to shatter the incoming body, but the result could well be a devastating meteor shower of fragments bombarding our planet. Better to divert the danger than to multiply it.
COMETS AND TNOs/KBOs
Comets appear in our skies fairly often. In ancient times the appearance of a comet was regarded with superstitious dread. Hanging in the sky night after night like a pointing, accusing finger, comets were associated with death and catastrophe.
Comets are actually kilometers-wide icebergs laced with carbon-rich dust. As they come closer to the Sun they begin to heat up; their ices start to boil away and they develop the long tails of gases and dust that in Latin were called coma, meaning “hair.”
No matter which direction the “hairy star” is moving, a comet’s tail always points away from the Sun, its gases and dust driven off the main body of the comet by the ethereal but very real solar wind flowing out of the Sun.
Comets have crashed into the Earth, as well as asteroids. Being composed mainly of water ice, they usually explode into steam before they reach the ground as they are heated by their plunge through our atmosphere. It was probably a comet that produced the spectacular fireball of June 30, 1908, near Tunguska, in western Siberia. The “falling star” was seen across much of Europe as it flashed through the sky, exploding in the remote Siberian forest with a force estimated to equal a ten-megaton blast. Had it entered the atmosphere a few minutes later, it could have exploded over a densely populated European city and caused immense damage. Although trees were flattened over an area of more than 2,000 square kilometers, there was no crater at “ground zero,” which reinforces the belief that the object was an icy comet and not a metallic or stony asteroid.
Recent calculations have estimated that the Earth is hit by a Tunguska-sized impact (ten megatons) on an average of once every thousand years.
Far out in the cold and darkness at the edge of our solar system, beyond the orbit of Neptune, lies the region where comets abound. (We will discuss this region in more detail in Chapter 16.) Millions of vast, silent icebergs have been there since the solar system first came into being. Astronomers call them, prosaicly, Trans-Neptunian Objects (TNOs). They are also called Kuiper Belt Objects (KBOs), since the region beyond Neptune is populated by a swath of icy cometary bodies called the Kuiper Belt. The first steps in the chemistry of life may well have taken place in those icy bodies.
Every now and then some gravitational disturbance nudges one of those primeval icebergs into a trajectory that sends it toward the inner reaches of the solar system. Feeling the warmth of the Sun, it begins to boil off some of its icy body and we on Earth see a comet hanging in our night sky.
Some comets, like the famous Halley, return to Earth’s vicinity on a regular orbit. Others pass through the inner solar system once and are never seen again. Still others have whipped around the Sun so many times that their ice has completely boiled away—all that are left of them are the grains of dust and pebbles that were once imbedded in the ice. These continue faithfully on the old comet’s orbit, and when they happen to intersect the Earth’s position in space, we see a meteor shower: hundreds, even thousands, of “shooting stars” hurtle through the sky, a heavenly spectacle that awes and delights watchers.
As we will see in the next chapter, comet impacts may well have brought most of the water we have on Earth. Some scientists have suggested that comets and asteroids may have carried biological materials to Earth, organic molecules that “seeded” our planet and began the processes that led to life arising here. A few thinkers, such as the British cosmologist Fred Hoyle (1915–2001), have even suggested that comets carry living organisms on them, which have been deposited on Earth.
Was life itself carried to Earth by comets and asteroids?
9
The Birth of Life
The evolutionary route that led to life [in the universe] seems to have taken the way with the fewest obstacles and chosen the most abundant construction materials available.
—Armand Delsemme
Our Cosmic Origins
THE SEARCH FOR EXTRATERRESTRIAL LIFE is intimately tied to the question of how life began on Earth. Did life arise on our world as a consequence of natural forces, or was life specially created on our one world alone? If the origin of life can be explained as a natural occurrence, then there is the possibility that life has arisen on other worlds. If life is the result of a special creation, however, then the search for extraterrestrial life may well be futile.
LIFE FROM NONLIFE?
If life is a natural phenomenon, then how did it start? How could living organisms spring up from nonliving matter? In earlier centuries this was known as the problem of spontaneous generation. Does life arise spontaneously out of materials that are not alive?
In the Middle Ages, spontaneous generation was accepted as an observable fact. Dead animals spontaneously generate maggots. Piles of hay lead to mice. Life obviously arose from nonlife.
It wasn’t until 1665 that the Italian physician Francesco Redi (1626–1697) tested the idea with an experiment. He put out a slab of meat and covered it with a fine gauze. Flies were attracted to the rotting meat but could not get through the protective gauze. The meat putrified, but no maggots appeared. Redi showed that maggots are not spontaneously generated but are the larval form of flies, hatched from eggs that the flies deposit in decaying meat.
Some ten years later the Dutch inventor of the microscope, Anton van Leeuwenhoek (1632–1723) discovered that droplets of water, human blood, and everything else he could examine were permeated with microscopic “animalcules.” Leeuwenhoek had discovered the bacteria that swarm unseen all around us (and even inside us).
In 1861, the great French chemist Louis Pasteur (1822–1895) put the final nail in the coffin of the spontaneous generation concept. In a simple yet elegant experiment, Pasteur boiled a meat extract and left the resulting broth exposed to the air in a goosenecked glass flask. Air could reach the sterilized broth, but the dust particles that carry yeast, bacteria, and other microbes could not get past the flask’s double-curved neck. The dust particles—and their passengers—settled in the bottom curve of the flask’s neck and never reached the highly nutritious broth.
The broth remained sterile. “Life comes from life” was the conclusion that Pasteur came to. Spontaneous generation does not occur. He was a religious man and believed that his experiment showed that life can only be explained as the special act of the Creator. This scientific evidence, backed by Pasteur’s enormous reputation, seemed to show quite conclusively that life cannot arise from nonliving matter.
Pasteur even examined the interior of a meteorite that had recently fallen near the village of Orgueil, France. Taking special precautions to make certain that the contents of the meteorite were studied under sterile conditions, he found that there were no discernable organisms inside the meteorite.
Although it took almost another century to realize it, what Pasteur had actually proved was that life does not arise from nonlife today. In the Earth of some 4 billion years ago, conditions were very different. Astrobiologists now believe that long before our world even began to form, the first steps leading to life on Earth were taking place in interstellar space. The ingredients of life were already present in the ice-covered dust grains of the original interstellar cloud that eventually became our solar system.
Nature is lazy. Our solar system was formed out of the elements that happened to be available in interstellar space—the debris of aged stars that exploded and spewed their atoms into the void. The forces of gravity, angular momentum, and chemistry produced our planet Earth, together with the rest of the solar system. Life
on Earth was likewise produced by those simple forces, working upon the building materials that existed in the cloud of gas and dust that gave birth to our Sun and solar system. The processes that led to life on Earth used the most abundant ingredients available and the easiest pathways for putting those ingredients together.
Nature is lazy, but fascinatingly clever.
PREBIOTIC CHEMISTRY AND PAHs
In 1963, radio astronomers at MIT’s Lincoln Laboratory were startled to find that hydroxyl radicals existed in interstellar clouds of gas and dust. The hydroxyl radical, OH, is two-thirds of the water molecule. In quick succession, radio telescopes detected many other life-associated molecules in interstellar clouds, including water, ammonia (NH3), and organic chemicals such as formaldehyde (H2CO), hydrocyanic acid, and the more complex polycyclic aromatic hydrocarbons (PAHs).
This was a surprise because astronomers had previously assumed that the ultraviolet light and other hard radiation emitted by the stars in deep space would break up complex molecules. But they found that within the shelter of interstellar clouds, such molecules could safely exist. To date, more than a hundred molecular species have been detected in interstellar space.
The chemical precursors of life exist far out in space, embedded in those thick interstellar clouds of gas and dust that swirl through the spiral arms of the Milky Way galaxy. There is every reason to expect that much the same ingredients existed in the protostar cloud that produced our Sun and solar system.
Biologists interested in the origins of life on Earth began to talk of prebiotic chemistry: chemical reactions that could build up organic molecules that become the building blocks for living organisms.
Of particular interest to astrobiologists are the PAHs, which seem to be ubiquitous. They have been detected in the interstellar clouds, in the gas and dust tails streaming out of comets, and in the meteorites that have fallen to Earth—including meteorites that originated on Mars. They have even been detected in the light coming from other galaxies, far beyond the Milky Way.
As we have seen, PAHs might have played a key role in the formation of the Earth and other planets.
AMORPHOUS ICE
Meanwhile, in a laboratory crammed with equipment and reverberating with the chugging of pumps and the hum of electrical machinery, the first steps in the beginning of life are being duplicated inside a stainless-steel chamber no bigger than a shoebox.
In that small chamber, Louis J. Allamandola and his colleagues at the Ames Research Center simulate the conditions of hard vacuum and near-absolute-zero temperature found in the clouds of gas and dust particles that exist deep in interstellar space. Out of such a cloud was born our solar system, our world.
Inside that shoebox-sized chamber, the NASA scientists create the type of ice grains seen by astronomers in the interstellar clouds. The ice is mostly frozen water, together with up to 10 percent of simple molecules such as carbon dioxide, carbon monoxide, methane, methanol, and ammonia.
When the ice grains are exposed to ultraviolet light, as they are in deep space, these molecules break apart. This is expected, and for generations astronomers and biologists had assumed that not even the earliest steps in the prebiotic chemistry of life could take place in space because the ultraviolet radiation from the stars would break up even the simplest molecules.
That was a wrong assumption.
Because these chemicals are trapped inside the ice grain, they cannot waft away into space. They remain in the ice. And they begin to recombine into new molecular forms. Complex chemical compounds arise, including ethers, alcohols, PAHs, and even amino acids. Amino acids are the building blocks of proteins; proteins are what living creatures are made of.
But how can chemistry take place in ice? If you sprinkled an ice cube with carbon, nitrogen, etc. and kept it in a freezer, no such chemical reactions would occur. The ice is a solid, a crystalline lattice; the atoms cannot move around in it and recombine into more complex compounds.
David Drake and Peter Jenniskens, also at Ames, have shown that the ice grains in those interstellar clouds are not the same as normal ice. The interstellar ice has a different structure. It is called amorphous ice. Amorphous ice has been known since 1935. It is formed when water vapor is deposited slowly on a substrate (such as grains of silicate dust) in a vacuum.12 The ice-covered dust grains in space are made of amorphous ice.
Amorphous ice is not a crystalline lattice; its internal structure is not rigid. Instead, it is somewhat like the makeup of glass, which is structurally a fluid that happens to remain solid at normal room temperature. When amorphous ice is exposed to the intense ultraviolet radiation coming from the stars in deep space, it can flow like a liquid, even though it remains a bare few degrees above absolute zero. Atoms can mix and mingle in amorphous ice. The chemistry of life can begin in the dust grains of interstellar clouds, grains that are sheathed in amorphous ice.
Not far down the California coast from Allamandola’s ice lab, chemist David W. Deamer of the University of California, Santa Cruz, found that some of the molecules in the Ames cloud chamber apparatus form microscopic droplets, mini-capsules similar to those found in meteorites that carry organic materials to Earth. The membranes that encapsulate these droplets are similar to the membranes that surround living cells. Moreover, within the droplets is a photoluminescent chemical capable of absorbing light and converting it to chemical energy. The chemical’s molecular structure is similar to that of chlorophyll, the light-absorbing molecule in the heart of every green plant.
These experiments show that the first steps toward life were possibly made in interstellar space, even before the Earth and our solar system were formed.
The same steps may have been taken countless times, whenever one of the Milky Way’s 100 billion or more stars was formed. Those steps may be taking place now, deep in interstellar space, as new stars and planets are being created out of clouds of gas and dust.
PREBIOTIC CHEMISTRY AND PANSPERMIA
The Swedish Nobel laureate Svante Arrhenius (1859–1927) proposed in 1908 that life originated elsewhere in the universe and was wafted as spores through the vast interstellar distances to arrive eventually here on Earth. This panspermia hypothesis envisions life pervading the universe, but it does not tell us how life began.
In 1961, Juan Oró of the University of Houston revived the idea that life on Earth began in space. By the 1990s, observations of comets showed that they bore organic chemicals. And we have seen that radio telescopes have detected more than 100 types of organic molecules in interstellar clouds, including complex compounds such as polycyclic aromatic hydrocarbons (PAHs).
The work of Allamandola and his colleagues at NASA’s Ames Research Center has shown how the first steps in organic chemistry can take place in the ice grains of interstellar clouds. Even amino acids, the building blocks of proteins, have been produced in Allamandola’s laboratory and, separately, by an international team in Europe.
When the dust-laden interstellar cloud began to collapse and begin the processes that would create our solar system, the amorphous ice that coated the cloud’s dust particles already contained organic molecules.
In the violent “heavy bombardment” period when the planets were being built through accretion, the intense collisions between the growing planetesimals must have destroyed much of that organic material in their high-temperature smash-ups. But out on the fringes of the solar system some of the original ice-covered particles eventually became comets, hovering far enough from the Sun to remain frozen. They held a treasure trove of organic chemicals within them. Now and then, gravitational perturbations nudged one of these icebergs inward, toward the fledgling planets hugging the Sun. Those comets brought the chemicals of life with them.
Even today we see organic chemicals flowing into space from the tails of passing comets, chemicals that have existed in those giant chunks of ice since before the beginning of our solar system.
Prebiotic chemistry very likely began in space, in amorp
hous ice.
“ORGANIZED ELEMENTS” IN METEORITES
The first suspicion that organic chemicals existed in space came as early as 1834, when the Swedish chemist Jöns Jakob Berzelius (1779–1848) discovered carbon in some stony meteorites. Meteorites, remember, are the remains of asteroids that have fallen to Earth.
By the 1930s astronomers realized that a certain type of meteorite, dubbed the carbonaceous chondrites, was very ancient and contained both carbon compounds and water—not liquid water, but water in the form of hydrates, where the H2O molecules are chemically linked to the molecules of stone. This type of meteorite (asteroid) is composed of tiny spheroids of stone, which are called chondrules. They apparently were formed very early in the solar system’s history and have not changed structurally since the era of accretion. The Earth undoubtedly began as a collection of chondrules but grew to such a size that the little spheroids were crushed and melted. In the chondritic meteorites, the original chondrules are unmelted and therefore still present.
These meteorites, then, offered scientists a priceless glimpse into the earliest times of the solar system’s history.
In 1960, the American Nobel laureate Melvin Calvin (1911–1997) and Susan Vaughn announced that they had found “complex organic materials, some of them apparently uniquely pertinent to life processes” in a carbonaceous chondritic meteorite that had fallen near Murray, Kentucky, in 1950.
The following year, a team led by the Hungarian-born geochemist Bartholomew Nagy of Fordham University reported finding “biogenic” hydrocarbons in the Orgueil meteorite, which had fallen in France in 1864. Moreover, when examined under polarized light, the Orgueil hydrocarbons showed a right-handed spiral structure. Virtually all terrestrial organic molecules spiral to the left. The hydrocarbons in Orgueil were not of Earthly origin.