by Ben Bova
But before the world we know could come into being, our planet had to survive the violence of the solar system’s childhood.
SCARS FROM THE HEAVY BOMBARDMENT
For more than a billion years the solar system was filled with deadly collisions, as boulders crashed into mountains and planetesimals smashed into one another. Slowly, despite many shattering collisions, the planets grew.
We can see the violence that marked those early days on the battered face of our own Moon. The giant craters that pockmark the Moon were formed no later than 3 billion years ago, according to the evidence of the rocks brought back to Earth by the Apollo astronauts. Planetary astronomers call this time of massive violence (and planet-building) the “heavy bombardment” era. Scars of the heavy bombardment also mark the other worlds of our solar system. Mars, Mercury, and Venus all bear their craters. So do the moons of distant gas giant worlds.
Even on Earth, where wind and weather have erased most of the primordial craters, there are huge structures called astroblemes in Canada, Africa, and elsewhere. To date, more than 160 impact craters have been identified on Earth. Hudson’s Bay might be the result of a large meteor impact. The Ashanti Crater in Ghana certainly is. The famous Barringer Meteor Crater in Arizona is of much later origin; it was blasted out of the desert about 50,000 years ago by one of the chunks of rock or metal still floating through the solar system, leftover bits and pieces of the original creation of the worlds. Unless protected, the Arizona crater will be scrubbed away by weathering in a few million years or less.
DEATH-BRINGER: THE MOON
Our Moon was most likely created in those violent early years. The best available evidence indicates that the Earth was blasted by a huge planetesimal, about the size of Mars, more than 4 billion years ago. The collision was a grazing one, yet it blew an enormous amount of the Earth’s crustal rocks into orbit around our planet. The dust cloud raised by that collision must have blotted out the Sun for thousands, if not millions, of years. It created a mini–accretion disk around our world.
If life had already begun on Earth, that titanic collision and the Sun-blotting dust cloud that covered our planet would most likely have wiped out any living organisms. The birth of the Moon may well have sterilized the Earth, temporarily.
Gradually, the debris cloud hanging over the Earth coalesced into the Moon. Our natural satellite was much closer to our world when it formed, and the Earth’s spin rate—our day—was only a matter of a few hours. Over the course of some 4 billion years the Moon has slowly spiraled away from us, and Earth’s day has gradually increased to its present 23 hours, 57 minutes, and 4.09 seconds. The Moon’s spin has become gravitationally “locked” so that it turns on its axis in exactly the same time it takes to orbit our planet: 27 days, 7 hours, and 27 minutes. Thus the Moon always presents the same face to us.
But the solar system is a dynamic association of moving bodies, and the Moon is still creeping away from us at a rate of 3 centimeters per year. Eventually, billions of years from now, the Moon will reach the limit of its drift and begin coming closer to the Earth. At some point it will get so close that Earth’s gravity will break it up, and our world will have a ring of tiny particles orbiting around it, like Saturn and the other outer planets. Could life on Earth survive the Moon’s demise? Its breakup would shower the Earth with meteors and very likely our world will become as dead and pockmarked as our Moon is today, sterilized once again by our own Moon.
ROCKY WORLDS AND GAS GIANTS
The farther away you stand from a fireplace, the less heat you feel. That simple fact shaped the worlds of the solar system.
Close to the Sun, temperatures were too high for ices to remain frozen; they vaporized and most of their volatile gases were blown to the farther reaches of the solar system. The grains of metals and silicates remained, eventually to form the iron-rich cores of the rocky inner planets.
Earth formed in the heart of the Sun’s thermally habitable zone. The planets nearest to us, Venus and Mars, are on the fringes of the habitable zone. Yet, as we will see, they have developed in ways that are quite different from our world.
On Earth, free hydrogen does not exist in nature; hydrogen exists linked to heavier atoms, such as oxygen or carbon. Free hydrogen atoms float toward the top of the atmosphere and eventually escape into space. Helium, the second-lightest element, is very rare on Earth because it, too, floats up and boils away into interplanetary space.10
Our planet is literally too hot to retain light gases such as hydrogen, unless the hydrogen atoms are bound to heavier atoms such as oxygen or carbon to form water and methane. The rare pockets of helium that have been found trapped underground are the result of the breakdown of radioactive elements such as uranium and thorium.
Mercury, Venus, Earth, and Mars (in order of their distance from the Sun) are rocky worlds. They have metallic cores, mostly iron, sheathed in layers of silicate rock. Because of the enormous pressure of many, many gigatons of rock weighing on them, the lower layers of these planets’ rocky mantles and even the outer crusts can become quite hot, liquefied into red-hot magma, the stuff that volcanoes spew out. The heat from the decay of radioactive elements also adds to the interior heat of these planets.
Yet these four inner planets have rocky crusts. Earth is far enough from the Sun to have liquid water covering three-fifths of its surface. Mars is so cold that the only water on its surface is frozen in the polar caps. Venus and Mercury are far too hot to retain liquid water.
These four inner planets are called the terrestrial worlds because they somewhat resemble the Earth. As we will see in Chapters 12 through 14, the resemblance is superficial only; each of these planets is a unique place.
Farther from the Sun, temperatures were low enough for planetesimals to draw in massive amounts of the accretion disk’s gaseous hydrogen and helium. Thus were formed the gas giant worlds of Jupiter, Saturn, Uranus, and Neptune (again, in order of their distance from the Sun). Since hydrogen and helium were the most abundant elements in the accretion disk, they were plentiful enough to build very large worlds. As a result the gas giants are entirely different from the rocky inner planets. They are truly giants, ranging from about four to more than eleven times Earth’s size. Because they are composed mainly of the lightest elements, their densities are much lower than the densities of the terrestrial planets. Saturn, for example, would float on water, if anyone could construct a swimming pool ten times bigger than Earth!
Although Jupiter and the other gas giants are completely covered with clouds, measurements of Jupiter’s size and mass have clearly indicated that the planet is composed almost entirely of hydrogen and helium. While these are gases on Earth, under the titanic pressures within Jupiter they become liquid and even solid. Beneath all those layers of liquefied and solidified hydrogen, planetary astronomers deduce, is a rocky core about five to twenty times the mass of the Earth.
The giant planets have giant moons, plenty of them. Several of the moons of Jupiter and Saturn are prime targets in the astrobiologists’ search for life, because thermally habitable zones need not depend on the Sun’s heat. There are other possible sources of heat energy. Massive Jupiter is slowly contracting, and this generates heat. Measurements have shown that Jupiter is heated from within; the planet is warmer than it would be if sunlight were its only heat source.
The Galilean satellites of Jupiter are very likely heated by tidal flexing. As they revolve around Jupiter, the giant planet’s immense gravitational pull literally bends the moons out of shape, slightly, and that constant flexing generates frictional heating inside the moons.
There could also be thermally habitable zones inside Jupiter and its Galilean moons.
8
Nature’s Enforcers
Comets and meteoroids are nature’s enforcers: bringers of water and protocells, destroyers of species, enablers of intelligence, harbingers of change.
—Lynn Harper
NASA Ames Research Center
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bsp; IN TRYING TO PUZZLE out the origins of life on Earth, astrobiologists have come to understand that our planet did not come into existence in isolation. The Earth was born together with all the other components of our solar system, large and small. Moreover, the earliest steps toward the creation of life may well have taken place on the smaller bodies of asteroids and comets.
The creation of life on Earth depended at least in part on the materials, the energies, and the chemical processes brought to our planet by these smaller objects.
Even today the solar system still harbors millions upon millions of rocks and ice chunks, debris left over from the accretion disk from which the planets were built. The rocks are called asteroids; the icy objects are comets. In the solar system’s earliest days, comets and asteroids brought water and organic chemicals to Earth, possibly the precursors of living cells or even viable organisms themselves.
Although the solar system is calm and placid now compared to its violent early days, asteroids and comets still strike the Earth. On any clear, moonless night, you can see a meteor or two blazing across the sky. We call them meteors or “shooting stars,” but they are really the final funeral pyres of dust motes that have been wandering in space since the very beginning of our solar system. Every five minutes an asteroid about the size of a pea flashes through our atmosphere and burns up, trailing its bright streak. It has been estimated that some 40,000 tons of extraterrestrial material (mostly microscopic dust particles) bombard the Earth each year.
In 2002, an asteroid the size of a football field whizzed to within 119,229 kilometers of Earth, less than a third of the distance to the Moon. If it had struck, its impact would have released more energy than several megaton-sized hydrogen bombs.
Arizona’s 1.2-kilometer-wide Meteor Crater was caused by the impact of a nickel-iron asteroid some 50,000 years ago. A strike of that size could destroy a city; military experts worry that such an impact could be mistaken for a missile attack and trigger a nuclear war.
Much bigger strikes have peppered Earth’s history. Some 65 million years ago, an asteroid about 10 kilometers in diameter (roughly half the size of Manhattan) slammed into the Earth in the region now called Yucatán, which is part of Mexico. The explosion was like millions of hydrogen bombs going off together. The shock wave must have flattened everything in its path for millions of square kilometers. The dust cloud it raised blanketed the Earth for weeks, if not months. In that global darkness, plants could not get enough sunlight to make photosynthesis work. Whole species died away, driven into extinction in what paleontologists call “the time of great dying.” Fully one-third of all the life-forms on Earth—land, sea, and air—were wiped out, including all the dinosaurs. (For details, see Appendix 9.)
Such catastrophes have happened even earlier in Earth’s history. Massive waves of extinctions have dotted the course of life on our planet, going back hundreds of millions of years, as deep into the past as the fossil records in the rocks can tell us. Were each of these extinction events caused by the impact of an asteroid or comet? The evidence is unclear.
METEOROIDS, METEORS, AND METEORITES
Yet life is opportunistic and very tenacious. These extinction events kill off many whole species, but they also open the door to the rise of new life-forms. When the dinosaurs were annihilated, for example, suddenly every animal species larger than about 10 kilograms was erased from the Earth. Our world was abruptly depopulated; not only the huge earth-shakers such as Tyrannosaurus rex went extinct, but uncountable numbers of smaller dinosaurs and other land- and sea-dwelling creatures were wiped out.
Imagine all of today’s large mammals suddenly disappearing. Not only the elephants and bears and whales, but the horses, pigs, antelopes, lions, monkeys, apes—all the animals larger than about 10 kilograms abruptly gone, leaving the world to the mice and squirrels and smaller creatures. That is the depth of the catastrophe that struck the Earth when that asteroid hit.
Yet that disaster presented new possibilities to the species that survived. Enormous ecological niches were abruptly emptied and became available for any organisms that could take advantage of the opportunity. The birds and the mammals seized the moment: They multiplied and prospered, so much so that they now dominate the planet just as fully as the dinosaurs once did.
Human beings are mammals, of course. Could our species have come into being in a landscape ruled by dinosaurs? Did the asteroid that killed off the dinosaurs lead to the rise of intelligence on Earth?
Is there another asteroid out in the depths of space that will one day destroy our civilization and drive us into extinction?
MOUNTAINS FLOATING IN SPACE
Millions of asteroids—chunks of rock and metal—float silently, endlessly, through the deep emptiness of interplanetary space. They have been called “mountains floating freely in space.” Although a few of them are quite a bit larger than terrestrial mountains, most of them are more the size of pebbles or even grains of dust. The largest of them, Quaoar (discovered in 2002), is 1,200 kilometers wide, almost half the size of our Moon. It is more than 30 astronomical units from the Sun, out near the orbit of the planet Neptune.
To date, astronomers have catalogued more than 26,000 asteroids, and telescopic surveys indicate that there must be at least 700,000 asteroids larger than 1 kilometer across. They contain more metals and minerals, more natural resources, than the entire Earth can provide. There are hundreds of millions of billions of tons of high-grade ores in them. They hold enough real wealth, in terms of natural resources, to make each man, woman, and child of the entire human race into a millionaire many times over.
THE DISCOVERY
The first asteroid was discovered shortly after midnight on January 1, 1801, by a Sicilian monk who happened to be an astronomer. While others were celebrating the arrival of the new century, Giuseppe Piazzi was naming the tiny point of light he saw in his telescope Ceres, after the pagan goddess of Sicily. Perhaps a strange thing for a Catholic monk to do, but Piazzi was a Sicilian.
Technically, they are planetoids, little planets, chunks of rock and metal floating in the dark void of space, leftovers from the creation of the Sun and planets some 4.5 billion years ago. Piazzi correctly referred to them as planetoids, but in 1802 William Herschel (who had earlier discovered the giant planet Uranus) called them asteroids, because in the telescope their pinpoints of light looked like stars rather than the disks of planets. Piazzi was correct, but Herschel was far more famous and influential. We call them asteroids to this day.
Thousands of asteroids are in orbits that come close to the Earth, called Near-Earth Asteroids (NEAs). A recent survey by the Lincoln Near-Earth Asteroid Research program (LINEAR) has determined that there must be about 1,200 1-kilometer-long NEAs. If a 1-kilometer-long asteroid hit the Earth, the damage could be enough to destroy the fabric of civilization. Tens of millions might die in the blast and firestorms caused by the impact, and the rest from widespread starvation, disease, and other afteraffects of such a disaster.
By far the largest number of known asteroids, probably many millions in all, circle around the Sun in a broad swath in deep space between the orbits of Mars and giant Jupiter. This Asteroid Belt is centered more than 600 million kilometers from Earth, four times farther from the Sun than our home world. Disturbances caused by the gravitational pull of Jupiter and occasional collisions among these asteroids jostle some of them inward toward the Earth to eventually become NEAs or fall into Earth’s gravity well and hit our planet.
THE ASTEROID BELT
Although this region is called the Asteroid Belt, the asteroids are not strewn so thickly that they represent a hazard to space navigation. Far from it. The so-called Belt is a region of vast emptiness, dark and lonely and very far from human civilization. A half dozen space probes have sailed easily through the Asteroid Belt without coming anywhere near an asteroid. NASA, however, has sent probes to photograph several of them.
In February 2001, the NEAR Shoemaker11 probe not only photo
graphed the asteroid Eros, it landed gently on its rocky, rubble-covered surface. For seven days the spacecraft’s gamma-ray spectrometer examined the asteroid’s surface, showing that it has remained unaltered since the formation of the solar system. Eros and asteroids like it are museums of ancient history waiting to be studied in detail.
NEAR Shoemaker’s camera picked up features as small as 1.2 centimeters across. The asteroid is covered with a coating of fine dust, some of it filling the floors of craters and other depressions with “dust ponds.”
Once it was thought the Asteroid Belt might be the remains of a planet that broke apart, like the fictional planet Krypton, home of Superman. Soon enough, though, astronomers determined that the asteroids in the Belt do not contain enough mass to build a planet of even the size of Mars. More likely, the asteroids represent material that might have coalesced into one or more Earth-sized planets, but the gravitational disturbance caused by Jupiter’s nearby bulk not only prevented planet formation, but continually kicked asteroids out of the Belt, sending many of them spiraling in toward the Earth and others out of the solar system entirely.
SAFETY PATROL
Astrobiology is concerned with the future of life, as well as its past. It is inevitable that giant asteroids will strike our Earth again, with devastating results, unless we use our wits and our space technology to detect asteroids that may be on a collision course with Earth and divert them in time to avert the catastrophe. This is not an easy proposition. The orbits of the asteroids in the Belt are constantly being perturbed by the gravitational influence of Jupiter, so that a constant watch is necessary.
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.