Origins: Fourteen Billion Years of Cosmic Evolution (16 page)

For the moons that orbit the giant planets, this scenario seems to work quite well. All four giant planets have families of satellites that range in size from the large or extremely large (up to the size of Mercury) down to the small or even minuscule. The smallest of these moons, less than a mile across, may again be naked planetesimals, deprived of any further collisional growth by the presence of nearby objects that had already grown much larger. In each of these four families of satellites, almost all of the larger moons orbit the planet in the same direction and in nearly the same plane. We can hardly refrain from explaining this result with the same cause that made the planets orbit in the same direction and nearly the same plane: Around each planet, a rotating cloud of gas and dust produced clumps of matter, which grew to planetesimal and then to moon sizes.

In the inner solar system, only our Earth has a sizable moon. Mercury and Venus have none, while Mars’ two potato-shaped moons, Phobos and Deimos, each span only a few miles, and should therefore represent the earliest stages of forming larger objects from planetesimals. Some theories assign the origin of these moons to the asteroid belt, with their present orbits around Mars the result of Mars’ gravitational success in capturing these two former asteroids.

And what of our Moon, more than two thousand miles in diameter, surpassed in size only by Titan, Ganymede, Triton, and Callisto (and effectively tied with Io and Europa) among all the moons of the solar system? Did the Moon also grow from planetesimal collisions, as the four inner planets did?

This seemed quite a reasonable supposition until humans brought lunar rocks back to Earth for detailed examination. More than three decades ago, the chemical composition of the rock samples returned by the
Apollo
missions imposed two conclusions, one on either side of the possibilities for the Moon’s origin. On the one hand, the composition of these Moon rocks resembles that of rocks on Earth so closely that the hypothesis that our satellite formed entirely apart from us no longer seems tenable. On the other hand, the Moon’s composition differs from Earth’s just enough to prove that the Moon did not entirely form from terrestrial material. But if the Moon did not form apart from Earth, and was not made from Earth, how did it form?

The current answer to this conundrum, amazing though it may seem on its surface, builds upon a once popular hypothesis that the Moon formed as the result of a giant impact, early in the history of the solar system, that scooped material out of the Pacific Basin and flung it into space, where it coalesced to form our satellite. Under the new view, which has already gained wide acceptance as the best available explanation, the Moon
did
form as the result of a giant object that struck Earth, but the object striking Earth was so large—about the size of Mars—that it naturally added some of its material to the matter ejected from Earth. Much of the material thrown into space by the force of the impact might have vanished from our immediate vicinity, but enough remained behind to coagulate into our familiar Moon, made of Earth plus foreign matter. All of this occurred 4.5 billion years ago, during the first 100 million years after the formation of the planets began.

If a Mars-sized object struck Earth in that bygone era, where is it today? The impact could hardly have knocked the object into pieces so small that we cannot observe them: our finest telescopes can find objects in the inner solar system as small as the planetesimals that built the planets. The answer to this objection takes us to a new picture of the early solar system, one that emphasizes its violent, collisional nature. The fact that planetesimals built a Mars-sized object, for example, did not guarantee that this object would endure for long. Not only did this object collide with Earth, but the good-sized pieces produced by that collision would also have continued to collide with Earth and the other inner planets, with each other, and with the Moon (once it had formed). In other words, collisional terror reigned over the inner solar system during its first several hundred million years, and the pieces of giant objects that struck the planets as they formed themselves became part of these planets. The Mars-sized object’s impact on Earth merely ranked among the largest in a rain of bombardment, an epoch of destruction that brought planetesimals and much larger objects crashing down on Earth and its neighbors.

Seen from another perspective, this death-dealing bombardment simply marked the formation process’s final stages. The process culminated in the solar system we see today, little changed during 4 billion years and more: one ordinary star, orbited by eight planets (plus icy Pluto, more akin to a giant comet than to a planet), hundreds of thousands of asteroids, trillions of meteoroids (smaller fragments that strike Earth by the thousands every day), and trillions of comets—dirty snowballs that formed at dozens of times Earth’s distance from the Sun. We must not forget the planets’ satellites, which have moved, with few exceptions, in orbits with long-term stability ever since their birth, 4.6 billion years ago. Let’s take a closer look at the debris that continues to orbit our Sun, capable both of bringing forth life and of destroying life on the worlds such as ours.

CHAPTER 12

Between the Planets

F
rom a distance, our solar system looks empty. If you enclosed it within a sphere large enough to contain the orbit of Neptune, then the Sun, together with all its planets and their moons, would occupy little more than one trillionth of all the space in that sphere. This result, however, assumes that interplanetary space is essentially empty. Viewed close up, however, the spaces between the planets turn out to contain all manner of chunky rocks, pebbles, ice balls, dust, streams of charged particles, and far-flung man-made probes. Interplanetary space is also permeated by immensely powerful gravitational and magnetic fields, invisible but nonetheless quite capable of affecting the objects in our neighborhood. These small objects and cosmic force fields present a serious ongoing threat to anyone who attempts to travel from place to place in the solar system. The largest of these objects likewise pose a threat to life on Earth, if they happen—as they certainly do on rare occasion—to collide with our planet at speeds of many miles per second.

Local regions of space are so not-empty that Earth, during its 30-kilometer-per-second orbital journey around the Sun, plows into hundreds of tons of interplanetary debris per day—most of it no larger than a grain of sand. Nearly all of this matter burns in Earth’s upper atmosphere, slamming into the air with so much energy that the incoming particles vaporize. Our frail species evolved beneath this protective blanket of air. Larger, golf-ball-size pieces of debris heat rapidly but unevenly, and often shatter into many smaller pieces before they vaporize. Still larger pieces singe their surfaces but otherwise make their way, at least in part, down to the ground. You might think that by now, after 4.6 billion trips around the Sun, Earth would have “vacuumed” up all possible debris in its orbital path. We have made progress in this direction: things were once much worse. During the first half billion years after the formation of the Sun and its planets, so much junk rained down on Earth that the impact energy generated a strongly heated atmosphere and a sterilized surface.

In particular, one hunk of space junk was so substantial that it led to the formation of the Moon. The unexpected paucity of iron and other high-mass elements in the Moon, deduced from the lunar samples that the
Apollo
astronauts brought to Earth, indicates that the Moon most likely consists of matter spewn from Earth’s relatively iron-poor crust and mantle by a glancing collision with a wayward, Mars-sized protoplanet. Some of the orbiting flotsam from this encounter coalesced to form our lovely, low-density satellite. Apart from this newsworthy event about 4.5 billion years ago, the period of heavy bombardment that Earth endured during its infancy was similar to that experienced by all the planets and other large objects in the solar system. They each sustained similar damage, with the airless, erosionless Moon and Mercury still preserving most of the craters produced during this period.

In addition to the flotsam left from its epoch of formation, interplanetary space also contains rocks of all sizes thrust from Mars, the Moon, and probably Earth as their surfaces reeled from high-energy impacts. Computer studies of meteor strikes demonstrate conclusively that some surface rocks near ground zero will be thrown upward with enough speed to escape the object’s gravitational tether. From discoveries of Martian meteorites on Earth, we can conclude that about 1,000 tons of rocks from Mars rain down on Earth each year. Perhaps the same amount of debris reaches Earth from the Moon. Thus we did not have to go to the Moon to retrieve Moon rocks. A few dozen of them have come to us on Earth, although they are not of our choosing, and we had not yet learned this fact during the
Apollo
program.

If Mars ever harbored life—most likely billions of years ago when liquid water flowed freely on the Martian surface—then unsuspecting bacteria, stowed away in the nooks and crannies (especially in the crannies) of the rock ejected from Mars, could have traveled to Earth for free. We already know that some varieties of bacteria can survive long periods of hibernation, as well as high doses of the solar ionizing radiation to which they would be exposed en route to Earth. The existence of space-borne bacteria is neither a crazy idea nor pure science fiction. The concept even has an important-sounding name: panspermia. If Mars spawned life before Earth did, and if simple life traveled from Mars on ejected rocks and seeded Earth, we may all be descendants of Martians. This fact might seem to obviate environmental concerns over astronauts who sneeze on the Martian surface, spreading their germs on the alien landscape. In reality, even if we are all Martian in origin, we would dearly like to trace life’s trajectory from Mars to Earth, so these concerns retain vital importance.

Most of the solar system’s asteroids live and work in the “main belt,” a flattened region around the Sun between the orbits of Mars and Jupiter. By tradition, asteroid discoverers get to name their objects as they choose. Often pictured by artists as a cluttered region of rocks floating in the plane of the solar system, though in fact spread out over millions of miles at different distances from the Sun, the objects in the asteroid belt have a total mass less than 5 percent of the Moon’s, which itself has barely more than 1 percent of Earth’s mass. Sounds insignificant at first, but the asteroids quietly pose a long-term cosmic threat to our planet. Accumulated perturbations of their orbits continually create a deadly subset of asteroids, perhaps a few thousand in number, whose elongated paths carry them so close to the Sun that they intersect the orbit of Earth, creating the possibility of collision. A back-of-the-envelope calculation demonstrates that most of these Earth-crossing asteroids will strike Earth within a few hundred million years. The objects larger than about a mile across carry enough energy to destabilize Earth’s ecosystem and to put most of Earth’s land species at risk of extinction. That would be bad.

Meanwhile, asteroids are not the only space objects that pose a risk to life on Earth. The Dutch astronomer Jan Oort first recognized that within the cold depths of interstellar space, much farther from the Sun than any planet, a host of frozen leftovers from the solar system’s earliest stages of formation still orbit our star. This “Oort cloud” of trillions of comets extends to distances halfway to the closest stars, thousands of times larger than the size of the Sun’s planetary system.

Oort’s Dutch-American contemporary Gerard Kuiper proposed that some of these frozen objects once formed part of the disk of material from which the planets formed, and now orbit the Sun at distances considerably greater than Neptune’s but much less than those of the comets in the Oort cloud. Collectively, they compose what astronomers now call the Kuiper Belt, a comet-strewn swath of circular real estate that begins just beyond the orbit of Neptune, includes Pluto, and extends several times as far again outward from Neptune as Neptune’s distance from the Sun. The most distant known object in the Kuiper Belt, named Sedna after an Inuit goddess, has two-thirds of Pluto’s diameter. Without a nearby massive planet to perturb them, most of the Kuiper Belt comets will maintain their orbits for billions of years. As in the asteroid belt, a subset of the Kuiper Belt objects travel on eccentric orbits that cross the paths of other planets. The orbit of Pluto, which we may regard as an extremely large comet, as well as the orbits of an ensemble of Pluto’s small siblings, called Plutinos, cross Neptune’s path around the Sun. Other Kuiper Belt objects, perturbed from their usual large orbits, occasionally plunge all the way into the inner solar system, crossing planetary orbits with abandon. This subset includes Halley, the most famous comet of them all.

The Oort cloud is responsible for the long-period comets, those whose orbital periods far exceed a human lifetime. Unlike Kuiper Belt comets, Oort cloud comets can rain down on the inner solar system from any angle and from any direction. The brightest comet of the past three decades, comet Hyakutake (1996), came from the Oort cloud, high above the plane of the solar system, and will not return to our vicinity any time soon.

If we had eyes that could see magnetic fields, Jupiter would look ten times larger than the full Moon in the sky. Spacecraft that visit Jupiter must be designed to remain unaffected by this powerful magnetism. As the English chemist and physicist Michael Faraday discovered in 1831, if you move a wire across a magnetic field, you will generate a voltage difference along the wire’s length. For this reason, fast-moving metal space probes can have electrical currents induced within them. These currents interact with the local magnetic field in a way that retards the space probe’s motion. This effect might explain the mysterious slowing down of the two
Pioneer
spacecraft as they exit the solar system. Both
Pioneer
10
and
Pioneer
11
, launched during the 1970s, have not traveled quite so far into space as our dynamical models of their motions predict. After taking into account the effects of space dust encountered en route, along with recoils of the spacecraft arising from leaky fuel tanks, this concept of magnetic interaction—in this case with the Sun’s magnetic field—may provide the best explanation for the slowdown of the
Pioneers
.

Better detection methods and close-flying space probes have increased the number of known planetary moons so rapidly that counting moons has become almost obsolete: they seem to multiply as we speak. What matters now is whether any of these moons are fun places to visit or to study. By some measures, the solar system’s moons are far more fascinating than the planets they orbit. Mars’ two moons, Phobos and Deimos, appear (not with those names) in Jonathan Swift’s classic
Gulliver’s Travels
(1726). Problem is, these two small moons were not discovered until more than a hundred years later; unless he was telepathic, Swift was presumably interpolating between Earth’s single moon and Jupiter’s (then known) four.

Earth’s Moon has about 1/400 of the Sun’s diameter, but is also just about 1/400 as far from us as the Sun, giving the Sun and the Moon the same size on the sky—a coincidence not shared by any other planet-moon combination in the solar system, and one that grants earthlings uniquely photogenic total solar eclipses. Earth has also locked onto the Moon’s period of rotation, leaving the Moon’s rotation period equal to its period of revolution around Earth. The capture has arisen from Earth’s gravity, which exerts greater amounts of force on the denser parts of the Moon’s interior and makes them always face toward Earth. Wherever and whenever this happens, as it does for Jupiter’s four large moons, the locked moon shows only one face to its host planet.

Jupiter’s system of moons stunned astronomers when they obtained their first good look. Io, the large moon closest to Jupiter, has been tidally locked and structurally stressed by its gravitational interactions with Jupiter and with the other large moons. These interactions pump enough energy into Io (about the same size as our Moon) to melt some of its rocky interior, making Io the most volcanically active object in the solar system. Jupiter’s second large moon, Europa, has enough H
2
O that its internal heat, which arises from the same interactions that affect Io, has melted its subsurface ice, leaving a liquid ocean below an icy covering.

Close-up images of the surface of Miranda, one of Uranus’ moons, reveal badly mismatched patterns, as though the poor moon had been blown apart, and its pieces hastily glued back together. The origin of these exotic features remains a mystery, but may also be due to something simple, like the uneven upwelling of ice sheets.

Pluto’s lone moon, Charon, is so large and so close to Pluto that Pluto and Charon have tidally locked onto each other—both objects have rotation periods equal to their periods of revolution around their common center of mass. By convention, astronomers name planets’ moons after significant Greek personalities in the life of the god whose name the planet bears, though they use the Roman counterpart’s name for the planet itself (Jupiter rather than Zeus, for example). Because the classical gods led complicated social lives, no shortage of characters exists from which to draw names.

Sir William Herschel was the first person to discover a planet beyond those easily visible to the naked eye, and he was ready to name this new planet after the king who might support his research. Had Sir William succeeded, the planet list would read: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and George. Fortunately, clearer heads prevailed, so that some years later the new planet received the classical name Uranus. But Herschel’s original suggestion to name the planet’s moons after characters in William Shakespeare’s plays and Alexander Pope’s poem
The Rape of the Lock
remains the tradition to this day. Among Uranus’ seventeen moons we find Ariel, Cordelia, Desdemona, Juliet, Ophelia, Portia, Puck, and Umbriel, with two new moons, Caliban and Sycorax, discovered as recently as 1997.

The Sun loses
material from its surface at a rate of 200 million tons per second (which happens to closely match the rate at which water flows through the Amazon Basin). The Sun loses this mass in the “solar wind,” which consists of high-energy charged particles. Traveling up to 1,000 miles per second, these particles stream through interplanetary space, where they are often deflected by planetary magnetic fields. In response, these particles spiral down toward a planet’s north and south magnetic poles, colliding with atmospheric gas molecules to produce colorful auroral glows. The Hubble Space Telescope has spotted aurorae near the poles of both Saturn and Jupiter. On Earth, the aurorae borealis and australis (the Northern and Southern lights) serve as intermittent reminders of how sweet it is to have a protective atmosphere.

Earth’s atmosphere technically extends much farther above Earth’s surface than we generally conceive. Satellites in “low-Earth orbit” typically travel at altitudes of 100 to 400 miles and complete an orbit in about 90 minutes. Although no one can breathe at these altitudes, some atmospheric molecules remain—enough to drain orbital energy slowly from unsuspecting satellites. To combat this drag, satellites in low orbit require intermittent boosts, lest they fall back to Earth and burn up in the atmosphere. The most sensible way to define the edge of our atmosphere is to ask where the density of its gas molecules falls to the density of gas molecules in interplanetary space. With this definition, Earth’s atmosphere extends thousands of miles into space. Orbiting high above this level, 23,000 miles above Earth’s surface (one tenth of the distance to the Moon), are the communications satellites that carry news and views around Earth. At this special altitude, a satellite finds not only that Earth’s atmosphere is irrelevant but also that its speed in orbit, thanks to the diminished pull from Earth at this greater distance from our planet, falls to the point that it takes twenty-four hours to complete each revolution around our planet. Moving in orbits that precisely match Earth’s rotation rate, these satellites appear to “hover” above a single point on the Equator, a fact that makes them ideal for relaying signals from one part of Earth’s surface to another.