按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
t not so fast…moving as to escape from the planet's gravity—may form； for a time； a new ring。 It's made of whatever the colliding bodies were made of； but usually more of the target moon than the rogue impactor。 If the colliding worlds are icy； the net result will be rings of ice particles； if they're made of organic molecules； the result will be rings of organic particles （which will slowly be processed by radiation into carbon）。 All the mass in the rings of Saturn is no more than 'would result from the plete impact pulverization of a single icy moon。 The disintegration of small moons can likewise account for the ring systems of the three other giant planets。
Unless it's very close to its planet； a shattered moon gradually reaccumulates （or at least a fair fraction of it does）。 The pieces； big and small； still in approximately the same orbit as the moon was before the impact； fall together helter…skelter。 What used to be a piece of the core is now at the surface； and vice versa。 The resulting hodgepodge surfaces might seem very odd。 Miranda； one of the moons of Uranus； looks disconcertingly jumbled and may have had such an origin。
The American planetary geologist Eugene Shoemaker proposes that many moons in the outer Solar System have been annihilated and reformed—not just once but several times each over the 4。5 billion years since the Sun and the planets condensed out of interstellar gas and dust。 The picture emerging from the Voyager reconnaissance of the outer Solar System is of worlds whose placid and lonely vigils are spasmodically troubled by interlopers from space； of world…shattering collisions； and of moons re…forming from debris； reconstituting themselves like phoenixes from their own ashes。
But a moon that lives very close to a planet cannot re…form if it is pulverized—the gravitational tides of the nearby planet prevent it。 The resulting debris； once spread out into a ring system； might be very long…lived—at least by the standard of a human lifetime。 Perhaps many of the small； inconspicuous moons now orbiting the giant planets will one day blossom forth into vast and lovely rings。
These ideas are supported by the appearance of a number of satellites in the Solar System。 Phobos； the inner moon of Mars； has a large crater named Stickney； Mimas； an inner moon of Saturn； has a big one named Herschel。 These craters—like those on our own Moon and； indeed； throughout the Solar System—are produced by collisions。 An interloper smashes into a bigger world and makes an immense explosion at the point of impact； A bowl…shaped crater is excavated； and the smaller impacting object is destroyed。 If the interlopers that dug out the Stickney and Herschel craters had been only a little larger； they would have had enough energy to blow Phobos and Mimas to bits。 These moons barely escaped the cosmic wrecking ball。 Many others did not。
Every time a world is smashed into； there's one less interloper—something like a demolition derby on the scale of the Solar System； a war of attrition。 The very fact that many such collisions have occurred means that the rogue worldlets have been largely used up。 Those on circular trajectories around the Sun； those that don't intersect the orbits of other worlds； will be unlikely to smash into a planet。 Those on highly elliptical trajectories； those that cross the orbits of other planets； Will sooner or later collide or； by a near miss； be gravitationally ejected from the Solar System。
The planets almost certainly accumulated from worldlets which in turn had condensed out of a great flat cloud of gas and dust Surrounding the Sun—the sort of cloud that can now be seen around young nearby stars。 So； in the early history of the solar System before collisions cleaned things up； there should have been many more worldlets than we see today。
Indeed； there is clear evidence for this in our own backyard： If we count up the interloper worldlets in our neighborhood in space； we can estimate how often they'll hit the Moon。 Let us make the very modest assumption that the population of interlopers has never been smaller than it is today。 We can then calculate how many craters there should be on the Moon。 The number we figure turns out to be much less than the number we see on the Moon's ravaged highlands。 The unexpected profusion of craters on the Moon speaks to us of an earlier epoch when the Solar System was in wild turmoil； churning with worlds on collision trajectories。 This makes good sense； because they formed from the aggregation of much smaller worldlets—which themselves had grown out of interstellar dust。 Four billion years ago； the lunar impacts were hundreds of times more frequent than they are today； and 4。5 billion years ago； when the planets were still inplete； collisions happened perhaps a billion times more often than in our becalmed epoch。
The chaos may have been relieved by much more flamboyant ring systems than grace the planets today。 If they had small moons in that time； the Earth； Mars； and the other small planets may also have been adorned with rings。
The most satisfactory explanation of the origin of our own Moon； based on its chemistry （as revealed by samples returned from the Apollo missions）； is that it was formed almost 4。5 billion years ago； when a world the size of Mars struck the Earth。 Much of our planet's rocky mantle was reduced to dust and hot gas and blasted into space。 Some of the debris； in o：…bit around the Earth； then gradually reaccumulated—atom by atom； boulder by boulder。 If that unknown impacting world had been only a little larger； the result would have been the obliteration of the Earth。 Perhaps there once were other worlds in our Solar System—perhaps even worlds on which life was stirring—hit by some demon worldlet； utterly demolished； and of which today we have not even an intimation。
The emerging picture of the early Solar System does not resemble a stately progression of events designed to form the Earth。 Instead； it looks as if our planet was made； and survived； by mere lucky chance；* amid unbelievable violence。 Our world does not seem to have been sculpted by a master craftsman。 Here too； there is no hint of a Universe made for us。
* If it had not； perhaps there would today be another planet； a little nearer to or farther from the Sun； on which other； quite different beings would be trying to reconstruct their origins。
THE DWINDLING SUPPLY of worldlets is today variously labeled： asteroids； ets； small moons。 But these are arbitrary categories—real worldlets are able to breach these human…made partitions。 Some asteroids （the word means 〃starlike；〃 which they certainly are not） are rocky； others metallic； still others rich in organic matter。 None is bigger than 1；000 kilometers across。 They are found mainly in a belt between the orbits of Mars and Jupiter。 Astronomers once thought the 〃main…belt〃 asteroids were the remains of a demolished world； but； as I've been describing； another idea is now more fashionable： The Solar System was once filled with asteroid…like worlds； some of which went into building the planets。 Only in the asteroid belt； near Jupiter； did the gravitational tides of this most massive planet prevent the nearby debris from coalescing into a new world。 The asteroids； instead of representing a world that once was； seem to be the building blocks of a world destined never to be。
Down to kilometer size； there may be several million asteroids； but； in the enormous volume of interplanetary space； even that's still far too few to cause any serious hazard to spacecraft on their way to the outer Solar System。 The first main…belt asteroids； Gaspra and Ida； were photographed； in 1991 and 1993 respectively； by the Galileo spacecraft on its tortuous journey to Jupiter。
Main…belt asteroids mostly stay at home。 To investigate them。 we must go and visit them； as Galileo did。 ets； on the other hand； sometimes e and visit us； as Halley's et did most recently in 1910 and 1986。 ets are made mainly of ice； plus smaller amounts of rocky and organic material。 When heated； the ice vaporizes； forming the long and lovely tails blown outward by the solar wind and the pressure of sunlight。 After many passages by the Sun； the ice is all evaporated； sometimes leaving a dead rocky and organic world。 Sometimes the remaining particles； the ice that held them together now gone； spread out in the et's orbit； generating a debris trail around the Sun。
Every time a bit of etary fluff the size of a grain of sand enters the Earth's atmosphere at high speed； it burns up； producing a momentary trail of light that Earthbound observers call a sporadic meteor or 〃shooting star。〃 Some disintegrating ets have orbits that cross the Earth's。 So every year； the Earth； on its steady circumnavigation of the Sun； also plunges through belts of orbiting etary debris。 We may then witness a meteor shower； or even a meteor storm—the skies ablaze with the body parts of a et。 For example； the Perseid meteors； seen on or about August 12 of each year； originate in a dying et called Swift…Tuttle。 But the beauty of a meteor shower should not deceive us： There is a continuum that connects these shimmering visitors to our night skies with the destruction of worlds。