Stones From Space Help (page 2)
Stones From Space
An intense meteor shower is an unforgettable spectacle. So is a single large meteor as it lights up the sky, creates a sonic boom, and leaves a lingering trail. The Native Americans called these objects shooting stars . Other people called them falling stars . This is not surprising; in the absence of information to the contrary, that is what they look like. In the more recent past, some European immigrants to North America in the 1700s theorized that meteorites formed in Earth’s atmosphere as a by-product of the lightning in thunderstorms. We know today that meteors and meteorites do indeed come from space.
When the Solar System was much younger and the planets had formed but had not yet reached their present stable states, there was more debris in inter-planetary space than is the case now. The most popular theories of the origin of the Solar System involve the accretion of the Sun and planets from a rotating disk of gas and dust. Planets formed at certain distances from the central Sun. Some of the planets developed satellite systems of their own, like Solar Systems in miniature; Jupiter and Saturn are the most notable of these.
Most of the material in the primordial gas/dust disk made its way into the Sun and the planets. Some of the stuff, however, was left over. Some simply remained as gas and dust; some congealed into pebbles, rocks, boulders, and asteroids. Many of these smashed into the young planets and moons, forming craters. However, there are still plenty of space rocks out there. They, like their predecessors, are engaged in an incredibly complicated gravitational dance among themselves and the planets. Jupiter is the “conductor” of a vast “orchestra” of such rocks. Unlike a human orchestra conductor, though, Jupiter does not always maintain rhythm and harmony among its subjects. Often a rock is thrust into an orbit that puts it in a path where it has the potential to strike the Earth. Then it becomes a meteoroid. When we find one of these objects on the surface of our planet, we know the rest of the story!
Craters provide dramatic evidence of past meteorite bombardment on planets having little or no atmosphere. The Moon is the most familiar example. Mars is another. Mercury is still another. Most of the moons of the outer planets have craters.
We don’t see many craters on our planet because blowing dust and sand and falling rain have eroded them entirely or at least beyond recognition. No craters have ever been seen on Jupiter, Saturn, Uranus, or Neptune; these gas giants are not believed to have solid surfaces on which craters can form. This is not to say that these planets haven’t escaped bombardment by space debris but only that rocks from space leave no signatures on them.
Meteorite craters tend to be much larger than the objects that make them. This is because of the tremendous force that accompanies the crash landing of an object at several kilometers per second. Large meteorites form craters with central hills or small mountains. The rims of large craters form circular mountain ranges that can rise well above the surrounding terrain and several kilometers above the crater floor.
Some meteorite craters have rays , which appear as streaks radiating outward from the point of impact, and which can extend from the rim of the crater out to several times the diameter of the crater. Rays are produced by debris hurled up into the sky by the force of the impact and are especially prominent in craters produced by meteorites that struck the surface at a sharp angle.
Figure 11-6 A is a cross-sectional diagram of a typical large meteorite crater of the type commonly found on the Moon or Mercury. Figure 11-6 B is a top-view diagram of a similar crater.
Some meteorites cause fractures in the surface of the object on which they land. These fractures can take the form of concentric rings around the crater. Jupiter’s moon Callisto has a crater with these features. In other cases, there is no particular pattern to the fracture lines; the surface apparently cracks along the weakest points in the crust. Sometimes the fractures cover the whole surface of the object. They can occur as canyons or as escarpments (cliffs).
The impact of an especially massive meteorite or a small asteroid can produce volcanic activity all over the object it strikes, provided that there is hot lava inside the object that can rise up through the crust. The maria , or “seas,” of the Moon are believed to have formed when lava from the interior spread over the surface following one or more violent meteorite landings. The same thing is thought to have taken place on the Earth from time to time, but the features have been modified by weather erosion and by the constant shifting of the tectonic plates in the Earth’s crust.
In the extreme, a catastrophic impact can shatter a moon or planet. Nothing of this sort is thought to have taken place on any of the known planets or moons in the recent past, but in the early evolution of the Solar System, such events were commonplace. Going all the way back to the formation of the Sun and planets from the rotating disk of gas and dust 4.6 billion years ago, major collisions were the rule and not the exception. The ultimate long-term effect of meteorite landings is a sweeping up of space debris into major objects. Thus, as time passes, catastrophes become less and less frequent.
If you look at the sky for a long enough time on any given night, eventually you will see the bright glow and trail of at least one meteor. They usually look like silvery or gold-colored streaks of light lasting from a fraction of a second to perhaps 2 or 3 seconds. An especially brilliant meteor leaves a trail that is visible for several seconds after the object itself has disintegrated or reached the surface.
In theory, approximately 33 percent more meteors should fall near the equator than near the poles. This is simply a matter of the geometry of the spherical Earth versus its orbit around the Sun. However, this rule changes slightly depending on the time of year. At and near the September equinox, the north pole receives slightly more meteors than the south pole. At and near the March equinox, the opposite is true. The poles receive roughly equal numbers of meteors at and near the solstices.
There are certain brief events, known as meteor showers , that take place at the same time every year. These showers seem to originate from particular places in the sky. Generally, meteor showers are named according to the constellation (or, in some cases, a star within the constellation) from which the meteors appear to be coming. Part or all of the name of the particular constellation or star is followed by the suffix -ids . For example, near the end of October there is a shower called the Orionids ; these meteors seem to come out of the constellation Orion. Table 11-1 lists some well-known meteor showers that take place each year.
Why do meteors seem to come from a particular spot in the sky during a shower? Why don’t they just fall at random? The reason is that we see them from a certain perspective and the fact that during a shower the meteors tend to fall toward Earth in more or less parallel paths.
Have you ever lain down flat on your back during a rain shower when there was no wind? Bundle up and try it sometime. The raindrops fall down in nearly parallel paths, but they seem to emanate from a point directly overhead as you watch them come toward you. The same thing happens with meteors during a shower. The point from which the raindrops or meteors seem to come is called the radiant ( Fig. 11-7 ). The names of meteor showers derive from the positions of the radiants in the sky, which tend to be the same, year after year, for any given meteor shower.
Meteor showers are almost always associated with comets. As its ice evaporates with each apparition, a comet gradually deteriorates, leaving swarms of meteoroids (the rocky stuff of which comets are partly made) that gradually spread out along the comet’s orbital path. As the Earth passes through or near the comet’s orbit, these meteoroids fall as meteors, and we see a shower.
The most spectacular meteor showers take place when the Earth’s orbit precisely intersects a concentrated swarm of meteoroids. This does not always happen for any particular meteor shower. The orbit of the Earth and the orbit of the comet and associated meteoroids might not exactly cross each other, and even if they do, there might not be very many meteoroids in the comet’s orbit at that particular point. This is why, for example, the Leonids are spectacular in some years, but in other years they’re just so-so.
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