Stones From Space Help
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.
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