The Formation of the Moon and the other Planets
THE MOON——
The origin of the Moon, the only large body in orbit around the Earth, poses one of the oldest puzzles in planetary science. The Moon is one of the largest bodies in the solar system—larger than the planets Mercury and Pluto, and almost as large as Mars. The Moon's density and chemical composition, subjects of intense study by astronauts of the Apollo lunar missions, are quite different from the Earth as a whole, though they are remarkably similar to those of the Earth's mantel. The problem of the Moon's origin can be stated simply: How could it have arisen in the same region of space at the Earth, when its composition is so different?
There have been various attempts to solve this problem. Some astronomers have argued that the Moon was formed elsewhere in the solar system in a region of low-density, mantle–like material and later captured by the Earth. Other scientists have suggested that the Moon was somehow thrown out of a rapid rotating Earth after differentiation took place. Indeed, in the early twentieth century, astronomers often pointed to the Pacific Ocean basin as the likely "birth scar" of the Moon. Neither of these theories cam meet rigorous testing. However, it turns out to be very difficult for a planet to capture a large body in a nearly circular orbit, and even harder for a planet to throw off a Moon–sized chunk.
The current theory is the "big splash." In this theory, the Earth underwent differentiation but while it was still in a formative, state, it was struck by an object about the size of Mars. This collision blew a huge quantity of mantel rocks out into orbit around the Earth. The Moon then formed from the material, which explains the Moon's unusually composition, density and large size. Meanwhile, the great crater created by this epic impact was filled and eventually weathered away , so no evidence of it appears today on Earth.
THE OTHER PLANETS——
The natural process that occurred during the Earth's formation affected the other planets. Mercury, Mars and the Moon, for example, display surface cratering that suggests that large chunks of rock bombarded all of these planets late in their formation. The Earth undoubtedly looked like this over 4 billion years ago, but all evidence of those early craters have weathered away. On Mercury and the Moon, which have no atmosphere, no weathering affected the craters and they are still there.
The early bombardment could have affected other characteristics of terrestrial planets. The direction of rotation of Venus, for example, is opposite that of the Earth.(Planets revolve around the Sun, but rotate about their axes.) The Earth's axis of rotation, furthermore, is tilted at 23° to the plane of its orbit, while Uranus has its axis rotation close to the plane of its orbit, a full 90° from an upright orientation. current thinking is that these differences resulted from the more or less random collisions with large objects, perhaps hundreds of kilometers in diameter, expect that the details of these late–stage collisions were different for each planet. Thus the nebular hypothesis not only explains how it is that the planet all have their orbits in the same plane and move in the same direction around the Sun, but also allows us to explain why the rotations of individual planets can be so different.
WHEN COULD LIFE BEGIN?——
This scenario for the Earth's formation also has important implications for the origins of life on our planet. During his period of the great bombardment, the Earth was constantly being hit by huge objects–chuncks of rocks the size of a state or even a country. At the least, the tremendous energy released by such collisions would be enough to vaporize any oceans that formed. Each collision would, quite literally, sterilize the planet. Even if life had come into existence during this period, it would have been wiped out by the impacts. Thus the ancestors of modern living things could not have received their start until the end of the bombardment, which occurred between 4.2 and 3.8 billion years ago.
