SCIENCE ESSENTIALS 7 Australian Curriculum Edition
Some of these are as high as the Himalayas on Earth. The highlands are very old and exactly the same as they were when they were formed. This is because there is no air or liquid water on the moon, so no weathering and erosion can occur. The whole surface of the moon is covered by a thick layer of broken rock and dust. This too has been produced by meteor collisions. The footprints left by the Apollo 11 astronauts are still there in the moon dust after more than 30 years, because there is no wind or rain to erase them.
How was the moon formed? sea craters
by friction between the meteor and the air. However, the moon does not have an atmosphere to protect it from meteors. The near side has fewer craters than the far side, because the Earth gives it some protection. Craters such as Copernicus have bright rays radiating from them. Astronomers infer this is material spattered around when the meteor thudded into the moon. The large dark rounded areas are called maria (MA-ree-a) or seas. The early astronomers who studied the moon thought they were the same as the seas on Earth, but they are not. The Sea of Serenity is about the size of Victoria. The Apollo missions brought back rocks from the moon and many of these are basalt, which is formed from lava. Astronomers infer that the areas we call seas were once huge craters which filled with lava. This may have happened when meteors crashed through the moon’s crust, opening up cracks that allowed lava to flow out over the moon’s surface. After the lava hardened into rock, smaller meteors crashed into it, forming many smaller craters in its surface. There are also long narrow valleys called rills. One of these is more than 500 km long. Astronomers are not sure how rills were formed. One theory is that they are cracks partly filled by molten material. Another theory is that they are lava tunnels, like those on Earth, whose roofs have collapsed. The light areas of the moon’s surface are mountains or highlands, probably formed by volcanic activity.
Scientists infer the moon was formed about 4.5 billion years ago, at about the same time as the Earth. This is the age of the oldest rocks so far found on the moon. There have been three theories to explain the formation of the moon. The first theory says the moon was formed at the same time as the Earth when gas and dust clumped together to form the solar system. The second theory says that the moon was somehow split off from the Earth, perhaps pulled out by the tidal force of a passing star. For example, some people have suggested the moon may have come from the area now filled by the Pacific Ocean. The third theory says that the moon was captured by the Earth’s gravity. However, as a result of studying the moon rocks, scientists now favour a fourth theory—the ‘Big Whack’ theory. A Mars-sized body collided with the Earth, throwing huge amounts of debris into orbit around the Earth. Some of this debris eventually came together to form the moon, as shown below.
A Mars-sized body collided with the Earth, throwing huge amounts of debris into orbit. Some of this debris came together to form the moon.
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Science Essentials 7 AC edition sample pages