Because there is no atmosphere, any liquid water would have long ago evaporated into space. Any water remaining on the moon must be frozen. Evidence of ice near the moon's north and south poles was detected by space craft observations. Sunlight at the poles is less intense, so temperatures are low enough there to keep ice from sublimating (changing directly from a solid to a gas).

Surface Features

Exploration of the moon's surface has revealed the existence of a variety of geological features. The major surface features of the moon are maria, highlands, and craters.

Maria and Highlands

Do you sometimes think you can see a face when you look up at the Moon? What you are really seeing is a pattern of lighter and darker regions on the moon's surface, as shown in the top photo of Figure 6.

One of the first people to look at the moon with a telescope was Galileo. When Galileo aimed his telescope at the moon 400 years ago, he thought that the dark, smooth regions on the moon's surface resembled the seas on Earth. So, he called them maria (MAH ree uh), the Latin word for “seas.” In reality, maria are low, flat plains formed by ancient lunar lava flows. They cover about 15 percent of the moon's surface. The maria are surrounded by light-colored areas called lunar highlands. The lunar highlands, which cover most of the moon's surface, are rough, mountainous regions.

Craters

Much of the moon's surface is covered with impact craters, such as those shown in Figure 6. These craters are round depressions in the surface caused by the impact of high-speed meteoroids. Meteoroids are chunks of rock that move through the solar system. Although meteoroid collisions still occur, most craters formed more than 3.8 billion years ago.

The thousands of craters on the moon are evidence that the moon is geologically dead. On Earth, most old craters disappeared as the surface was altered by erosion or collisions between tectonic plates. On the moon, however, there is little erosion and no plate motion to alter craters. Notice in the top photo of Figure 6 that the lunar highlands are much more heavily cratered than the maria. Older craters within the maria were covered by more recent lava flows. The few craters within the maria formed after the lava that covered them had solidified.

Figure 6 The photos above show features of the moon. The large, dark areas in the top photo are maria. The lighter areas surrounding the maria are lunar highlands. The moon is covered with craters that formed from the impacts of smaller objects.

Why are the lunar highlands more heavily cratered than the maria?

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