Craters, Maria, and Highlands: The Moon's Surface Features

The surface of Earth is ever changing, but the surface of the Moon looks much the same as it did when Galileo first pointed his telescope at it some 400 years ago. Why hasn't the Moon changed? The Moon has no atmosphere, so wind does not erode it. It has no rivers and it never rains, so moving water cannot scar the landscape. As a result, craters (large excavated areas created by meteor impact) formed millions of years ago remain unaltered today. Galileo saw smooth, dark areas that he named maria (singular, mare), meaning "seas." Today, we know that Galileo's maria are actually huge craters that long ago were filled with lava, which solidified.

In this project, you will make models of impact craters to see how object size affects crater size. You will learn how to map the surface of the Moon, and you will investigate the history of Moon mapping. You will also find out about the composition of maria and the surface's lighter areas, called highlands.

Getting Started

Purpose: To model the formation of a crater.

Materials

• paper cereal bowl
• plaster of paris
• tap water
• craft stick
• 2 or more sheets of newspaper
  golf ball-size rock
• petroleum jelly

Procedure

1. Fill the bowl with plaster of paris
2. Gradually add water and stir with the craft stick until the mixture resembles a thick batter. The batter should hold its shape if you make a hole with the craft stick. Note: Discard the craft stick. Do not wash plaster down the drain. It can clog pipes.
3. Place the newspaper on the floor. Set the bowl in the center of the paper.
4. Cover the rock with a thick coating of petroleum jelly.
5. Standing and holding the rock chest-high above the center of the bowl, drop the rock (see Figure 20.1).
6. Carefully remove the rock from the plaster, disturbing the crater as little as possible.
7. Let the plaster sit until firm. Keep the bowl for display.

Results

The rock forces the plaster away from the point of impact. A ridge forms around the edge of the crater.

Why?

The Moon's surface is pitted with craters. A crater is a circular depression of any size. Large craters that are several miles (km) in diameter are called basins. Some of the smallest craters, at 10 to 20 microns (1 micron is equal to one-millionth of a meter) have been found etched on crystalline rocks brought from the Moon back to Earth by the Apollo astronauts. Most lunar craters are bowl-shaped impact craters caused by the impact of solid bodies. Both large and small craters are formed when meteorites slam into the Moon's surface. While in orbit about the Sun, these solids from celestial bodies are called meteroids. When a meteoroid strikes the surface of a celestial body it is called a meteorite. Because the Moon has no atmosphere, any object that hits it collides at full speed. In this investigation, you removed the rock so you could examine the impact crater. But on the Moon, the high-speed impact of larger meteorites produced an explosion upon impact. This explosion created an enormous amount of heat, which vaporized (changed to a gas) much of the meteorite and even some of the lunar material, leaving bowl-shaped craters. Small objects hitting with lesser force simply pushed the surface material aside, as was the case in this investigation. For small craters, if all the debris and crater walls were put back into the hole, they would fill it up.

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