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People have had a lot of ideas about how the Moon formed.
Most scientists believe a small planet-like body smacked into Earth about 4.5 billion years ago. Our planet was still young then. It was only 100 million years old! The crash threw pieces of rock into orbit. The pieces joined and became the Moon. Both Earth and the Moon spent the next 700 million years being hit by big meteorites. Earth’s scars from that time have mostly healed. That’s because of the weather, erosion, and plant life on Earth. But the Moon still has big pockmarks, called craters.
The last part of the Moon’s formation took place over the next 2 billion years. Hot lava came up from below the surface. It formed dark areas called maria (MA-ree-uh), or seas. As this process slowed down, the surface got harder. It turned into the grayish-green crust we see today.
◀ Scientists don’t know exactly what’s under the Moon’s hard outer crust. They think it has a mantle, made of dark rock. Below it is an area partly full of hot liquid rock. A small core is probably made of iron.
◀ The Moon is shrinking. Why? Because it’s slowly cooling from the inside out. As the interior cools, it shrinks. As it shrinks, it pulls everything in toward the center. The motion creates faults on the surface. The faults produce quakes (in this case, moonquakes). This is the conclusion scientists published in May 2019. Their thinking is based on eight years of data recorded by devices left on the Moon during the Apollo missions (1969 to 1972). By scientists’ estimates, the Moon is about 150 feet leaner than it was several hundred million years ago. The shrinking and the quakes continue.
A weather report from the Moon would be short. There is no weather on the Moon! It has no clouds. It has little or no atmosphere, or gases that surround a planet. The sky is always black and starry. The Moon has only a tiny bit of wind, caused by the Sun. That’s why craters that formed billions of years ago have stayed almost the same. ▶
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◀ The Moon’s dusty soil is called regolith. It has no life, but it does have minerals. They include aluminum, iron, and titanium. Astronauts brought back three main types of rocks. Basalts (blue, pictured) are dark, hardened lava. Anorthosites (red, pictured) are light rocks in the highlands. Breccia are composites, made as other rocks got crushed together when meteorites hit.
The gravity on the Moon is one-sixth of that on Earth. So people weigh a lot less on the Moon than they do on Earth. A person who is 180 pounds on Earth would weigh just 30 pounds on the Moon! ▶
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