This week’s Cosmic Quiz winner is Matt Raphael, who asked, “How many craters are on the moon?”
The quick and uninteresting answer is, “Too many to count.”
Here’s the longer and hopefully more interesting answer:
Almost as soon as the telescope was invented astronomers began counting craters on the moon. Knowing the number of craters on the moon lets you make an estimate of how old the moon is and how often objects in the solar system get smacked by other objects in the solar system. If you’re a scientist trying to understand the origins of planets and moons, that’s important information to know.
The first thing you notice when you look at the moon through a telescope is that it is covered with craters.
When the best telescopes in the world could see features on the moon as small as a few miles in diameter the number of craters on the moon measuring at least 30 kilometers in diameter (~20 miles) was estimated to be about 100,000.
As telescopes got better astronomers began to see smaller and smaller craters, and a lot more of them.
For every crater that was 100 km or so in diameter, there were roughly a hundred craters that were about 10 km in diameter. For ever crater that was about 10 km in diameter, there were about 100 craters that were on the order of 1 km in diameter… and so on. The harder you looked on the moon for craters, the more you saw.
For every big lunar crater, there are many more smaller craters.
Earth has about 120 impact craters visible on its surface. Well-known among these are the Barringer Crater in Arizona, Wolf Crater in Australia and Utah’s Upheaval Dome.
Why are there so many craters on the moon, but not so many on Earth? There are two reasons for this, and they both have to do with Earth’s atmosphere.
First, the moon lacks a thick atmosphere to burn up or at least slow down the smaller bits of rock that enter Earth’s atmosphere. Space rocks smaller than a kitchen table are slowed down by our atmosphere so much that they don’t impact the ground with enough speed to blast out a crater.
The second reason Earth has so few visible craters on its surface is because our weather slowly erodes ancient meteor craters, effectively erasing them from the surface.
Arizona’s Barringer Crater is only about 50,000 years old, and erosion hasn’t had time to do much erasing.
Arizona's Barringer Crater is a young, well-preserved impact crater.
Australia’s Wolf Creek Crater is a few hundred thousand years old, and is considerably more weathered.
The Wolf Creek Crater in Australia is about 300,000 years old and is still distinctive as an impact crater, but shows weathering.
Utah’s Upheaval Dome is well over 100 million years old, and erosion has worn it away to the point that it took considerable geological research to establish that it was in fact an impact crater.
Upheaval Dome near Utah's Canyonlands National Park
Earth is a bigger target for space rocks floating around in our solar system, and has had many more collisions than the moon. Because there is no atmosphere on the moon, there can be no weathering and erosion, so the moon’s craters are preserved for the ages. On Earth, however, a few million years is all it takes for erosion to obliterate a crater.
Without an atmosphere to protect its surface or erase ancient craters, the moon’s surface is saturated with craters, and many of them can be extremely small. Soil samples returned by Apollo astronauts include tiny beads of glass that are the sites of even tinier craters – some about 1/10th the width of a human hair.
Impact crater on a microscopic piece of lunar glass
If craters can be just any size on the moon, including so small you need a high-powered microscope to see, how many craters do YOU think there are?