Where Are Earth's Craters? An Astronomy Club Investigates

Craters You Can't See

1On the first clear night of the school year, the Wrenfield Middle School astronomy club set up a telescope on the deck of the town observatory and pointed it at a half-full Moon. Near the line between the bright and dark parts, long shadows made the craters stand out: round bowls with raised rims, small craters inside bigger ones, and bright streaks of rock splashed out around a few young craters. Club member Yemi Adebanjo asked a question that the club spent the rest of the month on: if Earth is the Moon's next-door neighbor, where are Earth's craters?

2The club's adviser, Mr. Casimir Halvorsen-Pike, said that scientists think Earth has been struck by space rocks at least as often as the Moon. Earth's air burns up most small space rocks, but rocks big enough to blast out a crater wider than about 1 km pass through the air. Even so, scientists have confirmed only about 200 impact craters on Earth, while the Moon has hundreds of thousands of craters wider than 1 km. Rocks brought back from the Moon show that many of its surfaces are 3,000 to 4,000 million years old, and many craters on them still have clear rims. The Moon has no liquid water, almost no air, no living things and no moving plates.

3Next, Mr. Halvorsen-Pike handed out Table 1, which describes five craters on Earth. The numbers are rounded, but each crater is like a real one. Crater A was blasted out in a few seconds when a space rock hit a desert, and since then wind has blown sand into its bowl. Crater B lies in hilly country. Streams have cut gullies into its rim, and in 2019 an earthquake shook a rockslide loose from its inner wall in less than a minute. Crater C formed on a shallow sea floor near a coast and was then buried by mud and sand that kept settling for 30 million years; geologists found it only by drilling. Thick ice-age glaciers scraped across Crater D, leaving a ring-shaped lake in the low ground. Crater E has been worn down so far that only a ring of low hills of shattered rock is left.

4Mr. Halvorsen-Pike added one more fact. Ocean floor covers about 70% of Earth's surface, and almost all of today's ocean floor formed within the last 200 million years. Older ocean floor has slid back down into the mantle at deep-sea trenches, as Earth's plates move a few centimeters a year.