What are the main reasons there are so few impact craters on Earth compared to the Moon?

Why does the Moon have so many craters?

The Moon is a museum, a showcase of ancient craters, some of them more than 150 km (nearly 100 miles) wide. For centuries, scientists debated their origin. In Galileo’s time, and until the 1890s, it was accepted that the craters were the vents of huge but extinct volcanoes. This was a reasonable assumption, because they looked much like Earth's volcanoes: roughly circular and always ringed by a steep sided, mountainous ridge.

American geologist Grove Karl Gilbert was the first to question that theory, asking why the Moon’s craters were at ground level, not on mountain peaks. A few years later, in 1929, another American, astronomer Forest Moulton, argued successfully that meteorites hitting the Moon with explosive impact at 108 000 km/h (67 000 mph) would create circular craters, and throw up a mountainous ridge around them.

Most of the Moon’s craters date back about 4000 million years. Before that time, the Moon was constantly growing, because its gravity attracted debris floating around in space large and small rocks, dust and asteroids. Some of these space missiles were hundreds of kilometres in diameter, and they left craters so vast that we can see some from Earth with the naked eye.

When the bombardment lessened, radioactive elements in the Moon’s collection of debris caused massive flows of lava. Surface rocks melted to a depth of 200 km (125 miles). For 500 million years, lava flooded over the Moon’s battered face, wiping out some craters and forming those wide dark shadows, known as maria (seas), so obvious today.

About 3000 million years ago, the lava flows stopped. More recent meteor strikes have made fresh and sometimes quite massive craters but, had astronauts landed on the Moon in ancient times, they would have seen a landscape much as it is today.

The Moon isn’t alone in showing the scars of such a battering. Space probes to other planets reveal similar evidence. It’s a sure sign of worlds lacking air and water, which over millions of years can slowly chisel and mould a new face, as they have done on planet Earth.