How did the Moon get its craters?
Almost all of the Moon's craters were made by asteroids and comets hitting it; with no air, no weather and almost no geological activity today, the scars have survived for billions of years.
Look at the Moon through binoculars and you will see a surface covered in round scars, from huge plains to tiny pits. For a long time people argued about whether these were volcanoes or impacts. In 1892 the American geologist Grove Karl Gilbert made an early strong case for impacts, but the question was only settled in the 20th century, through laboratory experiments, studies of craters on Earth, and the rocks brought back by the Apollo astronauts between 1969 and 1972.
We now know that almost all lunar craters were blasted out by impacts of asteroids, comets and smaller fragments. Even a modest object hits so fast, often more than 15 km/s, that it explodes on contact, digging a crater roughly ten to twenty times its own size and throwing out debris in bright rays. That is why craters are round, even when the impactor arrived at an angle.
The Moon keeps its scars because it has no protecting atmosphere to burn up incoming objects, no wind or rain to wear craters down, and no plate tectonics to recycle its crust. Earth has been hit just as hard, but most of our craters have been erased. The Moon is therefore a kind of record book of the inner Solar System's history.
That record shows that impacts were far more frequent long ago. The largest scars, such as the South Pole–Aitken basin, around 2,500 km wide, formed early in the Moon's history. The dark patches visible to the naked eye, the maria, are giant impact basins that later flooded with lava between about 3 and 4 billion years ago. Impacts still happen today: telescopes occasionally catch tiny flashes as small meteoroids strike the Moon's night side.