Impact craters come in all sizes, because impactors come in all sizes. At the smallest size are microscopic particles of dust. They may be tiny, but as noted in the last post, they are traveling very fast, and so they impact with plenty of energy. On a world like the Earth, little bits of dust like this never make it to the surface, because they burn up in our atmosphere. But places like the Moon have no atmosphere to protect them, and the whizzing bits of dust impact right into the surface, creating small but very real impact scars.
As we move from impacts with lesser to greater energy, we see a change in the kind of impact craters that they create. Smaller impactors create “simple” impact craters. These are basic bowl or cone shaped depressions. But as the impactors get larger, they create craters with a different shape. These are called “complex” impact craters. They have a central peak or peak ring, with a flat floor and terraced walls. Check out last week’s post to see an illustration of both a simple and a complex crater from the side.
The biggest craters are called impact “basins.” These are the giants among craters. Craters of this size have two or more massive concentric ring structures. Basins are old, having formed within the first billion years of the solar system (which is now 4.5 billion years old). There are basins all over the solar system, including Hellas on Mars, and Caloris on Mercury.
“Huge” can be relative. An impact event that would form an otherwise unremarkable crater on the surface of the Moon could be a catastrophic event on another body. Mars’s moon Phobos suffered a major impact event that left an impact crater that takes up a good portion of its surface. Phobos is 27 km in length (longest dimension). The impact crater Stickney is 9 km in diameter. Had this impact event been much more energetic, it would have shattered Phobos.