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.

A photomicrograph of a tiny sphere of lunar impact glass (such spheres can be less than 1 millimeter in size) with an even tinier circular impact feature on its surface.
Credit: NASA

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.

On the left is a simple crater, and on the right is a complex crater. Note that the complex crater is much larger. It has a classic central peak, flat floor, and terraces along the inside rim. Citation: Trang, D. (August, 2016) What Made the Doughnuts Inside Lunar Concentric Craters? PSRD

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.

An image of the lunar Orientale basin. Note it that it has multiple rings, instead of one simple rim structure. Orientale is about 1,100 km in diameter, or 684 miles across. Credit: NASA

“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.

This image is of Mars’s moon Phobos. Crater Stickney is on the left. Credit: NASA Viking orbiter.