What Are Moon Craters? How Were They Formed?

Moon craters are bowl-shaped landforms created by two processes: volcanism and cratering. There are hundreds of thousands of moon craters ranging from less than a mile across to giant basins called mare, which were once thought to be seas.

How Did Moon Craters Form?

For a long time, scientists did not know how the craters on the Moon were formed. Although there were several theories, it wasn't until astronauts actually went to the Moon and got rock samples for scientists to study that suspicions were confirmed.

The detailed analysis of Moon rocks brought back by the Apollo astronauts showed that volcanism and cratering have shaped the Moon's surface since its formation, about 4.5 billion years ago, shortly after Earth was formed. Giant impact basins formed on the infant Moon's surface, which caused molten rock to well up and create giant pools of cooled lava. Scientists called these "mare" (Latin for seas). That early volcanism deposited the basaltic rocks.

Impact Craters: Created by Space Debris

Throughout its existence, the Moon has been bombarded by comets and asteroid chunks, and those created the many impact craters we see today. They are in pretty much the same shape they were after they were created. This is because there is no air or water on the Moon to erode or blow away the crater edges.

Since the Moon has been pounded by impactors (and continues to be bombarded by smaller rocks as well as the solar wind and cosmic rays), the surface is also covered by a layer of broken rocks called regolith and a very fine layer of dust. Beneath the surface lies a thick layer of fractured bedrock, which pays testament to the action of impacts over billions of years.

The largest crater on the Moon is called South Pole-Aitkin Basin. It's about 1,600 miles across (2,500 kilometers). It's also among the oldest of the Moon's impact basins and formed just a few hundred million years or so after the Moon itself was formed. Scientists suspect that it was created when a slow-moving projectile (also called an impactor) crashed into the surface. This object was probably several hundred feet across and came in from space at a low angle. 

Why Craters Look the Way They Do

Most craters have a pretty characteristic round shape, sometimes surrounded by circular ridges (or wrinkles). A few have central peaks, and some have debris scattered around them. The shapes can tell scientists about the size and mass of the impactors and the angle of travel they followed as they smashed into the surface.

The general story of an impact follows a pretty predictable process. First, the impactor rushes toward the surface. On a world with an atmosphere, the object is heated by friction with the blanket of air. It starts to glow, and if it's heated enough, it may break apart and send showers of debris to the surface. When impactors strike the surface of a world, that sends a shockwave out from the impact site. That shock wave breaks up the surface, cracks rock, melts ice, and digs out a huge bowl-shaped cavity. The impact sends material spraying out from the site, while the walls of the newly created crater may fall back in on themselves. In very strong impacts, a central peak forms in the bowl of the crater. The surrounding region may get buckled and wrinkled into ring-shaped formations.

The floor, walls, central peak, rim, and ejecta (the material scattered out from an impact site) all tell the tale of the event and how powerful it was. If the incoming rock breaks up, as it usually does, then pieces of the original impactor can be found among the debris. 

Impact Cratering on Earth and Other Worlds

The Moon isn't the only world with craters dug out by incoming rock and ice. Earth itself was pummeled during the same early bombardment that scarred the Moon. On Earth, most craters have been eroded away or buried by shifting landforms or sea encroachment. Only a few, such as Meteor Crater in Arizona, remain. On other planets, such as Mercury and the surface of Mars, craters are quite obvious, and they haven't been eroded away. Although Mars may have had a watery past, the craters we see there today are relatively old and still look in fairly good shape.

Sources

• Castelvecchi, Davide. “Gravity Maps Reveal Why the Moon's Far Side Is Covered with Craters.” Scientific American, 10 Nov. 2013, www.scientificamerican.com/article/gravity-maps-reveal-why-dark-side-moon-covered-in-craters/.
• “Craters.” Centre for Astrophysics and Supercomputing, astronomy.swin.edu.au/~smaddiso/astro/moon/craters.html.
• "How Craters are Formed", NASA, https://sservi.nasa.gov/articles/how-are-craters-formed/