Astronomical Feature of the Month
Lunar Craters

    Essentially all lunar craters, regardless of their size, were produced by the impact of cosmic objects or meterorite impact. These objects strike the surface at a very high speed (70,000 km/hour) and are shattered. Some of the material is vaporized, some is melted, and some is tossed out of the target area, piling up around the hole produced. A small amount of the fragmented material can be great thrown distances from the impact area along straight paths, creating an effect called “rays.” The surface of the moon is scarred by millions of impact craters. Since there is no atmosphere on the Moon to help protect it from bombardment, craters remain unchanged until a new impact happens. Craters range in size up to many hundreds of kilometers. The most enormous craters have been flooded by lava leaving only parts of the outline visible. The low elevation maria (seas) have fewer craters than other areas because these areas formed more recently, and have had less time to be hit. The size, mass, speed, and angle of the falling object determine the size, shape, and complexity of the resulting crater. Small, slow objects have a low energy impact and cause small, simple craters. Large, fast objects release a lot of energy and form large, complex craters. Very large impacts can even cause secondary cratering, as ejected material falls back to the ground, forming new, smaller craters, or a series of craters. Most of the craters on the Moon are circular. The few craters that are not circular, like Messier and Messier A (pictured at the left) in the Mare Fecunditatis, are an enigma. Scientists have not yet determined how these oddly-shaped craters were formed. Most craters on the Moon that have diameters less than about 15 kilometers have a simple, bowl-like form. Lunar craters with a diameter over about 15 kilometers have more complex forms, including shallow, flat floors made of solidified lava, central uplifting (a single peak, multiple peaks, or a ring), and terraces on the inner-rim walls. Simple impact craters have bowl-shaped depressions, mostly with smooth walls. This type of crater generally has a diameter less than 9 miles (15 km). Their depth is about 20% of the diameter. Complex impact craters have a single or multiple peaks in the middle of the crater. These craters have diameters between about 12 and 110 miles (20 and 175 km), and the central uplift is usually one or a few peaks. Craters with a diameter over 110 miles (175 km) can have more complex, ring-shaped uplifts within the crater. A impact basin is an impact crater that has a rim diameter greater than 185 miles (300 km). There are over 40 impact basins on the Moon. These catastrophic impacts produce faulting and other crust deformations.

Cover of an old Analog magazine illustrating the article “Mercury’s Missing Divot,” theorizing that the impacting object which formed the Caloris Basin pretty well exploded outward and disappeared. Not sure if that’s true or if I recall the premise correctly, but the craters were fun to do.

Copyright © 2001
International Association of Astronomical Artists