A Square Corner

Did you ever notice just about every crater is at least a little rounded, some a bit elongated sure but rounded and most are just plain circular. Well “square” corners do occur and while they can be fodder for conspiracy theories, there are perfectly reasonable explanations for their formation.

The lunar crater above (NASA/GSFC/Arizona State University) is a good example. The name of the crater is Lavoisier, named for the famous French Chemist Antoine Lavoisier.

The crater is about 70 km across (42 miles), pretty large and can be seen with even a small telescope – yhe image width is only about 10 percent of the entire crater. The thing about Lavoisier is the location, Longitude: 81.253° West, Latitude: 38.169° North puts it on the northwestern limb so you need something steady to see it well. Yes binoculars would work but not for a decent examination. Plus the moon probably needs to be pretty well full. I will definitely have a look in a couple of weeks.

So how do we get to the squared off corners? Here’s what the LRO / NASA website had to say:

Why are most craters circular (even craters found on Earth)? By hurtling objects together at many miles per second in large laboratories, scientists have shown that only the most oblique impacts (less than 10° from the horizon) produce elliptical craters. The kinetic energy of an impactor behaves much like the energy from a nuclear bomb. The energy is transferred to the target material by a shock wave, and shock waves produced by an impact, whether oblique or head-on, propagate hemispherically. This shape means that energy is being delivered equally in all directions; resulting in a hemispherical void and thus circular craters. However, conditions in nature do not always mirror the laboratory. In fact some craters are nearly square! A portion of the rim of Lavoisier A crater tells a story of the geology before impact. Lavoisier A is a square-ish crater with a diameter of ~26 km (16 miles) found in the northwestern portion of Oceanus Procellarum.

Much of Lavoisier A’s shape is thought to be due to preexisting joints or faults in the target rock. These discontinuities create zones of weakness, affecting how the shock wave travels through the material. We find square craters on other planetary bodies such as on the asteroid Eros and here on Earth! An example of a square crater that has been thoroughly studied is Meteor Crater in Arizona. This crater formed on layers of sedimentary rocks that have orthogonal vertical joints running below where the crater formed. The joints disrupted the shock wave flow in certain directions, preventing the formation of a circular crater. Another indication of weaknesses within the target layers is the appearance of the northeastern portion of the crater rim. It appears as if a layer of rock has been peeled back.

Leave a Reply