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Most impacts that make a crater make a round crater. This is because explosions expand in all directions equally. But when an incoming meteorite strikes the ground at a shallow angle — less than 15° above the local horizon — the resulting crater is no longer circular.

Low-angle impacts produce craters with an oval outline. And instead of a debris apron all around the crater, two "wings" of debris are flung to either side, making a butterfly pattern. (This is similar to what happens at double-impact craters.) On occasion, there is a smaller crater in line with the oval one and the wings; these may be caused by the impactor breaking apart just before hitting — or possibly the top of the impactor splits off and travels a little farther downrange before hitting.

Elliptical craters with butterfly ejecta patterns make up roughly 5 percent of all craters on Mars. Similar craters are also seen with about the same abundance on the Moon and Venus. While scientists have studied crater-making explosions in detail, their understanding of what happens in an oblique impact is much less certain. Lots of work needs to be done still to fully understand this process.

When a meteorite strikes at a shallow angle (less than about 15° above the surface), the crater is not round as usual. Instead, it is oval or asymmetric like this crater, which is 5 km (3 mi) long and made by a meteorite traveling toward the lower left. The ejected debris flies mostly on both sides, making "wings". The small crater downrange from the main crater may have been caused by the impactor breaking apart before impact. Or perhaps it was "decapitated" in the impact, with part of the impactor traveli
Oblique Crater
(NASA/JPL-Caltech/Arizona State University)