Rio Cuarto craters

The Río Cuarto craters are a purported group of impact craters located in Córdoba Province, Argentina. However, they have also been considered as just a result of Aeolian processes.

In 1990, Captain Ruben Lianza of the Argentine Air Force, an amateur astronomer, provided a report to an astronomy publication that included aerial pictures of a set of odd teardrop-shaped depressions near the city of Río Cuarto, Córdoba in north-central Argentina. The depressions seemed very similar to the sets of craters produced in laboratory simulations of impacts taking place at low angles. Such features exist on the Moon, Mars, and Venus, but had not been seen on Earth up to that time.

The depressions had long been known to Argentine geologists, but until Lianza, nobody had seriously investigated them. Samples of materials obtained from the depressions indicated the presence of shocked materials, as well as pebbles that were clearly of meteoritic origin. A team of American researchers went to Argentina to investigate, collaborating with Captain Lianza and Argentine academics to study the strange depressions.

There were ten depressions, four of them of substantial size. One crater, named the "Drop", was about 200 meters wide and 600 meters long. Two more large craters, the "Eastern Twin" and "Western Twin", both about 700 meters wide and 3.5 kilometers long, were located 5 kilometers to the northeast. Another major crater, the "Northern Basin", about half and again as big as one of the Twins, was sited 11 kilometers further to the northeast. The long axes of the craters all pointed to the northeast.

The craters were thought to be due to a grazing impact of a set of objects at a very low angle, which calculations show to be a rare occurrence. Most impacts will strike at an angle of 45 degrees to the horizontal or greater, and the impact craters will always be close to circular, since the shock wave that results from the impact propagates symmetrically.

A grazing impact, however, will form an elliptical crater, with sprays of debris that look like butterfly wings. This has been confirmed by high-velocity guns used for impact experiments, and more recently by computer simulations. On impact, the object may shed chunks of itself that fly further downrange to perform secondary impacts.

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Wikipedia