The Rio Cuarto craters are a group of depressions located in Argentina at lattitude S 32° 52', longitude W 64° 14' There is currently some controversy as to whether these structures are actually produced by impacts, or by aeolian surficial processes, which form many similar features in that region.
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 Rio Cuarto 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 been long known to Argentine geologists, but until Captain Lianza came along, 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 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 clearly 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 less, 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.
Models of the Rio Cuarto event suggest that the object struck at an angle of no more than 15 degrees from the horizontal, with the impact itself having 10 times more explosive energy than the Barringer Crater event and 30 times more than the Tunguska event. Although the age of the craters has not yet been determined precisely, it is believed they are about 10,000 years old.
The object came in from the northeast, bright as a second Sun. The object hit ground at the Northern Basin, creating a mountain of fire about 10 kilometers wide and 50 kilometers long, and scattered off pieces that went downrange to form the Twins and the Drop. The fireball incinerated all life downrange in a firestorm with a parabolic-shaped footprint that created hurricane-force winds, erasing the butterfly-shaped pattern of debris characteristic of such low-angle strikes.
The object was clearly a "carbonaceous chondritic" asteroid, largely made up of simple carbon compounds and resembling something like a big lump of soot. The impact probably released huge clouds of toxic carbon monoxide that killed off wildlife in the area, assisted by heavy concentrations of toxic nitric oxides created through ionization by the object's fiery passage through the atmosphere. It is likely the impact resulted in serious atmospheric effects and may have even had a short-term effect on global climate.