Recent grazing impacts on the Earth recorded in the Rio Cuarto crater field, Argentina

Abstract

THE most probable angle of a meteoroid impact on a planet is 45°, and an impact at 15° from the horizontal or lower is as likely as at 75° or higher ^1,2 . Yet little direct evidence for oblique impacts exists on the Earth, for two reasons. Unless the impact angle is very low, any asymmetry created during the initial transfer of energy from impactor to target is lost as the crater is formed ³ ; moreover, the shallow craters formed by oblique impact are more easily obscured by subsequent erosion. During routine flights two years ago, however, one of us (R.E.L.) noticed an anomalous alignment of oblong rimmed depressions (4 km x 1 km) on the otherwise featureless farmland of the Pampas in Argentina. We argue here, from sample analysis and by analogy with laboratory experiments, that these structures resulted from a low-angle impact and ricochet of a chondritic body originally 150–300 m in diameter.