When a meteor strikes a planet or moon, it creates a crater -- a smooth bowl shape that may or may not contain fragments of the meteor. Impact craters retain their shape on bodies such as Mars, Venus and the Earth's Moon because these bodies lack major geologic or meteorologic processes which can deform their shapes. Craters which retain their smooth walls are called simple craters; if crater walls collapse inward, the crater is called complex.
Over the course of its planetary life, the Earth has received more meteor impacts than its moon, yet its surface shows relatively few crater formations. This is due to Earth's powerful, active geological processes, which keep its surface and atmosphere in motion and conceals the traces of meteor strikes. Earthquakes, climate changes, and erosion by wind and rain can change the shape of craters or conceal them entirely under bodies of water or vegetation.
On Earth, wind is the major agent of erosion, particularly in desert areas with little surface water and vegetation to hold soil in place. Wind carries sand grains, which are heavy and abrasive enough to cut, polish and pit stone. In addition, strong winds shift sand and soil, resulting in constantly changing shapes, which obscure features on the earth underneath. In this way, crater shapes can be eroded, cut, or partially covered by the effects of wind.
Although craters in wetter areas have been obscured by water or overgrown with vegetation, craters in dry areas such as the Wolfe Creek Crater in Australia and the ancient site at Aurounga in Chad, Africa, show clear effects of wind erosion. The Wolfe Creek Crater has been partially buried under repeated onslaughts of windblown sand and is only partially recognizable as an impact crater. The Aurounga Crater, primarily visible from the air, shows dark valleys across its center, cut by blowing sand for billions of years.