On 23 August 2002 a team of US scientists led by Gary Byerly (Louisiana State University) and Donald Lowe (Stanford University) announced their discovery of a 3.47-billion-year-old asteroid impact on Earth. They had studied ancient rock samples from Australia and South Africa and analysed the spherules contained within. These tiny particles are a common byproduct of meteoritic impact and were used to date the impact, along with local zircon crystals. The 20-30 cm-thick (8-12 inches) spherule beds in the rock indicate that the impacting body had a rough diameter of around 20 km (12 miles) across. However, no crater has been found as Earth's geological processes have had plenty of time to erase it. Although many impacts would have occurred prior to this, including the postulated impact of a Mars-sized body with the early Earth, this is the oldest impact that has direct evidence from geological samples.