Calculating a cosmic collision
About 50,000 years ago, a nickel-iron meteorite, just 50 meters across, screamed through Earth's atmosphere at a speed of around 12.8 kilometers per second (29,000 mph). The resulting impact in the Arizona desert released energy estimated at 10 megatons of TNT. This explosion excavated 175 million metric tons of rock, creating a crater 1.2 kilometers (0.75 miles) wide and 170 meters (560 feet) deep. For decades, the origin of the crater was debated, with many attributing it to volcanic activity. The definitive proof of an impact event came from the rocks themselves.
The way to calculate the immense energy of the impact lies in a process called shock metamorphism. The extreme pressure and temperature generated by the impact permanently altered the atomic structure of the local Coconino Sandstone. Geologists discovered that quartz crystals within the sandstone had been transformed into much denser, high-pressure minerals named coesite and stishovite. These mineral transformations can only occur under instantaneous, intense shock pressures that are not produced by volcanic eruptions. The presence of coesite and stishovite is considered definitive evidence of a meteorite impact.
Reading the mineral pressure gauge
The discovery of coesite at Meteor Crater in 1960 by Eugene Shoemaker and Edward C. T. Chao was a significant moment. Coesite had been created in a laboratory in 1953, and scientists knew it required immense pressure to form. Finding it in nature confirmed that the crater was formed by an impact that generated extreme pressures and temperatures. Later, the even higher-pressure mineral stishovite was also found.
Computer simulations, recreating the first billionths of a second of the impact, show that the sandstone experienced temperatures around 3,000°C and pressures exceeding half a million atmospheres. Under these conditions, quartz transforms into stishovite in less than a nanosecond. By understanding the specific pressure and temperature thresholds required to form these minerals, at least 8 to 30 gigapascals—scientists can calculate the minimum energy released during the impact event. The mineralogical evidence points to the 10-megaton explosion and explains the fate of the impactor. The energy was so great that the 50-meter iron asteroid mostly vaporized, leaving only scattered fragments around the rim and the surrounding plains.