Asteroid impact
craters on Earth

When a large object strikes at tens of thousands of kilometres per hour, it does not simply make a hole. The kinetic energy is enormous - a 1-kilometre asteroid arriving at 20 km/s carries roughly the energy of 100,000 nuclear weapons. That energy is released almost instantaneously.

The impact generates a shockwave that compresses the target rock to pressures far beyond its strength, melting and vaporising material in microseconds. As the pressure wave expands and rebounds, the surrounding rock is excavated outward and upward, forming a crater with a raised rim. In larger impacts, the crater floor rebounds to create a central peak or ring - a frozen wave in rock. The melted material may splash out as droplets, solidify in the air, and fall back across a wide area as glassy particles called tektites.

The resulting crater is typically 10 to 20 times the diameter of the impactor. A 10-kilometre object produces a structure 100-200 kilometres across. The impactor itself is largely or entirely vaporised in the process - what remains is the crater, not the rock that made it.

The Chicxulub crater, buried beneath the Yucatán Peninsula and the Gulf of Mexico, is the product of an impact 66 million years ago. The object - estimated at 10-15 kilometres across, roughly as tall as Mount Everest - struck with energy equivalent to billions of nuclear weapons. The immediate region was incinerated.

The global effects were worse. Ejecta thrown into the upper atmosphere blocked sunlight for months to years, collapsing food chains that depended on photosynthesis. Sulphur released from the vaporised limestone and ocean floor caused acid rain across the planet. Wildfires spread across continents from re-entering ejecta. The resulting Cretaceous-Paleogene mass extinction eliminated roughly 75% of all species on Earth, including all non-avian dinosaurs.

The crater itself was confirmed only in the 1990s. Buried under roughly a kilometre of sediment and partly offshore, it had gone unrecognised despite being one of the largest impact structures on the planet. The confirmation came through geophysical surveys and the identification of shocked quartz and tektites across the global geological record at the 66-million-year boundary layer.

Barringer Meteorite Crater in Arizona formed roughly 50,000 years ago from a 50-metre iron meteorite travelling at approximately 12 km/s. The impactor was entirely vaporised by the energy of the explosion - no significant fragment survived. The crater is the product of the blast, not of excavation by the rock itself.

At 1.2 kilometres across and 170 metres deep, with a raised rim 45 metres above the surrounding plain, it is the best-preserved simple impact crater on Earth. At 50,000 years old, erosion has had little time to work on it. The arid Arizona climate has helped further. Visitors can walk to the rim and look down into the bowl - a scale model of exactly what a small impact event leaves behind.

The site played a role in establishing that lunar craters are impact structures. Geologist Eugene Shoemaker mapped the shock metamorphism features at Barringer in the late 1950s and early 1960s, establishing the geological signature of impacts - work that later confirmed the origin of craters across the solar system.

Earth is not less heavily struck than the Moon. The two bodies share roughly the same gravitational environment in the inner solar system. The difference is preservation.

Plate tectonics continuously recycles Earth's crust. Oceanic crust is subducted and replaced every 200 million years or so, erasing any impact structures on the ocean floor. Continental crust lasts longer but is subject to erosion: water, ice, and wind grind down crater rims over geological timescales. Vegetation obscures the shapes of structures that remain. Many ancient craters are now identifiable only through geophysical surveys - circular gravity anomalies or patterns of shocked rock - not through surface morphology.

The Moon has no plate tectonics, no erosion, no liquid water, and no atmosphere. Its surface preserves a cratering record extending back 4 billion years. Earth retains only the youngest and the largest - large enough that even billions of years of erosion have not entirely flattened them.