It’s no surprise that the four ‘rocky’ planets of the Solar System – Mercury, Venus, Earth and Mars – feature their fair share of impact craters.
Over the last 4.6 billion years countless natural space objects left over from the formation of the Solar System have hurtled through the galaxy and smashed into these planets and their moons.
While the dense gas of our atmosphere protects us from small impacts, larger comets, asteroids and meteoroids have broken through and crashed into Earth in the past. A space rock’s typical velocity is between 40 and 70 kilometres (25 and 45 miles) per second.
The air ahead of the speeding projectile is therefore suddenly compressed, and when gas is compressed the temperature rises. Indeed, when the leading edge of the object encounters air molecules at these speeds, temperatures as hot as 1,650 degrees Celsius (3,000 degrees Fahrenheit) are generated.
The violent collision of atoms causes the surface of the projectile to become incandescent. When a glowing meteoroid shoots through the atmosphere it becomes known as a meteor (or ‘shooting star’).
While the smallest meteors boil up and melt in our planet’s atmosphere, larger ones can break through and crash into the surface as meteorites. Much larger projectiles, however, will hurtle straight through the atmosphere and smash hard and fast into the ground creating huge impact craters. A ten-kilometer (six-mile)-wide projectile can produce a crater more than ten times that size.
When an impact occurs, the projectile’s kinetic energy is instantly transferred to the target’s surface in other forms, including heat and sound. Large meteorites travel many times faster than the speed of sound and, when they strike the ground, energy is released over a small area in a very short space of time.
The heat energy generated usually vaporizes the meteorite itself. The impact, meanwhile, excavates the crater, forcing surface material (ejecta) up into the air. The initial seismic shockwave travelling out from the collision site is closely followed by a secondary ‘release’ wave generated by the reflection of the first wave. A central mound then often forms in the floor of the crater when the downward pressure of the impact rebounds back up.
Did a mega impact kill off the dinosaurs?
It’s believed that 65.5 million years ago Earth may have been struck by a number of massive asteroids whose impacts triggered the K-T (Cretaceous-Tertiary) global extinction event thought to have wiped out around 80 per cent of Earth’s animal and plant species – most notably all the non-avian dinosaurs.
As well as the immediate annihilation caused by such an asteroid impact, other fatal consequences include wildfires ignited by the burning rock, kilometre-high mega-tsunamis triggered by the shockwaves, global clouds of dust and other ejecta capable of blocking out the Sun for a decade, as well as endless other knock-on effects from the strike.
Evidence that supports the asteroid theory includes the discovery of the 180-kilometre (111-mile)-wide Chicxulub Crater in Mexico’s Yucatan Peninsula. What’s amazing about Chicxulub is that it’s not actually visible above ground; its structure lies almost a kilometer (0.6 miles) below today’s surface. The crater was only discovered in the Seventies by people prospecting for oil.