Theia, a Mars-sized body that formed over 4 billion years ago, collided with Earth, creating a volume of molten rock that formed the Moon. The largest asteroid to hit Earth was about 6 miles in size, with the largest verified crater being Vredefort in South Africa. The Wilkes Land anomaly in Antarctica may indicate an even larger impact structure.
The largest asteroid to ever hit Earth was probably Theia, a Mars-sized body that formed over 4 billion years ago, during the early days of the solar system. Theia formed at a Lagrange point, a gravitationally stable region in Earth’s orbit, located on the opposite side of the Sun. Eventually, orbital fluctuations swung this body closer and closer to Earth, and eventually collided , throwing up a volume of molten rock greater than all present-day continents combined. Some of this molten rock was ejected with such force that it went into orbit, clumped together, cooled, and became the Moon. This scenario is called the Giant Impact Theory and it accurately explains most features of the Earth-Moon system, such as why the chemical composition of the Moon is similar to that of the Earth’s crust.
From Theia, the largest asteroids known to impact Earth were much smaller, reaching a maximum size of about 6 miles (about 10 km). The larger ones may have hit the Earth during the period called the Late Heavy Bombardment, which occurred between 4.1 and 3.8 billion years ago, but since most of the crust from this period was subducted into the mantle or covered by layers of sediments and volcanic rock, the craters may be hidden.
The largest verified crater on Earth today is Vredefort Crater in South Africa, also known as the Vredefort Dome or Vredefort Impact Structure. The crater is more than 186 miles (about 300km) wide, about twice the size of Chicxulub crater, left behind by the asteroid that wiped out the dinosaurs. Vredefort crater is thought to have been caused by the largest impact other than the one that created the Moon – strictly speaking, Theia was a planetoid rather than an asteroid – of about 6 miles (10 km).
The Wilkes Land anomaly, a concentration of mass under the Wilkes Land ice sheet in Antarctica, could indicate an even larger impact structure, but its nature is uncertain.
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