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However, because plate tectonics constantly changes and revamps the crust, the first rocks have long since been recycled, melted down and reformed into new outcrops.
In the early 20th century, scientists refined the process of radiometric dating.
Gravitational interactions coalesced this material into the planets and moons at roughly the same time.
By studying other bodies in the solar system, scientists are able to find out more about the early history of the planet.
More than 70 meteorites have fallen to Earth to have their ages calculated by radiometric dating.
The oldest of these have ages between 4.4 and 4.5 billion years.
Research groups in Australia found the oldest mineral grains on Earth.
These tiny zirconium silicate crystals have ages that reach 4.3 billion years, making them the oldest materials found on Earth so far. The rocks and zircons set a lower limit on the age of Earth of 4.3 billion years, because the planet itself must be older than anything that lies on its surface.
Although no rocks have been deliberately returned from Mars, samples exist in the form of meteorites that fell to Earth long ago, allowing scientists to make approximations about the age of rocks on the red planet.