The solar system once experienced a meteor shower of epic proportions: Asteroids whizzed around the inner planets, crashing down in a rain of fire that left their surfaces scarred for billions of years. Astronomers typically call this period the Late Heavy Bombardment.
But exactly when that fiery assault happened has been a matter of intense debate. The answer has big implications for the evolution of the solar system as a whole, and even for the timeline of life on Earth.
Finding evidence of such a bombardment here on Earth is difficult. Our planet regularly melts and recycles its crust, destroying detailed evidence that might give us a concrete age for the period of heavy impacts. Farther off, on Mercury, Mars, and the rocky or icy moons of the outer solar system, scientists are left to count craters, an imprecise dating method. The other option is to use an objective dating method – radiometric rock dating, for instance – on bodies that have kept cleaner records than Earth. The moon and asteroids – or the meteorite pieces of them that fall to Earth – are the most accessible.
