Our solar system’s orbit through the Milky Way helped form Earth’s first continents

The transit of our solar system through the spiral arms of the Milky Way. Major geologic events are displayed on the overlay timeline to highlight the increased frequency of events associated with passing through dense star regions. The + indicates the number of passages of the Earth in the spiral arm. Credit: Kirkland et al., 2022.

Earth is unique among planets known to have continents, and their formation has fundamentally influenced the habitability of our planet. It is generally accepted that the Earth’s continental crust was formed by internal processes. However, new research from a team at Curtin University challenges that theory by attributing the incipient formation of continents to high-energy comets bombarding early Earth.

Led by Chris Kirkland, the team studied the mineral zircon from Earth’s oldest continents, the North American Craton in Greenland and the Pilbara Craton in Western Australia. Crystals were mined from cratons because they are the oldest and most stable parts of a continent. Zircon crystals are ideal timekeepers because they contain trace amounts of uranium (U), isotopes that decay at a known rate to become lead (Pb). When zircon is analyzed, the amount of Pb produced by the decay of U corresponds to the time that has elapsed since the crystallization of the zircon. Additionally, hafnium (Hf) isotopes in zircon follow episodic influxes of magma that contribute to the formation of continents. Analyzes of U decay reveal that the two cratons formed between 2.8 and 3.8 billion years ago, while analysis of the time spectrum of Hf isotopes shows that they have underwent periods of increased continental crust formation every ~170 to 200 million years. This periodicity corresponds to the “galactic year”, which is the time required for the Sun to orbit around the Milky Way galaxy. During this orbit, our solar system moves through the spiral arms of the Milky Way, where star density is high, resulting in an increased frequency of comet bombardments on Earth. Kirkland and his team hypothesize that when these high-energy comets hit Earth, they initiate increased melting in the mantle, acting as nuclei for the formation of new continental crust. These findings oppose the mainstream theory that the Earth formed in isolation and present an exciting connection between Earth’s geological processes and the motion of our solar system. READ MORE

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