Comets from the Milky Way’s other arms helped form Earth’s continents.

Science News winds back the astronomical clock to the early days of Planet Earth, when waves of comet collisions seem to have slammed the planet’s solid crust into shape in a regularly repeating cycle. Geologists think comets are responsible because our solar system’s journey through our galaxy’s arms lines up with the formation of solid land masses:

To peer back in time, geochronologist Chris Kirkland and his colleagues turned to geologic structures known as cratons…. These relics of Earth’s ancient continental crust are some of the planet’s oldest rocks. Using material from cratons in Australia and Greenland that are billions of years old, the team measured the chemistry of more than 2,000 bits of rock. The analysis let the researchers determine the exact ages of the rocks, and whether they had formed anew from molten material deep within the Earth or from earlier generations of existing crust.

When Kirkland and his colleagues looked for patterns in their measurements, the team found that new crust seemed to form in spurts at roughly regular intervals. “Every 200 million years, we see a pattern of more crust production,” says Kirkland, of Curtin University in Perth, Australia.

That timing rang a bell: It’s also the frequency at which the Earth passes through the spiral arms of the Milky Way…. The solar system loops around the center of the galaxy a bit faster than the spiral arms move, periodically passing through and overtaking them. Perhaps cosmic encounters with more stars, gas and dust within the spiral arms affected the young planet, the team suggests.

You can read more of the team’s comet research here, in Geology.