Here’s a question I never thought about: where did the oceans come from?

Scientists have wondered and this is what they think, according to Science Advisor.

Billions of years ago, asteroids bombarded Earth, bringing with them bits of water that coalesced into the ocean and helped make our planet habitable. But the details of how so much water could arrive in such small packages have been fuzzy.

In 2018, Japanese spacecraft Hayabusa2 visited Ryugu, a near-Earth asteroid studied to show us what materials could have been brought to Earth from other bodies in the solar system. The samples the craft returned from Ryugu’s surface were tiny: only a few grams in total. But when researchers analyzed two key isotopes used as geological clocks within them, lutetium-176 and hafnium-176, they noticed far higher levels than expected. This indicated that fluid, likely water, was washing out the isotopes from the rocks’ interior.

The researchers hypothesize that Ryugu’s larger asteroid parent was in a space collision, triggering buried ice to melt and seep into its outer layers, chunks of which later broke off, like Ryugu. While researchers believed watery asteroids only occurred in the very young solar system, this theory would suggest they retained ice for a billion years. That in turn suggests that when asteroids like Ryugu’s parent crashed into Earth, they were carrying two to three times more water than we gave them credit for.

“Suddenly we have evidence that these [asteroids] were wetter than we previously thought, which meant that they can more reasonably explain the origin of the Earth’s oceans when they hit the early planet,” astronomer Jonti Horner, who was not involved in the work, told New Scientist.