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Where Did Earth's Water Come From?
Credit: Lars Jensen | umich.edu

The exact origin of our planet's water, which covers about 70 percent of Earth's surface, is still a mystery to scientists. Many researchers think that, instead of water forming at the same time as Earth, objects in the outer solar system delivered water to Earth in violent collisions shortly after its formation.

Researchers speculate that any water conglomerating on the surface of the planet as it formed 4.5 billion years ago would have most likely been evaporated away by the young, blazing sun. That means that water probably came here from somewhere else. The inner planets (Mars, Mercury, Venus), were also probably too hot to house water during the solar system's formation, so our water didn't come from them, either.

Outer planetary bodies, however, such as the moons of Jupiter and comets, were far enough away from the Sun to retain ice.

During a period around 4 billion years ago called the Late Heavy Bombardment, massive objects, probably from the outer solar system, hit Earth and the inner planets. It's possible that these objects were filled with water, and that these collisions could have delivered gigantic reservoirs of water that filled Earth.

So what were the objects that delivered the water?

For a long time, astronomers thought that comets chunks of ice and rock with tails of evaporating ice and with long, looping orbits around the Sun were the likely culprit. However, remote measurements of the water evaporating off of several major existing comets (Halley, Hyakutake, and Hale-Bopp) revealed that their water ice was made of a different type of H20 (containing a heavier isotope of hydrogen) than Earth's, suggesting that these comets could not be the source of our water.

With major comets crossed off the list, astronomers began to wonder if clues to our water's past may lie in the asteroid belt. This region of hundreds of thousands of asteroids orbiting between the inner and outer planets was believed by astronomers to be too close to the sun to house water, but astronomers recently found the first evidence of ice on the asteroid 24 Themis.

This discovery and others of ice on asteroids suggest that there might be far more ice in the asteroid belt than originally thought and provide another possibility for the origin of ocean water. Probes sent to explore asteroids, such as the DAWN spacecraft, in the upcoming years will reveal more about their mysterious water ice, potentially help us understand the beginnings of Earth's water.

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