I don't know if you've noticed by now, but the earth is a little wet. How the earth got all of its water is one of the greatest puzzles in the formation of the solar system, and a team of Japanese researchers just found an important clue. But not on Earth – the reference is to Mercury.
Here is the traditional history of the early solar system, as well as we know it. The sun is formed with a disk of dusty gas that surrounds it. At a certain distance from the sun, called the snow line, solar radiation is too hot and too intense to support the formation of ice or lighter elements. Hence the rocky planets are formed.
Beyond the ice line, which is somewhere around today's orbit of the asteroid belt, ice and light elements can stick together to form the giant bulky planets of the outer system.
In between is a kind of no man's land of stones, ice, rubble and basically a whole range of other junk.
Once the giant planets are formed, they rearrange themselves, and their gravitational influence causes chunks of random junk to plow into the inner solar system, providing all sorts of goodies like water. These treats end up on the surfaces of the rocky worlds where they sit billions of years.
However, a team of Japanese researchers are challenging this view by looking at the crater balance on Mercury. To explain the abundance of lighter elements in the mercury crust, there must be at least three times as many impacts as we see in the crater record. (And if you're wondering why Mercury fascinates us so much, it's because this airless, dead world has no erosion and can preserve memories of bombings billions of years ago.)
To explain the discrepancy, the researchers believe the bombardment was strong enough to literally chew up the mercury crust and turn it into molten mud. In this way, most of the lighter and more volatile elements that the bombardments provided were buried deep underground.
And what about the earth? A similar process could have taken place, with most of the water supply coming from these early bombardments that were sunk deep below the surface. Fortunately, they provided enough water to leave Earth with a healthy supply of oceans.
The European Space Agency's upcoming BepiColombo mission, which is currently en route to Mercury, will provide even more answers.