unnamedWe know more about the surface of Mars than we do the mantle of the planet we live on. As little as 30kms (19 miles) below the surface – the distance between the centre of London and Heathrow Airport – the continental crust turns into the Earth’s mantle, a layer some 2,900km (1,800 miles) thick that surrounds the Earth’s molten outer core. Underneath the Earth’s oceans, the crust can be as little as five kilometres (three miles) thick. And yet this trifling distance might as well be light years, for we know very little about this vital component of our planet. For example, is there more water down there than we thought? For decades, scientists have postulated that the Earth’s oceans were created by comets striking the planet’s surface. But now some, like Professor Steven Jacobsen of Northwestern University, think that the rocks in the Earth’s mantle might have had a part to play as well; specifically a magnesium-rich silicate called ringwoodite. More