Where Earth got its water
Sean Raymond writes: When Carl Sagan famously called Earth the “pale blue dot,” he was judging a book by its cover. Even though three quarters of our planet’s surface is covered by oceans, our planet is actually very dry. Water makes up about one part in a thousand of Earth’s mass (most of it is simply rock and iron). I say “about” because we don’t know exactly how much water is trapped in Earth’s interior. Estimates range from less than…