In the beginning, all was dust and gas. For a while, anyway — until a great gas cloud, called the solar nebula, in what would become our solar system gradually condensed. Then came the Sun. Eventually, the closer, hotter protoplanets collected heavy metals floating around and became the terrestrial planets (Mercury, Venus, Earth and Mars) and the more distant protoplanets, in turn, became the gas giants (Jupiter, Saturn, Uranus and Neptune).
How Earth ended up with its volatile chemicals — composed of elements like hydrogen that can easily transform into lightweight gases and vapors — has therefore been something of a mystery. In particular, Earth’s persistent supply of a rare form of helium, called helium-3, has posed a challenge. Most of it was created shortly after the Big Bang, and its supply isn’t replenished on Earth. Yet mid-ocean ridges steadily leak about 4 pounds of it into the atmosphere each year.
