An artwork of the early Earth around 3.8 billion years ago, when its oceans were forming. Both the Moon (right) and the Earth are heavily cratered. There are two possible sources for Earth's water: it may have separated out from the rocks shortly after the Earth formed, or it may have been delivered to Earth during the late heavy bombardment period, in the form of comets or water-rich, chondritic meteorites. It is most likely that both processes contributed - the chemical make-up of the water found in comets and asteroids differs from that found in our oceans, so impacts alone are an unlikely explanation.