William sent in this link – https://phys.org/news/2019-05-formation-moon-brought-earth.html … the formation of the Moon brought water to Earth, the latest idea on why there is so much water on earth compared to the other planets. The new theory involves a collision of the Earth with a proto planet – which is called Theia. This is the same collision that has been proposed as causing the actualy creation of the Moon – but with a difference. It brought lots of water to Earth rather than just gouge a great big hole out of the planet in which the bits reassembled themselves into the Moon. Until now, models indicated Theia existed in the inner solar system. The new theory is that it originated in the outer solar system – which is supposed to be water rich. The research is published by the journal Nature Astronomy (May 2019). Apparently, the innder solar system is dry – apart from the Earth.