Water on Earth
Scientists believe that just after the Earth formed, it was very hot and dry. Theory also suggests that millions of water-rich comets and asteroids bombarded our planet around 3.8 billion years ago, neatly explaining why oceans later appeared.
What's more, the ratio of deuterium – or "heavy hydrogen" because it contains a neutron in addition to a proton – to hydrogen in our sea water matches the value found in water-rich asteroids, suggesting a common origin.
But Hidenori Genda and his colleague Masahiro Ikoma from the Tokyo Institute of Technology suggest another possibility. They say the Earth could have had a thick atmosphere of hydrogen, which reacted with oxides in the Earth's mantle to produce copious water.
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