Earth is a famously wet planet, but where all that water came from in the first place remains a mystery. The most commonly-accepted theory is that comets and asteroids delivered it via impacts during the early days of Earth, and now a NASA study has found new evidence to support that idea. Observations of a comet that whizzed by close to Earth a few months ago show that it contains “ocean-like” water – and this may apply to other previously-dismissed comets too.
Some theories suggest that water has been here more or less since the beginning, when early Earth was just a huge ball of churning, warm, wet mud. Other evidence points to it being delivered when a Mars-sized proto-planet crashed into the young Earth and spawned the Moon.
That might sound like an open-and-shut case, but it’s more complicated than that. Most comets studied so far have been found to harbor the wrong type of water. The stuff we’re used to here on Earth is, as you know, made up of two hydrogen atoms and an oxygen atom. But there’s also “heavy” water, which contains a hydrogen atom with an extra neutron.
