A comet starts to tell its watery tale

It’s widely accepted that when the Earth formed approximately 4.6 billion years ago, it was so hot that any water present in its component parts must have boiled away. Today, however, over two thirds of the Earth’s surface is covered with water; how this got here is the subject of many a steamy debate.

The most likely scenario is that it arrived with a collection of comets and asteroids that collided with the Earth once it had cooled down, but how much each object might have contributed to the Earth’s water supply is unknown. Now, mass spectrometry measurements taken by instruments onboard the European Space Agency’s (ESA) Rosetta spacecraft have collected data that may help us eventually answer this question.