What did early missions like Giotto and Rosetta reveal about the D/H ratio in comets compared to Earth's oceans?

Answer

The cometary ratio was roughly double that of Earth’s oceans

Early analyses of water locked within specific comets indicated a much higher concentration of deuterium, suggesting these comets could only supply a minor fraction of Earth's total water.

What did early missions like Giotto and Rosetta reveal about the D/H ratio in comets compared to Earth's oceans?
OriginEarththeorywatercomet