One piece of evidence supporting the idea that water on Earth may have come from comets is the measurement of the ratio of deuterium to hydrogen in Earth's oceans. This ratio is found to be similar to that of comets, suggesting a common origin. Additionally, the chemical composition of water found in comets matches the composition of water on Earth, further supporting the theory. The detection of water-rich plumes emanating from comets, such as those observed on comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft, also provides direct evidence of water being present on comets. Finally, the presence of ice and organic compounds, which are essential ingredients for the formation of water, have been observed on comets.
