One piece of evidence supporting the idea that water on Earth may have come from comets is the striking similarity between the composition of water found on comets and that of water found on Earth. The ratio of deuterium to hydrogen in water is one key indicator, as it is found to be similar on both comets and Earth. Furthermore, the detection of complex organic molecules, such as amino acids, in comets strengthens the case for cometary impact delivering water to our planet. Lastly, the presence of isotopes of the element xenon in Earth's atmosphere, which are also found in comets, further supports the theory.
