Comets Might Have Brought Water to Earth

One theory suggests that much of Earth’s water came from comets. In the early solar system, Earth was too hot to hold onto its water, so comets—icy visitors from the outer solar system—may have delivered some of it through countless impacts. Some comets have a similar ratio of deuterium (heavy hydrogen) to regular hydrogen as Earth’s oceans, but Rosetta found that comet 67P’s water has more than three times Earth’s ratio, which suggests asteroids may have supplied more of our water than comets.