How Earth was watered
Early Earth’s accidental deluge via water-carrying comets has long been a stumbling block for those interested in life on other planets. Scientists agree that life needs water to evolve. But if water only arrives through chance impacts with comets, then life elsewhere might indeed be rare. Water is common among the meteorites and other small bodies whose collisions formed the Earth, but scientists have long believed that the intense heat of the events dried out the young planet. Water must have arrived later, splashing down from comets after the planet was formed. New research, however, is changing that view. Evidence is mounting that the planet’s water arrived early, during formation, aboard meteorites and small bodies called “planetesimals.” The work also suggests that though the planet-forming collisions were so energetic that they led to oceans of magma and widespread melting, even the intense heat would not have dried out the planet completely. The emerging view...