Water is rather ubiquitous, cosmically speaking, composed of hydrogen, the most common element in the universe, and oxygen, the third most common. In between is helium, a noble gas that doesn’t play well with other elements.
Water was present in great quantity 4.6 billion years ago when the planets of our solar system took shape. Venus, Earth and Mars formed with copious amounts of water. But Venus’ 900-degree atmosphere boiled it away and broke it down into its constituent parts or combined it with sulfur dioxide gas to create the planet’s thick sulfuric acid cloud cover. Mars’ small size meant weak gravity, so its atmosphere leaked into space, and its water evaporated, which it does with no air pressure above it, and also leaked into space.
Strong, active volcanism, aided by massive asteroid impacts boiled into space most, if not all, of remaining water on all three planets. And yet, today, three quarters of Earth is covered with water. Where did it come from?
For years, astronomers assumed that comets, the most common water-bearing objects in our solar system, brought the water to Earth. But comets formed much farther from the sun than Earth did. The isotopic composition of water differs with distance from the sun.
Thanks to the Rosetta spacecraft, we now know that cometary water doesn’t match terrestrial water. But asteroid water does. Today, asteroids are quite parched, but 4.5 billion years ago, when many asteroids impacted the planets and moons of the inner solar system, water represented a much larger fraction of their mass. And, it appears, they are the source of Earth’s oceans.
It turns out that asteroid impacts that, today, could wipe out life on Earth, made it possible for life to flourish here in the first place by providing the precious water.