There are two main theories as to how water first appeared on Earth.
The first posits that Earth’s water didn’t start here, but rather arrived with some extraterrestrial help. No, not aliens – icy comets and asteroids. When the Earth was being bombarded by such objects during its early life, some 4.5 billion years ago, enough water may have been carried by asteroids and comets to populate the world’s oceans. Asteroids are the most likely candidates to bear water to Earth since they have a similar deuterium-hydrogen (D/H) ratio to that of our oceans. The two 4.5-billion-year-old meteorites with traces of liquid water discovered here on Earth lend credence this hypothesis. Comets are typically passed over as water delivery candidates. Their D/H ratio is markedly different from that of our oceans. However, there are theories that claim it is possible that the D/H ratio of the oceans evolved over time, which means that we cannot fully dismiss comets.
The second theory of water generation claims that some of Earth’s water may have actually been created here. To investigate this idea, scientists have interrogated someone who has been here since shortly after Earth’s birth: the moon. The moon is currently understood to be formed from the debris split off from Earth when a Mars-sized body collided with our young planet. Therefore, moon rocks are actually 4.5-billion-year-old Earth rocks. The D/H ratios of moon rocks suggest that they shared a common water source with Earth, which, in turn, suggests that that Earth had water since its formation. This water could be the remnants of planet formation, the result of water-vapor-producing volcanism on early Earth, or the result of water-containing minerals (hydrates) being slowly squeezed of their water.
Now that we’ve filled our bottles with our precious Earth water, it’s time to start our journey outwards, to find the rest of the water in the universe. Drawing on the fact that the average human should drink 4 fluid ounces of water per mile walked, we can calculate just how much we will need to take on this walking tour of the universe.
Distance to next destination: 33.9 million miles (at the closest approach)
To stay hydrated, drink: 1.1 million gallons (or, 1.7 Olympic-sized swimming pools)