Drinkability: Highest.

There are two main theories as to how water first appeared on Earth.


In 2018, flyby observations by the spacecraft OSIRIS-REx confirmed that this asteroid, Bennu, has hydrated minerals. Bennu is one of several asteroids which show evidence of water very similar in composition to our own oceans. While it does not now have liquid water, the parent asteroid it split from billions of years ago likely did.

Photo by: NASA/Goddard/University of Arizona

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)

About The Author

Madelyn Broome

Madelyn was the 2018 Editor-in-Chief of Innovation, and a former writer and editor for the Space/Physics section. Her piece "Where's the Water?" won the 2019 Gregory T. Pope Prize for Science Writing. She is passionate about science communication and about making science engaging and accessible for people of all ages - though she especially enjoys working to ignite excitement for the sciences in young girls and other underrepresented communities in STEM. When she's not trying to share her enthusiasm for the sciences, she can usually be found exploring, practicing mixed martial arts, archery, lifting, playing soccer, or just generally trying to make up for the dessert she just ate.