The longer I do science communications, the more I come to appreciate that some questions just don’t have good answers, and people are going to publish ideas – sometimes contradictory ideas – on a regular basis as we work toward answers.

Where is life? Where ever it feels like being, apparently. What is needed for life? Probably water or at least a solvent like water. Where does water come from? That is a very good question we can’t answer. Personally, I side with the researchers saying it came from impacting meteors and asteroids, but a new paper appearing in Physical Review Letters has another idea: water came to Earth from Earth.

How could that possibly work with a liquid metal core, asteroids constantly bombarding the planet, and even a collision with a Mars-sized object that led to the formation of the Moon? Well, it turns out that there actually wasn’t a liquid metal core. And so the water could have been locked up in magnesium hydrosilicate, which is over 11% water by weight and can also stably exists at pressures in excess of two million atmospheres and extremely high temperatures, such as in the Earth’s core. Co-author Artem R. Oganov explains: There was no core at the time. In the beginning of its existence, the Earth had a more or less evenly distributed composition, and it took the iron roughly 30 million years from when the planet formed to seep down to its center, pushing the silicates up into what we now call the mantle.

So all that potential water was safely stored deep below the heavily bombarded surface of Earth. Then the iron slowly crept down into the core, pushing all the silicates up, which broke down into the much more common magnesium oxide and magnesium silicates we see today — plus water. All of that became the mantle, and then the water spent the next 100 million years working its way to the actual surface of the planet, at a time when all the bombardment had slowed down and there was no longer a bunch of surface heat to evaporate the water.

One of the interesting side effects of this new hypothesis is that it can also be used to change our perspective on the origin of water on other worlds. Oganov notes: Mars, for example, is too small to produce pressures necessary to stabilize magnesium hydrosilicate. This explains why it is so dry and means that whatever water exists on Mars, it likely came from comets.

Or how about exoplanets? Co-author Xiao Dong explains: There was an estimate that for an Earth-like planet of any size to be habitable, it should have no more than 0.2% water by weight. Our results imply that for large Earth-like planets, called ‘super-Earths,’ the story is likely different: In such planets, pressures stabilizing the magnesium hydrosilicate must exist even outside the core, locking up large amounts of water indefinitely. As a result, super-Earths can have a much greater water content and still support the existence of exposed continents.

Water, water everywhere, and probably a lot to drink and from different sources than we originally thought.

More Information

Skoltech press release

Mineral Candidates for Planet Interiors (Physics)

“Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth,” Han-Fei Li et al., 2022 January 21, Physical Review Letters