The Enduring Mystery of Earth’s Water

You have to go to extreme lengths to find places on Earth that don’t reveal that they’re part of a water-rich planet. Even the highest and driest deserts, like the Atacama Plateau in South America, still get a minimum of a couple of millimeters of annual precipitation on average (although there are places where we don’t yet know what the average is because it’s simply not rained for years). And if you whip out your handy mass spectrometer on a desert walkabout the chances are that you’ll be able to detect at least a few atmospheric water molecules.

Go elsewhere, and it’s hard to imagine anything but a water-logged world. More than 70 percent of Earth’s surface is covered in oceans and roughly 97 percent of the surface water is in those oceans, leaving a scant 1 percent as freshwater. Water is also seldom static, whether it’s flowing in ocean currents or being evaporated and precipitated. Averaged out over the planet there is about 100 centimeters of rainfall a year, but that’s across a total surface area of around 5.1×10¹⁸ square centimeters. In other words, doing the back-of-the-envelope calculation, some 510 trillion metric tons of water gets evaporated and then re-precipitated every year on Earth.

But the catch is that we don’t really know where all of this water came from in the first place. For a long while our picture of the formation of a rocky planet like the Earth has involved a violent, hot assembly some 4.5 billion years ago out of comparatively dry material in the inner solar system. Water would have come along later, with proposals for possible delivery by comets from the chill, frozen outer solar system, or by rocky but still volatile-rich meteoritic infall. But these options have proven tricky to justify completely for a variety of reasons. Comets, for instance, often (but not always) have a deuterium concentration that doesn’t match what we see in Earth’s water—limiting their likely contribution. Similarly, water-rich rocky meteoritic material—so-called carbonaceous chondrites—have isotopic differences that could also limit how much they contributed to a young planet.

At the same time, the representative type of material for building the entirety of a rocky Earth (and matching the planet’s overall isotopic composition in elements like oxygen and calcium) seems to closely resemble what’s called enstatite chondrite. Chunks of enstatite chondrite are still around in the solar system, and occasionally fall as meteorites. But they’ve been thought to be too dry to be involved in Earth’s water supply.

Now, in a work reported by Piani, et al. in the journal Science, an analysis of the composition of 13 enstatite chondrite meteorite samples reveals a much higher than expected hydrogen content. Extrapolating from these numbers the researchers claim that if this is the type of protoplanetary material that built Earth, it could have resulted in a total, initial water content of at least three times the present mass of water in our oceans. The same material could have also provided a starter mix of atmospheric nitrogen to the young planet.

This possibility is enormously appealing for its relative simplicity: Our wet world was simply made this way from the very beginning, with little need to invoke any more complex evolution except for a small drizzling from comets and other outer solar system material. Whether or not this idea holds up to further scientific scrutiny, it’s a beautiful reminder that even the simplest things in our lives, like a glass of water or a shower in the morning, are actually windows into the deepest origins of everything we know.