The Earth is full of water. Not just lakes, river, streams, and oceans on the crustal surface, or even aquifers close to the surface—the planet literally holds water inside itself.
Deep inside the mantle, where the temperature and pressure are so high you would think it impossible, viscous crystalline rocks potentially trap the equivalent of the Pacific Ocean.
Last year, scientists found a diamond with the tiniest speck of an olivine mineral called ringwoodite in it that was 1.5 percent water by weight. Ringwoodite only exists at great depths, some 550-660 km beneath the surface of the Earth, where phase transitions alter the structure of olivine into something that is more capable of holding water.
Convection within the mantle could conceivably bring water held in olivine back to the surface. In the case of the ringwoodite-containing diamond, the process was rapid and explosive, but it is more likely to be slow and gradual. The inner-Earth's water cycle is thought to take on the order of 250-500 million years.
There are chemical processes at work around undersea vents and volcanoes by which water gets incorporated into rock in the Earth's crust. The crust is constantly moving, with separate plates jockeying for position, rubbing up against one another, and sometimes getting subsumed underneath each other.
When one crustal plate dives beneath another, that's called subduction. This process is thought to take rocks, and the water held in them, down into the mantle.
At about 100-150 km down, the rocks start to break down under the pressure and increasing temperature. Water gets released during the breakdown process, but it's not entirely efficient. A lot of water remains tied up in the minerals as they break apart, and recombine through chemical reactions. They head ever deeper.
We know that ringwoodite can hold water, but it has been determined that below 660 km, ringwoodite transitions into yet another form of olivine called bridgmanite, which can't hold much water. However, seismic mapping experiments have detected areas of melt, melted material held within the crystalline solids that differ in their chemical composition, and which are possibly indicative of water, at depths of 760 km. This is 100 km deeper than water should be able to venture.
So, how does the water get there? And, how is there still water deep down there if the cycle keeps taking the stuff back to the surface? What is the missing step?
Wendy Panero, PhD, an Assistant Professor at Ohio State University, has been addressing these questions with her graduate student, Jeff Pigott. Together, they created computer models of the lower mantle, and came up with an answer.
Garnet. The burgundy-colored mineral is stable at depths beyond what ringwoodite can handle, and well into the lower mantle. It is possible that garnet could be a water-carrying missionary into the land of bridgmanite.
If the mechanism is correct, it puts another link in the chain of Earth's inner water cycle, and when connected to the oceans and atmosphere, it puts the duration of a complete cycle on the order of billions of years. Additionally, it constrains the amount of water that could be contained within the Earth by specifying the minerals that are in the chain, and determining their respective contributions to the cycle. Whereas previous estimates have put the amount of water in the mantle at 1-3 times the amount of water on the surface, this study brings that quantity down to a single ocean. Regardless of the reduction, this is still substantial considering that all of the water could have originated from geochemical processes alone.
The interior of our planet is something we can't touch, unless it spits itself out at us. Our technological abilities allow us to mimic it ever more precisely with each passing advancement. The lab Dr. Panero has created contains a piece of equipment called a diamond anvil cell, which squeezes minerals between two diamonds in order to apply immense amounts of pressure, and then fires a laser to bring up the heat to subterranean levels. Her lab just might have the burn marks to prove it. She also gets to take that diamond anvil cell to a synchrotron where she and a team of physicists fire high-energy x-rays at it.
Dr. Panero is also a planetary scientist in addition to investigating our Earth's geochemical pathways, and she suggested that plate tectonics might be the key to Earth's abundance of water. The mantle probably plays an influential role in the amount of water that is in our oceans, and consequentially the amount of carbon that it can store. However, Panero mentioned that this finding raises more questions than it answers, as we know very little about the content of these melts at great depth, the other things they could carry within them, and exactly how they move through the mantle's circulation.
If we want to get a better idea of our planet's formation and modern state, rather than simply considering the Goldilock's zone as the place where water can be liquid, we need to look at our and other planets with a broader eye toward what Panero called the "geochemical Goldilock's zone"… that place where all the chemical and physical processes on and in a planet can allow for the culmination of something like oceans and an atmosphere.
Maybe we are even more lucky than we thought.
