PRINCETON, N.J. — From the surface to the inner core, Earth has several layers that continue to be a mystery to science. Now, it turns out one of these layers may consist of material from an entirely different planet!
Deep within our planet lies a mysterious, patchy layer known as the D” layer. Located a staggering 3,000 kilometers (1,860 miles) below the surface, this zone sits just above the boundary separating Earth’s molten outer core from its solid mantle. Unlike a perfect sphere, the D” layer’s thickness varies drastically around the globe, with some regions completely lacking this layer altogether – much like how continents poke through the oceans on Earth’s surface.
These striking variations have long puzzled geophysicists, who describe the D” layer as heterogeneous, meaning non-uniform in its composition. However, a new study might finally shed light on this deep enigma, proposing that the D” layer could be a remnant of another planet that collided with Earth during its early days, billions of years ago.
The findings, in a nutshell
The research, published in National Science Review and led by Dr. Qingyang Hu from the Center for High Pressure Science and Technology Advanced Research and Dr. Jie Deng from Princeton University, draws upon the widely accepted Giant Impact hypothesis. This hypothesis suggests that a Mars-sized object violently collided with the proto-Earth, creating a global ocean of molten rock, or magma, in the aftermath.
Hu and Deng believe the D” layer’s unique composition may be the leftover fallout from this colossal impact, potentially holding valuable clues about our planet’s formation. A key aspect of their theory involves the presence of substantial water within this ancient magma ocean. While the origin of this water remains up for debate, the researchers are focusing on what happened as the molten rock began to cool.
“The prevailing view,” Dr. Deng explains in a media release, “suggests that water would have concentrated towards the bottom of the magma ocean as it cooled. By the final stages, the magma closest to the core could have contained water volumes comparable to Earth’s present-day oceans.”
Is there a hidden ocean inside the Earth?
This water-rich environment at the bottom of the magma ocean would have created extreme pressure and temperature conditions, fostering unique chemical reactions between water and minerals.
“Our research suggests this hydrous magma ocean favored the formation of an iron-rich phase called iron-magnesium peroxide,” Dr. Hu elaborates.
This peroxide, which has a chemical formula of (Fe,Mg)O2, has an even stronger affinity for iron compared to other major components expected in the lower mantle.
“According to our calculation, its affinity to iron could have led to the accumulation of iron-dominant peroxide in layers ranging from several to tens of kilometers thick,” Hu explains.
The presence of such an iron-rich peroxide phase would alter the mineral composition of the D” layer, deviating from our current understanding. According to the new model proposed by Hu and Deng, minerals in the D” layer would be dominated by an assemblage of iron-poor silicate, iron-rich (Fe,Mg) peroxide, and iron-poor (Fe,Mg) oxide. Interestingly, this iron-dominant peroxide also possesses unique properties that could explain some of the D” layer’s puzzling geophysical features, such as ultra-low velocity zones and layers of high electrical conductance — both of which contribute to the D” layer’s well-known compositional heterogeneity.
“Our findings suggest that iron-rich peroxide, formed from the ancient water within the magma ocean, has played a crucial role in shaping the D” layer’s heterogeneous structures,” says Dr. Hu.
The researchers propose that the peroxide’s strong affinity for iron creates a stark density contrast between these iron-rich patches and the surrounding mantle material. This density difference acts as an insulator, preventing the iron-rich regions from mixing with the rest of the mantle.
“This model aligns well with recent numerical modeling results, suggesting the lowermost mantle’s heterogeneity may be a long-lived feature,” Deng adds.
In other words, the D” layer’s patchiness could be an enduring imprint from Earth’s tumultuous birth, a relic of the extreme conditions that existed within the ancient magma ocean formed by the giant impact that shaped our world.
StudyFinds Editor Chris Melore contributed to this report.