Ayu Aulia
New scientific investigations have profoundly altered the understanding of how Earth secured its vital water supply during its tumultuous formation, revealing a subterranean sanctuary within the planet’s deep mantle that sequestered vast quantities of H2O through an epoch of extreme heat and planetary-scale volcanism. This groundbreaking research posits that Earth’s most abundant deep-seated mineral acted as a formidable reservoir, preserving the essential ingredient for life even as the young world was consumed by a global ocean of molten rock. This paradigm shift in geoscience offers a compelling explanation for the planet’s eventual transformation from a hostile, incandescent body into the blue, life-sustaining world observed today.
During the Hadean Eon, approximately 4.6 billion years ago, Earth’s nascent existence was characterized by conditions inimical to liquid water. The early planet was subjected to an unrelenting barrage of asteroid and protoplanet impacts, generating immense kinetic energy that maintained its surface and much of its interior in a persistently molten state. This epoch was defined by a pervasive magma ocean, where temperatures soared to such extremes that any water present would have been instantly vaporized, existing only as superheated steam in a dense, suffocating atmosphere, or lost to space. The prevailing scientific consensus has long grappled with the apparent contradiction between this fiery genesis and the subsequent emergence of vast surface oceans, which now cover approximately 70% of the globe. The fundamental question—how did Earth retain enough water to facilitate the development of these aquatic environments, an indispensable prerequisite for life—has remained a central enigma in planetary science for decades.
Previous hypotheses regarding Earth’s water inventory often focused on two primary mechanisms: either water was delivered relatively late in Earth’s accretion history, perhaps by icy comets or carbonaceous chondrites after the planet had sufficiently cooled, or it was always present but largely outgassed from a relatively dry, initially forming mantle. However, these models faced challenges in fully accounting for the sheer volume of water observed and its consistent presence throughout geological time. The complexity lay in understanding the fate of water during the intense early phases, particularly how it could be sequestered within the planet’s evolving structure rather than being expelled or lost.
A recent study, published in Science, provides a novel and compelling resolution to this long-standing puzzle. The research, led by Prof. Zhixue Du of the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences (GIGCAS), reveals that substantial quantities of water could have been effectively stored deep within Earth’s mantle as the planet progressively cooled and transitioned from a molten state to a largely solid body. This finding fundamentally redefines our understanding of Earth’s deep-water cycle and its role in planetary evolution.
The cornerstone of this new explanation lies in the properties of bridgmanite, the most abundant mineral within Earth’s lower mantle, constituting approximately 70-80% of its volume. This magnesium silicate perovskite mineral, stable under immense pressures and temperatures, has now been identified as a critical "water container." The study demonstrates that bridgmanite possesses an exceptional capacity to structurally incorporate significant amounts of water into its crystal lattice, acting as a microscopic sponge. This intrinsic ability of bridgmanite, previously underestimated, could have enabled early Earth to sequester a vast, subterranean reservoir of water beneath its surface as the global magma ocean gradually solidified. This internal water vault, the researchers contend, played an instrumental role in guiding Earth’s transformation from an uninhabitable, incandescent world into a dynamic, hydrated planet capable of sustaining diverse ecosystems.
Prior experimental investigations into bridgmanite’s hydrous capacity had suggested a relatively limited ability to store water. However, a critical limitation of these earlier studies was their execution at comparatively lower temperatures, failing to accurately replicate the extreme thermal conditions characteristic of the deep mantle during Earth’s formative years. To address this methodological gap and re-evaluate the question with greater fidelity, the research team confronted two formidable experimental challenges. Firstly, they needed to accurately reproduce the extraordinary pressures and temperatures prevalent at depths exceeding 660 kilometers beneath Earth’s surface – conditions where pressures can exceed 23 gigapascals and temperatures can soar into the thousands of degrees Celsius. Secondly, the detection of minuscule traces of water, often mere hundreds of parts per million, within mineral samples often thinner than a human hair, demanded unprecedented analytical precision.
To overcome these obstacles, the team engineered a sophisticated experimental setup centered around a diamond anvil cell system. This specialized apparatus, capable of generating immense pressures by compressing samples between two diamond anvils, was integrated with advanced laser heating technology. This allowed for precise and localized heating of the minute mineral samples to temperatures as high as approximately 4,100 °C, accurately mimicking the thermal environment of the deep lower mantle. Concurrently, high-temperature imaging techniques were employed to precisely monitor and measure equilibrium temperatures within the sample chamber. By meticulously reproducing these extreme deep mantle conditions and accurately quantifying the thermal states, the researchers were able to investigate the profound influence of temperature on bridgmanite’s water absorption mechanisms, a critical variable missed in previous analyses.
The analysis of the experimental results necessitated the deployment of state-of-the-art analytical facilities at GIGCAS. The scientists leveraged techniques such as cryogenic three-dimensional electron diffraction, which provides atomic-scale structural information, and NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry), a powerful tool for high-resolution isotopic and elemental mapping. Furthermore, in collaboration with Prof. LONG Tao from the Institute of Geology of the Chinese Academy of Geological Sciences, the team incorporated atom probe tomography (APT). This cutting-edge technique provides atomic-scale three-dimensional chemical imaging, allowing for the precise visualization and identification of individual atoms within the mineral structure. Collectively, these advanced methodologies functioned as ultra-high-resolution "chemical CT scanners" and "mass spectrometers" for the microscopic realm. This integrated analytical approach enabled the team to meticulously map the distribution of water within the minute mineral samples and, crucially, confirm that water was not merely adsorbed on surfaces but structurally dissolved within the bridgmanite crystal lattice itself, forming hydroxyl groups.
The findings from these rigorous experiments were transformative. They unequivocally demonstrated that bridgmanite’s capacity to incorporate water, quantified by its water partition coefficient, exhibits a dramatic increase at elevated temperatures. This pivotal observation implies that during Earth’s hottest magma ocean phase, when temperatures were at their peak, newly formed bridgmanite crystals would have been capable of storing substantially more water than previously estimated. This discovery directly challenges the long-standing geological assumption that the lower mantle, due to its extreme conditions, must be largely anhydrous. Instead, it suggests a much wetter deep mantle, particularly during the planet’s formative period.
Building upon these experimental results, the research team developed sophisticated numerical models simulating the cooling and crystallization process of Earth’s magma ocean. Their simulations yielded compelling evidence that, owing to bridgmanite’s exceptional efficiency in trapping water under extreme thermal conditions, the lower mantle emerged as the single largest water reservoir within the solid Earth following the solidification of the magma ocean. The model projections indicate that this colossal subterranean reservoir could have been between five and 100 times larger than earlier estimates, potentially holding a total water volume ranging from 0.08 to 1 times the volume of Earth’s current surface oceans. Such a vast internal repository fundamentally alters the global hydrological budget and the understanding of Earth’s initial water distribution.
The implications of this deeply stored water extend far beyond mere volumetric considerations; it represents a critical factor in Earth’s long-term geological and environmental evolution. This sequestered water did not simply remain inertly trapped; instead, it acted as a profound "lubricant" for Earth’s internal geodynamic engine. By significantly lowering the melting point and reducing the viscosity of mantle rocks, the presence of water facilitated crucial internal processes such as mantle convection and, critically, the initiation and sustained motion of tectonic plates. Plate tectonics, a defining characteristic of Earth, is fundamental for regulating the planet’s climate over geological timescales through the carbon cycle, recycling nutrients, and even contributing to the generation of Earth’s protective magnetic field. The deep water, by influencing mantle rheology, provided the necessary conditions for this long-term geological energy and activity.
Over immense spans of geological time, a portion of this deeply stored water was gradually returned to the surface through processes of volcanic outgassing and magmatic activity. This slow but continuous release of volatiles contributed substantially to the formation of Earth’s early atmosphere and the eventual condensation of water vapor to form the primordial oceans. The researchers postulate that this buried "spark of water," meticulously preserved within the deep mantle, may have been the decisive factor that steered Earth’s trajectory from a molten, desolate inferno towards the vibrant, blue, and profoundly life-friendly planet that exists today.
This groundbreaking research necessitates a re-evaluation of fundamental planetary formation models, particularly those concerning the retention of volatile elements in rocky planets. It suggests a more robust and efficient mechanism for terrestrial planets to sequester water internally, even if their early stages involve extreme thermal events. For the burgeoning field of exoplanet research, these findings hold significant implications. If Earth-like planets can indeed harbor vast internal water reservoirs, it expands the criteria for planetary habitability. A planet appearing dry on its surface might, in fact, possess a significant subterranean hydrological cycle, potentially extending the window for sustaining life or providing a mechanism for replenishing surface water over billions of years. Future research will undoubtedly focus on refining these models, exploring the precise kinetics of water release from bridgmanite, investigating the isotopic signatures of deep mantle water, and examining the hydrous capacities of other deep-seated minerals, thereby further illuminating the intricate hydrological history of Earth and potentially countless other worlds. This study stands as a testament to the intricate and often hidden processes that shaped our planet’s destiny, underscoring the profound role of deep Earth in forging a habitable world.
Ridwan Kamil
Safa Marwah
Aura Kasih
Lisa Mariana