Where does the water on Earth come from? my country's latest scientific research results provide new ideas for understanding the evolution of life

Image source: Tuchong Creative

There are many different theories about where Earth's water came from, and solving this mystery is crucial to understanding how life emerged and how the dynamics of Earth's interior have evolved over time.

Recently, Dr. Li Hanfei, Associate Professor Dong Xiao and their collaborators from the School of Physical Sciences of Nankai University discovered two new hydrated magnesium silicate (Mg2SiO5H2) structures that can exist as storage media for water in the early Earth. After the core-mantle separation, it released a large amount of water. This provides new ideas for the origin of the ocean on the early Earth. The relevant paper was published in the physics journal Physical Review Letters on January 21.

Screenshot of the paper.

Image source: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.035703

The origin of Earth's water remains controversial

Currently, there are two views on the origin of water: one is the "hell origin theory", which holds that water comes from deep within the earth, that is, the earth acquired a large amount of water during the accretion period and stored it inside the earth; the other is the "heaven origin theory", which holds that after the formation of the earth, the bombardment of water-rich meteorites provided a large amount of water.

When did water appear? There is also controversy. In the past, scientists found traces of water in the analysis of meteorites about 4.5 billion years ago, indicating that water may have existed at the beginning of the Earth's origin.

The ocean covers 71% of the Earth's surface, making the Earth a veritable "water ball" and a vibrant planet.

Image source: Tuchong Creative

Recently, more and more evidence supports the first hypothesis. The ratio of deuterium to hydrogen is considered to be the fingerprint of the origin of water. A discovery pointed out that the deuterium-hydrogen ratio of the Earth's deep mantle is very low, close to the basic components of the early Earth, enstatite chondrites and the original solar nebula, and much lower than comets and other outer solar system materials. This provides strong evidence for the view that the water in the Earth's interior may have come directly from the original solar nebula.

Dong Xiao explained: "The deuterium-hydrogen ratio is considered to be a sign of the origin of water. With the 'ignition' of the sun and the subsequent action of the solar wind, the deuterium-hydrogen ratio in the outer solar system is different from that in the original solar nebula. The deuterium-hydrogen ratio of the Earth is the same as that in the original solar nebula, but different from that in the outer solar system. This is strong evidence that the water on Earth is not an 'extraterrestrial'."

However, there are considerable problems with this hypothesis. Compared with other planetary materials such as iron and silicates, water has much lower melting and boiling points, so on the thousands of degrees hot surface of the newborn Earth, water will evaporate into space. It can be seen that water can only exist deep inside the newborn Earth and be released when the newborn Earth evolves to a certain extent. However, the form of material storage in this process is still unclear.

Discovery of a new stable hydrous mineral

Through first-principles calculations and structural prediction methods, the research team found that two new undiscovered stable hydrated magnesium silicate structures appeared under millions of atmospheres of pressure, and named them α phase and β phase. Among them, the stable pressure range of the α phase is 262-338 GPa, and the stable range of the β phase is above 338 GPa. Today, the pressure at the core-mantle boundary is 136 GPa, and the pressure at the center of the earth is 364 GPa. The main difference between the α phase and β phase structures is that there are different numbers of oxygen atoms around the magnesium ions.

Image source: "Nankai Physics" (Official WeChat public account of the School of Physics of Nankai University)

First-principles calculations show that at a pressure of 300 GPa, hydrated magnesium silicate has a very high density and extremely high water content, containing 11.4% water by weight, which is higher than the water content of most other reported hydroxide minerals.

"Theoretical calculations show that this hydrated magnesium silicate is far more heat-resistant than other hydrated minerals, and shows no signs of decomposition or melting even at a high temperature of 8,000 Kelvin," said Dong.

Image source: "Nankai Physics" (Official WeChat public account of the School of Physics of Nankai University)

In the early Earth, because the core and mantle had not yet separated, silicates and excess magnesium oxide may have penetrated deep into the Earth's interior, thereby enduring pressures far higher than they are today. For example, when the pressure is higher than 262 GPa, they can store water in the form of hydrated magnesium silicate.

Calculations show that under ideal conditions, the early Earth's interior could store up to eight times the mass of today's oceans in the form of hydrated magnesium silicate.

As the core-mantle separation progressed, the iron core gradually grew, which lifted the silicates and reduced their pressure, forcing the hydrated magnesium silicate to decompose and release water. The released water reached the surface through complex geophysical and chemical processes. At this time, the surface was cool enough to ensure the existence of liquid water and form a primitive ocean.

The decomposition products of hydrated magnesium silicate, magnesium silicate and magnesium oxide, are retained in the lower mantle and still play an important role today.

"The discovery of hydrated magnesium silicate is also of great significance for human understanding of material cycles in other terrestrial planets, especially super-Earths." Dong Xiao said that this research fills the gap in the existence form of matter in hydrated silicate systems under pressures of hundreds of gigapascals, opens up a new perspective on the circulation of water and light elements in the early Earth, and deepens people's understanding of the existence and circulation of matter during the core-mantle separation process.

Commenting on the research, Swiss planetary scientist Ravit Helled said: "The origin of water is one of the important open questions about the formation of our planet. We are still not sure how much water there is deep inside the Earth now. If the core of the Earth can act as a carrier of water in the early Earth as predicted by Dong Xiao and his collaborators, then similar water storage processes will occur in other terrestrial planets and affect their evolution."

The origin and evolution of life are all inseparable from water.

Image credit: NASA

Further reading:

The fourth form of water on Earth

Most of the water on Earth does not exist in the three forms of ice, water, and gas that we are familiar with. Water also exists in another form that is unusual - that is water trapped in rocks.

These rocks are like a huge reservoir, containing at least as much water as all the rivers, oceans and glaciers on Earth combined, and perhaps four, six or ten times the amount of water in the oceans. But they are buried 410 kilometers below our feet.

This strange "fourth form" of water may also be hidden in your kitchen. If the green stone stove in your kitchen is made of serpentine, assuming that a serpentine stove top weighs about 90 kilograms, there will be about 10 kilograms of water in this stone, that is, 10 liters of water may be dissolved in the stone. However, this fusion is not like mixing eggs in a thin batter, but the water is melted into every molecule of the ore, that is, wrapped in the lattice structure of magnesium, silicon and oxygen atoms that make up serpentine. Almost all the ores at a depth of 410 kilometers underground have been melted into water in this way - under the combined effect of the gravity generated by the superposition of 410 kilometers of thick rocks and the high temperature of about 1093.33 degrees Celsius, a hydrogen atom will leave the water molecule, leaving a hydroxyl group, and this hydrogen atom will be integrated into the ore molecule. Scientists call this kind of ore that has been dissolved in water "hydrated minerals", that is, "water rocks".

It can be explained in this way: under the right temperature and pressure, a certain mineral does absorb water into its molecular structure, just like a sponge absorbs water. When water molecules enter the mineral, they decompose into a hydrogen atom and a hydroxyl group. So there is definitely water in the mineral.

Scientists believe that this water rock is spread 400-650 kilometers underground and is 240 kilometers thick, which is thicker than the water layer on the earth's surface. Even if the water content of this ore is only 1%, its water content is very large, in fact, it is equivalent to several times the amount of water in the earth's oceans.

There are still many unsolved mysteries about the relationship between water and life, waiting for scientists to further explore.

Paper link:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.035703

Comprehensive sources: Science and Technology Daily, Xinhuanet, Nankai University of Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, etc.