As far as we know, vast oceans appeared on Earth at least 3.8 billion years ago. Since the Earth is not a closed system in the universe, it is reasonable to assume that the amount of water on Earth may also change over time.
So, after 3.8 billion years, has there been less water on Earth? Scientists are actually interested in this question, and have carried out related research, in fact, in the past days, scientists have found the answer in a stone. Let's take a look at what exactly this is all about.
We know that water is made up of two elements, hydrogen and oxygen, and we can simply describe it as a water molecule, which contains two hydrogen atoms and one oxygen atom, which are held together by covalent bonds, so if there is some mechanism that can break the covalent bonds between them, then the water molecules will break down into hydrogen and oxygen, and since the earth's gravity is not strong enough to effectively bind hydrogen, it is possible for hydrogen to escape from the top of the earth's atmosphere once such a situation occurs.
In the natural environment on Earth, there are such mechanisms, such as short-wave radiation (mainly ultraviolet rays) in sunlight, which has the ability to break down water molecules into hydrogen and oxygen, a process also known as "photolysis".
It can be seen that the water vapor in the earth's atmosphere will have a certain probability of being "photolyzed" into hydrogen and oxygen, and the earth's water will also be reduced because of this, fortunately, the earth loses very little water because of "photolysis", in addition, the earth will also get some "supplements" in the universe, such as cosmic dust and small celestial bodies that fall to the earth, can bring some water or hydrogen to the earth, so in general, the water loss of the earth because of "photolysis" can be compensated.
However, deep in the earth, there is also a mechanism that allows water molecules to break down into hydrogen and oxygen. It is important to know that the seawater in the ocean has the possibility to enter the depths of the earth, such as plate subduction, the cracks in the lithosphere of the seabed, which can allow seawater to enter the depths of the earth, and once this happens, the high-temperature magma in the depths of the earth and the crystalline bedrock in it may have a series of reactions with the seawater, and finally decompose the water molecules into hydrogen and oxygen (the net effect is shown in the figure below).
It can be seen that this mechanism will continuously split the water on the earth into hydrogen and oxygen, because the atmosphere of the modern earth contains a large amount of oxygen, so when this hydrogen diffuses into the atmosphere, it is basically oxidized and then regenerated into water, so that there is no loss of water on the earth.
However, it is important to know that the large amount of oxygen in the Earth's atmosphere actually appeared after the "Great Oxidation Event" about 2.6 billion years ago, so it is reasonable to assume that before that, the hydrogen produced through this mechanism would escape from the top of the Earth's atmosphere in large quantities, resulting in less water on Earth.
We know that there are three isotopes of hydrogen, namely "deuterium", "deuterium", and "tritium", among which only "deuterium" and "deuterium" can exist stably.
Scientists have found that in the process of producing hydrogen by this mechanism, the efficiency of "deuterium" is significantly higher than that of "deuterium", which means that the proportion of "deuterium" in the hydrogen produced will be more than that of "deuterium", so the ratio of "deuterium" and "deuterium" in seawater will also change over time.
Since most of the water on the earth is in the ocean, we only need to know the ratio of "deuterium" and "deuterium" in seawater 3.8 billion years ago, and then compare it with modern seawater, and then analyze how much less water on the earth has become after 3.8 billion years through theoretical models.
But the problem is that we can't go back to the past, how can we know the ratio of "deuterium" to "deuterium" in the seawater 3.8 billion years ago? In fact, there is a way, that is, to find some special stones, such as "serpentine".
"Serpentine" is a water-bearing magnesium-rich silicate mineral, they are usually formed in the earth's seabed, mid-ocean ridge or subduction zone and other areas, because "serpentine" will absorb a large amount of water during the formation process, which is equivalent to "locking" a part of the water in its interior, so this stone may preserve the water when it was formed for a long time.
This means that if we can find a well-preserved "serpentine" that was formed on the seabed 3.8 billion years ago, we can know the ratio of "deuterium" to "deuterium" in the seawater at that time.
In fact, scientists have actually found such a stone. The stone was found in the "Isua Supercrustal Belt" in southwestern Greenland, and according to scientists, it was formed on the ocean floor about 3.8 billion years ago, and what is even more surprising is that it is a well-preserved "serpentine".
(↑ "Serpentine" under the microscope)
So scientists extracted the water from this stone, and after careful study, the answer given by scientists is: compared with 3.8 billion years ago, the volume of the oceans on the modern earth has shrunk by about 26%, that is, after 3.8 billion years, there is indeed less water on the earth.
As you can see, in the past few days, the amount of water lost by the earth was still very considerable. However, we don't have to worry about this, as mentioned earlier, because the atmosphere of the modern earth contains a lot of oxygen, even if there is still some water in some areas of the earth that is constantly decomposing, we do not have to worry about a large decrease in the amount of water on the earth.