The Earth is a planet filled with liquid water, about 71% of its surface area is covered by sea water, and this number is getting bigger as global warming sea levels rise.
Many people may have wondered what the bottom of the sea looks like. The simple answer is that it is almost the same as the land. There are mountains, canyons, and plains, but they are all in lower places.
But no one still has a complete picture of the underwater world because the satellite radar technology we use to map landforms doesn't work well in the water.
In fact, the maps of the moon and even Mars are now clearer and more reliable than the maps of the Earth's oceans, because the oceans are more difficult to explore than those of the moon and Mars.
Although the underwater world still leaves us with many fantasies, what is actually more magical is why only the earth has oceans, and where does the earth's massive water resources come from?
Where did the water resources on the earth come from?
Although the earth is filled with liquid water everywhere, compared with the composition of the entire earth, the total volume of all water resources on the earth's surface - including frozen, evaporated, and liquid - is only about the current volume of the earth. One thousandth of the volume.
Most of the water resources are in the ocean, accounting for about 96.5% of all water resources on the earth. Only about 3.5% of the earth is fresh water, and about 68% of the fresh water is in glaciers, so save water, fresh water resources It's really stressful.
The reason why seawater is very salty is that it is affected by the earth's plate tectonics, which results in a large amount of mixing of liquid water with materials inside the earth.
In other words, most of the earth's water resources are actually "polluted", which brings a lot of trouble to the study of the source of the earth's water resources.
Currently, there are three main hypotheses about the source of water resources on the earth. The first is that the earth collected water resources during accretion in the early stages of its formation; the second is that some water-rich meteorites were formed during and after the formation of the earth. Gradually bringing water resources to the earth, these meteorites may be similar to the carbonaceous chondrites that we can still see today; the third type is also from the later growth, but it is some icy asteroids, which is us Now we see comet , gradually sending water resources to the earth.
If you want to know the correct source of the earth's water resources, you must understand the current and past composition of the earth's water resources, as well as the composition of meteorite and comet water resources.
Carbonaceous chondrite meteorites, source: Mario Müller
First of all, it is certainly impossible for meteorites to transport too much of the earth’s water resources.
This is because carbonaceous chondrites are basically rich in xenon, a noble gas. This gas is rarely found on the earth now, so most people may not have heard of it.
In fact, if one of the main sources of water resources on the earth includes meteorites, then the xenon content in the earth's atmosphere will be at least 10 times more than it is now.
Picture: Halley’s Comet
Secondly, comet cannot transport too much water to the earth.
This is because by studying the water ice of Halley's Comet and Comet Hyakutake (these two are the only comets whose water molecules can be studied in detail), it was discovered that the comet's ice is rich in deuterium atoms - hydrogen atoms A heavy isotope.
If the earth's water resources mainly come from comets, then the current deuterium-to-hydrogen ratio (D/H ratio) of the earth's water molecules will be very high, but the actual situation of the earth now is not high.
In fact, the deuterium-to-hydrogen ratio of water resources will continue to increase during the formation of the earth. This is because deuterium is heavier than hydrogen. After water molecules are photolyzed, ordinary hydrogen is more easily lost. This is why One of the main reasons why the current deuterium-to-hydrogen ratio on Venus is unusually high.
However, there is currently no known process on Earth that can reduce the deuterium-to-hydrogen ratio. Even if an asteroid transports water resources, at present, the deuterium-to-hydrogen ratio of its water resources is basically the same as that of the Earth's oceans.
The early days of the solar system, picture source: NASA/JPL-Caltech
Therefore, the water resources transported by comets can only be secondary. The existing water resources on the earth mainly come from the formation of the primitive planets. The highest possibility is .
Of course, Earth may have initially received less of its own water molecules and more of hydrogen—the most ubiquitous element in the universe and the most dominant element in any early nebula. After
, hydrogen combined with various other elements to form our current water, as well as all other hydrogen-containing compounds.
However, one more thing must be mentioned here, that is, the earth's water resources are often "polluted" during subsequent crustal movements, and the amount of water resources contained on the earth's surface is now much lower than that of the earth's interior. Studies have shown that the total water content of the mantle 3 times that of the ocean.
Figure: Earth structure
Therefore, people hope to learn more about the source of the planet's water resources by understanding the water resources on Mars, because many researchers believe that Mars is rarely affected by plate tectonics.
How were the Earth’s oceans formed?
After we understand the source of the earth's water resources, it is actually clear that the formation of the ocean is a matter of course. As long as the earth cools enough to support liquid water, the ocean will naturally form.
Current research and observation data show that the formation of primitive planets gradually grew through continuous accretion and collision, but both processes would release huge amounts of heat.
Early Earth, Picture source: SwRI/Simone Marchi
Therefore, the early Earth was very hot, so hot that any material was in a molten state, so there was no question of the existence of liquid water.
As the planet stabilizes and no asteroids continue to collide with it, it will gradually cool down or cool down. The earth is just in the habitable zone of the solar system, and the solar radiation it receives can support the existence of liquid water.
When the earth itself cooled down, liquid water naturally appeared, but it took at least millions of years to form the earth's oceans - This is what is called the earth's rain that created the oceans for millions of years. Say .
When the earth is very hot, water resources mainly exist in the form of water vapor in the earth's atmosphere. After the earth cools down, it starts to rain. This important turning point for the earth occurred about 3.8 billion years ago .
However, the rainwater is evaporated again the moment it hits the ground or before it hits the ground. At the same time, the earth continues to cool down as it continues to turn water into water vapor, until liquid water can exist on the surface of the earth.
This process has been going on for millions of years. At that time, the earth was experiencing strong convection weather all the time, which was very bad. However, the interesting thing is that the lightning caused by strong convection may help to form life in the ocean later.
Millions of years of rainfall have allowed rainwater to wash away on the surface of the earth, eventually creating rivers and accumulating water on original basins to form oceans.
The emergence of the ocean has completely changed the earth, including its transformation of the composition of the earth's atmosphere - mainly the absorption of carbon dioxide by seawater, and the emergence of life possible.
Finally
You may also be curious, Venus and Mars are other planets in the habitable zone of the solar system. Why is it that only the earth finally formed an ocean? It can only be said that everything on the earth is just right.
The size of the Earth is just right, but Mars is too small, and Mars does not have enough ability to preserve its core temperature for a long time; the Earth is also just the right distance from the sun, but Venus is too close. After Venus was formed, the extremely high proportion of carbon dioxide in its atmosphere created Due to the global greenhouse effect, water resources can never exist in liquid form under sunlight.
But then again, it is now a common consensus that the ocean is the cradle of life on earth. But if the water resources of the earth’s oceans basically come from the water collected when the planet was formed, will life on earth still be unique?
