Why do we say that the Earth once had rain for millions of years before the ocean was formed?

The Earth is a planet filled with liquid water, with about 71% of its surface covered by sea water, and this figure is increasing as sea levels rise due to global warming.

Many people may have been curious about what the seabed looks like. The simple answer is that it is almost the same as on land. There are mountains, canyons, and plains, but they are all located in lower places.

But no one has yet been able to fully depict the underwater world because the satellite radar technology we use to map land topography doesn't work well in water.

In fact, maps of the Moon and even Mars are now clearer and more reliable than maps of Earth’s oceans, because the oceans are more difficult to explore.

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 vast water resources come from?

Where does the water resources on Earth come from?

Although the Earth is filled with liquid water everywhere, compared to the entire composition of the Earth, the total volume of all water resources on the Earth's surface - including frozen, evaporated, and liquid - is only about one thousandth of the Earth's current volume.

Most of the water resources are in the ocean, accounting for about 96.5% of all the 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 let's save water, as fresh water resources are indeed quite tight.

The reason why seawater is so salty is that it is affected by the Earth's plate tectonics, which causes a large amount of liquid water to mix with the material 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.

Now, there are three main hypotheses about the source of water resources on the earth. The first is that the earth collected water resources together during accretion in the early stage of its formation; the second is that during and after the formation of the earth, some water-rich meteorites gradually delivered water resources to the earth. These meteorites may be similar to the carbonaceous chondrites that we can still see today; the third is also from later growth, but it is some icy asteroids, which are the comets we see now, that gradually delivered water resources to the earth.

If we want to know the exact origin of Earth's water resources, we must understand the current and past composition of Earth's water resources, as well as the composition of meteorite and cometary water resources.

Carbonaceous chondrite, Image 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 generally rich in xenon, an inert gas that is rare on Earth today, so most people may not have heard of it.

In fact, if one of the main sources of Earth's water resources included meteorites, then the xenon content in the Earth's atmosphere would be at least 10 times higher than it is today.

Image: Halley's Comet

Secondly, it is impossible for comets to deliver too much water to the earth.

This is because studies of the water ice of Comet Halley and Comet Hyakutake (the only comets whose water molecules have been studied in detail) have shown that cometary ice is enriched in deuterium atoms, a heavy isotope of hydrogen atoms.

If the Earth's water resources mainly come from comets, then the deuterium-hydrogen ratio (D/H ratio) of the Earth's water molecules would be very high, but the actual situation on Earth is not that high.

In fact, the deuterium-hydrogen ratio of water resources on Earth would have continued to increase during its formation. This is because deuterium is heavier than hydrogen, and ordinary hydrogen is more easily lost after water molecules are photolyzed. This is one of the main reasons why the deuterium-hydrogen ratio on Venus is abnormally high now.

However, there is currently no known process on Earth that can reduce the deuterium-to-hydrogen ratio. Even if asteroids transport water resources, the deuterium-to-hydrogen ratio of their water resources is currently basically the same as that of the Earth's oceans.

The early solar system. Image source: NASA/JPL-Caltech

Therefore, the water resources transported by comets can only be secondary. The most likely source of the Earth’s existing water resources is the water brought by the formation of the protoplanet.

Of course, Earth probably didn't start out with its own water molecules, but more hydrogen - the most common element in the universe and the most dominant element in any early nebula.

Later, hydrogen combined with various other elements to form the water we have today, as well as all other hydrogen-containing compounds.

However, one point must be mentioned here, that is, the earth's water resources are often "polluted" in subsequent crustal movements, and the amount of water resources on the earth's surface is now far lower than that in the earth's interior. Studies have shown that the total water content of the mantle is three times that of the ocean.

Figure: Earth's structure

So, people hope to learn more about the source of planetary water resources by understanding the water resources on Mars, because many researchers believe that Mars is rarely affected by plate tectonics.

How were Earth's oceans formed?

Once we understand the source of the Earth's water resources, the formation of the ocean becomes a natural and obvious process. As long as the Earth cools down enough to support liquid water, the ocean will naturally form.

Current research and observational data show that the formation of protoplanets is a gradual growth process through continuous accretion and collision, but both processes release huge amounts of heat.

Early Earth. Image credit: SwRI/Simone Marchi

Therefore, the early Earth was very hot, so hot that any substance was in a molten state, so there was no question of whether liquid water existed.

As the planet becomes stable and no asteroids continue to hit it, it will gradually cool down. The Earth is located in the habitable zone of the solar system, and the solar radiation it receives can support the existence of liquid water.

Liquid water naturally appeared as the Earth itself cooled, but it took at least millions of years for the Earth's oceans to form - this is the so-called theory that the Earth rained for millions of years to create the oceans.

When the Earth was very hot, water resources mainly existed in the Earth's atmosphere in the form of water vapor. After the Earth cooled down, it began to rain. This important turning point for the Earth occurred about 3.8 billion years ago.

However, the rain is evaporated again the moment it hits the ground or before it hits the ground, and the Earth cools down further as it continues to turn water into water vapor until liquid water can exist on the Earth's surface.

This process continued for millions of years. At that time, the Earth was experiencing severe convective weather all the time, which was very severe. But the interesting thing is that the lightning caused by severe convection may have helped the subsequent formation of life in the ocean.

Millions of years of rainfall caused rain to wash over the earth's surface, eventually creating rivers and accumulating water in existing basins to form oceans.

The emergence of the ocean has radically changed the Earth, including its transformation of the composition of the Earth's atmosphere - mainly the absorption of carbon dioxide by seawater - and making the emergence of life possible.

at last

You may also be curious about why only Earth eventually formed an ocean when there are other planets in the habitable zone of the solar system, such as Venus and Mars? All we can say is that everything on Earth is just right.

The Earth is just the right size, while Mars is too small, and Mars does not have the ability to maintain its core temperature for long.

The distance between the Earth and the Sun is just right, while Venus is too close. After Venus was formed, the extremely high proportion of carbon dioxide in its atmosphere created a global greenhouse effect, and water resources could never exist in liquid form under the sunlight.

But then again, it is now a general consensus that the ocean is the cradle of life on Earth. But if the water resources in the Earth's oceans basically come from the collection when the planet was formed, then would life on Earth still be unique?

Source: Weird Luo