The Earth is within the solar system’s “snow line,” where water is rare. Earth has a lot of water compared to other inner planets due to several theories, including the addition of water from asteroids and biological processes. Most of Earth’s water comes from asteroids.
The Earth lies within the solar system’s “snow line,” the region closest to the Sun where H2O is mostly in liquid or gaseous form, if at all. The snow line is located in the outer asteroid belt, between the orbits of Mars and Jupiter. The snow line phenomenon is reflected in the water content of planets such as Mercury, Venus and Mars. Water is absent on Mercury. On Venus, H2O exists only as a trace element in the atmosphere. Mars has only a thin layer of ice in its polar regions. In general, water is rare within the snowline.
Why does Earth have so much water compared to other inner planets? 71% of the surface is covered by oceans, more than half of which is deeper than 3,000 meters (9,800 feet), with an approximate total volume of 1.3 billion cubic kilometers (310 million cubic kilometers). However, the oceans make up only 0.023% of the total mass of the Earth.
There are various theories as to where all of Earth’s water comes from, but several theories have fared better than others. We know that the oceans already existed 100 million years after the formation of the Earth. When the Earth was forming, with a radius just 40% smaller than it is today, it would have had enough gravity to hold a tenuous atmosphere with water vapour. The first water vapor on the planet would have come from the planet’s interior, where volatile (low weight) chemicals would have a tendency to float to the top and heavy chemicals (iron and nickel) would sink.
Although Earth’s first water arose through volcanism, that alone probably didn’t produce enough to form stable pools on the surface. More water has been added to the planet during several hypothesized large impacts by asteroids from the outer asteroid belt. Comparison of the isotope ratios of water on Earth and water from comets and asteroids revealed that most of Earth’s water comes from asteroids.
Throughout its history, the Earth’s water has increased in volume due to biological processes. In the early seas of the Earth, hydrogen sulfide would have been in large quantities, which, when reacted with carbon dioxide from the atmosphere during photosynthesis in sulfide-reducing bacteria, would have produced hydrogen, sulfur and water. Many geologists believe that most of the Earth’s water is generated through this process.
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