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How did water come to Earth?

Sophia Xu
Updated: September. 5, 2023 

Water is essential for life on Earth, but how did it get here? This is a fascinating question that scientists are still trying to answer. In this blog post, I will summarize some of the main theories and evidence about the origin of Earth’s water.

The Early Earth

Earth formed about 4.5 billion years ago from a disk of gas and dust that surrounded the young Sun. At that time, the disk was too hot for water to turn into liquid, so any water molecules would have evaporated and escaped into space. Moreover, Earth did not have an atmosphere to protect it from the solar wind and radiation, which could also strip away any water vapor.

Therefore, most scientists agree that Earth’s water must have been delivered later, after the planet had cooled down and developed an atmosphere. But what was the source of this water?

Comets vs Asteroids

Two possible candidates are comets and asteroids, which are both remnants of the early solar system. Comets are icy bodies that orbit far from the Sun, while asteroids are rocky bodies that orbit closer to the Sun. Both comets and asteroids occasionally collide with Earth, and some of them may contain water ice.

One way to test whether comets or asteroids brought Earth’s water is to compare their chemical composition with that of Earth’s water. One important clue is the ratio of two isotopes of hydrogen: normal hydrogen (H) and heavy hydrogen (D), which has an extra neutron. Water molecules can contain either H or D, and the ratio of D to H is called the deuterium-to-hydrogen ratio (D/H).

Different regions of the solar system have different D/H ratios, depending on their temperature and history. For example, comets tend to have higher D/H ratios than asteroids, because they formed farther from the Sun where it was colder and more D was available. Earth’s water has a D/H ratio of about 1.5 x 10-4, which means that for every 10,000 hydrogen atoms, there are 1.5 deuterium atoms.

By measuring the D/H ratio of comets and asteroids that visit Earth, scientists can see if they match with Earth’s water. So far, most comets have been found to have much higher D/H ratios than Earth’s water, which suggests that they are not the main source of our oceans. For example, comet 67P/Churyumov-Gerasimenko, which was visited by the Rosetta spacecraft in 2014-2016, had a D/H ratio of about 5.3 x 10-4 1.

On the other hand, some asteroids have been found to have similar or lower D/H ratios than Earth’s water, which suggests that they could be the main source of our oceans. For example, asteroid Bennu, which was visited by the OSIRIS-REx spacecraft in 2018-2020, had a D/H ratio of about 1.4 x 10-4 2. However, not all asteroids have low D/H ratios, and some may have lost their water due to heating or impacts.

Other Sources

Besides comets and asteroids, there may be other sources of water for Earth. One possibility is that some water was trapped inside the rocks that formed Earth, and later released by volcanic activity or tectonic movements. Another possibility is that some water was produced by chemical reactions between hydrogen from the solar wind and oxygen from the rocks on Earth’s surface.

A recent study by researchers from Arizona State University and Caltech proposed a new model that combines these two possibilities 3. They suggested that in the early stages of Earth’s formation, when it was still covered by a magma (hot lava) ocean, some hydrogen from the solar wind could have dissolved into the molten rock. Then, as the magma ocean cooled and crystallized, some of the hydrogen could have reacted with oxygen in the minerals to form water. This water would have been trapped inside the mantle until it was released by volcanism or plate tectonics.

The researchers estimated that this process could have produced enough water to fill at least one ocean on Earth. They also argued that this process could explain why Earth’s water has a similar D/H ratio to some asteroids, because both sources would have been influenced by hydrogen from the solar wind.

Conclusion

The origin of Earth’s water is still a mystery that scientists are trying to solve. There may not be a single answer, but rather a combination of different sources and processes that contributed to our planet’s hydration. By studying comets, asteroids, and other celestial bodies, as well as Earth’s own geology and chemistry, we may be able to unravel this puzzle and learn more about our home in the cosmos, or maybe not, who knows ? 

Anyhow its always nice to leave a comment below.