What is the Age of Water on Earth?
The question of where Earth’s water came from has long been a mystery, with various ideas and theories being proposed and supported by evidence. However, recent research suggests that the presence of water in protoplanetary disks, which are found in many young solar systems, may provide a clue to the origin of water on Earth. Specifically, the high levels of heavy water on Earth indicate that our planet’s water is around 4.5 billion years old, which suggests that it was present during the formation of our solar system. In other words, it is likely that water was present during the early stages of the development of the planets in our solar system.
“We Drink Good 4.5-Billion-Year-Old Water” is a research article written by Cecilia Ceccarelli, an Italian astronomer at the Institute for Planetary Sciences and Astrophysics in Grenoble, France, and Fujun Du, an astronomer at the Purple Mountain Observatory in Nanjing, China. The article discusses the formation of solar systems, which begins with a giant molecular cloud made up primarily of hydrogen, the main component of water. The cloud also contains helium, oxygen, carbon, silicate dust, and carbonaceous dust in varying amounts. The article explores the history of water in the solar system, starting with the giant molecular cloud.
In the cold conditions of a molecular cloud, oxygen tends to freeze and stick to dust grains. Meanwhile, hydrogen molecules are free to move around and eventually encounter the frozen oxygen. When this happens, they react and form water ice, which can be either “regular” water (made up of protium, or normal hydrogen, and oxygen) or heavy water (made up of deuterium, a form of heavy hydrogen, and oxygen). Deuterium is an isotope of hydrogen that has a proton and a neutron in its nucleus, while protium is the most common form of hydrogen and has only a proton in its nucleus. Both deuterium and protium can combine with oxygen to form water, and both isotopes still exist today.
According to the authors of the article, the formation of a water ice mantle on dust grains marks the beginning of the “cold phase,” which is the first step in the process they outline for the evolution of water in the solar system. As gravity causes matter to clump together in the center of the molecular cloud, it begins to form a protostar. Some of the energy from gravity is converted into heat, and within a few astronomical units (AU) of the protostar, the gas and dust in the surrounding disk reach a temperature of 100 Kelvin. This marks the start of the next phase in the process, in which the temperature in the disk begins to rise.
During the “protostar phase,” which is the second step in the process outlined by the authors, the protostar is surrounded by a large amount of water vapor. According to the authors, a typical hot corino (a region around the protostar where gases are excited and emit radiation) contains about 10,000 times the amount of water found in the Earth’s oceans. As the protostar begins to rotate, the surrounding gas and dust form a flattened, rotating disk called a protoplanetary disk. This disk contains all the material that will eventually become the planets and other features of the solar system.
The young protostar is still gathering mass, and its life of fusion on the main sequence is still well in its future. The young star generates some heat from shocks on its surface, but not much. So the disk is cold, and the regions furthest away from the young protostar are the coldest. What happens next is crucial, according to the authors.
The water ice that formed in step one is released into gas in step two but recondenses again in the coldest reaches of the protoplanetary disk. The same population of dust grains is again covered in an icy mantle. But now, the water molecules in that icy mantle contain the history of the water in the solar system. “Thus, dust grains are the guardians of water inheritance,” the authors write.
This process continues with step three, in which the solar system begins to take on a more recognizable form, with the formation of planets, asteroids, and comets. These celestial bodies originate from the dust grains and their water molecules that have undergone freezing twice. Currently, astronomers are studying young solar systems to learn more about this process and are also examining the ratio of heavy water to regular water on Earth for clues about the history of water in the solar system.
Additionally, during step one, when water ice is formed, the low temperature causes a phenomenon called super-deuteration, which results in a higher concentration of deuterium in the water ice compared to other temperatures. Deuterium is a type of hydrogen that was formed shortly after the Big Bang and is relatively rare, with a ratio of one deuterium for every 100,000 protium atoms. If this deuterium were evenly distributed throughout the solar system’s water, the abundance of heavy water would be expressed as 10-5. However, there are additional factors that contribute to the complexity of this process.
The abundance of deuterium in hot corinos, or regions containing gas and dust that are heated by young stars, is significantly higher than expected based on the ratio of deuterium to protium formed after the Big Bang. In these regions, the ratio of HDO (water molecules containing two deuterium isotopes) to H2O (regular water containing two protium isotopes) is roughly 1/100, and the ratio of D2O (doubly deuterated water) to H2O is about 1/1000, or approximately 107 times higher than what would be estimated based on the elemental abundance ratio of deuterium to protium.
The high concentration of deuterium in these regions is due to the process of super-deuteration, which occurs when ice forms on the surfaces of dust grains and results in an enhanced number of D atoms compared to H atoms. The authors suggest that this abundant heavy water is a characteristic of water synthesis in cold molecular cloud clumps during the first stage of the process described in this article. They also note that there are two distinct episodes of water synthesis: one that occurs when the solar system is still a cold cloud and another that occurs when planets are forming.
These episodes occur under different conditions and leave an isotopic imprint on the water, with the water from the first synthesis being approximately 4.5 billion years old. To determine how much of this ancient water made its way to Earth, the authors observed the overall amount of water and the amount of deuterated water, specifically the ratio of HDO to H2O.
The researchers found that the HDO/H2O ratio in hot corinos is significantly different from the ratio in the initial cloud, and that hot corinos are the only place where HDO has been observed in any solar-type planetary systems that are still forming. By comparing the HDO/H2O ratios in hot corinos with those in objects in our own solar system, such as comets, meteorites, and Enceladus (an icy moon of Saturn), the scientists determined that the HDO/H2O ratio on Earth is about 10 times higher than the elemental D/H ratio in the universe and at the beginning of the solar system.
Based on these findings, the researchers estimate that between 1% and 50% of Earth’s water may have come from the initial phase of the solar system’s formation, with most of it likely having been inherited from planetesimals (precursors to asteroids and planets) rather than comets that may have delivered water to Earth from the frozen Oort Cloud beyond the frost line. The hypothesis that comets may have delivered water to Earth from this distant region is another possibility that has been proposed to explain the presence of water on our planet.
This study suggests that the water on Earth may not have all come from the same source, and that some of it may have formed at the beginning of the solar system. However, there are still many questions that need to be answered about how the water reached Earth and how it has been affected by other factors such as carbon, molecular oxygen, and the magnetic field. These factors are all interconnected and play a role in the origins of life and the formation of planetary bodies. The authors of the study have provided a starting point for further research by showing that some of Earth’s water dates back to the early solar system. Further investigation is needed to understand the full story of how Earth’s water came to be.
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