The isotopic properties of a compound can be used to study its origin and evolution, and we can apply this technique to studying the origin of water on Earth. So what do we know about water on our planet? First of all, other than Earth, there is no other place in the solar system or beyond that we know for sure has liquid water.
We know that there is ice made of water on the Moon and on Europa and Enceladus (the moons of Jupiter and Saturn, respectively), or on comets like 67P/Churyamov-Gerasimenko. We also know the presence of water vapor in cryovolcanoes on these moons and in the interstellar medium, especially near star formation zones. So is all this water the same – does it have the same isotopic composition?
It turns out that there is a paradox in the origin of water on Earth. The environment in which the Sun and Earth were created was quite dry, despite the fact that water is one of the most abundant compounds in the star-forming zones where the Sun and Earth developed. . Indeed, according to scientific modeling, rocky planets like Earth appeared in an area of the solar system close to the Sun. Here, the high temperature prevented the type of atmosphere from forming where water could expand beyond the gaseous state. In this way, the water formation escaped the gravitational pull of the planet.
The presence of carbon, the other basis of life on Earth, also contains a paradox. Carbon is the fourth most abundant element in the universe after hydrogen, helium and oxygen and the second most abundant element in our body (nearly 20% of our body mass is carbon). And yet, carbon is 10 times less abundant on Earth than in the universe in general.
Yet, what is the relevance of carbon here?
Well, a small portion (about 5%) of the meteorites that reach our planet today are carbon-rich. They are called “carbonaceous chondrites” and, interestingly, they also contain large amounts of water. This means that they must have formed in areas far from the Sun, beyond the so-called “freezing line”, where temperatures were already much lower, which at the beginning of the solar system , allowed the formation of water, methane or ammonia ice. . This is one of the reasons why it is hypothesized that the water arrived on Earth via a bombardment of these meteorites during a period when the Earth had already cooled considerably since its formation.
Indeed, another question is when the water arrived. There is evidence of its existence on our planet 4.4 billion years ago, just over 100 million years after its formation, when the surface temperature of our planet must have been cold enough to freeze the water. This proof is based on the study of certain minerals such as zircon, which resists geological changes and atmospheric action quite well, thus giving us information on the origins but not so much on the evolution of water on Earth. .
The study of the “isotopic abundances” of water present in carbonaceous chondrites, at least in those as old as the solar system itself, gives results very close to those of terrestrial water. In particular, the amount of deuterium relative to protium is usually studied, since the ratio of these isotopes for Earth’s water is quite similar for chondrites in the vicinity of Jupiter, some of which come from the asteroid Vesta. Farther out (for example, in comets from the outer reaches of the solar system), deuterium abundances are much higher, occurring in what is known as the Oort cloud.
So what do Jupiter and the Moon have to do with the whole story of water on Earth? In the case of Jupiter, its influence in matter comes from its intense gravitational action in the Solar System, which agitates the orbits of a multitude of asteroids. Some evolutionary models suggest that at some point in the history of the solar system, Jupiter may not have had the same orbit as it does today – instead, it may have been closer to the Sun before. to migrate to its current position. This excursion of Jupiter would have caused it to sweep up objects along the way, which in turn could have been launched en masse into inner orbits closer to the Sun, thus reaching Earth. This is called “late massive bombardment”, as evidenced, for example, by the concentration of meteorite impacts on the Moon about 3.9 billion years ago.
This is where the role of the Moon comes in. To understand this, we must return to the study of isotopes, but this time it is molybdenum, a much rarer element. Molybdenum is a metal that has 42 protons (for comparison, iron has 26) and dozens of isotopes. It turns out that the relative abundances of these isotopes on Earth are in the middle of the abundances observed for carbonaceous chondrites and chondrites from the outer reaches of the solar system.
Taking into account that molybdenum is denser than iron (a small one centimeter cube of metal weighs 10 grams, compared to seven grams if it were iron and one gram if it were water ), and most of the iron on our planet is in its core, it wouldn’t be strange to think that the molybdenum that reached Earth early in its history flowed all the way to the Earth’s core. Surface molybdenum, in the crust or upper mantle, may have a more recent origin, and its isotopic composition indicates areas where there was plenty of carbon and water. The timing works to associate this arrival of molybdenum and water with the impact of Theia, the protoplanet that caused the formation of the Moon after colliding with Earth 4.5 billion years ago, mixing much of its material with the Earth’s mantle. According to these “molybdenic studies”, Theia would be a planet resulting not from the zone of the rocky planets, but from the zone of the gaseous planets (Jupiter, Saturn) and/or the icy planets (Uranus, Neptune), which are full of water .
Thus, although the evidence is inconclusive, it may well be that the planetary cataclysm caused by Theia, with the consequent formation of the Moon, possibly with the mediation of Jupiter, had a fundamental effect on the appearance of life for several reasons, including representing most of the water existing on our planet today.
So, when we are thirsty, consider that our life may be more connected to the stars than we realize, and that in addition to stardust, we are the result of a clash of giants.