New theory explains how water got to Earth

At the very beginning of its history our planet was dry, how did it become blue? This is an absolutely fundamental question to which a new theory could provide an answer.

At the time of its formation, the Earth was too hot to contain water ice. All the water present on Earth must therefore have an extraterrestrial origin. The study of ancient terrestrial rocks shows that there was liquid water on our planet very early (on the time scales of astrophysicists in any case) around 100 million years after the formation of the Sun. This water is now more than 4.5 billion years old, under a cycle maintaining its permanent renewal. With my team, we have just proposed a new theory to explain the arrival of water on Earth.

A billion-year-old question

Astrophysicists have been trying to answer the question of the arrival of water on our very young planet for several decades. One of the first hypotheses considered that terrestrial water was a direct by-product of the formation of the Earth, which could be released via magma during volcanic eruptions where the vast majority of the gas produced is water vapor.

However, by analyzing the composition of terrestrial water, this hypothesis evolved in the 1990s with the discovery of the potential role of icy comets, suggesting a contribution of extraterrestrial origin. Comets are balls of ice and rock that form quite far away in the solar system and are sometimes ejected towards the Sun. They can become spectacular when, heated by the Sun, they form a tail of dust that can be observed from Earth. Asteroids, which are objects located in the asteroid belt between Mars and Jupiter, are also discussed as potential progenitors of water on Earth.

The analysis of cometary rocks and asteroids, via meteorites (small fragments of asteroids or comets that fell on Earth), in particular by measuring their D/H ratio (which is the quantity of heavy hydrogen, called deuterium, by compared to standard hydrogen), made it possible to see that terrestrial water corresponds more to that of so-called “carbonaceous” asteroids (those which contain traces of the presence of past water), thus directing research towards these last.

Recent work has therefore focused on the search for the best celestial mechanism capable of causing these asteroids to crash into our young dry Earth, at the beginning of its history, in order to supply it with water. A plethora of scenarios have thus been published to theorize the “upheaval” of planetesimals, that is to say large icy bodies present in the asteroid (between Mars and Jupiter) and Kuiper (beyond Neptune) belts. so that they can be dislodged to be sent to Earth. But this underlies a game of gravitational billiards which is not trivial and imposes the idea of ​​a complicated history of the Solar System. It is clear that there must have been upheavals and impacts to form the planets. However, it may be that things happened more peacefully and more naturally when it came to bringing water to Earth.

A “simpler” hypothesis

I started from the principle that asteroids are icy when they emerge from their formation cocoon (also called “protoplanetary disk”). This cocoon is a massive disk of gas, mainly composed of hydrogen and filled with dust, in which planets and belts initially form. It therefore encompasses the entire planetary system in the making. Once the initial protective cocoon disappears (after a few million years), the asteroids heat up and the ice melts, or more precisely sublimates. In other words, their ice turns into water vapor. In the space where the pressure is almost zero, the water remains in vapor form.

A disk of water vapor is then superimposed on the asteroid belt orbiting the Sun. As the ice sublimates, the disk fills with water vapor and naturally spreads inwards, that is to say towards the Sun, as a result of complex dynamic processes. Along the way, it encounters the internal planets which find themselves bathed in this disk of water vapor. In a way, the water disk “waters” the telluric planets that are Mars, Earth, Venus and Mercury. The bulk of water capture by the planets happens around 20-30 million years after the formation of the Sun, at a stage where the latter saw its luminosity increase sharply over a short period of time, which increased the rate of outgassing of asteroids.

Once water is captured by the gravitational pull of the planets, a lot can happen. However, on Earth, there is a protective mechanism that explains why the total mass of water has not changed much, since the end of capture until now. Indeed, if water goes too high in the atmosphere, it condenses and forms clouds, which are found a little later in the form of rain on the Earth’s surface: this is the water cycle.

The quantities of past and present water on Earth are therefore well known. Our model which, starting from the original asteroid belt, and proceeding with the degassing of ice, manages to bring in the right quantity of water, which is then used to form oceans, rivers, lakes, and explains buried water deep in the Earth’s mantle. Fine measurements of the D/H ratio of water in the oceans can also be explained using our model. Finally, the quantity of water present in the past on other planets (and even on the Moon) is also well explained with our theory.

One might wonder how I came up with the idea of ​​proposing this new theory. This does not come out of nowhere and is based on recent observations, in particular with ALMA, a radio telescope made up of more than 60 antennas deployed in Chile, on a plateau at an altitude of 5 km. Indeed, by observing extrasolar systems that have belts similar to that of Kuiper, we now discover that planetesimals in these belts sublimate carbon monoxide (CO). For belts closer to their star such as the asteroid belt, CO is too volatile to be present and it is rather water that should be degassed.

Future observations to support the hypothesis

It was therefore on the basis of this observation that the initial idea was formed. Furthermore, thanks to recent results from the Hayabusa 2 and OSIRIS-ReX probes that left to explore in situ asteroids similar to those which could have participated in the formation of the initial water vapor disk, we were able to confirm (because we also observed for a long time with ground-based telescopes) the presence of large quantities of hydrated minerals on these bodies, which can only form in contact with water. The prerequisite to explain these observations is that these asteroids were initially icy, although they are no longer so today (apart from the most massive ones like Ceres).

The foundations of the model were in place and it was then necessary to build a digital simulation that could follow this degassing, the spreading of the gas, then its capture by the planets. By carrying out these simulations, I immediately realized that this could explain the quantity of water on Earth. For the other planets, I did a little research to find the constraints on the quantities of water spent on Mars and the other terrestrial planets. That worked too. All that was needed was to publish everything!

As a researcher, you don’t just develop a model that works and seems to explain everything; we must go further and test the theory on a large scale. If it is now too late to detect the initial disk of water vapor (on which everything is based) which made it possible to “water” the telluric planets, it is appropriate to look at extrasolar systems with young belts of exo-asteroids to see if we can indeed distinguish these water vapor disks. According to our calculations, these water disks, although not very bright, could be detectable with ALMA and our team has just obtained time on ALMA to test all of this on very specific systems.

We may be at the beginning of a new story…