How did water get to Earth ? A new theory proposed by scientists that calls everything into question

Une nouvelle théorie avance que notre planète a pu capturer le précieux élément dans un bain de vapeur, peu après la formation du système solaire. Dimitris66/Getty Images

How Water Got to Earth ? A new theory suggests that our planet may have captured the precious element in a steam bath shortly after the solar system formed, according to a study published Tuesday in Astronomy & Astrophysics.

According to the dominant theory, water arrived on Earth mainly via asteroids and comets, coming from outside the solar system, in the first hundred million years.

A bombardment that had all the makings of a “gravitational billiards game”, astrophysicist Quentin Kral, first author of the study, described to AFP, who for his part proposes a process “a little more natural and a little simpler to set up”.

Less random therefore, and especially applicable to other rocky planets in the solar system, such as Mars or Mercury, which we know contain water, just like the Moon.

Everything starts with the asteroid belt, a ring of small celestial bodies, located between Mars and Jupiter, which was much more massive at the time of the formation of the solar system, 4.6 billion years ago.

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“We know that initially the asteroids were icy”, explains the researcher at the LESIA laboratory of the Paris-Meudon Observatory PSL.

These ices, “we don't see them much anymore” today, except on Ceres, the most massive of the asteroids. But traces of it are detected on others with the presence of hydrated minerals. Like those identified in the samples of the asteroid Ryugu, recently brought back by a Japanese mission.

The idea of ​​the LESIA team, with an astronomer from the Paris Institute of Globe Physics, is that the Earth has indeed recovered water from asteroids, but without the latter bringing it directly to it.

“Water vapor lives its life as water”

In this scenario, the Sun has just formed and is heating the asteroid belt, with a peak at around 25 million years. This heating “sublimates the water ice” and then forms a “water vapor disk at the asteroid belt”, describes Quentin Kral.

From there, this disk spreads out in the solar system, all the way to the Earth, which will gradually capture this resource as it cools. Once accreted (capture the matter under the effect of gravity) on the planet, this “water vapor lives its life as water”, and is found there in liquid form.

The model developed by Quentin Kral and his colleagues works just as well with a massive asteroid belt, as they assume that the one in our system was, as with a thinner belt, but over a longer period of time.

This is the first time that such a hypothesis has been put forward. But it “doesn't come out of nowhere”, the astrophysicist specifies. It owes much to the observations of the ALMA radio telescope, a leading specialist in detecting clouds of gas and dust in the Universe.

“For ten years we have known that there are disks of carbon and oxygen gas in belts of planetesimals”, in other words asteroids and miniplanets, “extra-solar systems”.

Before that we only saw dust, where now we see the presence of gas. Or water ice in the asteroid belt of HD 69830, a solar system with at least three planets.

So how can we “test the theory ? thoroughly”, asks Quentin Kral. By looking for slightly younger systems “that still have their water gas disk”.

The LESIA team obtained observation time with ALMA on “somewhat unusual, interesting” systems. And is now waiting for the results.