How did water get to Earth? A new theory suggests that our planet was able to capture the precious element in a steam bath, shortly after the formation of the solar system, 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 having everything of a “gravitational billiard game”, described to AFP the astrophysicist Quentin Kral, first author of the study, who for his part proposes a process “a little more natural and a little simpler to implement place”.
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.
It all starts from 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.
“We know that initially the asteroids were icy,” explains the researcher at the LESIA laboratory at the Paris-Meudon PSL Observatory.
These ices, “we don’t see them too much” today, except on Ceres, the most massive of the asteroids. But we detect traces of it on others with the presence of hydrated minerals. Like those identified in samples from the asteroid Ryugu, recently reported by a Japanese mission.
The idea of the LESIA team, with an astronomer from the Institute of Globe Physics in Paris, 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 heats the asteroid belt, peaking around 25 million years ago. This heating “sublimates the water ices” and then forms a “disk of water vapor at the level of the asteroid belt”, describes Quentin Kral.
From there, this disk spreads out into the solar system, all the way to the Earth, which will gradually capture this resource as it cools. Once accreted (capturing matter under the effect of gravitation) 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 our system's belt did, 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 “does not come out of nowhere”, specifies the astrophysicist. It owes a lot to the observations of the ALMA radio telescope, a major specialist in the detection of gas and dust clouds in the Universe.
“For ten years we have known that there are carbon and oxygen gas disks in planetesimal belts”, 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 “thoroughly test the theory?”, questions Quentin Kral. By looking for slightly younger systems “which still have their water gas disk”.
The LESIA team obtained observation time with ALMA on “somewhat unusual, interesting” systems. And now waiting for the results.
