Water on Earth, Mars and everywhere within the inner Solar System can be traced back to the rapid waist-expanding growth of Jupiter and Saturn, which knocked inwards a local population of icy planetesimals. This is according to a new model, which could also explain the current makeup of our modern asteroid belt.

Whilst Earth is often described as the blue marble, with over 70% of its surface covered in oceans, seas, rivers and lakes, water actually makes up less than 0.1% of our planet by mass.

The majority of H 2 O in the inner Solar System actually can be found in the asteroid belt — particularly within the outer belt’s carbonaceous asteroids. Meteorites from these so-called C-type asteroids have contained up to 10% water by mass.

However, for scientists, a bigger surprise is the fact that water exists at all this far inside the orbit of Jupiter, where temperatures should have restricted planetary bodies to grow out of little more than rock and iron.

Debates over the origin of this inner solar system water go back decades, focusing initially on icy comets.

However, at the dawn of the 21st century that preference has to be rethought after comparisons of a particular chemical fingerprint of water, known as its D-H ratio, between samples from Earth and those collected from asteroids, comets and free solar system gas showed it was the C-type asteroid water that matched most closely with Earth’s.

“It suggested that C-type asteroids and whatever delivered Earth’s water came from the same population,” says astronomer and solar system modeler Sean Raymond at the Laboratoire d’Astrophysique de Bordeaux in Bordeaux, France.

“They are brothers and sisters from the same parent population.”

Now all that was lacking was an origin of these celestial siblings and an explanation of why they packed up and left their colder outer solar system home.

Ten years ago when this question was first posed, Raymond was more concerned with Mars.

In particular, he was looking at the red planet’s relatively small mass compared to Earth and Venus, an anomaly that couldn’t be resolved in any classical models of planetary formation.

Raymond’s search for ways to starve Mars of building materials led to his Grand Tack model involving an inward migrating Jupiter clearing material at Mars’ current orbit.

However, Raymond’s model, like classical ideas before, glossed over a potentially important part of the story — Jupiter and Saturn’s rapid early growth.

Ten years later Raymond’s missing chapter is to be published in the journal Icarus and unexpectedly provides a pleasingly simple explanation for our inner watery worlds.

We know Jupiter and Saturn grew at a time when remnants of the gas disk still surrounded our young Sun, a situation that only lasted for a few million years. However what hadn’t been looked at is how this rapid period of growth may have affected any icy planetesimals in the gas giants’ local neighborhood.

So Sean rewound the clock back to the earliest stages of planetary formation, placed the cores that would eventually form Jupiter and Saturn in their current positions along with a smattering of nearby icy planetesimals, and enveloped them all in a slowly dissipating gas disk.

Then he pressed play.

In his model planetesimals near either Jupiter or Saturn become destabilized as the two planet’s rapid growth increases their sphere of gravitational influence. The orbits of these smaller bodies become stretched out and begin to cross the gas giants. Here they receive big gravitational kicks which cause some to be lost to the Solar System entirely. However, many have their orbits stretched towards the inner Solar System.

Here a new effect, gas drag, takes hold as the destabilized inward orbiting planetesimals plough through a diminishing but still present gas disc. The gas drag causes their orbits to shrink again, initially trapping them in the asteroid belt where today’s C-type asteroids can be found. Later on, as the gas disk dissipates further and the drag effect reduces more, newly destabalized planetesimals carry on towards the inner planets.

“Jupiter and Saturn’s growth naturally pollutes the inner Solar System with water-rich planetesimals. In my mind the mechanism is very clear,” says Raymond, though he admits at present the gas giant’s growth actually works too well in his simulation.

“You end up dumping a significant fraction of outer planetesimals in the asteroid belt but the present day asteroid belt doesn’t have that much stuff in it,” he adds.

“Other models do suggest the asteroid belt might have loss some mass over time, but this still needs to be worked out and combined with our model.”

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Sean N. Raymond & Andre Izidoro. 2017. Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion. Icarus, in press; arXiv: 1707.01234