• The composition of Saturn's moons would be determined by the distance that separates them from the Sun, according to our partner The Conversation.

  • The precise place where temperature forces their transition is called the “ice line,” and it is this space that scientists have simulated to better understand this process.

  • The analysis of this phenomenon was carried out by Sarah Anderson, doctoral student in Sciences of the Universe at the University of Franche-Comté.

Besides being one of the most fascinating planets with its impressive rings, Saturn is home to more than 80 moons, including Enceladus and Titan.

These moons are of different composition: Enceladus is completely covered with water ice, while Titan harbors an atmosphere of methane and nitrogen.

Where does this variety come from?

To understand this, we must study their training.

In the absence of a time machine, the researchers simulate the process with digital models.

How to make a moon?

Our solar system formed about 4.5 billion years ago from a dense cloud of gas and dust.

The cloud collapsed, forming a solar nebula - a spinning, swirling disk of matter.

At the center, gravity attracted more and more matter until, finally, the pressure in the nucleus was so great that the hydrogen atoms began to combine and form helium, releasing a huge amount of energy and giving birth to the Sun.

We are now dealing with an accretion disk rotating around our protostar.

The most detailed image of a proto-stellar disc, around the star HL Tau, acquired by the ALMA telescope (Atacama Large Millimeter / submillimeter Array) in the Atacama desert in Chile © ALMA / NRAO / ESO / NAOJ, CC BY-SA (via The Conversation)

This disc is composed of grains of matter which agglutinate under the effect of electrostatic force to form small blocks of a few kilometers.

These blocks, called "planetesimals", can collide with others to gradually form larger and larger objects.

Some of them get big enough that their gravity takes over and attracts more and more matter, shaping them into spheres and forming planets.

At the same time, this process was taking place on a smaller scale around the planets themselves, forming a multitude of moons.

The "leftovers" that could not result in objects large enough have become our asteroids and comets - or can form rings around the larger planets.

But it is not just any molecules that will form these planets or moons: their composition will depend on the molecules available;

and this availability depends on the temperature, therefore on their distance from the sun.

The temperature of these discs decreases over time, and as we move away from the hot body in the center.

Each type of molecule behaves differently: at a certain temperature, a molecule will be too cold to remain in a gaseous state and will change to a solid state (jumping the liquid state) to become ice.

The place where temperature forces this transition is called the “ice line”: it is the distance from the central body that determines whether a molecular species exists in solid or gaseous form.

Each molecule has its own transition temperature, and therefore its own ice line.

Near the sun, the disk is too hot for volatile molecules such as water or methane to remain under the solid state, and these molecules will only come to form planetesimals beyond their ice line.

This is why there is a big difference in composition between the internal planets - those closest to the sun which are rocky and are said to be "telluric" (Mercury, Venus, the Earth, and Mars) - and the external planets, which are "gas giants" (Jupiter, Saturn, Uranus, Neptune).

The line of water ice sculpted these two populations, lying between them at the time of the formation of these bodies.

The successive lines of ice define the composition of the planets around their star © A. Aguichine, O. Mousis, B. Devouard & T. Ronnet (via The Conversation)

We can imagine this nebula as a big bucket of matter and water to understand why heavy elements "sink" faster down than small particles left in suspension.

The same thing happened in these nebulae: heavy elements like iron, nickel, and silicate rocks, attracted by gravity, approach the central heavy body while the light elements remained outside. the ice line, because the water - in gaseous form - which hinders their approach pushes them outwards.

And around Saturn?

It is believed that the same thing happened: the composition of the moons would have been sculpted by these lines of ice.

We therefore need to find the famous lines of ice for each molecule involved in order to understand the formation of these so different moons.

A fully digital mini planetary system

We fill a virtual space with all the ingredients that make up these moons: water ice, carbon monoxide, methane and nitrogen.

With these ingredients in place, we apply the laws of gravity as well as the laws of thermodynamics, and we trigger time: our simulation observes the position of the elements, the temperature and pressure of the disk, then calculates their subsequent positions, which allows to estimate the evolution of the disc over time.

To match what we see in moons today, the building blocks had to come from a location between the carbon monoxide and dinitrogen ice lines at their outer boundary, and the methane ice line as the boundary. interior.

The lines of ice of different molecules around Saturn (which also heats up much less than the Sun but still), and the position of Titan and Enceladus © Sarah Anderson (via The Conversation)

However, during our simulations, Saturn "devoured" all the particles so quickly that it did not give the dust time to grow large enough to build moons.

We have had to continually replenish the systems with new solids to form the moons.

At the end of this last simulation, we observed the position of these lines of ice, and of course that of the “building blocks” of our moons: these are at the end of the simulation more distant than the real position of the moons. current moons.

This adds to the theory that Titan may have formed further and drifted inward over the millennia.

Enceladus and the rings of Saturn

Given that Enceladus is too small to handle the stress of such a trip (notably it would have been torn apart by tidal forces), it seems more likely that it formed much later than Titan, perhaps at from the same cosmic catastrophe that formed the rings of Saturn… if this scenario is correct.

This is because Saturn's rings are made up of billions of small pieces of ice and rock covered with other materials such as dust.

Initially, astronomers believed they were pieces of comets, asteroids or moons that had shattered before reaching the planet, torn apart by the powerful gravity of Saturn.

But a theory based on observations from the Cassini probe (between 2004 and 2017) wants these rings to be much younger than Saturn.

The rings are said to be millions of years old, not billions, suggesting that a catastrophic event would have destroyed all ice moons (Enceladus type): the ones we see today are from a second generation.

Our "SATURNE" file

Only another mission to Saturn could provide more detail and bring precision to these results.

The last one - Cassini-Huygens, which ended in 2017 - brought us a huge amount of data;

we are still studying them.

But it will take a while for a next probe to be sent to the Saturnian system.

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This analysis was written by Sarah Anderson, doctoral student in Sciences of the Universe at the University of Franche-Comté.


The original article was published on The Conversation website.

Declaration of interests

Sarah Anderson does not work, advise, own shares, receive funds from any organization that could benefit from this article, and has not declared any affiliation other than her research organization.

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