How the young Titan escaped its destruction by Saturn

Until now, all numerical simulations have led to the formation of several Saturn moons the size of Titan, which were all quickly swallowed by the giant or survived, thus leaving at least two large satellites. For the first time, a scenario allows Titan to appear in the simulations while escaping, alone, from destruction.

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Build the cosmogony of Solar system does not only need to account for the formation of the planets but also for their moons and rings they could own. The origin of the Moon Earth, just as much as those of Saturn and its rings, interrogates. We would like to understand in particular how this gas giant acquired its satellite Titan.

Remember that this is the largest moon in Saturn and the second largest moon in our Solar System after Ganymede, the moon of Jupiter, which is only 2% larger. With a radius of about 2,575 kilometers, Titan is not only about 50% larger than our Moon but also larger than the planet Mercury.

To explain its birth, planetologists generally involve training in some way in situ of Titan in a accretion disc containing dust and especially gas around the young Saturn, similar to the genesis of the planets in the protosolar disc around the young Sun. This Saturnian sub-nebula, as researchers call it, is even at the origin of other Saturn moons such as Encelande and Iapetus. For the record, if Titan was discovered by Christian Huygens, Tethys, Dioné, Rhea and Iapetus were by Jean-Dominique Cassini who named them Sidera Lodoicea (“The stars of Louis”) in honor of King Louis XIV.

But this scenario had a problem. The numerical simulations sure computers hitherto conducted to reproduce the Titan formation, either failed to produce a celestial body of such a large size, or produced several which survived, in clear contradiction to the observations. In fact, to be more precise, large moons formed well, comparable in size to Titan, but they all ended up being quickly engulfed by the gas giant.

The physics of the accretion disk better modeled

But today two astrophysicists Japanese, Yuri Fujii, professor at the University of Nagoya, and Masahiro Ogihara, professor at the National Observatory of Astronomy of Japan (NAOJ), announce having found a solution to this enigma which is the subject of a published article in Astronomy and Astrophysics Letters and also available on arXiv.

The accretion disc of the sub-nebula behaves like a gas, with a temperature and a pressure but also radiation and sources of opacity for this radiation, which influences the distribution of temperatures in the disc. Astrophysicists have taken better account of the sources of opacity for radiative transfer in the form of icy and silicate dust. In the end, the simulations were better able to account for the effects of the gas on the forming moons in the calculations, which obviously also took into account the gravitational forces of attraction present between all the bodies.

The researchers then discovered that there was a kind of safety zone, a ring-shaped region around Saturn, in which a planet the size of Titan could form, migrate outward under the effect of the pressure of a hotter gas than in previous simulations, but staying in this area while the others, closer to Saturn, ended up being swallowed.

For the first time, a digital simulation automatically led to the existence of a single giant moon like Titan around Saturn. But astrophysicists are careful. It is not yet possible to conclude that this is how the satellite was formed.

To find out, it would probably be necessary to observe a large number of exolunes aroundexosaturns and discover that almost every time there is only one exotitan for each exosaturn. We are not there yet in the near future.