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Saturn's Formation and Early Evolution at the Origin of Jupiter's Massive Moons


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Abstract

The four massive Galilean satellites are believed to have formed within a circumplanetary disk during the last stages of Jupiter's formation. While the existence of a circum-Jovian disk is supported by hydrodynamic simulations, no consensus exists regarding the origin and delivery mechanisms of the building blocks of the forming satellites. The opening of a gap in the circumsolar disk would have efficiently isolated Jupiter from the main sources of solid material. However, a reservoir of planetesimals should have existed at the outer edge of Jupiter's gap, where solids were trapped and accumulated over time. Here we show that the formation of Saturn's core within this reservoir, or its prompt inward migration, allows planetesimals to be redistributed from this reservoir toward Jupiter and the inner Solar System, thereby providing enough material to form the Galilean satellites and to populate the Main Belt with primitive asteroids. We find that the orbit of planetesimals captured within the circum-Jovian disk are circularized through friction with gas in a compact system comparable to the current radial extent of the Galilean satellites. The decisive role of Saturn in the delivery mechanism has strong implications for the occurrence of massive moons around extrasolar giant planets as they would preferentially form around planets within multiple planet systems.

Source:  Saturn's Formation and Early Evolution at the Origin of Jupiter's Massive Moons - The Astronomical Journal, Volume 155, Number 5, May 2018 (free issue)

I thought this paper was very interesting.  I've read lots of material about how Saturn and Jupiter migrated towards the inner solar system during the early formation days of our solar system, but that material focused only on the movement of planets.  This is the first paper I've read that focused on the Galilean satellites and how they could have formed.

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54 minutes ago, T. McGrath said:

I thought this paper was very interesting.  I've read lots of material about how Saturn and Jupiter migrated towards the inner solar system during the early formation days of our solar system, but that material focused only on the movement of planets.  This is the first paper I've read that focused on the Galilean satellites and how they could have formed.

Wasn't the hypothetical notion that Jupiter and Saturn may have migrated inwards and then again outwards in those violent early days of the solar system, due to the discovery and observations of the many exoplanets that were "hot Jupiters" ? 

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56 minutes ago, beecee said:

Wasn't the hypothetical notion that Jupiter and Saturn may have migrated inwards and then again outwards in those violent early days of the solar system, due to the discovery and observations of the many exoplanets that were "hot Jupiters" ? 

The discovery of those "hot Jupiters" was certainly the impetus behind the theory that our gas giants also moved inward initially.  I do not recall seeing anything on the subject until after 1995. 

The theory would explain many things, such as the small size of Mars and of course the asteroid belt.  If the theory is valid, then we were very fortunate.  Those other "hot Jupiters" did not have a Saturn to pull them back out again.  They suggest that Jupiter initially formed somewhere beyond the frost line, > ~3 AU, and then migrated inward until it got to about where Mars is now (~1.5 AU).  When Saturn got into a certain orbital resonance with Jupiter it managed to pull Jupiter back out to its current location of 5.2 AU.

This would also cause Uranus and Neptune to first move inward, and then outward.  How all this planetary movement effected the creation and orbits of the Galilean satellites and other "natural" satellites (as opposed to captured) I don't think will ever be known, but it is certainly interesting to speculate.

Edited by T. McGrath
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