The formation of the Galilean moons and Titan in the Grand Tack scenario
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abstract
In the Grand Tack (GT) scenario for the young solar system, Jupiter formed
beyond 3.5 AU from the Sun and migrated as close as 1.5 AU until it encountered
an orbital resonance with Saturn. Both planets then supposedly migrated outward
for several $10^5$ yr, with Jupiter ending up at ~5 AU. The initial conditions
of the GT and the timing between Jupiter's migration and the formation of the
Galilean satellites remain unexplored. We study the formation of Ganymede and
Callisto, both of which consist of ~50% H$_2$O and rock, in the GT scenario. We
examine why they lack dense atmospheres, while Titan is surrounded by a thick
N$_2$ envelope. We model an axially symmetric circumplanetary disk (CPD) in
hydrostatic equilibrium around Jupiter. The CPD is warmed by viscous heating,
Jupiter's luminosity, accretional heating, and the Sun. The position of the
H$_2$O ice line in the CPD, which is crucial for the formation of massive
moons, is computed at various solar distances. We assess the loss of Galilean
atmospheres due to high-energy radiation from the young Sun. Ganymede and
Callisto cannot have accreted their H$_2$O during Jupiter's supposed GT,
because its CPD (if still active) was too warm to host ices and much smaller
than Ganymede's contemporary orbit. From a thermal perspective, the Galilean
moons might have had significant atmospheres, but these would probably have
been eroded during the GT in < $10^5$ yr by solar XUV radiation. Jupiter and
the Galilean moons formed beyond 4.5 (+/- 0.5) AU and prior to the proposed GT.
Thereafter, Jupiter's CPD would have been dry, and delayed accretion of
planetesimals should have created water-rich Io and Europa. While Galilean
atmospheres would have been lost during the GT, Titan would have formed after
Saturn's own tack, because Saturn still accreted substantially for ~$10^6$ yr
after its closest solar approach, ending up at about 7 AU.
