Vision and Voyages for Planetary Science in the - Solar System ...

How Did the Giant Planets and Their Satellite Systems Accrete
and Is There Evidence that They Migrated to New Orbital Positions?
The terrestrial planets grew only to relatively small sizes owing to the scarcity of metal and
silicate grains in the inner solar nebula. However, the ices that condensed from the nebular gas beyond
the snow line were more abundant. We witness similar processes ongoing in exoplanetary systems.
Thus, the planetary embryos of the giant planets grew rapidly in the first few million years until they
became massive enough to capture directly the most abundant elements in the solar nebula, hydrogen and
helium. Jupiter’s enormous size is very likely correlated to its position just outside the snow line: water
vapor driven out across this boundary would rapidly condense and pile up; solid particles orbiting outside
the snow line experienced a low pressure zone and sped up owing to the reduced drag, thus slowing their
migration inward. In this way, Jupiter’s feeding zone was extremely well supplied.
The regular satellites of Jupiter, Saturn, and Uranus orbit in their equatorial planes, suggesting
that they formed in subnebular disks like miniature solar systems. 13 Neptune’s coplanar satellite system
was likely destroyed by the capture of Triton, its large retrograde satellite, probably a renegade Kuiper
belt object. Too small to capture much gas gravitationally, the satellites accreted mainly from icy and
rocky solids. They might have captured gases in clathrates (i.e., water-ice cages) or in amorphous ices. If
their icy solids came directly from the solar nebula, they would retain nebular volatile abundances.
Cassini-Huygens data suggest this to be the likely case for Saturn’s moons and Titan in particular. 14,15 If
they were formed in the gas-giant subnebulae, dependent on the radial temperature profile, some regions
would be hot enough to vaporize ices, resetting isotopic thermometers and phases before they recondensed.
Such subnebula processing is speculated for Jupiter’s regular satellites, but crucial
measurements are lacking. Untangling nebula versus subnebula processes requires knowing the internal
structures of the satellites, volatile ice abundances, stable isotope ratios of carbon, hydrogen, oxygen, and
nitrogen, and abundances of the noble gases. Addressing these key questions will require precise
geophysical, remote sensing, and in situ measurements across the outer planet satellites of their internal
structures and their compositions from their plumes, sputtered atmospheres, co-orbiting tori, and surfaces.
Many unknowns remain as to how the outer planets formed out of the solar nebula and if and
when they migrated into different orbits. This is also an important question with regard to exoplanets.
The Galileo probe sent into Jupiter’s atmosphere showed quite surprisingly that the noble gases argon,
krypton, and xenon are much more abundant there than in the Sun. Suggested explanations for their
concentration include: condensation of noble gases on extremely cold nebular solids, capture of clathrate
hydrates, evaporation of the protoplanetary disk before Jupiter formed, and outgassing of noble gases
from the deep interior enriching them in the atmosphere. 16 Each of these hypotheses leads to testable
predictions for noble gas abundances in the other giant planets—definitive answers will require in situ
probe measurements—critical data that we lack for Saturn, Uranus, and Neptune.
Resolving a second major puzzle also mandates probe measurements. Solar system models that
placed the formation of Uranus and Neptune at their current positions were unable to produce cores of the
ice-giants rapidly enough. Modelers concluded that the giant planets must have migrated to new orbits
since their formation. It is now thought that during the first half billion years of the solar system, Uranus
and Neptune orbited in the region much closer to the Sun; it is even possible that Neptune was inside
Uranus’s orbit. 17,18 The models suggest that at about 4 billion years ago Saturn and Jupiter entered a 2:1
orbital resonance, increasing Saturn’s eccentricity and thereby driving Uranus and Neptune out into the
Kuiper belt that in turn was driven out to its current location.
Many variations of such scenarios have been hypothesized. However, key evidence is lacking,
and a complete understanding of formation and migration of the four planets that account for 99 percent
of the mass in the solar system awaits key measurements at Saturn, Uranus and Neptune. To distinguish
between the array of theorized scenarios for formation and migration of the giant planets and their
satellite systems, we need to know detailed composition—deuterium/hydrogen and hydrogen/helium
ratios, other isotopic ratios, and information about noble gases that can only be obtained in situ from
giant-planet atmospheric probes. To address these questions, detailed in situ measurements as acquired
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