A new model describing the origin
of the four large moons of Jupiter — the so-called Galilean satellites —
can reconcile the moons’ major properties with the formation of the
satellites from a disk of gas and small particles orbiting Jupiter during
the very end stages of the planet’s growth. This model may represent a
breakthrough in understanding how the large satellites of Jupiter formed.
Calculations performed by two researchers at Southwest Research Institute
(SwRI) and funded by NASA show that a prolonged period of satellite growth
from a “gas-starved” disk accounts for several key satellite properties. The
moons’ bulk compositions, the unusual internal structure of the outermost
satellite, Callisto, and the survival of the satellites against inward decay
caused by interaction with the disk are properties resolved by the new
model. The findings appeared in the December issue of The Astronomical
Journal.
Researchers have long known that the Galilean satellites have bulk densities
that decrease with distance from the planet. This trend indicates an
increasing proportion of low-density ice, with the outer two, Ganymede and
Callisto, containing approximately 50 percent rock and 50 percent ice by
mass. Spacecraft encounters by NASA’s Galileo mission have more recently
suggested that the interior of Callisto is not divided into a distinct
central core and outer mantle, which means it had to have formed slowly —
over more than 100,000 years — to stay cool enough to avoid large-scale
melting. The combined mass of the satellites indicates that the amount of
gas and solids (ice plus rock) necessary to form them was about 2 percent of
Jupiter’s mass.
Previous models assumed that the satellites formed from a disk orbiting
Jupiter that contained 2 percent of Jupiter’s mass all at one time. The
underlying assumption was that the disk formed first, and the satellites
formed within it. However, expected temperatures in such a massive and
gas-rich disk would be too high to retain ices in the region of Ganymede and
Callisto, resulting in very short satellite formation times of only about
1,000 years. In addition, the gravitational interaction of the satellites
with such a disk would have caused them to decay inward onto the planet,
likely leading to the complete loss of the newly formed satellites on a time
scale similar to the one forming them.
The SwRI model does not require that all of the mass needed to form the
satellites be present in the disk all at once. Instead, material is
delivered to the disk slowly over a prolonged period of time. This would be
expected to occur as Jupiter itself grows, as the planet channels gas and
small particles from solar orbit into orbit around itself. This inflow would
provide an ongoing source of material for the satellite disk.
Thus in the new model, the satellites grow gradually as material is supplied
to the disk. Solids entering the disk rapidly accumulate in orbit around
Jupiter and buildup over time, while the inflowing gas spreads radially and
maintains a low density. In this “gas-starved” disk, temperatures low enough
for ice in the region of Ganymede and Callisto naturally result. The new
model predicts that the satellites also form slowly — over 100,000 to 1
million years — consistent with the theorized internal state of Callisto.
“The strength of this model is that it can tie together a wide variety of
compositional and dynamical properties of the Galilean satellite system with
a single, simple set of origin conditions,” says the paper’s lead author,
Dr. Robin M. Canup, assistant director of the SwRI Space Studies Department.
Co-author Dr. William R. Ward, an Institute scientist at SwRI, adds, “We
find that the well-known interactions between planets and gas disks, thought
to cause orbital decay and to lead to hot Jupiters in extrasolar systems,
also have important ramifications for the formation of satellites in gaseous
disks.”
One example is a prediction under the new model that the satellites would
migrate inward somewhat during their formation, each at a rate proportional
to its mass. A separate work by Peale and Lee (2002, Science 298) has shown
that this inward migration can lead to the establishment of the so-called
Laplace resonance as the satellites form. The Laplace resonance is a locked
configuration that exists among the inner three Galilean satellites — Io,
Europa, and Ganymede — in which innermost Io completes four orbits to every
two of Europa, and every one of Ganymede. This new means of establishing the
Laplace resonance would occur on shorter timescales than previously favored
explanations that require a much later outward orbit evolution of the
satellites caused by interaction with Jupiter. The existence of the
resonance leads indirectly to internal heating of the satellites, causing
Io’s extensive volcanism, for example.
The conditions for Galilean satellite formation are important not only for
understanding the compositions and thermal histories of the moons
themselves, but also for the potential constraints that such models provide
on the growth of jovian planets.
SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700 employees and
an annual research volume of more than $319 million.
