At the planet’s very
heart lies a solid rocky core, at least five times larger than Earth, seething
with the appalling heat generated by the inexorable contraction of the
stupendous mass of material pressing down to its centre. For more than four
billion years Jupiter’s immense gravitational power has been squeezing the
planet slowly, relentlessly, steadily, converting gravitational energy into
heat, raising the temperature of that rocky core to thirty thousand degrees,
spawning the heat flow that warms the planet from within. That hot, rocky core
is the original protoplanet seed from the solar system’s primeval time, the
nucleus around which those awesome layers of hydrogen and helium and ammonia,
methane, sulphur compounds-and water-have wrapped themselves.
- Ben Bova, Jupiter
(2000)
+-+1.jpg)
Figure 1: Artist’s depiction of Jupiter and the 4 Galilean
moons - Io, Europa, Ganymede and Callisto.
Gas giant planets such as Jupiter and Saturn are believed to
have formed from the rapid accretion of hydrogen and helium gas around an
initial solid core of rock and ice material. Such a protoplanetary core is
expected to containing approximately 10 times the mass of Earth. So much
hydrogen and helium is eventually accreted that the core of rock and ice
material only forms a small fraction of the gas giant planet’s total mass. For
instance, Jupiter and Saturn have respectively 318 and 95 times the mass of
Earth. Following the formation of a gas giant planet, the icy component of the
planet’s core is expected to dissolve into the overlying layers of hydrogen and
helium. However, the fate of the rocky component of the planet’s core is less
understood. Surrounding the core of a gas giant planet like Jupiter is a layer
of metallic hydrogen. This is a state of hydrogen that is formed when hydrogen
is crushed by the titanic gravitational compression in the planet’s deep
interior.
Calculations have shown that the intense temperatures and
pressures at the core of a gas giant planet can cause the rocky component of
the planet’s core to partially or fully dissolve into the overlying layer of
metallic hydrogen. For example, magnesium oxide is a major constituent of
Jupiter’s core and it is soluble in metallic hydrogen at the intense temperatures
and pressures found in the heart of Jupiter. One can imagine the core of
Jupiter dissolving like an antacid tablet plopped into a glass of water. Over
time, the dissolved rock material is expected to be redistributed throughout
the entire bulk of the planet. The redistribution of dissolved rock material is
consistant with the observed enhancement of heavy elements in the atmosphere of
Jupiter.
The solubility of rock material increases with temperature. More
massive gas giant planets have higher interior temperatures and are expected to
have higher solubility. This may explain why Saturn, with only 30 percent the
mass of Jupiter, seems to have a heftier core than Jupiter. Saturn’s core is
either not dissolving at all or is dissolving a lot slower because conditions
inside Saturn are not as extreme as inside Jupiter. A gas giant planet more
massive than Jupiter can have its core completely dissolved and redistributed
throughout the planet. “I suspect that for very large ‘super-Jupiters’, you
would have no core at all,” says Hugh Wilson, one of the researchers involved
in the study. “If so, this should boost the concentration of heavy elements in
their atmospheres, which future telescopes might be able to detect.”
Reference:
Hugh F. Wilson and Burkhard Militzer (2013), “Rocky Core Solubility
in Jupiter and Giant Exoplanets”, arXiv:1111.6309 [astro-ph.EP]