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)
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.
Figure 2: Artist impression of a gas giant planet.
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.”
Hugh F. Wilson and Burkhard Militzer (2013), “Rocky Core Solubility in Jupiter and Giant Exoplanets”, arXiv:1111.6309 [astro-ph.EP]