3 The Interior of Jupiter Tristan Guillot

Jupiter, owing to its large mass and rapid formation, played a crucial role in shaping the solar system as we know it today. Jupiter mostly contains hydrogen and helium (more than 87% by mass), and as such bears a close resemblance to the Sun. However, the Sun has only 2% of its mass in elements other than hydrogen and helium (the heavy elements), whereas Jupiter has between 3 and 13%. The exact amount of these heavy elements in the planet and their distribution are keys to understanding how the solar system formed. Yet, it would seem that since the first Jupiter book was published, more than twenty five years ago, there has been little qualitative change to our vision of the interior of Jupiter, as a planet with a central dense core and a surrounding hydrogen and helium envelope (Stevenson and Salpeter 1976, Hubbard and Slattery 1976). Fortunately, several factors have led to significant quantitative improvements to that picture. Jupiter's gravity field has been measured with a better accuracy by the Voyager flybys in 1979, thereby yielding stronger constraints on the interior models. Our understanding of its atmosphere has been steadily improved, in particular by the in situ measurements of the Galileo probe in 1995, but also by the Galileo and Cassini missions, and by more accurate ground-based observations. On the experimental side, hydrogen (actually deuterium) has been successfully compressed to pressures up to several Mbar. Although the latest experiments remain controversial, this has generally led to the calculation of improved equations of state, a crucial ingredient for the calculation of interior models of the giant planet. Last but not least, the discovery of giant planets in orbit around other stars and of the related brown dwarfs has motivated more detailed studies of the evolution of substellar objects, with direct applications to Jupiter. To first order, Jupiter's interior can be described by simple arguments. Jupiter is a hydrogen-helium planet in hydrostatic equilibrium. Its interior is warm ( cv20000 K) because it formed from an extended gas cloud whose gravitational energy was converted into heat upon contraction. (It is still contracting at the rate of cv3 em per year while its interior cools by ""'1 K per million year.) This has several important consequences: The relatively warm conditions imply that Jupiter's interior is fluid, not solid. The cooling and contraction yield a significant intrinsic energy flux (revealed by the fact that Jupiter emits more energy than it receives from the Sun) that drives convection in most parts of the interior. Convection ensures the planet's homogeneity and generates the observed magnetic field through a dynamo mechanism.