The past decade has seen significant progress in our understanding of giant planets. In our solar system, Juno and Cassini have measured with exquisite precision Jupiter’s and Saturn’s gravity and magnetic fields. Oscillations have been detected in both planets, tentatively for Jupiter, definitely for Saturn. The depth to which the zones and belts of both planets extend has been constrained. Thanks to spectroscopy, the abundance of some species, in particular carbon in the form of CH4, has been shown to increase from Jupiter to Saturn to Uranus and Neptune. In parallel, observations of transiting giant exoplanets have shown that, consistently with the situation for the solar system giant planets, the bulk abundance of heavy elements inferred from the mean planetary density is negatively correlated with planet mass.
On the other hand, the presence of clouds has hindered our ability to truly determine their composition. Water, probably the most crucial building block of giant planets, is still elusive because it is hidden below the planets’ visible clouds. In Jupiter, direct and indirect evidence from spectroscopic data and Juno microwave observations point to an oversolar abundance, but the fact that the Galileo probe found a low abundance even down at 20 bars in Jupiter remains still at best partially explained. The situation is not clear in Saturn, Uranus and Neptune where we must rely on indirect inference. In exoplanets, the determination of abundances is made difficult both by the quality of the data (improving) and by the presence of clouds which may or may not be masking the troughs of the molecular absorption bands.
Juno’s microwave radiometer provides us with the possibility to peer through Jupiter’s clouds, reaching depths beyond 300km below the visible atmosphere at pressures of a 100 bars and more. While one would have expected to see that this convectively active planet is well-mixed below the cloud decks, we see that this is far from being the case: Jupiter’s equatorial zone is ammonia rich while other latitudes are generally depleted with variable abundances down to great depth, 30 bars or more. A theoretical explanation is that during strong storms able to loft water ice crystals to high altitudes, ammonia can melt the ice crystals to form partially liquid “mushballs” that then fall towards the interior at great speeds. The consequence of that model that the abundance of ammonia, but also of water should be highly variable down to great depths.
In fact, we are realizing that the meteorology of giant planets differs from what we expected based on Earth models. But giant planets are different to Earth in two respects: They have no surface and condensates are much more dense than the surrounding gas (even in vapor form). This implies that anything that is condensing (water, ammonia, iron...) can sink more easily, and we do not know to how deep! This has important consequences for the meteorology of these planets, but also the planetary interior and evolution, and for the interpretation of exoplanetary spectra.
We must acknowledge that we poorly understand the mechanisms that govern hydrogen planetary atmospheres. In fact, we should take advantage of the unique laboratories that are Uranus and Neptune. Their atmospheres are characterized by abundant methane clouds that can be easily observed from a spacecraft. A mission with an orbiter and a probe to one of these planets would provide key elements to understand the mechanisms governing these atmospheres and interiors. It will be crucial to constrain the composition of giant planets in our Solar System, to interpret spectroscopic observations of exoplanets and to understand the formation of giant planets in general.