Figure 1: Artist’s impression of a gas giant planet.
Brown dwarfs and gas giant planets such as Jupiter and Saturn have hydrogen-helium dominated atmospheres that are very different from the atmospheres of Earth-like planets. In the hydrogen-helium dominated atmospheres of brown dwarfs and gas giant planets, a wide variety of molecular species can condense to form clouds. This is expected to produce multiple cloud layers in the atmosphere, where each cloud layer is made up of a particular type of condensable species. By comparison, only water clouds produced from the condensation of water exist in the atmospheres of Earth-like planets.
Gas giant planets are known to exist over a wide range of distances from their parent stars, from “star-hugging” hot-Jupiters to isolated Jupiter-mass planets in the frigid depths of interstellar space. Also, depending on its age, a gas giant planet is much hotter when it is young and it will gradually cool over billions of years. As a result, gas giant planets can have a wide range of atmospheric temperatures. The atmospheric temperature of a gas giant planet strongly influences the number of cloud layers it has and the position of its cloud layers. With decreasing atmospheric temperatures, the more refractory cloud layers form at progressively greater depths in the planet’s atmosphere and cloud layers composed of more volatile condensable species become present at the upper portions of the atmosphere.
Figure 2: Illustration of cloud layers in the hydrogen-helium dominated atmospheres of gas giant planets. The three panels correspond roughly to effective temperatures of approximately 120 K (Jupiter-like, left), 600 K (middle) and 1300 K (right).
When a condensable species forms a cloud layer, the condensate is removed from the cooler overlying atmosphere and is no longer available to react with other molecular species higher up in the atmosphere. For example, the detection of hydrogen sulphide (H2S) in Jupiter’s atmosphere by NASA’s Galileo entry probe indicates that iron (Fe) must be sequestered into a cloud layer much deeper in the planet’s atmosphere because the presence of Fe will lead to the formation of iron sulphide (FeS) instead of H2S. In another example, the detection of monatomic potassium (K) in the atmospheres of some brown dwarfs suggests that rock-forming elements such as aluminium (Al) and silicon (Si) are removed from the atmosphere due to cloud formation deeper down in the atmosphere. If Al and Si were not removed, the potassium would have condensed into silicate minerals such as orthoclase (KAlSi3O8) which would remove the presence of monatomic potassium from the observable atmosphere of the brown dwarf.
In the atmosphere of Jupiter, the highest clouds are composed of a cirrus-like layer of ammonia (NH3) ice crystals. Further down into the atmosphere are ammonium hydrosulphide (NH4SH) and water (H2O) cloud layers. The troposphere of Jupiter’s atmosphere is commonly defined from the 0.1 bar level (approximately 50 km above 1 bar level) down to the 10 bar level (approximately 90 km below 1 bar level). At the top of the troposphere, the temperature is about 110 K and at the bottom of the troposphere, the temperature is about 340 K. The NH3 (0.6 to 0.9 bar), NH4SH (1 to 2 bar) and H2O (3 to 7 bar) cloud layers are all located within the troposphere of Jupiter. Although methane (CH4) also exists in the atmosphere of Jupiter, the temperatures are too warm for it to condense to form clouds.
Deeper down into Jupiter’s atmosphere are alkali, halide and sulphide cloud layers. These are then followed by silicate and iron cloud layers at increasing atmospheric pressures and temperatures. Finally, the deepest cloud layers consist of refractory species such as perovskite (CaTiO3) and aluminium oxide (Al2O3) crystals. In fact, in the atmospheres of the hottest gas giant planets, only cloud layers consisting of refractory species exist in their atmospheres because the temperatures are too high for other molecular species to condense. As a result, the atmospheres of the hottest gas giant planets resemble the deepest regions of Jupiter’s atmosphere.
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3. Loadders et al. (2006), “Chemistry of Low Mass Substellar Objects”, arXiv:astro-ph/0601381