Aerosols in the atmospheres of the Giant Planets

<p>Many observations have been made in recent years of the visible/near-IR spectra of the Giant Planets: Jupiter, Saturn, Uranus and Neptune (Fig. 1).  Spectroscopic observations in reflected sunlight are complemented by studies of aerosol opacity at thermal wavelengths, from 5 µm into the mid- and far-infrared.  Observations in reflected sunlight can be inverted to infer vertical atmospheric structure using retrievals algorithms such as NEMESIS. In this paper, we will review recent observations and compare and contrast the aerosol structures derived to be present in these planetary atmospheres. Common themes that will be explored are:</p><ul><li>Cloud condensation requires cloud condensation nuclei (CCN), small particles that can ‘seed’ the condensation process. Such materials are common in Earth’s atmosphere, blown up from the surface or ocean, but to understand formation in Ice Giant atmospheres, which have no surface, we are reliant on photochemistry in the upper atmosphere to photolyse gases such as ammonia and methane to generate hydrocarbon and nitrile hazes. This has a strong effect on where clouds/hazes can form.</li> <li>These photochemically-produced hazes are significantly absorbing at some wavelengths, which affects the observed colours of these planets and also how the reflectance varies with solar and observing zenith angle, i.e., limb-darkening.</li> <li>Moist convection on the giant planets is very different from that in the Earth’s atmosphere. Moist air in the Earth’s atmosphere is naturally buoyant since water vapour has a lower molecular weight than the surrounding N<sub>2</sub>/O<sub>2</sub> air. On giant planets, however, moist air containing ammonia, water vapour or methane, has a significantly higher molecular weight than the surrounding H<sub>2</sub>/He air and hence tends to sink. Precipitation of condensed phases of ices, such as ammonia-water ‘mushballs’ on Jupiter, can be responsible for changing the vertical distributions of condensable species considerably, compared to equilibrium condensation models.</li> <li>Regions of cloud condensation can lead to a significant decrease of mean molecular weight with height, leading to regions of significant static stability that may help suppress convection, and potentially separate atmospheric circulation into multiple stacked layers of differing properties.</li> </ul><p>While reviewing these recent measurement and retrieval studies we will also outline how degenerate the solutions are: since we have very little prior knowledge and limited data there are a wide range of solutions that can fit the observations equally well. Fortunately, we have found that the Minnaert limb-darkening model gives us a means of reducing this degeneracy and we shall show how this approach has greatly improved the robustness and reliability of our recent retrievals.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.d6e6c056128260652962561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=08ad42422f5c74b7a51b315fec59ad93&ct=x&pn=gepj.elif&d=1" alt=""></p><p>Figure 1. Visible appearance of the giant planets in our solar system: Jupiter (upper-left), Saturn (upper-right), Uranus (lower-left) and Neptune (lower-right)</p><p> </p><div> <div> <div> </div> </div> </div>