Aerosol layers, clouds, spots and the colours of Uranus and Neptune

In the last twenty years, spectroscopic imaging observations of Uranus and Neptune, the solar system’s ‘Ice Giants’, have revolutionised our understanding of the atmospheres of these cold, distant worlds. In spectroscopic imaging observations, each pixel in the resolved image of the planet contains a continuous spectrum, which can be used to probe gaseous abundances as well as the precise vertical distribution of scattering particles, which is something that filter imaging alone cannot achieve. For example, observations made near 800 nm with the STIS instrument on Hubble Space Telescope have determined that the abundance of methane varies strongly with latitude in these atmospheres, with roughly a factor of two depletion at polar latitudes compared to the equator. At longer wavelengths (~1.5 μm), observations made with the NIFS instrument at Gemini-North have revealed not only the presence of hydrogen sulphide, but also hints of its latitudinal variation.

In this presentation we will highlight recent advances made with spectral imaging observations, using HST/STIS and also the MUSE instrument at the ESO Very large Telescope. On both planets the weight of evidence supports an atmospheric aerosol structure comprised of: 1) a deep layer of aerosol/H₂S ice near the H₂S condensation level at p > 5 bar;  2) a middle layer of aerosol/CH₄-ice near the CH₄ condensation level at p = 1 – 2 bar; and 3) an upper layer of photochemical haze. Variation in opacity and scattering properties of the middle aerosol layer near 1 – 2 bar are found to be responsible for the bulk difference in colour between Uranus and Neptune, and also for the seasonal cycle of Uranus’s colour. Meanwhile, variations in the reflectivity of the particles in the deep layer are found to be responsible for the dark spots seen in Neptune’s (and occasionally Uranus’s) atmosphere and in Neptune’s dark South Polar Wave near 60°S. In addition, a new class of deep bright cloud has been identified in Neptune’s atmosphere using VLT/MUSE, which hints at deep, vigorous convection.

While it is important that HST/STIS and VLT/MUSE monitoring observations will continue, the James Webb Space Telescope has recently observed both Uranus and Neptune using the NIRSpec instrument in Integral Field Unit (IFU) mode (i.e., spectroscopic imaging) at even longer wavelengths from 1.6 to 5.2 μm. These observations will advance even further our understanding of these distant worlds, although we note that extending such observations to NIRSpec’s shorter wavelengths would allow JWST to also recover the latitudinal variation of hydrogen sulphide, a key tracer of deep convection.