Free Astronomy Magazine

JULY-AUGUST 2022 T his diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, as modeled by a team of scientists led by Patrick Irwin. The height scale on the diagram represents the pressure above 10 bar. The deepest layer (the Aerosol-1 layer) is thick and composed of a mix- ture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The key layer that affects the col- ors is the middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. Above both of these layers is an extended layer of haze (the Aerosol-3 layer) similar to the layer below it but more tenuous. On Neptune, large methane ice particles also form above this layer. [International Gemini Observatory/NOIRLab/NSF/AURA, J. da Silva/NASA /JPL-Caltech /B. Jónsson] that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow. Because Neptune has a more active, turbulent atmos- phere than Uranus does, the team believes Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, meaning the blue color of Neptune looks stronger. “We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” commented Mike Wong, an astronomer at the Univer- sity of California, Berkeley, and a member of the team behind this re- sult. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!” To create this model, Irwin’s team analyzed a set of observations of the planets encompassing ultravio- let, visible, and near-infrared wave- lengths (from 0.3 to 2.5 micro- meters) taken with the Near-In- frared Integral Field Spectrometer (NIFS) on the Gemini North tele- scope near the summit of Mau- nakea in Hawai’i — which is part of the international Gemini Observa- tory, a Program of NSF’s NOIRLab — as well as archival data from the NASA Infrared Telescope Facility, also located in Hawai’i, and the NASA/ESA Hubble Space Telescope. The NIFS instrument on Gemini North was particularly important to this result as it is able to provide spectra — measurements of how bright an object is at different wavelengths — for every point in its field of view. This provided the team with detailed measurements of how reflective both planets’ at- mospheres are across both the full disk of the planet and across a range of near-infrared wave- lengths. “The Gemini observatories continue to deliver new insights into the nature of our planetary neighbors,” said Martin Still, Gem- ini Program Officer at the National Science Foundation. “In this experi- ment, Gemini North provided a com- ponent within a suite of ground- and space-based facilities critical to the detection and characterization of atmospheric hazes.” The model also helps explain the dark spots that are occasionally vis- ible on Neptune and less commonly detected on Uranus. While as- tronomers were already aware of the presence of dark spots in the at- mospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less re- flective. The team’s research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Nep- tune and perhaps Uranus. !