Uranus in HST/STIS 2002-2022: the latitudinal structures of methane depletion and aerosol opacity

The Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope provided image cubes of Uranus in 2002, 2012, 2015, and 2022. Each image cube covers both spatial dimensions and over 1000 wavelengths from 530 to 1020 nm. STIS is ideally suited to distinguish between variations of methane and variations of aerosols because it can accurately probe the hydrogen quadrupole absorption near 820 nm, where hydrogen absorption dominates over methane absorption. It also can probe methane absorption at many different strengths allowing relatively fine altitude probing. For the 2022 STIS observations, we added observations of the hydrogen absorption feature with nine times higher spectral resolution and five times higher signal-to-noise ratio than the other STIS observations. This gave us the ability to accurately describe the latitudinal structure of methane and aerosols. We find that the methane structure has stayed constant. It is almost perfectly symmetric with respect to the equator. Low latitudes show the maximum mixing ratio of methane, up to about 30° latitude. High latitudes show the strongest depletion of methane, down to about 50° latitude. The transition region in between is very smooth without any fine structure. The only small-scale structure is a small depletion near the equator, centered on 2° north. If methane is fully indicative of the circulation of Uranus, it means that Uranus has a Hadley circulation with upward flows at low latitudes and downward flows at high latitudes, with a slight modification due to a tiny downward flow near 2° north. We find that the aerosol structure has two components. One component has no small-scale structure, but varies strongly with time. The clearest regions are those that just emanated from the decade-long darkness of winter. They approach full-scale after about a decade of illumination by the sun. This hemispherical asymmetry became close to symmetric two years after the equinox, indicating that this is the time scale that aerosols last or stay in probed altitudes after they have been photochemically produced at high altitudes.

The other component of aerosol structure is a complicated small-scale structure that has not shown significant changes over 20 years. It may be related to extremely small latitudinal variations in the circulation. Considering that aerosols settle with speeds on the order 1 mm/s, vertical motions on the order of 0.1 mm/s can cause significant variations in the aerosol density. This research was supported by STScI grant GO 16663.