Chavez et al. have examined ~26 years’ worth of HST near-infrared observations with the WFPC2 and WFC3 instruments (see top row of the image, where monochrome images have been artificially coloured), along with archival observations from the W. M. Keck and Lick observatories that cover the period 1994–2022 in total. Clouds appear bright against the dark planetary disk, and are often smeared longitudinally by strong Neptunian winds (for instance, in the 2002 and 2015 observations). The authors have measured two quantities from the images: the fractional cloud coverage of the disk and the average cloud brightness. The former involves setting a (wavelength-dependent) threshold by which to separate bright features (clouds) from dark (not clouds). The latter required taking into account the point-spread function to recover reflected light from outside the planet’s disk and careful calibration. Both quantities turn out to be correlated and show the same periodic pattern. Two peaks occur in 2002 and 2015, with three minima in 1996, 2007 and 2020.
The 2020 minimum corresponds to the solar minimum in 2019 (see lower portion of the image, where the solar Ly-ɑ irradiance has been plotted in grey points). At the time of writing, cloud coverage on Neptune is low, with a few clouds clustered around Neptune’s south pole, and according to the authors’ predictions, we can expect to see the number of Neptunian clouds increase as solar maximum approaches in 2025. A connection to the solar cycle is not entirely unexpected, since a study last year (M. T. Roman et al. Planet. Sci. J. 3, 78; 2022) linked Neptune’s stratospheric temperature to the Sun’s activity. The mechanism linking clouds to solar activity requires further study, but it may be that relatively high levels of solar UV increases the photodissociation rate of methane in Neptune’s upper atmosphere. The resulting hydrocarbons may then descend and condense, forming clouds and hazes. However, such a process should cool the stratosphere rather than heat it, because methane (a greenhouse gas) is removed. This apparent conundrum hints that there is a complex interplay between photochemical and radiative processes that will require modelling to unravel.
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