Neptune cloud cover over three decades

This sequence of NASA/ESA Hubble Space Telescope images chronicles the waxing and waning of the amount of cloud cover on Neptune. This long set of observations shows that the number of clouds grows increasingly following a peak in the solar cycle – where the Sun’s level of activity rhythmically rises and falls over an 11-year period.

The theory is that the increased ultraviolet radiation from the Sun, during its peak of activity, causes chemical changes deep in Neptune’s atmosphere. After a couple years this eventually percolates into the upper atmosphere to form clouds.

In 1989, NASA’s Voyager 2 spacecraft provided the first close-up images of linear, bright clouds, reminiscent of cirrus clouds on Earth, seen high in Neptune’s atmosphere. They form above most of the methane in Neptune’s atmosphere and consequently are not blue, but reflect all colours of sunlight. Hubble picks up where the brief Voyager flyby left off by continually keeping an eye on the planet yearly.

The findings are published in the journal Icarus.

[Image description: This sequence of Hubble Space Telescope images chronicles the waxing and waning of the amount of cloud cover on Neptune taken in the years 1994, 1998, 2000, 2002 (top row) and 2006, 2010, 2015, 2020 (bottom row). The planet is blue (due to methane absorption of red light in its atmosphere) and the high-altitude, cirrus-like clouds are white. A comparison of Neptune’s cloud cover corresponds to peaks in the 11-year-long repeating solar cycle where the Sun’s level of activity rhythmically rises and falls.]