Suddenly the sky became clear on Neptune and this is (most likely) the sun’s fault

Normally, clusters of white clouds drift north and south of Neptune’s equator. It’s not always very visible, but in 2019 it almost completely disappeared in a relatively short time. Only close to the South Pole can a cloud now be seen occasionally.

in Journal article Icarus A group of astronomers points to the sun as the cause of the disappearance of clouds. Neptune has been watched closely in recent decades, both by the Hubble Space Telescope and by major telescopes on Earth. It turns out that if the Sun is more active than average once every eleven years, then there are many clear clouds, but with less solar activity there are even fewer.

This is a remarkable discovery: Neptune is located 4.5 billion km from the Sun, and the sunlight there is almost a thousand times less bright than on Earth. Therefore, many scientists doubted that the Sun could have such a great influence on the distant planet.

chimical interaction

How exactly it works is still not clear. The sun’s activity fluctuates every eleven years. During maximum activity, the sun displays more dark spots and bright bursts and also emits high-energy ultraviolet radiation. This extra ultraviolet radiation would trigger chemical reactions in Neptune’s methane-rich atmosphere, eventually leading to cloud formation — with a delay of a year or two. After a few years of minimal activity, those clouds are disappearing again, as in 2019.

If there really is a causal relationship, new clouds should soon appear in Neptune’s atmosphere. Recently solar activity has greatly increased again; The next maximum is expected next year.

“It’s a comprehensive analysis, and it looks very convincing,” said Heidi Hamill, a NASA planetary expert. She herself was not involved in the new research, but has previously indicated a link between solar activity and weather on Neptune. “But the mechanism behind it is very complex, and not yet well understood.”

Voyager 2

Weather systems on Earth, dust storms on Mars, and even large storms in Saturn’s atmosphere are a major cause of the changing seasons. Neptune also has seasons, but they last for forty years instead of three months. It is also possible that there are seasonal effects, but they happen at a much slower pace than on Earth.

According to Hamel, there could also be a relationship between solar activity and cloud formation on the planet Uranus, which is a kind of twin brother of Neptune. But because Uranus’ axis of rotation is tilted and the planet is on its side, so to speak, the seasonal effects are much stronger, she says. It is likely that they will completely dominate the planet’s weather.

Neptune has only had one brief visit from a spacecraft so far: Voyager 2, in August 1989. That was also a few years after the solar minimum. So many bright clouds were not visible in the Voyager images.

At the moment, there are no concrete plans for a new space flight to Neptune. For now, planetary experts rely on images from telescopes on Earth and in space, including the James Webb Space Telescope.