Something very strange has happened to Neptune. Over the past few years, the faint streaks of clouds that typically adorn its blue atmosphere have all but disappeared.

What's more, images dating back to 1994, when the Hubble Space Telescope first began documenting Neptune, show that this is not the first time. The fluctuations also appear to be linked to another periodic change: The 11-year activity of the solar cycle.

Given that Neptune is about 4.5 billion kilometres from the Sun, or just over 30 times the average distance between the Earth and the Sun, this discovery has both surprised and intrigued astronomers. And the latest transformation has been particularly dramatic. In 2019, clouds in the mid-latitudes began to fade. By 2020, the planet looked almost cloudless.

"Even after four years, the images we took last June showed that the clouds have not returned to their former levels," says astronomer Erandi Chavez of Harvard University, who led the research at UC Berkeley.

"This is extremely exciting and unexpected, especially since Neptune's previous period of low cloud activity was not as dramatic and long-lasting."

Neptune is the most distant of the major planets of the Solar System, and as a result is not as well studied or understood as some of Earth's near neighbours. But the information we do have points to a complex and dynamic atmosphere driven by processes that we cannot fully comprehend.

Studying the atmosphere remotely is relatively limiting, but it allows long-term atmospheric trends to be determined. Chavez and colleagues focused their work on data collected by Hubble since 1994, Keck Observatory since 2002, and Lick Observatory in 2018 and 2019.

These combined datasets revealed that the amount of cloud cover on Neptune fluctuates over roughly 11-year cycles that appear to be synchronised with solar activity.

Roughly every 11 years, the Sun's magnetic field changes polarity, which is marked by flares, coronal mass ejections and a peak in sunspots. After the pole reversal, the Sun goes quiet for a while, then rises again to reach another solar maximum.

When the Sun reaches its maximum, it emits more intense ultraviolet light, irradiating the Solar System. The team's 29-year analysis shows that clouds begin to appear on Neptune about 2 years after the strong UV radiation begins. There is also a positive correlation between Neptune's cloud cover and its albedo, the amount of sunlight it reflects.

"These remarkable data give us the strongest evidence yet that Neptune's cloud cover is related to the Sun's cycle," says UC Berkeley astronomer Imke de Pater.

"Our findings support the theory that the Sun's UV rays, when strong enough, may trigger a photochemical reaction that produces Neptune's clouds."

The team's analysis, spanning 2.5 solar cycles, shows that Neptune's cloud cover and albedo peaked in 2002 (consistent with a previous study) and then fell to a low in 2007. Since then, cloud cover and albedo peaked again in 2015 and then declined again. The two most recent solar maxima occurred in 2001 and 2015.

However, while the results point to some kind of photochemical activity, scientists will need to break down some of the data to find out what that activity might be. For example, one possibility of UV interference is that clouds darken, not brighten, which would lower albedo, not raise it. And storms from the depths of Neptune would have nothing to do with photochemically induced clouds, which is a complication.

Observations continue, and we have obtained new data from the James Webb Space Telescope. Both are consistent with the team's findings as we head towards the next solar maximum, expected in 2025.

"In the latest images, we saw more clouds, especially at northern latitudes and high altitudes, as expected from the observed increase in solar UV flux over the past ~2 years," says de Pater.

Of course, it would be even better to send a spacecraft to study Neptune up close. But in the meantime, we'll have to make do.

 

Source: https://www.sciencealert.com/