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Neptune's Clouds Have Disappeared and It's the Sun's Fault
The Sun is alternately feeding and starving Neptune's cloud systems.
The Sun is such a constant presence, bathing us consistently in light and warmth, that we can easily forget how much influence it has. From the weather to the food chain, it all traces back to the Sun. So, it stands to reason that if something goes really wrong or really weird, the Sun may have had something to do with it. That’s the thinking behind the 2006 disaster flick Solar Attack (streaming now on Peacock) and, despite its wacky premise, the filmmakers may have been onto something.
The story hinges on a coronal mass ejection (CME) interacting with methane in Earth’s atmosphere and threatening to set the sky on fire. Now, real world scientists have found a similar (if less destructive) process gobbling up the clouds in the skies of Neptune. The results were published in the journal Icarus.
The Mysterious Missing Clouds of Neptune
When Voyager returned the first detailed pictures of Neptune in the summer of 1989, we got our first glimpse of the cloud features in its distant sky. Since then, astronomers have followed up those observations with telescopes on the ground and in space, and they noticed something weird.
Over the years, astronomers noticed a characteristic ebb and flow in the Neptunian cloud cover, but it wasn’t clear what was causing it. Now, more than three decades after Voyager sent back its first grainy images, astronomers may have figured it out and, you guessed it, it’s the Sun.
To figure out what was going on, astronomers pored through 29 years of observations from NASA’s Hubble Space Telescope, Hawaii’s Keck Observatory, and the Lick Observatory in California. Those observations included a series of eight Hubble images (below) collected between 1994 and 2020. In them, you can clearly see the disappearance and reappearance of clouds from image to image. Critically, recent observations reveal an almost complete absence of clouds.
The important clue was the frequency of change, with cloud activity peaking in 2002 and 2015 and slumping in 2007 and 2020. Those dates didn’t immediately line up with any celestial phenomena, but it all fell into place when astronomers realized there was a two-year delay in activity. Through that lens, the changes in cloud cover start to reveal themselves as a consequence of solar activity, with clouds peaking two years after solar maximum of each solar cycle.
How the Sun Forms Clouds in Neptune’s Atmosphere
The degree of effect is surprising, considering how far away Neptune is. We’re orbiting about 93 million miles away from the Sun. Neptune, by contrast, orbits at an average distance of 2.78 billion miles. At that great vantage point, Neptune receives only about 0.1% of the sunlight Earth enjoys, but that’s enough.
Solar radiation impacts weather patterns on Earth too, but in a different way. While the Sun feeds terrestrial weather systems largely through heat, on Neptune the relationship is subtler. As solar radiation strikes the atmosphere, it interacts with molecules to steal or donate an electron. Those ionized molecules in the upper atmosphere then serve as condensation points around which clouds can form.
That photochemical relationship also explains the two-year delay in solar maximum and maximum cloud production. While the travel time between the Sun and Neptune is only about 4 light-hours, actual cloud production can take years to spin up, resulting in partly cloudy skies in the Neptunian weather report roughly two years following solar maximum. Of course, 2.5 solar cycles of data is a pretty small sample size and astronomers will be looking to confirm this finding as the current solar cycle wraps up.
There aren’t any clouds in the Neptunian skies right now, but if researchers are correct, there will be in 2026 or 2027, on the other side of the Sun’s upcoming temper tantrum.
Want to prep for a worst case scenario? Catch Solar Attack, streaming now on Peacock!