All of Neptune's clouds have mysteriously disappeared, and the sun may be to blame

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Earth has often been described as a pale blue marble in space, but the planet Neptune is much deeper blue. It's been known since the 1800s when it was separately discovered by John Couch Adams (1843) and Urbain Le Verrier (1845-46). The name "Neptune" comes from the Roman god of the sea, which coincidentally matches its deep blue color, even though that was not known at the time. Voyager 2 provided the first close-up photos in 1989, and that was when the high-altitude clouds of 80% hydrogen and 19% helium were seen. But since then, their appearance has changed considerably, and the reason for that is only now coming to light.

Picture of Neptune by Voyager 2 (NASA/JPL-Caltech)

First of all, why is Neptune blue -- essentially all blue? The trace amount of methane in the upper atmosphere absorbs red wavelengths of light from the sun and reflects back blue. 

Second, we know that on Earth, clouds form when water vapor in the air (which comes from evaporation on the surface) runs into dust particles. These collect until the vapor changes (condenses) into liquid form and eventually clouds. Evaporation from lakes, oceans, rivers, pools, and other places is driven by heat from the sun as part of the water cycle.

Cloud formation (from ClimateKids.NASA.gov)

But Neptune doesn't have liquid water. The average temperature there is -353 Fahrenheit (-214 Celsius). It's not a uniform temperature all over the planet, of course. In fact, scientists have used infrared telescopes to measure changes at the south pole during its long summer. Yes, Neptune has seasons like Earth because it is tilted nearly the same as our planet. But because it takes 165 years to revolve around the sun, each season is about 40 years long! But in the pictures below, you can see that instead of being dark red (hot), the south pole cooled down (white) during summer. The difference is 46-52 degrees F (26-29 degrees C). The reason is unknown.

Infrared images of Neptune. Note cold white on the south pole growing during summer (EarthSky.org)

So, what makes clouds on Neptune if there's no liquid water? It's the frozen methane (not the methane mixed evenly with hydrogen and helium) in the upper atmosphere that looks white and casts shadows on the blue background of the lower atmosphere. The lower atmosphere is still very cold, so it may be made of hydrogen sulfide, ammonium sulfide, ammonia, and even water ice, all under great pressure.

Neptune is known to have the most violent storms in the solar system. Winds up to 1,305 miles per hour (2,100 km per hour) have been measured. Those are 9 times faster than the fastest on Earth. But how are winds created? On Earth, the sun heats the air, land, and waters but unevenly. As the hot air from land rises, it is replaced by heavier cold air; thus, that influx of air moves, and we call that wind.

Images from scijinks.gov

But at 2.8 billion miles from the sun (compared to the 93 million miles for Earth), Neptune receives far less energy from the sun, about 1% of what Earth gets. And, there is almost no significant rocky core (and no liquid water) on Neptune compared to Earth. So how does wind form there, let alone blow so strongly? The first thing to keep in mind is that they blow only in the upper 600 miles (1,000 km) of its atmosphere.

Cross section of Neptune (Wikipedia)

There are two theories why winds are created on Neptune. One is that some process in the upper layer reduces friction. The other is that there is some unknown heating coming from the core.

The newest data from the University of California at Berkeley concerns a related phenomenon: clouds. From instruments on Earth and the Hubble space telescope, they noticed that in the 40-year summer season, there was a change in cloud patterns during an 11-year cycle. They are disappearing.

Cloud patterns dying off 2002-2023 (UC Berkeley/W.M. Keck Observatory)

That same 11-year cycle coincides with the sun's activity. It isn't a constant source of heat, light, and radiation. Every 11 years, its magnetic poles flip, so north becomes south and vice versa. That changes the amount of sunspots and solar flares. More of each occur at the maximum solar activity, and each one sends out strong magnetic radiation into the solar system. They can cause beautiful views like the aurora borealis on Earth when the radiation interacts with the planet's own magnetic field, or they can disrupt electronic communications (radio blackouts). 

Solar activity cycle (skyjinks.gov)

Out on Neptune, these bursts of energy are thought to cause more clouds. The thought nowadays is that, unlike cloud formation on Earth, somehow the energy from sunspots and flared (maybe also from UV light) interacts with the chemicals in the upper atmosphere slowly over 2 years to create enough condensation to form clouds. When the activity from the sun is low, the reaction slows or stops, and clouds disappear. 

Here's a 5-min YouTube video on Neptune's winds. Note the mention of diamond rain!