From 4.5 billion km away, Neptune’s clouds seem to be in sync with the solar cycle

August 22, 2023   Issue #607

Astronomy News

It’s a big Universe. Here’s a thing about it.

[With a hint of climate change thrown in, too]

There’s been a lot of interesting astronomy news over the past week, but a story that caught my eye right away was a surprising one about the solar system’s outermost official planet: Neptune.

Astronomers looked at archival (that is, from previous observations already taken) data of Neptune from Hubble and the Keck, and Lick observatories to document cloud cover on the distant gas giant, and found something pretty surprising: the clouds seem to wax and wane following the Sun’s magnetic cycle! You’d expect they would grow and diminish with Neptune’s seasons, but they found a much stronger correlation with the Sun.

Wow. That’s super cool.

Neptune is a gas giant, meaning it’s a huge planet with a very thick atmosphere, and no real solid surface to speak of. The atmosphere is thousands of miles thick, blending into a denser liquid farther down. There’s likely a solid core buried deep in the planet’s center, too.

The upper atmosphere is gaseous hydrogen, helium, and methane. That last is a great absorber of red light, so the planet strongly reflects bluer light, giving it its striking sky-blue appearance at the eyepiece.

Voyager 2 passed Neptune in 1989, taking the first close-up images of it. They revealed (among a lot of other things) the presence of thin cirrus clouds in the upper atmosphere, possibly made of methane ice crystals. This ice reflects light at nearly all wavelengths, making them appear white.

They also reflect a lot of infrared light, so what the astronomers did was look at infrared observations of Neptune from between 1994 and 2022, covering an amazing 29 years [link to paper]. Assuming that how bright Neptune appears in the infrared is due to the presence or absence of these clouds, they see Neptune getting brighter and dimmer over a long cycle of very roughly 13 years (very roughly; a second peak in brightness around 2015 wasn’t as strong as the earlier 2002 one, so nailing down the exact timing isn’t possible).

Neptune’s year is 165 Earth years long, so its seasons are about 40 years each. Clearly the cloud cover cycle isn’t due to the seasons! And 13 years is suspiciously close to the Sun’s 11-year magnetic cycle. So they plotted Neptune’s infrared brightness over time on top of the Sun’s magnetic activity (they used the brightness of the Sun in ultraviolet light as a proxy for its cycle; I explain why this makes sense below) and got a strong correlation, meaning they appear to be related. The cloud cover gets stronger around two years after the Sun’s magnetic activity peaks, and dips about two years after the Sun’s activity dips as well.

What does this mean?

The Sun’s cycle has to do with complicated internal mechanisms, but basically the Sun’s overall magnetic field gets really strong and then weakens to essentially zero over a course of a little over 5 years. Then the magnetic field gets stronger again, peaking once again roughly 11 years after the last peak (I describe this in more detail in BAN Issue 582). This activity triggers the creation of sunspots, more and more up until the 11-year peak.

Sunspots are dark in visible light, but around their edges are regions called faculae (“little torches”) which are actually a little bit brighter than average, and emit a lot of ultraviolet light. So, when the Sun’s activity peaks and you get lots of sunspots it actually sends out more UV light than usual.

Even though Neptune is about 4.5 billion kilometers from the Sun, this extra UV has an effect. It hits the methane in the upper atmosphere and breaks it apart, which then recombines to form more complex molecules. And pun intended, what happens next isn’t clear; these molecules can fall down into a lower part of the atmosphere and form a haze, making that part of the air less transparent.

Haze is also a little bit brighter than the air normally is, which could contribute to Neptune’s changing brightness, but Hubble also clearly showed the clouds increasing and decreasing with the solar cycle, so somehow these are tied together. The chemistry of a gas giant’s atmosphere is complicated, so it’s not understood exactly why the clouds change. It’s not mentioned in the paper, but clouds can form around “condensation nuclei”, particles where vapor can condense. I wonder if the more complex molecules made by UV light might be acting as these seeds?

More work is needed here. It would be interesting to see if Neptune’s maximum brightness in infrared gets higher when the solar activity is higher than usual even for the average peak. We’re currently heading toward a 2025 solar max, and the Sun has been a busy little star, putting out a lot of activity. It’s not clear how strong this peak will be, though it’s already above predictions, but no matter what this will provide more data for the astronomers to mull over.

… and I have to add something. Climate science deniers, always eager to go anomaly hunting, have tried to tie global warming on Earth to the Sun for a long time. It’s obviously BS, because there’s no big swing in Earth’s temperature seen as the Sun cycles every 11 years. There’s some marginal data that the Earth warms a teensy bit at the solar peak due to the extra UV from faculae, but that’s a drop in the ocean compared to the huge and ongoing upward trend in Earth’s temperature.

Right here on this very newsletter a bloviating denier left some ridiculous comments after I posted about the Maui fires being amplified by climate change. They even mentioned the Sun getting brighter, which, to be clear, is absolute garbage. The Sun’s output over time is extremely steady. Over a several million year timescale it gets brighter, but we’ve only seen global warming for the past century or so, which — very much not coincidentally — is when we started dumping gigatons of carbon dioxide, a well-understood greenhouse gas, into our atmosphere. Many decades ago, scientists predicted this would warm the atmosphere. Even ones who worked for fossil fuel companies said this, though those companies have made a huge effort to suppress that information.

I imagine they’ll be jumping on this Neptune news screaming “SEE?!” and blaming the Sun for what is entirely our own damned fault.

We’ll see. I hope they don’t, since that would despoil this otherwise pretty cool (so to speak) news about Neptune. And if they do, well, send ‘em here. The links above will be unlikely to set them straight — deniers deny, after all — but at least others can see why they’re full of hot air.

Et alia

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