The Tropic of Ice: how cold might it get in a long-lasting planetary ring shadow?

Question

I am working on my second novel set on a ringed Earth-like planet. Compared to Earth it has a slow orbit (19 Terran years) and a low axial tilt resulting in minimal seasonal variance. The low axial tilt also means that the latitude where the rings eclipse the sun, casting a band of shadow onto the surface, is more or less the same for years at a time, moving slowly away from the equator until it switches direction every solstice, crossing the equator every equinox. Here is a photo of ring shadows on Saturn.

My question is this: is such a long period of perpetual darkness enough to induce a freezing climate at that one latitude? Even if it's quite warm to the north and south of the shadowed area? Are there any particular geographic, oceanic, or atmospheric features that could make it more plausible to have a transient "Tropic of Ice?"

Answer 1

I'm going to say no

Without more detail, the answer is entirely up to you: it's believable either way. But let's run with the idea.

Planetary rings aren't as dense as Hollywood shows. Light passes through them, just less light. You also have a low axial tilt, which means you won't see the shadow like you do on that Saturn picture. Saturn has a significant axial tilt (almost 27°). Someone has photoshoped the image in your link to make the rings entirely horizontal, but that's not in alignment with the sun — so the image is leading you a bit astray.

With a low axial tilt (and, thus, minimal seasons) your shadow will be much, much closer to the equator. Your rings would need to be enormous to cast a significantly wide shadow. But let's ignore ring width.

Let's say "Earth-like" means "our solar system, but Earth has rings."

OK, would you get a band of freezing weather under the shadow?

Probably not

I say this because the shadow is at the equator and the heated air around it will interact with the colder air in the shadow. Weather is a LOT more complex than this, but generally speaking, areas of high heat have low pressure zones from gas expansion and hot air rising into the upper atmosphere and areas of cold air have high pressure zones due to gas contraction and air settling near the surface. The result is that the air would mix and there's a lot more hot air than there is cold air due to the shadow.

So what you would get is a greater chance for storms along the edges of the shadow. But a freezing equatorial zone? My vote is no. The odds are pretty good you'd only see a 15°-20° maximum temperature variation below the equatorial daytime average of 31 °C (88 °F). Cooler, maybe even considered cold by the inhabitants. But hardly freezing.

Please keep something in mind. I'm not a fan of "as real as possible." That's a great goal for documentaries. It's only a useful guideline for good fiction. There's enough ambiguity in this situation that you could believably claim to have a freezing zone at the heart of the shadow that gradually warms toward the edges where you'll find storms. That would be a great idea for a world! So don't let my answer fool you into believing you need to throw your idea away.

Answer 2

You write:

I am working on my second novel set on a ringed Earth-like planet. Compared to Earth it has a slow orbit (19 Terran years) and a low axial tilt resulting in minimal seasonal variance...

...My question is this: is such a long period of perpetual darkness enough to induce a freezing climate at that one latitude?

Yes. The climate will be freezing in the shadow of the rings. In fact, the climate will be freezing outside the shadow of the Rings all over the planet. A planet with Earth like temperatures can't have a year 19 Earth years long.

Part One: A Planet with Earthlike temperatures and a 19-Earth-year-long Orbital period.

There is an old science fiction story where a habitable planet had a year many thousands of Earth years long, so that no native remembered any seasonal changes. It turned out that the planet orbited "the great and glorious S Doradus", which was the most luminous star known for several decades.

The Earth is about 4.6 billion years old, and it only developed a breathable by humans atmosphere rich in oxygen about 600 million years ago when it was already 4 billion years old. A planet needs fairly stable temperatures for a few billion years to develop a breathable atmosphere, which requires the star to remain on the main sequence for those billions of years.

S Doradus is a Luminous Blue Variable (LVB) type star.

Because of these stars' large mass and high luminosity, their lifetime is very short—only a few million years in total and much less than a million years in the LBV phase.[12]

https://en.wikipedia.org/wiki/Luminous_blue_variable#Evolution

So you need stars which will stay on the main sequence for thousands of times as long as S Doradus for their planets to become habitable, and that greatly restricts the lengths of the years of those planets.

I will discuss the problems and suggest several solutions later.

To be continued: