[Space] Climate & habitability of tide-locked planets

[Agemegos]

Originally Posted by YankeeGamer

I don't know if this is scientifically valid--I'm not enough of an expert--but it looks plausible--certainly sufficient for gaming. Here's a well thought out tide-locked world which keeps in mind the fact that orbits are usually not circular.
http://www.worlddreambank.org/L/LIB.HTM

Well, there are a lot of things about it that I would like to ask the author about. I would have thought that the tides that braked that world's rotation would also have eliminated its obliquity. The eccentricity (0.2) seems rather large (it's equal to Mercury's, with stronger tides), and large enough that I would have expected a 2:1 spin:orbit resonance rather than synchronous rotation. The pattern of winds on Librata has equatorial easterlies, whereas the modelling I have seen shows superrotation (i.e. equatorial westerlies).

http://www.worlddreambank.org/P/PLANETS.HTM has a lot of neat concepts :)

It has.

[Daigoro]

Originally Posted by Agemegos

Since permanent human settlement requires (a) photosynthetically active illumination, (b) an excess of precipitation over potential evaporation, and (c) an average temperature between 273 K and 303 K it seems to me that the habitation candidates among synchronously rotating planets are not those that are about as warm as Earth (settled in the twilight zone), but those that are on average a little cooler than Earth, which will be settled in the subsolar region where it rains and where the light is brighter.

Does that mean that there'd be a gradient of habitable worlds, with a habitable band located anywhere from the subsolar point to the twilight zone, depending on their average temperature (and hydro load, etc.)? Or is there absolutely going to be an arid band around the subsolar pole?

[Agemegos]

Originally Posted by Daigoro

Does that mean that there'd be a gradient of habitable worlds, with a habitable band located anywhere from the subsolar point to the twilight zone, depending on their average temperature (and hydro load, etc.)? Or is there absolutely going to be an arid band around the subsolar pole?

I haven't seen or done studies, so I don't actually know.

In the slowly-rotating limit (as illustrated in Merlis & Schneider (op. cit.)) there is a fairly simple pattern of winds and temperatures which seems to be readily understood in terms of a thermally-direct toroidal cell of circulation that is absolutely dictated by the fact that the subsolar region is hotter than what surrounds it. So you're definitely going to get a convergence and humid upwelling at the subsolar point, with a ring of downdrafts some way terminator-wards of it, and a pattern of brisk, relatively dry winds towards the subsolar region. In the case where the torrid zone is too hot for agriculture and permanent human habitation then the band of equable temperatures will be comparatively dry. But that doesn't necessarily mean that all the land will be non-arable. I live in the horse latitudes at 31°5' south, and the area around here is usually pretty green and well-vegetated. The is a bit of semi-arid grassland to the west, and an awful lot of desert west of that, but geographical features interrupt the straightforward patterns of winds and ocean currents — the South Equatorial Current that ought to flow to the west is diverted to the south by a continent in its way, the resulting East Australian current makes the seas here warmer and the air more humid than would be if the entire world were covered by a uniform ocean 5m or 4 km deep.

So I suspect that in the slowly-rotating case you start on a planet 30 K cooler than Earth with an "eyeball" world: only the subsolar region is free from ice. Then as you consider gradually warmer worlds the ice around the subsolar optimum gives way to an expanding ring of arid to semi-arid that is streaked and speckled with fortunate lands where local and geographical effects produce adequate rainfall. Once you get to a point a little warmer than Earth the subsolar region starts to become uninhabitable owing to heat — but there remain some favoured areas in the torrid zone that are cooled by diverted ocean currents etc and remain habitable and wet. At that stage, the zone of habitation is like, say, Australia: largely arid to semi-arid but with some good bits. Eventually the zone of equable temperatures gets pushed towards the terminator. In the twilight zone it isn't as windy, but the low slanting sunlight gets too dim to support agriculture.

Worlds that rotate rapidly enough for the Coriolois effects to be significant (and I don't know how fast that has to be) are more fortunate. The pattern of the winds and rainfall is a lot more complicated, yielding a warm/wet patch that is shaped like a lobster with its tail stretching towards, perhaps even across, the terminator east of the subsolar point. Screw that pattern up with a few mountain ranges and you have an excellent chance of finding pretty much any climate you need, albeit probably in a small area. Take a look at the map of surface temperature on p.4 of the paper by Merlis & Schneider, and the map of potential evaporation minus precipitation on p.5. The fast-rotating cases (on the right of Fig. 1 and Fig. 2 respectively) are charmingly quirky.

[David Johnston2]

Originally Posted by Agemegos

Since permanent human settlement requires (a) photosynthetically active illumination, (b) an excess of precipitation over potential evaporation, and (c) an average temperature between 273 K and 303 K it seems to me that the habitation candidates among synchronously rotating planets are not those that are about as warm as Earth (settled in the twilight zone), but those that are on average a little cooler than Earth, which will be settled in the subsolar region where it rains and where the light is brighter.

I've never had the impression in 4th edition that people who live on tide locked planets are living in the twilight zone anyway. More like in the northern and southern latitudes where things are cooler than they are at the east pole anyway. The only effect on habitability is the -25% to hydrosphere.

Which overlooks the biggest issue impacting habitability there of course, the periodic blasts of radiation from the sun.

[Agemegos]

Originally Posted by David Johnston2

I've never had the impression in 4th edition that people who live on tide locked planets are living in the twilight zone anyway.

Have you calculated the Habitability scores and Dayside temperatures of a lot of randomly-generated tide-locked worlds? I've had quite a lot of them drop out of my generator, and I don't like the look of them. The ones with high Habitability scores have uninhabitably hot day sides.

The thing that brought this issue to the forefront of my mind just now is trying to generate a society (and adventure) for Gliese 370 II "Persatuan", the highest-Habitability tide-locked world in Central Sector of my randomly-generated universe. It has an average temperature of 27 C, and a calculated dayside temperature of 67 C.

More like in the northern and southern latitudes where things are cooler than they are at the east pole anyway.

I don't understand this. What do you mean by "East Pole"? The subsolar point? The intersection of equator and terminator east of the subsolar point?

What is it in Space that makes you think that latitude makes a significant difference? Did I miss something big, or are you bringing in more recent discoveries from other sources?

Which overlooks the biggest issue impacting habitability there of course, the periodic blasts of radiation from the sun.

Yes, those x-ray flares and solar ion storms are nasty. The stripping of atmosphere by the intense solar wind is bad, too. Then there is the prolonged intense heating of the inner system as the star develops along the Hayashi track. Given those three challenges, the fact that the sunlight is rich in IR and poor in photosynthetically active radiation (meaning slow photosynthesis and delayed development from "Ocean" type to "Garden" type) is almost trivial by comparison.

There is still a lot to find out about the planets of M and late K -type dwarf stars — twenty years ago it was still not clear that they were common at all! — but I'm not really holding my breath on the discovery of planets habitable by humans in the system of anything cooler than about K5.

[David Johnston2]

Originally Posted by Agemegos

Have you calculated the Habitability scores and Dayside temperatures of a lot of randomly-generated tide-locked worlds? I've had quite a lot of them drop out of my generator, and I don't like the look of them. The ones with high Habitability scores have uninhabitably hot day sides.

Well of course your random generator produces a huge number of tidelocked worlds with much higher average temperatures than the by the book world generation technique. When I randomly generate systems for smaller M-Class dwarfs, on the rare occasion when I actually get a stanard-sized world in the habitable zone they are always on the cold side of things. I've never gotten a "normal", much less a "warm"

The thing that brought this issue to the forefront of my mind just now is trying to generate a society (and adventure) for Gliese 370 II "Persatuan", the highest-Habitability tide-locked world in Central Sector of my randomly-generated universe. It has an average temperature of 27 C, and a calculated dayside temperature of 67 C.

That's a bit on the warm side. I'm guessing that you rolled really high on hydrosphere.

I don't understand this. What do you mean by "East Pole"? The subsolar point? The intersection of equator and terminator east of the subsolar point?

Why would the intersection of the equator and the terminator be east of the subsolar point?

What is it in Space that makes you think that latitude makes a significant difference? Did I miss something big, or are you bringing in more recent discoveries from other sources?

The average temperature is roughly the seasonal average at a latitude of 45 degrees. Obviously it will be colder on average as you approach the poles and warmer as you get closer to the equator. But in the case of a tidelocked worlc that will actually be a roughly circular band of tolerable temperature all around the sun-side. Frankly I didn't didn't consider the possibility of a warm or even tropical average temperature sun locked world because it's apparently impossible to randomly generate them using the unmodified gurps method.

There is still a lot to find out about the planets of M and late K -type dwarf stars — twenty years ago it was still not clear that they were common at all! — but I'm not really holding my breath on the discovery of planets habitable by humans in the system of anything cooler than about K5.

Oh, honestly M0s aren't that bad.

[AlexanderHowl]

One possibility for a tide-locked planet that is not often talked about is a planet with a 90-degree axis that 'rolls' as it revolves around its star (the north pole is always pointed at the star, the south pole is always pointed away, and the equator is the terminus). In recent reports though, there are some rather massive differences in temperature between the night side and dark side of directly observed tide-locked planets with very dense atmospheres. I am not sure if the mathematical models of previous research actually reflect reality.

For example, there is a planet where molten iron replaces water, and it possesses a day side with temperatures 150% higher than the night side. The molten iron vaporizes in the dayside, rains down at the terminus, and freezes on the night side. Of course, iron gas is much less reflective than water vapor, so that would drive up the day side temperatures, but it is an amazing planet.

Another thing to consider is the placement of continents. A 'polar' ocean would moderate day side temperatures through evaporation and reflective cloud cover, as well as being an effective method of transferring heat to the night side. Conversely, a 'polar' continent would be an inferno, and the rest of the planet would likely be uninhabitable outside of the terminus because there would be insufficient heat transfer. Of course, the tidal bulge caused by the tide locking would likely result in a polar continent, meaning that most tide-locked planets are likely uninhabitable outside of the terminus.

[Agemegos]

Originally Posted by AlexanderHowl

One possibility for a tide-locked planet that is not often talked about is a planet with a 90-degree axis that 'rolls' as it revolves around its star (the north pole is always pointed at the star, the south pole is always pointed away, and the equator is the terminus).

No, that is not a possibility, because it violates conservation of angular momentum.

Another thing to consider is the placement of continents.

Yes. Hu & Yang acknowledged that in their paper, and I mentioned it in my original post. You will have continents and oceanic ridges and shallows deflecting currents, and mountain ranges deflecting winds, besides a patchwork of land where the models assume water. It's hard, perhaps impossible, to estimate the effects at the level at which the Space planet generator works, but I suppose that one near-universal is that geographical features will result in a greater variety of surface conditions than the models indicate.

A 'polar' ocean would moderate day side temperatures through evaporation and reflective cloud cover, as well as being an effective method of transferring heat to the night side. Conversely, a 'polar' continent would be an inferno, and the rest of the planet would likely be uninhabitable outside of the terminus because there would be insufficient heat transfer.

According to the modelling by Hu & Yang that I linked in the OP the westerly current at the equator is most important for advecting heat from the light side to the dark side. Flows in the polar regions are much less significant.

Of course, the tidal bulge caused by the tide locking would likely result in a polar continent

The mantle material conforms to the same geoid as the oceans do. Water conforms more quickly than mantle material does, producing the tides where applicable, but the mantle goes to the long-term average equipotential surface. You don't get a band of continents around the equatorial bulge of a planet like Earth, and you won't get continental bulges at the subsolar and anti-solar points on a tide-locked planet. Remember that tides affect the oceans.

meaning that most tide-locked planets are likely uninhabitable outside of the terminus.

What do you mean by "outside the terminus"?

I really don't believe that the twilit band near the terminator is likely to be highly habitable, for reasons that I have explained at length. That area is going to be poor in light to drive photosynthesis (a problem exacerbated by the low photosynthetic efficiency of the light from cool stars). And the modelling all shows that it is likely to be arid, too.

The results of global circulation modelling suggest to me that the best prospect for settling on tide-locked worlds is on the sunlit faces of comparatively cool ones.

[Daigoro]

Originally Posted by David Johnston2

Why would the intersection of the equator and the terminator be east of the subsolar point?

A tide-locked world would have 6 identifiable poles:

- The subsolar and anti-subsolar points (which I like to call the day-pole and night-pole). The equator passes through both these points.
- The north and south poles. These point up and down from the planet's orbital plane, and are located on the terminator. The planet's rotational axis, due to its orbiting its star, passes through these poles.
- The east and west poles (for lack of clearer names). These would be the leading and trailing points in the planet's orbit. They'd be on the intersections of the equator and terminus. They might be significant for climatic patterns to gather at due to Coriolis forces.

These points might vary due to orbital eccentricity, libration, etc.

I'm not sure if the academic literature uses these terms exactly though.

[Agemegos]

Originally Posted by David Johnston2

Well of course your random generator produces a huge number of tidelocked worlds with much higher average temperatures than the by the book world generation technique.

Why? It's a pretty damn faithful implementation.

The average temperature is roughly the seasonal average at a latitude of 45 degrees. Obviously it will be colder on average as you approach the poles and warmer as you get closer to the equator. But in the case of a tidelocked worlc that will actually be a roughly circular band of tolerable temperature all around the sun-side.

Ah. I thought you were talking about people living on tide-locked worlds, because that was the topic. Referring to the geography of a normally-rotating world was rather confusing in context. So you were saying that people live in the mid-latitudes rather than at the east pole. What's the east pole on a normally-rotating planet?

Why would the intersection of the equator and the terminator be east of the subsolar point?

There's got to be one such intersection to the east and the other to the west.