[Space] Weather on planets & moons

[Agemegos]

Originally Posted by martinl

Flat Black has plants that grow PVC plumbing.

This isn't a FLAT BLACK discussion.

[su_liam]

I thought the Brooks formula had more potential, but it just doesn't deal well with planetary parameters interestingly different from those of Earth. The information provided is pretty basic, even for rpg use: mean annual temperature for a latitude band. Given that, the, so far, enormous rigamarole of figuring out the value of the constants for what will still be a toy model seems less than worthwhile.

So, I've decided on the novel concept of looking at the book. The chart for tide-locked worlds on page 125 of GURP Space looked like a promising start. Past tense. So I calculated day face and night face temperatures based on the Earth. This has some problems.

First problem. If I'm reading this correctly, these multipliers are supposed to be applied to the surface temperature after the greenhouse adjustment. That means that a stronger greenhouse effect will lead to a greater range of variation. I'm trying to step back from obsessive detail, but the opposite of reality is a step too far.

Problem two. I figure the mean temperature of the coldest month of the coldest place on Earth is a pretty good upper bound for a night side temperature on a face-locked planet. The mean temperature at Vostok station in August is listed as -67.6ºC. The night face temperature from the chart on page 125 of Space is 0.8 times the average temperature of the planet, in Earth's case 288 K. This results in a temperature of -42.75ºC. Hmm... not good. How high is Vostok Station? Perhaps I should adjust for altitude. I get 3488 m(geeze, 11,433 feet!). With an adjustment of -6.5 Cº/km, I get -44.928ºC. Actually, this is close enough to good. The strong westerlies in the antarctic sea(the Roaring Forties, Furious Fifties and Screaming Sixties) generated by the huge temperature gradient between the ice sheet and the surrounding water, may isolate Antarctica so well that it is a good model at the height of its long, dark winter for the night side of a tidally-locked planet. Don't forget, when planet building, that the night side surface might be on top of 10,000+ feet of ice(again, wow!). I can't find an average temperature by month for Dallol or Al Aziziyah, but the average high for Aziziyah in August is 37.3ºC, this is well less than the 49.41ºC I calculate from the day face table, so that should be sufficient. It might be more reasonable to calculate from recorded extremes(which are also easier to find), but I think more extended averages represent heat exchange effects better. It looks like somewhat less than a six month night would work to closely approach night face temperatures. Antarctica could also be viewed as similar in summer to a twilight zone situation, although the extensive highly reflective ice cover accumulated during winter skews insolation down from what would be observed on a tidally-locked planet. Although the twilight zone may and probably will experience strong sustained winds due to nightside-dayside temperature gradient, the lack of coriolis force will preclude the formation of the kind of ring wind bands that so effectively isolate the atmosphere over Antarctica. This should result in more efficient heat transfer.

Okay, so not really problems, but there is no account taken of moderating effects due to hydrosphere(or the converse :) ), greenhouse effect(the opposite in fact) or obliquity(higher axial tilt moves the subsolar point north and south, spreading out insolation variation). I do think the variation is a little narrow for a standard atmosphere planet. Perhaps I might start with a day face adjustment of +.15 to +.16 and a night face adjustment of -.25 to as much as -.5 for planets with thin, standard or dense atmospheres.

As a first draft, I will try a moderation factor, M, such that

M = G * (1 + sqrt(H))

Where, G : greenhouse correction = Qcomp * Psurf / gSurf,
H : hydrosphere fraction = %hydrosphere / 100,
Qcomp : atmospheric composition modifier for greenhouse effect(same as the Greenhouse Factor from the temperature factors table on page 84 of GSpace,
Psurf : surface pressure in atmospheres,
gSurf : surface gravity in g's(9.806 m/s^2)

Td : day face temperature = Tsurf*(1 + D/M),

Tn : night face temperature = Tsurf*(1 + N/M),

where Tsurf : average surface temperature of the planet in Kelvins,
D : day side adjustment above,
N : night side adjustment above.

Working my way up from ground zero...

[Agemegos]

Originally Posted by su_liam

First problem. If I'm reading this correctly, these multipliers are supposed to be applied to the surface temperature after the greenhouse adjustment.

Yes, and they apply to the state of the planet after [some of] the water and air have frozen out into an ice-cap on the night side. I think they are based on computer models, and I think those computer models were based on an atmosphere of carbon dioxide (which is a lot more prone to freeze out than either nitrogen or oxygen). They are certainly not a good place to start for a planet that still has an apparent rotation, even thousands of hours long, becasue the accumulation of such a cap takes hundreds of thousands, perhaps even millions, of hours.

[su_liam]

Originally Posted by Brett

Yes, and they apply to the state of the planet after [some of] the water and air have frozen out into an ice-cap on the night side. I think they are based on computer models, and I think those computer models were based on an atmosphere of carbon dioxide (which is a lot more prone to freeze out than either nitrogen or oxygen). They are certainly not a good place to start for a planet that still has an apparent rotation, even thousands of hours long, becasue the accumulation of such a cap takes hundreds of thousands, perhaps even millions, of hours.

I think GURPS Space overstates the likelihood and extent of atmospheric collapse in the chart on pg. 125. Although the CO2 condensation factor might explain the seemingly relatively high night face temperatures. CO2 vapor would release quite a bit of latent heat from deposition. Not as much as water, but it would extend to lower temperatures.

My best reference for this subject is Haberle et al(1996). Their modeling runs showed that, all else being equal, any planet receiving enough insolation to be considered habitable by human beings won't experience freeze-out of atmosphere on the cold-side. The model they used was based on an atmosphere consisting entirely of water vapor and carbon dioxide. As you mention, this is going to be more sensitive to freeze-out than an atmosphere with a significant content of lower temperature volatiles such as nitrogen and, eventually, oxygen.

An interesting question is what happens if photosynthetic life does arise on a planet that has a significant amount of carbon dioxide frozen out on the night face. I suspect that, as plants remove carbon dioxide from the atmosphere and replace it with less-easily frozen oxygen, CO2 will continuously sublimate from the ice-cap and fail to be replaced. The ice cap will thus shrink and atmospheric pressure will increase, perhaps to approximately it's pre-freeze-out level (at least to the precision required for gaming purposes). An interesting point is that I haven't been able to find any equivalent research as to loss of atmospheric components from thermal ablation at the subsolar point. This may be because loss of molecules to space is more dependent on exospheric temperature, which is unlikely to vary due to relative rotation of the planet surface.

Anyway, I agree that formation of fully developed ice caps, even of water ice is unlikely in even a very long night. After all, the most important aspect of development of permanent ice pack on Earth isn't the cold period, it's the warm period's lack of sufficient heat to melt off accumulated ice.

Ultimately, though I'm not happy with the chart on GSpace pg.125 for its stated purpose, I do think it should be more than sufficient as a base for our purposes. With that in mind I think I shall use it as-is (for our purposes). Perhaps, as a first approximation something like:

Tc = Tavg - dTcs*([hours in year] / ([hours in year] + [hours in day]))

Th = Tavg + dThs*([hours in year] / ([hours in year] + [hours in day]))

Where, Tc: temperature on coldside(night minimum) of planet,
Th: temperature on hotside(day maximum) of planet,
Tcs: temperature of cold face of equivalent synchronously-rotating planet,
Ths: temperature of hot face of equivalent synchronously-rotating planet,
Tavg: the average temperature of the planet and its synchronously-rotating equivalent,
dTcs = Tavg - Tcs: difference between average temperature of synchronously-rotating planet and the cold face temperature of that planet.
dThs = Ths - Tavg: difference between average temperature of synchronously-rotating planet and the hot face temperature of that planet.

So what do you folks think?

* Haberle, R. M., McKay, C. P., Tyler, D., Reynolds, R. T.(1996). "Can Synchronously Rotating Planets Support an Atmosphere?" In Circumstellar habitable zones: Proceedings of the first international conference, ed. L. R. Doyle. Menlo Park, Calif: Travis House Publications, 29-33.

[Captain Joy]

What I'd done in the past, was get an idea what place on earth my not-on-earth location was most like, and then use weather data from the place on earth. To that end, this website might be of some help: http://weatherspark.com/