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This is where you fail, because the post you replied to is talking about oxygen levels, not carbon dioxide levels.


This seems counter-intuitive...


Hire someone! (I'm not volunteering myself, though I might try one day if I have time/energy to start drawing again.)


Is the gravity light enough that lighter-than-air flight would be possible for a relatively large creature?

According to scientists, the oxygen content was 28% in the Cretaceous, a third more than the current levels. Also, accoding to Wikipedia, the levels in tge mesozoic era were also about a quarter higher (26%). There was more oxygen in the atmosphere.

More importantly: Carbon dioxide is NOT inversely related to oxygen. If other gases increase in concentration, both oxygen and carbon dioxide will decrease. More carbon dioxide does not imply less oxygen.

(Semi-edit: what the other guys said.)

Hmm... I don't that LTA flight would be viable for any creature, small or large... I mean... what, hidrogen baloons? what terrestrial thing can evolve something like that? and perhaps more interestingly, for what reason.

however, OTOH, LTA flight is easier for people! did anyone around the bronze age figure out how to make a hot air baloon?

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Articles on Suenu - Amphitrite

I don't know, it was just an idea. xD But maybe you could have a bug that spends the few days of its adult life drifting in the wind or something like that.


They wouldn't have really had a light enough material, IMO (though that's from an Earth perspective!). Skins are too heavy, but maybe a certain plant fibre could make a light enough balloon. But they wouldn't be very practical, if they have only wood to fuel the fire.

1. Gracility: being robust has a cost. If you have massively thick legs, for instance, you need extra food and oxygen to maintain all that matter. This is only wise if that extra matter is actually useful - ie if it stops you falling over. In your world, a) there would be less threat of falling, and b) there would be less damage from falling anyway. So there would be less energy expended on not falling: ie creatures would be more gracile. Of course, if you also make it colder, that would work in the other direction. I think what you'd tend to get is animals with very short legs? Because they'd want to keep the mass of the legs minimal to reduce costs, but they'd also want to reduce the ratio of surface area to mass. In this world, bigger legs would be a big drain on resources. [Of course, there are reasons to the contrary, so they won't all be snakes...] Not sure about this, though...

2. Hopping: no, I don't see why it would be much more common compared to, say, galloping. Although cold climate might help, I suppose - though to my knowledge it hasn't done on earth. If you did have hopping, that would counteract the short-legs argument some, I suppose.

3. Flight: humans in such an environment would probably never discover LTA flight, since they'd discover HTA flight so early on in their history. Consider: on earth, it is possible for humans to fly through human muscle-power - but only just (no flights under 1km, iirc), and it wasn't until the 20th century that we could do this. It wouldn't take a much thicker atmosphere or much less atmosphere to make this seriously viable.

4. The bigger point: nobody can answer your question, because when you're talking about a planet SO dramatically unearthlike it's not really possible to sensibly extrapolate.

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1. Hmm... yeah, ceteris paribus legs should be thinner, at least for organisms that don't depend too much on speed and maneuverability, so we'd get elephants, cows and rhynoes with thinner legs, probably covered in fur. Heavily leg-robust designs, like elephants, would evolve into a more gracile form... say, antelope-like... something like a huge elephant-headed tapir. long legs are still useful, under these circumstances: remember, they help you move faster, so overall leg lenght would probably be similar as on earth, they would just tend to be thinner.

2. Yup, hopping would be as affected as other hoppy means of locomotion, like galloping. Overall we'd see stuff like elephants and rhynos galloping, probably moving more like cows and horses. I wonder if this would have an effect on, say, the comparative advantages of chasing over long distances versus ambushing... jumping over something would probably be a better idea, since falling is way less dangerous.

3. discovering HTA flight? sure, muscle power in these circumstances becomes more than enough to make a man fly, but its the technology of the thing which makes it hard, I think. Gliding, on the other hand, becomes a good idea, like jumping off a cliff or tower with some sort of winged contraption strapped to one's back and safely landing a few kilometers away, but from there to mustering human power into flight would require some means to turn leg movement into something that can provide thrust, like a propeller, cause bird-like flight is *waaaay* too complex for anyone without space-age tech to imitate. How would anyone with iron-age tech make a human-powered flying machine, I wonder... was the thrust-generating effect of a propeller known to, say, greeks?

4. Yeah, I know... its fun to try!

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Articles on Suenu - Amphitrite

This whole thread raises an interesting question.

Is there a general formula relating atmospheric pressure to composition, height, temperature, and gravity?

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There's the scale height: http://en.wikipedia.org/wiki/Scale_height.

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You can't read that, right? Yes, it says that.

Venus and Earth at one point had quite similar atmospheres, with lots of carbon dioxide. Venus, however, either lacked surface water or had all its surface water evaporate early on, while on Earth the CO2 tended to get dissolved in water and then precipitated out as carbonate minerals. Earth's CO2 didn't escape (it's a fairly heavy gas, and will escape slower than other common atmospheric gasses), it just had water as a catalyst to help it get deposited chemically in various minerals. If Venus had had/kept surface water, it would, all else being equal, likely have a thinner atmosphere than Earth. If Earth had lost/never had its water, then, all else being equal, it would likely have a heavier CO2 atmosphere than Venus (though perhaps to less of a degree, given the heaviness of CO2 and the fact that it's less prone to escape from either planet in the first place).

As for Titan, it's *much* colder. Earth, at that distance from the sun, would have a *much* thicker atmosphere. Titan, conversely, at our distance from the Sun, would have no atmosphere, and it's outer layers would also probably have evaporated and been lost to space (being composed of ammonia and water ice).

How about giant bugs, though? During the Carboniferous, arthropods were able to grow to much larger sizes (including a dragonfly with a nearly 3 ft wingspan), specifically because of higher oxygen content -- as arthropod respiration is quite sensitive to partial pressure of oxygen. With lower gravity, they could possibly grow even bigger, as gravity limits the size of an exoskeleton (too big and the shell collapses before it hardens -- makes molting quite unpleasant).

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George Corley
Producer and Moderating Host, Conlangery Podcast

Well Suenu is a cold place: even in the equator, near the coastline, it gets pretty cold, way below zero... say, -10. I'm not sure, but aren't bugs kind of bad at cold?

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Articles on Suenu - Amphitrite

They don't much like it, true.

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George Corley
Producer and Moderating Host, Conlangery Podcast

Is there a relationship between scale height and surface pressure? Assuming you knew the values of gravity, gas ratio, and temperature, how would you calculate the pressure at sea level?

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you wouldn't. You'd also have to know the total amount of gas in the atmosphere.

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Articles on Suenu - Amphitrite

I noticed a string of variables thrown about concerning atmosphere, but it apparently has been forgotten that incident solar winds significantly affect atmosphere. Being far away from a star, as well as being in orbit around a dimmer star helps, but the real kicker is the existence of a magnetic field. Earth and Venus both are protected from Solar wind by self-generated fields. Titan is also protected, at least by Saturn's (I'm not sure if it has its own field as well) field. I've seen it hypothesized that Mars used to have a significant atmosphere, but then lost its magnetic field, and the atmosphere with it.

Having said that, a planet half the size of Earth would have to be very young (imminently possible, since young is several millions of years) so that any molten core it has remains molten, or subject to powerful tidal forces, such as from a close-orbiting binary planet, or by turning the panet into a moon of a gas giant (remember that the moon was unable to maintain its molten core though it is subject to tidal forces from Earth) (also remember that very large gas giants, such as Jupiter, exude massive i.e. lethal radiation).

I'd say stick with young, it makes sense if this system is in a nebula, and it's simpler, too. And now that I think about it, the magnetic field from a gas giant could easily screw up terrestrial biology, depending on their sensitivity to it. Even if it doesn't answer your original question, it adds a piece of history to the place.

Since I'm here, though, I might as well tackle gracility: animals that can use more strength will develop more strength, and the lower grav will allow them to keep their speed (imagine a lioness hunting down elephants); animals that could use more speed will lose muscle mass, but that won't affect strength (you've already imagined elephants filling a gazelle niche). Here's the kicker, though: it's known that humans lose muscle mass in zero-g, this is because the muscles do not get used as heavily. The same may well hold true in a 1/2-g environment, but if you've specifically seeded/engineered megafauna, then the usual atrophy from transplantation might be ignored. Ultimately, if you can make a halfway-reasonable explanation, you can do what you want and the reader will fill in the gaps, or at least assume the gaps can be filled by some science that's over their head.

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The relationship between scale height and gravity is pretty much inversely proportional at the same temperature (though there are confounding issues since the altitude to temperature to pressure ratios are non-linear).

Basically, at 1 atm, doubling the gravity more or less halves the scale height, and vice-versa.

So a planet with 0.5 Earth gravity but similar surface pressure would have an atmosphere something around twice as deep. Also, since pressure falls off exponentially, a moderate increase in the surface pressure makes the entire atmosphere thicker by that factor, but doesn't increase the depth of the atmosphere by as much (mathematically, the change in depth in units of scale height is proportional to the log of the change in pressure).

I agree that heavier-than-air flight would be easier to discover with a thicker atmosphere/lighter gravity, but also lighter-than-air flight as well. Your lift per volume of lighter-than-air gas is proportional to both pressure and gravity, so if your gravity is 0.5x and your pressure is 2x, you get the same number of pounds of lift. But in 0.5x gravity, your airship only weighs half as much, so it's still a huge win.

Or to put it differently, lighter gravity and thicker air both help heavier-than-air flight. Thicker air helps lighter-than-air flight. But lighter-than-air flight is simpler - you don't have to discover how a wing works in order to do it.

My very first post on this board was concerning the same topic as in this thread, so I feel a bit nostalgic reading it. One piece of interesting advice that someone gave me was concerning blood biochemistry. On a world with lighter gravity, blood will need something heavier than Iron in order to be transported throughout the body just as efficiently. Your planet may need heavier elements on it to make this possible. However, given that it is lighter in gravity, you may need to stick with the 26 basic elements that Earth has. This may involve some research but I believe there are elements heavier than Iron that have lighter atomic numbers. I haven't read the whole thread in-depth, but after skimming it I have a few things to add which may or may not have already been mentioned:

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1. Smaller animals loose a lot more body heat than larger animals do because they have less body mass to trap the heat with. Hopping continuously would probably be a godsend for any small animal that does so much with it. Likewise, in adherence with Allen's Rule, any slender animal will also lose heat faster. They will be able to hop pretty well, too, without overheating.
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2. Animals won't necessarily need to hop; lower gravity favours bipedalism more than higher gravity does because it frees up more weight from being supported by the limbs. Your animals could simply be better at running bipedally than most animals are on Earth. A note of warning, though: earlier trends toward bipedalism will lead to more specialization for bipedalism later on. Your animals may be less likely to have any limbs or other parts of the body available for tool use.
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3. Lighter gravity favours taller and more gracile organisms because the evolutionary trend in lighter gravity is to grow as tall as you can, not as thick-bodied as you can. A thick body means more weight, which means the limit toward height happens earlier in the organism's evolution. Gracile animals become gracile because there is selection toward it, so it is more reasonable to assume they are not trying to challenge gravity by becoming bigger overall (body mass + body height) but by trying to cheat gravity by growing as tall as they can at the expense of weight. I know it sounds contradictory, but any organisms on a world with lighter gravity will want to become lighter themselves. Organisms don't evolve larger bodies for the sake of being larger. In herbivores, the larger size is a result of A) more food, or B) a larger stomache to digest the food. In carnivores, larger size is selected for to catch larger prey; as soon as the realli big animals are gone - elk, moose, bison, cape buffalo, etc., the carnivores start becoming smaller again. On a lighter gravity. A more slender animal doesn't require as much food so they are not restricted to the same evolutionary limits in size that thicker-bodied animals face. They are able to do much the same things - reach food higher up in trees, ward off the smaller predators, loose excessive body heat, etc. - but they don't have all that extra bulk slowing them down and requiring more calories to sustain. All in all, gracility is a very useful evolutionary trend on a world with lighter gravity.
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4. Having a colder or more distant star will not help very much if your planet has higher levels of Carbon Dioxide. CO2 in itself is a heavier gas so it traps heat a lot more readily and efficiently than Oxygen and (I think) Nitrogen does. To nullify the heating effect of CO2 you would need to move your planet far enough away from your star that all of the water on its surface will be permanently frozen. No liquid water, no life.

And now a couple of questions:

1. What is the mean global temperature on your planet?
2. What is the seasonal average per latitude?

I may have more later.

The two may balance each other out; i.e. you could have a smaller star or a more distant orbit but have more CO₂ to compensate. The trick is: CO₂ levels fluctuate over time with different geological processes and the evolution of life. You have to be close enough to the star that when (not if, but when - it happened to Earth several times over its first 4 billion years) the greenhouse effect collapses and your planet goes all "snowball", your atmosphere doesn't also freeze out onto the surface. And once life has more or less stabilized your planet's atmosphere, you don't want ice ages to become "snowball" periods. Theoretically, life could survive either for a period of time, and they're both recoverable (frozen atmosphere less so, but it helps if your planet is geologically active), but it would make things harder.

1, 2- terrestrial animals transplanted not long ago

questions:
1 5C°
2. no seasons. not significant ones at least, very small tilt

aas for CO2, irradiation, and stuff... I dunno, I'm gonna go with authorial fiat here; the planet's just cold, with a smaller, dimmer star. As for the exact figures, how strong the greenhouse effect is, and stuff.... hummm... yeah, no, I don have 'em.

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Articles on Suenu - Amphitrite

I'm just guessing, but I'm pretty sure that you can tune the variables (% CO2 of atmosphere, distance from star, intensity of star) to assure liquid water. After all, Earth has CO2 and liquid water (and this fact as persisted despite variations), so its obviously possible in at least some domains. This ought to be a fairly simple tuning considering the CO2 levels in question are variations from Earth norm (i.e. the atmosphere is still overwhelmingly N2-O2). A more precise tuning to achieve precise surface temperatures is not in my field, though.

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My Conlang Site which pretty much only has Tayéin.
Still under construction, but at least I did some photoshop.

Isn't carbon dioxide supposed to be dangerous to animals when taken in beyond a certain amount? I know that enough of it will give you hallucinagenic effects and a slight burning sensation in your lungs.

Low temperature and low axial tilt actually makes you much more prone to having your atmosphere solidify. You need to make sure the temperature at the poles doesn't get below -70F for any significant period over any significant area or you'll have CO2 start to freeze out. Sea ice creates a positive feedback loop towards colder temps. High ocean surface area and a bunch of greenhouse gas might help a bit. One of the advantages of higher axial tilt is that each pole gets some sun each year.

As for the toxicity of CO2, it's not a problem until it's many times the current atmospheric concentration. Current levels are <0.04%; 1% is where some mild effects start; you have to get even higher for it to really start to be dangerous to humans.

Well, more like -78C° if google is correct. Still, earth reaches those temperatures sometimes, but earth has a less dense atmosphere and less greenhouse effect, so temp shouldn't vary as dramatically on Suenu. Also there's no continents at the poles but there's one pangea that is near the south pole at one latitude. I figure that causes two superficial sea currents along the south coast of the pangea which tend to drag ice from the poles into the ocean. Also, since the rotation is so slow, ocean currents work differently: at the surface the dayside pushes warm water towards the nightside through the poles, and part of that current's bound to go through the poles, in turn warming them. not really warm, but probably enough to melt the eventual dry ice patch that forms... and indeed they form, I'm sure.

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Articles on Suenu - Amphitrite

I don't remember if this planet was colonized by humans and Earth fauna, but, wouldn't this also be moot if the native species were adapted to deal with the extra CO2?

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It's not so much whether the current setup has landmass at a pole, but rather if it ever did. However, a geologically active planet in the habitable zone with enough gravity to hold onto its volatiles isn't likely to stay in a crashed snowball state forever.

The slow rotation rate actually helps you, for the reasons you brought up. Single-belt rotation or even single-cell (nightside to dayside) circulation is possible in that situation, which allows for a bit better heat distribution across the poles. And you're also right that a little CO2 condensation isn't the end of the world as long as it's more or less in equilibrium with the atmosphere.