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In this context, my world assumes an infinite universe so parallel Earths are possible and probability isn't an issue. My planet has half the gravity of Earth but an atmosphere 12x denser, although, answers do not have to be restricted to this planet specifically and can be general in nature.

I've read that Insects, for example, walk very differently to larger creatures as gravity doesn't affect them in the same way - See this article. So on a planet with less surface gravity, how would it affect the ways things walk, look and evolve? Presumably, the weight of a creature would scale proportionally to the change in gravity. In this case, things could grow 1.26x larger at $0.5g$. A T-rex parallel, for instance, could be 15.5m long instead of 12.3m. And the weight limit of flight would be doubled (not accounting for Air Density).

I've outlined a few things I feel might be effected:

  • Would bipedals have more of a spring in their step so-to-speak?
  • Would animals in general evolve to move slower as each step would propel them forward with more force?
  • How would falling be affected with a lower terminal velocity? Would falling no longer be an issue?
  • If land-animals first evolved with 6 limbs, would the increase in stability make them more likely to retain the extra limbs through the evolutionary process?
  • Or would the reduced weight of these creatures counter-act the change in gravity? So a human with half the mass in $0.5g$ would look the same as a regular human on earth whilst walking (if you were watching a video per-se).

I'm thinking more about how it translates into a visual medium so I am specifically interested in how the motions of movement would be different. Bonus points if you can work Air Density into your answer.

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    $\begingroup$ "a terminal velocity of 4.9m/s/s" That's not a terminal velocity; that's the acceleration due to gravity. A velocity would be in m/s not m/s/s. Note that terminal velocity on Earth is roughly 54-80 m/s for a skydiver, depending on position. Air density would affect this a lot. You might split the terminal velocity into a separate question, as it is rather narrowly focused and may be easier to answer. $\endgroup$ – Brythan Aug 31 '16 at 5:36
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    $\begingroup$ I think the answers in this question will have similar answers to why-are-low-gravity-humans-depicted-as-tall $\endgroup$ – Sarfaraaz Aug 31 '16 at 5:58
  • $\begingroup$ @Sarfaraaz Updated. The answers were similar so I rephrased the question and added better clarification $\endgroup$ – Zac Walton Aug 31 '16 at 21:30
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The question already deals with possible answers. However, terminal velocity is more affected by air density than acceleration due to the reduced gravity. Expect terminal velocity to be fairly low. Drag forces increase proportionally to the mass density of air. The terminal velocity for a falling human, on Earth, is approximately 120 miles per hour. On your planet this would be reduced to about ten miles per hour.

Evolution would favour flight, so flying and gliding organisms will be in abundance. This could include creatures that simply fall as terminal velocity would be so low.

Slower rates of falling could result in slower reflexes and neural speeds. Creatures wouldn't need to correct so rapidly to falling so they would react in what would appear to be slower motion (not slow motion as seen on film or TV). They only need to act, react and save themselves more slowly than high-gravity creatures like humans from Earth.

Hopping and jumping forms of locomotion would appear to have an advantage in low gravity of 0.5 g, but the denser atmosphere would be an impediment. However, this is an impediment that lifeforms could take advantage of, by short-rang gliding.

Normally gliding animals drop from trees to gain the speed they need to glide. On a low gravity, high atmospheric density planet, gliding speed might be achieved at ground level enabling creatures to travel in short, low bursts. This might consist of a kangaroo-like jump, spreading its glider possum-like wings and gliding further than the jump normally allow.

Hexapods might be more probable on high-gravity planets. They are more likely the result of benthic lifeforms (ones that live sea bottoms). On Earth, most land animals are tetrapods because our remote ancestor was a teleost fish and its four fins eventually became our four limbs.

Whether low gravity planets have their animal life as tetrapods or hexapods depends on the quirkiness of the planet's evolutionary history independently of its gravity.

EDIT:

Almost so obvious it's easy to forget. The wind on a planet with twelve-times air density will be an exceptionally powerful force. This makes the possibility of wind-born lifeforms a high probability. On Earth there are many seeds and spores that are wind-borne. Even spiders, particularly social spiders, can create masses of web that can be carried on the wind.

Recently in Australia there were outbreaks of the Russian wheat aphid that had arrived from South Africa and carried by wind.

High-density winds will make wind-surfing lifeforms effectively a certainty. Compared to wind-borne organisms on Earth on your hypothetical plabet they will be reasonably large.

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  • $\begingroup$ wouldn't high-density air hinder flight (as we know it)? An avian would have to spend more energy to fly on said planet because of more drag. Also, the low-pressure pocket of air a bird creates above its wings would equalize more quickly, making it necessary to flap more times per second. I agree that gliders would be successful here, but I doubt avians. $\endgroup$ – Tony Aug 31 '16 at 7:34
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    $\begingroup$ @Tony. The lower terminal velocity should mean a lower stall speed. There's a good chance slower flight might be more practical. I mentioned air density as an impediment, but I did mainly in discussing gliding were it is an advantage. This might work for flight too. Avians would only need smaller wings. $\endgroup$ – a4android Aug 31 '16 at 12:34
  • $\begingroup$ > However, terminal velocity is more affected by air density than acceleration due to the reduced gravity < This is, unfortunately, not true. For thermal escape, the terminal velocity at the upper boundary of the atmosphere is relevant, and this is only determined by gravity. The atmosphere will loose all its lighter gases, leaving only carbondioxide as its main component. $\endgroup$ – jknappen Sep 1 '16 at 11:47
  • $\begingroup$ @jknappen The terminal velocity I was referring to is derived from the drag equation en.wikipedia.org/wiki/Terminal_velocity. The terminal velocity you referred to is formulas.tutorvista.com/physics/terminal-velocity-formula.html They are similar, but with a subtle difference. I was considering the role of drag on flight. Thermal escape of its atmosphere indicates problems with long-term survival of the planet. That is a different but crucial topic. $\endgroup$ – a4android Sep 3 '16 at 14:00
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Retaining air is largely dependent on the strength of gravity. Gases expand until they hit the wall of the container; planets have no container, so the only thing stopping the atmosphere from slowly bleeding off as individual molecules achieve escape velocity is gravity. This is why Mars' atmosphere is gone; at .38G, lightweight gases like water and oxygen will slowly waft away into nothingness. At .5G, this will probably still happen.

So one of the effects is that your superdense atmosphere is going to be composed almost entirely of denser-than-air gases. What those gases are will greatly affect evolution; the most common organic gasses I know of would be various hydrocarbons (propane, etc) or alcohols. There won't be much oxygen, so fire won't be a big concern. There won't be much water, because in gaseous form it will tend to waft away too. So any oceans will be exotic, like a hydrocarbon sea.

Life forms living there will be utterly alien biochemistry-wise.

As for size, .5G makes life much easier for organisms; it's easier to support your own weight around, it's easier to pump blood through your body, it's easier to move around and not as hard to get up to really high speeds.

Look at the biggest land-based lifeforms on Earth. They died out, because it's really hard to be a ginormous land animal. But huge sea animals are still relatively common. Because it's easy to be huge in the sea; you just have to be buoyant, and the water will literally support your weight for you. So I'd expect a lot more megafauna and megaflora.

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  • $\begingroup$ Actualy Mars add a thicker atmosphere but due to the end of her geological activity she lost her magnetic shield, then solar wind blow her atmosphere away. $\endgroup$ – Rigop Sep 1 '16 at 14:16
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Try walking chest-deep in water. This will reduce your weight and illustrate the effect of viscosity and drag. How do you change what you do to try and make progress?

When you walk, you fall forward. If that is happening slower, you must wait for it! Your normal gait would be impossible. Meanwhile you must reach forward with the limb, and this now has significant resistance.

I predict different useful modes:

① hopping with gliding. Once you loft, you can “swim” against the thick air and continue to control your motion.

② high-traction crawling. Think of a lizard walking. You don’t want to lose contact with the ground at all, and your reduced weight makes it more difficult to generate thrust, so you need large feet or gripping of items as you pull yourself forward.

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  • $\begingroup$ High-traction crawling is a nifty idea. $\endgroup$ – a4android Sep 1 '16 at 11:50
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In your description, the 12 atmosphere pressure is more appealing than the halve gravity.

Lighter than air flight

As everybody notice, the combination would favor flight. But maybe, they underestimate it.
12 atm means 12 time the Archimede force. There could be lighter than air animals.

Even for heavier than air, it is very possible that they'll get easily blown by the wind (especially on a standing position).

Also dense atmosphere means strong wind and land erosion. Your world may be super-flat making ground effect flight possible (like flying fishes or ekranoplan (yes, I love ekranoplan idea))

Insect rules

Arthropods have a different breathing system (actually, they do not breath) making them very dependent on oxygen partial pressure (is it also *12? ). In your world, dragonflies may prey on birds.

East to west

Dense atmosphere means strong winds.
On an earth-like planet, winds will blow toward west in one hemisphere and toward east in the other.
If the winds are strong, it may happen that animals continuously migrate in this direction. Especially if lighter than air.

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