What kind of atmosphere is needed for large floating organisms?

Question

Assume you have an identical Earth-clone planet, what kind of atmosphere is required to have floating organisms? What I define as a large floating organism is something about the size of a bat, but with a lighter-than-air-filled sac that allows it to float like a balloon. The organism can be an animal or plant, but bonus points if it's an animal. I am wondering if an atmosphere needs to be dense or have a unique composition to allow a difference in density.

Constraints if possible:

• The organism must be at least 6 inches (15 cm) in diameter
• It has to have controlled movement (cannot be a wind-born seed with no direction)
• Must be capable of some activity (cannot be dormant)
• Preferably able to exist with flying predators

Answer 1

It depends on your evolutionary path

The trick with evolution is there is no grand goal - there are incremental advances that at the time make sense, either sexually or on a survival basis.

So to consider what atmosphere would be required to meet your criteria the answer is really that it depends on the incremental evolutionary path up to that point.

I would imagine, for instance, that our current-day floating spiders (called ballooning) are good examples, where traditionally land-based creatures used techniques already at their disposal to eventually fly, and this then became evolutionarily locked in.

Your creatures would evolve similarly: there would need to be a pathway up to the point where they are already inflating, already can control to a degree their direction, and already around about the same size, for them to then make a 'small' leap to be then a large floating organism.

As far as the atmosphere goes, there are several characteristics I would imagine it could have for this 'small leap' to be evolutionarily advantageous:

• it should be desirable for your floating organism to be in. This means either it provides them with a mating advantage (they can float far away to spread the genetics further), or a survival advantage (they can evade land based predators)
• it should be relatively easy for your organism to be in. So no huge wind factors that would make it hard for your organism to make this leap. I guess a denser atmosphere also helps, as less inflation is needed to accomplish the same goal.
• it should be safe for them - ie. they won't die straight away if they do this. An absence of predators up there would help. Atmosphere could also be 'cloudy' so vision based predators have a difficult time finding them.
• it should be 'sexy' for them. If sexual reproduction is available on your world, which I believe it needs to be in order for the diversity of life to get up to the requirements you have outlined, then there needs to be competition and sexual selection. This means a female should really be 'awed' by the fact the male can do this, or the other way around, to promote competition within the same species and push these evolutionary developments. Perhaps the atmosphere can carry sound easily for mating rituals, or your organisms can carry scent to advertise their sexy presence (meaning not too much wind again).

Again there is no goal in evolution, there is no real reason why it hasn't happened on Earth, other than perhaps it isn't really sexy enough yet for our organisms that can 'almost' do this.

Answer 2

A denser atmosphere would help floating organisms, since it allows more lift to be generated for the same balloon volume. This could be done by having a high-pressure atmosphere or by having dense gases in the atmosphere (the latter also lets you use a wider range of gases for lift). Venus is an example of both of these: its atmosphere has over 90 times the pressure of Earth's and is almost entirely CO2. Notably, people have proposed using balloons to colonise its upper atmosphere: these balloons could be filled with nitrogen or even with breathable air and still float.

It would also be helpful if the atmosphere has a diverse composition, enough for organisms to get everything they need without descending to the ground. As an example, the six elements of carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur are essential for all Earth life, so it would help if these are all present in the atmosphere. These wouldn't need to be present as gases, necessarily, they could also be liquid droplets or solid dust particles suspended in the air. And they wouldn't necessarily have to be in high concentrations: plants do fine with an atmospheric CO2 concentration of ~400 ppm, so it's likely that similarly low concentrations of the vital elements would be enough to support life.