# How do you get a thick atmosphere with less than earth-like gravity?

Basically: I want a highly viscous atmosphere but I want people to be able to walk around. I was thinking earth's gravity * 0.6 or similar.

I was thinking that an atmosphere of Sulfur Hexafluoride would work, but I don't know what that would imply as far as temperature or feasibility. I also know that it's generally inert, so there would need to be some more reactive gases in solution if there is complex life.

Ideal situation: men are too heavy to actually "swim" through this environment, but if they move their arms right after a good jump, they'll be able to go pretty far. And if they fall a couple hundred feet it won't kill them.

• $SF_6$ is dielectric, wouldn't there be occasional discharges? – TheAutomaton Mar 16 '18 at 19:01
• @TheAutomaton Air is dielectric too. Occasional discharges occur too, they're known as lightning. – Samuel Mar 16 '18 at 19:46
• Possible duplicate of How to increase air density on a planet? – Renan Mar 16 '18 at 19:58
• @Renan that Q refers to Earth-like fauna and ecosystem. Needless to say, Mars isn't that Earth-like. – RonJohn Mar 16 '18 at 21:05
• I didn't get an answeer to my question, but I believe @Renan's right, if breathability is a requirement (and the phrase "...there would need to be some more reactive gases in solution if there is complex life" suggests it is), then this is a duplicate and, regrettably, Samuel's answer failed to answer the question. – JBH Mar 17 '18 at 0:12

How about Titan? It has 14% the gravity of Earth, but 1.45 times the atmospheric surface pressure. Titan can achieve this because the atmosphere it does have is much heavier than Earth's atmosphere. Although Titan does not have its own magnetic field to protect its atmosphere from solar wind, it's protected by Saturn's relatively large magnetic field.

If you're willing to have a non-breathable atmosphere and are ok with the planet or moon being protected by a magnetic field you can easily tune an atmospheric composition to be very dense despite the low gravity.

There is a xkcd covering this question.

• Holy cow! That's awesome. – cwallenpoole Mar 16 '18 at 18:29
• What about Venus? Gravity there is lower than Earth, but its atmospheric pressure at surface level is about 90 times that of Earth. Carbon dioxide likely produces a denser atmosphere than oxygen and nitrogen, but I've never yet seen a proper explanation for why the difference is so massive. Does Venus just have far more molecules of carbon dioxide around than Earth does of oxygen and nitrogen, or is there a limit to how much atmosphere of a given composition can be retained? In other words, could you just assume a copy of Earth with a hundred times as much oxygen and nitrogen in the air? – Palarran Mar 19 '18 at 18:30
• @Palarran Carbon dioxide is almost three times heavier than O2. Which explains the high pressure at the surface. In fact, Earth's air is a decent lifting gas on Venus, similar to helium in our own atmosphere. We could use air filled balloon habitats to float around the hospitable parts of the Venusian atmosphere. – Samuel Mar 19 '18 at 18:38
• @Samuel I was questioning the magnitude of the difference. Carbon dioxide is three times heavier than O2? Okay. But, even if we square that figure for exaggeration (whether or not we're actually supposed to) and account for N2, Venus should not have more than about ten times as much pressure at the surface compared to Earth. That's not even an eighth of the actual figure of ninety times the pressure of Earth. – Palarran Mar 19 '18 at 18:51
• @Palarran When calculating barometric pressure, the molar mass of the gas is in the exponential term. The atmosphere of Venus is 93 times more massive than Earth's. So not only is the atmosphere heavier per unit volume, there is a lot more of it. – Samuel Mar 19 '18 at 21:03

To expand on @Loren Pechtel 's answer, the escape velocity is more important than what the surface gravity (although all else being equal, a planet with more gravity will have a higher escape velocity, so surface gravity is a relevant concern). Suppose you have a planet that has one tenth the density of earth, but ten times the radius. Then the volume will be 1000x and the mass will be 100x. The surface gravity will be from 100x the mass, but it will be 10x the distance, so the surface gravity will be the same. However, it will take 10x as much altitude to reduce the gravity by the same factor on Earth. Thus, we should expect the atmosphere to extend 10x the height, increasing the surface pressure and density.

The ability to retain atmosphere is purely tied to escape velocity. Surface gravity is totally irrelevant.

In practice there is a limit to how low the surface gravity can go because the material from which planets are made.

• Surface gravity affects the escape velocity, unless I'm mistaken. If gravity is only 1/2 of Earth, that implies a lower velocity being sufficient to escape the planet's atmosphere, although I can't say offhand if the relation is linear/exponential/etc. – Palarran Mar 18 '18 at 3:41
• @Palarran What's important is the depth of the gravity well. A very dense planet could have 1g and yet not be able to hold an atmosphere, the outside of Dyson sphere around a globular cluster could be in microgravity and yet have no problem holding an atmosphere. – Loren Pechtel Mar 19 '18 at 4:26

To state the obvious, an astronomical body has to produce or acquire an atmosphere in order to have one. Thus the composition of a atmosphere is likely to be the product of the average atmospheric acquisition rate per time unit multiplied by the number of time units between starting to acquire an atmosphere and stopping acquiring an atmosphere.

Minus the average atmospheric loss rate per time unit multiplied by the number of time units between starting to lose an atmosphere and stopping losing an atmosphere. At any one time an astronomical body may be both gaining and losing atmosphere, gaining but not losing atmosphere, losing atmosphere and not gaining any, or neither gaining nor losing any atmosphere.

And of course the rates of atmospheric gain and loss can vary enormously.

One way for a planet to lose atmosphere is to have fast moving molecules, atoms, & ions escape into interplanetary space. Molecules, atoms, and ions escape from the outer layers of an atmosphere. I once read that it was calculated that if the escape velocity of the astronomical body was X times the average speed of the particles in the escape layer the astronomical body would retain atmosphere for billions of years without much escape. I don't remember exactly the value of X, but I think it was either 5 or 6.

I believe the velocity of particles in the escape layer of an atmosphere depends on their temperature, which is the result of direct solar radiation upon them plus any heat rising up from the astronomical body. Thus some astronomical bodies far from the Sun, like Titan and Triton, are able to retain more atmosphere than they would be able to if at Earth's distance from the Sun.

Astronomical bodies also lose atmospheric particles into space due to charged particles in the solar or stellar wind emitted by the star striking the atmosphere. Astronomical bodies with strong enough magnetic fields are protected from that process by deflecting or trapping the charged particles.

Atmospheres can also be lost by turning liquid or solid. On Earth, for example, carbon dioxide, a greenhouse gas that keeps Earth warm enough for life, is trapped into minerals and lost from the atmosphere. Over ages of time plate tectonics gradually pushes some plates of the crust beneath other plates and down into the magma layer. Eventually some of that carbon dioxide enriched magma reaches the surface and carbon dioxide is released back into the atmosphere, maintaining a rough equilibrium. Planets without plate tectonics will not have that carbon dioxide cycle.

On Earth, most plants and animals release carbon dioxide into the atmosphere, while plants also convert carbon dioxide and sunlight into biochemicals and oxygen, thus producing Earth's free oxygen in the atmosphere.

Thus it is relatively simple to calculate whether a specific astronomical body would retain a specified atmosphere for billions of years, and much lss easy to calculate whether it would ever acquire that specific type of atmosphere given the initial conditions of planetary formation.