Would a denser atmosphere hold in more moisture even if the temperature was exactly that of Earth's surface temperature? (Atmospheric Density)

(My world's details) I have a super-earth world. The surface of the world is shallower due to extra gravity, so shorter mountain ranges and fewer ocean depths. The world orbits a binary star system at a distance of the furthest region of the habitable zone, both of the stars having masses of 0.93x that of our sun's. The world is highly volcanic, approximately 15 to 16 times more volcanic than our planet.

Surface gravity: 1.35x that of Earth's.

Atmosphere density: 10x that of Earth's (a result of it being highly volcanic in comparison to earth.)

Atmospheric composition: Nitrogen 78%, 15% Oxygen, 5% Carbon dioxide, 2% Trace gases.

Temperature: Rather hot, the tropical regions being 90 to 110° Fahrenheit. (Based on what I've tested on Universe Sandbox 2)

Land coverage: 75% water, and 25% land (but less water being present on this world as the oceans are shallower mostly due to a lot of island formations)

Day/Night length/full rotation: 28 hours.

Axial tilt: 53 degrees.

Orbital period: 1.15 years

Eccentricity: 0.025

I've heard that a denser atmosphere has more capacity for humidity compared to a thinner atmosphere with similar temperatures, is this true? Would a denser atmosphere hold more moisture even if the temperature was exactly that of Earth's surface temperature?

• When you say thickness your refer to physical thickness or to viscosity? – L.Dutch - Reinstate Monica Jul 26 '18 at 10:48
• If this planet was in anyway like Earth, there would be a lot of sulfur compounds in the air due to the volcanic activity. You may want to either explain it or account for it. – Pᴀᴜʟsᴛᴇʀ2 Jul 26 '18 at 11:35
• If by "atmosphere thickness" you mean surface atmospheric pressure, then keep in mind that your 15% O2 gives an O2 partial pressure around 1500 millibar. For Earth-like life, oxygen becomes toxic somewhere around 400-500 millibar partial pressure. – user Jul 26 '18 at 11:40
• I mean by atmospheric density for the thickness. – Neuryte Jul 26 '18 at 20:06
• sorry to be pedantic, but density and thickness are two different things: density is how much mass you have in a given volume, thickness is either how many meters you have or, if it is viscosity, of much you can transmit shear forces – L.Dutch - Reinstate Monica Jul 27 '18 at 2:25

No

In a mixture of gases, every gas behaves independently from others. That means that every gas has its own saturation pressure at a given temperature that is not dependent on total atmospheric pressure. You may have a pressure chamber or a vacuum vessel - in each case it will hold exactly the same amount of water vapor before it starts to condensate.

Vapour pressure of water

• Not sure why somebody downvotes it. This is right, at least for absolute humidity. – Bob Jacobsen Jul 30 '18 at 1:52

The absolute amount of water per cubic meter will have the same range as air on earth at the same temperature. This is because liquid water only evaporates until it reaches the saturated partial pressure for that temperature. It doesn’t matter whether it’s evaporating into 10atm, 1atm, or a vacuum.

This means the mass fraction of water in the air is only 1/10th what it is on earth: same mass of water per cubic meter but a lot more air

But that smaller fraction won’t have a big effect on your residents. What your planet’s residents feel is relative humidity: the fraction from 0% to 100% of that maximum possible absolute. 100% fells muggy because water can’t evaporate. 0% dries out your sinuses because moisture evaporated easily.

10x atmosphere isn’t terra incognito: divers routinely work at that and higher pressures. Humidity is just another thing to control in the breathing mix. The deep-water breathing mixes start out very dry because if they start with a typical 70% humidity partial pressure and then compress it into a tank, the compression raises the partial pressure too high (you raised all pressures by compression) and the water condenses. That in turn causes various downstream problems. Instead, they start with a small pressure of water that compressed is at the right partial pressure.

Note another lesson from deep diving: oxygen at high partial pressure is a poison. Earth mammals can’t live at 1.5atm oxygen (10atm of 15%).

• Damn, how can I make a world really wet? – Neuryte Jul 30 '18 at 5:06
• @Neuryte Depends on what you mean. 100% humidity feels wet, has condensation on surfaces, etc. what are you trying to do? – Bob Jacobsen Jul 30 '18 at 6:58
• I'm looking to make the world really wet in terms of humidity, really humid, more than that of earths. – Neuryte Jul 30 '18 at 7:05
• @BobJacobsen 100% relative humidity also results in you drowning in clear air as water condenses on the inside surfaces of your lungs. – Ash Aug 1 '18 at 18:21
• @Ash it's not as dramatic, otherwise we'd be dying when there is a fog :) – Alexander Aug 1 '18 at 20:04

Moisture in the atmosphere is essentially a gaseous solution: water vapor into air.

If you look at how the solubility is usually expressed, you see that the units are $mass_{solute}/mass_{solvent}$ or $volume_{solute}/volume_{solvent}$, or a combination of the two.

Therefore if you have more mass of atmosphere available, you can dissolve more mass of water vapor in it at the same temperature.

Intuitively, where can you dissolve more sugar? In a glass full of water, or in a bushel full of the same water? Yours is the same situation.

• "Gaseous solutions" work differently. Basically every gas acts on its own in a mixture. – Alexander Jul 26 '18 at 17:03
• @Alexander, then why we have 100% relative humidity? – L.Dutch - Reinstate Monica Jul 26 '18 at 20:43
• But would 100% RH depend on air pressure or just the temperature? – Alexander Jul 26 '18 at 20:46
• L. Dutch may have a point, I'm not certain but I think the density of air determines the capacity of how much moisture could be pulled out via higher temperatures. – Neuryte Jul 26 '18 at 21:24
• @Willk In the video, the explanation for the condensation is TEMPERATURE DROP, not the pressure drop by itself. At the constant temperature no condensation would have occurred. – Alexander Jul 30 '18 at 18:36

In terms of the actual mass of water in a given volume of air a higher pressure atmosphere holds more water at a given humidity but the actual humidity is based on the partial pressure of water vapour in the air that's a percentage ratio dictated by water's saturation pressure and won't change with alterations in overall pressure. Anything higher than 100% relative humidity is mist, fog, or rain depending on just how much water is available.

Odds are that an atmosphere loaded with water vapour, carbon dioxide, and sulfur compounds from so much volcanic activity is going to have a constant acidic haze in the air made of Carbonic, Nitric and Sulfurous Acids, and water vapour. Moisture level will be almost permanently in excess of 100% relative humidity adding fog to the mix of noxious, opaque, gases.

Any alien species, like humans, coming to the world you're describing will need serious protective equipment; the atmosphere is going to be pretty corrosive, even when it isn't the high humidity would cause clear air drowning as the over-wet air caused water to condense on the lungs, and the total pressure up at 1013.25 kPa is going to be like parking a car on everyone's chest.

• The total pressure isn't that hard. The world record for freediving (descending into the water without any special breathing equipment) is 214 meters. At that depth, you have 2097 kPa of pressure, over twice as much as at the surface of this planet. – Jarred Allen Aug 2 '18 at 6:43
• @JarredAllen And your lungs are crushed flat and have to fill with blood to stop them from irreparable damage. – Ash Aug 5 '18 at 10:30