Let’s suppose that a Moon rover releases a small quantity of a heavy gas in a cave (lava tube) under the Moon’s surface.

Could you use sonar in the gas before it dissipates?

Look, a question that has been bugging me for a while is this.

If a gas is released in an enclosed cave under the Moon, gravity would tend to want to hold the gas in place but vacuum would want to suck it out. Turbulent diffusion would tend to want to push it out – so reduce turbulent diffusion to zero and only have the much smaller laminar diffusion. How long would it stay in a cave of volume V with small opening area A at the top?

This leads to another question that has been bugging me for decades. Titan is much smaller than Earth, lower gravity, but has a thicker atmosphere. Venus is smaller than Earth but has a much thicker atmosphere. What is the limit to the maximum density an atmosphere, eg. of nitrogen or CO2, can have at the surface of a planet or moon? For simplicity assume steady state – ignore erosion of the top of the atmosphere by sunlight and the solar wind.

mollwollfumble said:

Let’s suppose that a Moon rover releases a small quantity of a heavy gas in a cave (lava tube) under the Moon’s surface.

Could you use sonar in the gas before it dissipates?

Look, a question that has been bugging me for a while is this.

If a gas is released in an enclosed cave under the Moon, gravity would tend to want to hold the gas in place but vacuum would want to suck it out. Turbulent diffusion would tend to want to push it out – so reduce turbulent diffusion to zero and only have the much smaller laminar diffusion. How long would it stay in a cave of volume V with small opening area A at the top?

This leads to another question that has been bugging me for decades. Titan is much smaller than Earth, lower gravity, but has a thicker atmosphere. Venus is smaller than Earth but has a much thicker atmosphere. What is the limit to the maximum density an atmosphere, eg. of nitrogen or CO2, can have at the surface of a planet or moon? For simplicity assume steady state – ignore erosion of the top of the atmosphere by sunlight and the solar wind.

Yes you can use echolocation that way.

Answer to your last question depends on temperature: at higher temperatures it is harder to hold an atmosphere because it is easier for molecules to have escape vel. Also depends on atmos comp, obv.

dv said:

mollwollfumble said:

Let’s suppose that a Moon rover releases a small quantity of a heavy gas in a cave (lava tube) under the Moon’s surface.

Could you use sonar in the gas before it dissipates?

Look, a question that has been bugging me for a while is this.

If a gas is released in an enclosed cave under the Moon, gravity would tend to want to hold the gas in place but vacuum would want to suck it out. Turbulent diffusion would tend to want to push it out – so reduce turbulent diffusion to zero and only have the much smaller laminar diffusion. How long would it stay in a cave of volume V with small opening area A at the top?

This leads to another question that has been bugging me for decades. Titan is much smaller than Earth, lower gravity, but has a thicker atmosphere. Venus is smaller than Earth but has a much thicker atmosphere. What is the limit to the maximum density an atmosphere, eg. of nitrogen or CO2, can have at the surface of a planet or moon? For simplicity assume steady state – ignore erosion of the top of the atmosphere by sunlight and the solar wind.

Yes you can use echolocation that way.

Answer to your last question depends on temperature: at higher temperatures it is harder to hold an atmosphere because it is easier for molecules to have escape vel. Also depends on atmos comp, obv.

I can see how it would be strongly dependent on temperature. I wonder if I can compute it, using the same equations I used for computing the concentration of salt aerosol in the Earth’s troposphere as a function of height.

mollwollfumble said:

dv said:

mollwollfumble said:

Let’s suppose that a Moon rover releases a small quantity of a heavy gas in a cave (lava tube) under the Moon’s surface.

Could you use sonar in the gas before it dissipates?

Look, a question that has been bugging me for a while is this.

If a gas is released in an enclosed cave under the Moon, gravity would tend to want to hold the gas in place but vacuum would want to suck it out. Turbulent diffusion would tend to want to push it out – so reduce turbulent diffusion to zero and only have the much smaller laminar diffusion. How long would it stay in a cave of volume V with small opening area A at the top?

This leads to another question that has been bugging me for decades. Titan is much smaller than Earth, lower gravity, but has a thicker atmosphere. Venus is smaller than Earth but has a much thicker atmosphere. What is the limit to the maximum density an atmosphere, eg. of nitrogen or CO2, can have at the surface of a planet or moon? For simplicity assume steady state – ignore erosion of the top of the atmosphere by sunlight and the solar wind.

Yes you can use echolocation that way.

Answer to your last question depends on temperature: at higher temperatures it is harder to hold an atmosphere because it is easier for molecules to have escape vel. Also depends on atmos comp, obv.

I can see how it would be strongly dependent on temperature. I wonder if I can compute it, using the same equations I used for computing the concentration of salt aerosol in the Earth’s troposphere as a function of height.

How about that time you measured the viscosity of the Earths central liquid outer core using the permubility of liquid petrohydrochlorate?

Witty Rejoinder said:

mollwollfumble said:

dv said:

Yes you can use echolocation that way.

Answer to your last question depends on temperature: at higher temperatures it is harder to hold an atmosphere because it is easier for molecules to have escape vel. Also depends on atmos comp, obv.

I can see how it would be strongly dependent on temperature. I wonder if I can compute it, using the same equations I used for computing the concentration of salt aerosol in the Earth’s troposphere as a function of height.

How about that time you measured the viscosity of the Earths central liquid outer core using the permubility of liquid petrohydrochlorate?

That seems tamer than my usual forays, such as modular stirred nuclear meltdown to get relativistic spacecraft speeds. It’s an interesting question, though. How could you measure the viscosity of the Earth’s liquid outer core?

For salt aerosols it was upward motion by turbulent diffusion balanced by downward motion by gravity on saturated microdroplets.

For gas in the absence of turbulence it would be an upward motion due to Brownian motion balanced by downward force due to gravity on air molecules. You can see how the strong dependence on temperature fits in, Brownian motion is (roughly?) proportional to temperature. You can also see how slow wind speeds have an effect – the stronger the wind, the more the turbulent diffusion, which adds to the Brownian motion.

“I can see how it would be strongly dependent on temperature. I wonder if I can compute it, using the same equations I used for computing the concentration of salt aerosol in the Earth’s troposphere as a function of height.”

I wish this was straightforward but the temperature of the exosphere is going to depend on incoming radiation, the composition of the atmosphere, the albedo of the planet. You could probably knock up a roughy toy model but I don’t think it is going to come down to a simple equation.

dv said:

mollwollfumble said:

Let’s suppose that a Moon rover releases a small quantity of a heavy gas in a cave (lava tube) under the Moon’s surface.

Could you use sonar in the gas before it dissipates?

Look, a question that has been bugging me for a while is this.

If a gas is released in an enclosed cave under the Moon, gravity would tend to want to hold the gas in place but vacuum would want to suck it out. Turbulent diffusion would tend to want to push it out – so reduce turbulent diffusion to zero and only have the much smaller laminar diffusion. How long would it stay in a cave of volume V with small opening area A at the top?

This leads to another question that has been bugging me for decades. Titan is much smaller than Earth, lower gravity, but has a thicker atmosphere. Venus is smaller than Earth but has a much thicker atmosphere. What is the limit to the maximum density an atmosphere, eg. of nitrogen or CO2, can have at the surface of a planet or moon? For simplicity assume steady state – ignore erosion of the top of the atmosphere by sunlight and the solar wind.

Yes you can use echolocation that way.

Answer to your last question depends on temperature: at higher temperatures it is harder to hold an atmosphere because it is easier for molecules to have escape vel. Also depends on atmos comp, obv.

Is the atmosphere of Venus escaping into space ?
If it was I imagine it must be extremely slow as its still extremely thick

Related articles on Atmospheric escape

Atmospheric escape
https://en.wikipedia.org/wiki/Atmospheric_escape

Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and impact erosion. The relative importance of each loss process depends on the planet’s escape velocity, its atmosphere composition, and its distance from its sun. Escape occurs when molecular kinetic energy overcomes gravitational energy; in other words, a molecule can escape when it is moving faster than the escape velocity of its planet. Categorizing the rate of atmospheric escape in exoplanets is necessary to determining whether an atmosphere persists, and so the exoplanet’s habitability and likelihood of life.

Atmosphere_of_Venus
https://en.wikipedia.org/wiki/Atmosphere_of_Venus

Venus: The Atmosphere, Climate, Surface, Interior and Near-Space Environment of an Earth-Like Planet
https://link.springer.com/article/10.1007/s11214-018-0467-8

ESA – Mars Express – Leaky atmosphere linked to lightweight planet
https://www.esa.int/Our_Activities/Space_Science/Mars_Express/Leaky_atmosphere_linked_to_lightweight_planet

H+/O+ Escape Rate Ratio in the Venus Magnetotail and its Dependence on the Solar Cycle
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL079454

How Earth sheds heat into space
https://www.sciencedaily.com/releases/2018/09/180924153430.htm