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EDG
2011-Oct-15, 10:12 PM
I'm wondering what a habitable planet with a thicker atmosphere would be like, physically speaking. To that end, let's take the Earth, but say that the atmospheric pressure at sea level is 2 bars (about 200,000 Pa), instead of the 1 bar that it is now.

Let's also assume that the atmospheric composition is such that the atmosphere is as breathable as it is today for life - for this to happen, (I think) we need the oxygen percentage to be half of what it is today so the O2 partial pressure is the same as it is today (gas % * pressure = partial pressure (ATA), so we want the O2 ATA to be 0.21 atms). This means there'll be more Nitrogen (89.5% by volume) so the N2 partial pressure would be higher (1.79 atms), but I think that's still survivable without Nitrogen Narcosis etc. Regardless of the details, the atmosphere composition is such that life can continue to exist on it - I don't want people to be hung up on this part.

What I'm interested in are the physical differences caused by higher pressure/atmospheric density. Assume gravity is the same - the only thing that's changed is the atmospheric density (and associated atmospheric properties to make this happen).

One thing I've heard is that a thicker atmosphere would be harder to move around (because there's more mass of air to impart energy to). Given the same amount of solar energy as we received today, does that mean that weather would be calmer in this thicker atmosphere? IIRC winds move from high pressure to low pressure areas, so would the pressure difference have to be higher for winds of a given speed to exist than on Earth today? Would hurricanes and tornadoes be harder to form?

That said, I've also heard that once a wind gets going, it's ability to affect the surface is higher because there's more mass of air moving around (in other words, a breeze on Earth today doesn't move much, but a breeze at the same velocity on our thicker atmosphere Earth would be more damaging, picking up more material etc - and a hurricane would be way more destructive in a thicker atmosphere than it is here)

What about warming? Would a thicker atmosphere (with a scaled composition) mean that more heat is trapped and the surface temperature would be higher? Would convection occur more easily to transport the heat away? Would temperatures vary more or less on the surface?

How about lift? I think higher pressure (but same gravity) means that it's easier for wings to lift bodies off the ground? Would planes and birds be able to fly and glide more easily?

What about fire? If the oxygen pressure is the same as today, but the atmosphere is denser, what does that do to fires? Does the pressure make things easier/harder to ignite, or would it have no effect? Would fires last longer? (I think convection might be more vigorous in a denser atmosphere?).

Volcanic eruptions would change - the higher atmopheric pressure means that gases won't exsolve as easily from lava as they erupt, so they'd be less explosive. We might expect more fissure eruptions, and more runny lavas.

Would the ocean be affected? Would wave height be higher/lower in the higher atmospheric pressure? Would waves break more easily? Would there be more oxygen dissolved in the oceans?

What about insects? There was more oxygen in previous geological eras, which means that insects could be larger (oddly, I've not heard much about whether the atmospheric pressure was higher back then too), because of how their breathing surfaces worked. But if the pressure's higher and the oxygen pressure isn't, would that happen on our thicker atmosphere earth?

I know there's a lot of questions here, but I just want to get an idea of how higher pressure affects the environment from a physical perspective. Any (informed) ideas/thoughts/input would be appreciated!

we are not alone
2011-Oct-15, 11:09 PM
Venus has a thicker atmosphere. It also has a runway greenhouse effect. All that atmosphere soaks up heat. Dragonflies like Meganeura had a wingspan of a metre during the carboniferous period when the atmosphere was denser with oxygen but that was partly due to lack of predators I think. Breathing through the thorax does work better with a rich oxygen atmosphere. Fires were more commonplace during that era as a result of lightning and such.

I think the atmosphere is thickening thanks to CO2. Atmospheric pressure would make breathing more difficult. Your other claims sound feasible.

EDG
2011-Oct-15, 11:34 PM
I've covered pretty much all of that already, and also that the big insects of bygone eras were large because of higher oxygen percentage, not higher pressures. The atmosphere isn't thickening because of CO2 either (well, not enough to be remotely noticeable).

Tensor
2011-Oct-16, 04:47 AM
I'm wondering what a habitable planet with a thicker atmosphere would be like, physically speaking. To that end, let's take the Earth, but say that the atmospheric pressure at sea level is 2 bars (about 200,000 Pa), instead of the 1 bar that it is now.

What about fire? If the oxygen pressure is the same as today, but the atmosphere is denser, what does that do to fires? Does the pressure make things easier/harder to ignite, or would it have no effect? Would fires last longer? (I think convection might be more vigorous in a denser atmosphere?).

You have to be careful here. Oxygen makes up ~21% of the atmosphere by volume. This means that of the one bar (100 kPa) of atmospheric pressure, oxygen contributes ~21 kPa (Please note for simplification, I'm assuming the elements contribute equally to the pressure and I'm ignoring water vapor.). If you keep that same pressure for oxygen, but double the atmospheric pressure (with presumably, more of the other constituents of the atmosphere), the amount of oxygen, by volume, drops to ~10.5%. This would tend to make fire harder to start and harder to maintain due to the oxygen atoms being more scattered throughout the atmosphere.

Or, are you talking about maintaining the same pressure relative to the other elements in the atmosphere? To get to the same pressure, relative to the rest of the elements, oxygen, in a two bar atmosphere, has to be ~21% by volume. This would give oxygen, a pressure of ~42 kPa, in a two bar atmosphere, with the same percentage by volume. It's more about the percentage of oxygen in the atmosphere, by volume, rather than the pressure alone, of the oxygen when considering fire.

korjik
2011-Oct-16, 05:26 AM
You have to be careful here. Oxygen makes up ~21% of the atmosphere by volume. This means that of the one bar (100 kPa) of atmospheric pressure, oxygen contributes ~21 kPa (Please note for simplification, I'm assuming the elements contribute equally to the pressure and I'm ignoring water vapor.). If you keep that same pressure for oxygen, but double the atmospheric pressure (with presumably, more of the other constituents of the atmosphere), the amount of oxygen, by volume, drops to ~10.5%. This would tend to make fire harder to start and harder to maintain due to the oxygen atoms being more scattered throughout the atmosphere.

Or, are you talking about maintaining the same pressure relative to the other elements in the atmosphere? To get to the same pressure, relative to the rest of the elements, oxygen, in a two bar atmosphere, has to be ~21% by volume. This would give oxygen, a pressure of ~42 kPa, in a two bar atmosphere, with the same percentage by volume. It's more about the percentage of oxygen in the atmosphere, by volume, rather than the pressure alone, of the oxygen when considering fire.

Isnt it the partial pressure that determines reactivity, not the relative percentages? At 21kPa O2 in a 200kPa atmo, you would have just as many O2 molecules hit a surface per second as in a 21kPa O2 in a 100kPa atmo. You would just have alot more N2 hits than before. I dont really remember what that would do to reactivity rates if everything else was equal.

Tensor
2011-Oct-16, 06:02 AM
Isnt it the partial pressure that determines reactivity, not the relative percentages? At 21kPa O2 in a 200kPa atmo, you would have just as many O2 molecules hit a surface per second as in a 21kPa O2 in a 100kPa atmo. You would just have alot more N2 hits than before. I dont really remember what that would do to reactivity rates if everything else was equal.

I was thinking it was relative, but Zeus knows I've been wrong before. I'll have to look it up. EDG, hold off on my answer. I don't mind being wrong and getting corrected, but I don't want to give you wrong info.

chornedsnorkack
2011-Oct-16, 08:40 AM
One thing I've heard is that a thicker atmosphere would be harder to move around (because there's more mass of air to impart energy to). Given the same amount of solar energy as we received today, does that mean that weather would be calmer in this thicker atmosphere? IIRC winds move from high pressure to low pressure areas, so would the pressure difference have to be higher for winds of a given speed to exist than on Earth today? Would hurricanes and tornadoes be harder to form?

That said, I've also heard that once a wind gets going, it's ability to affect the surface is higher because there's more mass of air moving around (in other words, a breeze on Earth today doesn't move much, but a breeze at the same velocity on our thicker atmosphere Earth would be more damaging, picking up more material etc - and a hurricane would be way more destructive in a thicker atmosphere than it is here)

Ocean currents are slower than winds, but they are powerful. The winds on Venus are also slower than Earth, but effective in moving.

What about warming? Would a thicker atmosphere (with a scaled composition) mean that more heat is trapped and the surface temperature would be higher?
The warm layer of atmosphere would be thinner.
The wet adiabatic temperature gradient would be closer to dry adiabatic, for the same temperature. The reason would be that starting from the same ocean surface temperature, there would be equal water vapour pressure, but therefore lower percentage of water vapour. As the moist air rises, the condensing water vapour would have to warm more nitrogen, and therefore it would cool faster and rain out faster. Thus, water vapour would be confined to a thinner surface layer.
Since the concentration of greenhouse water vapour would be the same on the surface but be confined to a thinner layer, the greenhouse effect would be weaker. The transparent nitrogen would not trap heat - rather, it would hinder water from trapping heat.

How about lift? I think higher pressure (but same gravity) means that it's easier for wings to lift bodies off the ground? Would planes and birds be able to fly and glide more easily?

Yes.

Consider a plane. Lift is proportional to density, and square of speed. Also proportional to wing area.

In twice the atmospheric density, lift of a given plane would be doubled at a given speed, so it would exceed the requirements (of the unchanged weight in unchanged gravity).
One way could be to take off and fly at a slower speed - slower by square root of two times would give the same lift.

But power is product of force and speed. A plane at slower speed in denser air would have to overcome roughly the same drag - drag, like lift, scales with density times square of speed - but slower speed means lower power is needed to overcome the same drag. Power required would be square root two times smaller.
Therefore, the engine can be weaker and weigh less - the plane can have more carrying capacity left over for fuel and payload.

Alternatively, the plane might fly at the same speed, but have twice smaller area.
A smaller wing carrying the same weight would need less weight. So, smaller and lighter wings carrying the same weight mean that more weight is left over for fuselage, engine and useful loads.

Or combination of these approaches.
Likewise a bird could have weaker muscles and heart, or smaller and lighter wings.

What about fire? If the oxygen pressure is the same as today, but the atmosphere is denser, what does that do to fires? Does the pressure make things easier/harder to ignite, or would it have no effect? Would fires last longer? (I think convection might be more vigorous in a denser atmosphere?).

Harder to ignite.
At exact same temperature, and same oxygen concentration, reactions would have equal speed at equal temperature. So reactions at low temperature like breathing or rusting would be unaffected.
But in fire, much of the heat goes to heating the nitrogen in air. So increasing the nitrogen concentration means that a given amount of heat will not heat air to the same temperature. But at lower temperatures, the reactions will be slower. So nitrogen will suppress fires.

Would there be more oxygen dissolved in the oceans?

No. Concentration of dissolved oxygen depends on oxygen partial pressure in air, and is independent of total pressure.

EDG
2011-Oct-16, 11:26 PM
Tensor - yeah, it sounds like you're talking about Oxygen Pressure vs atmospheric pressure. All I'm concerned with here is that the pressure is higher but the atmosphere is still as breathable as it was (as much as it can be). Any other effects would be due to having more nitrogen, or higher pressure - not more oxygen.

Ocean currents are slower than winds, but they are powerful. The winds on Venus are also slower than Earth, but effective in moving.

Yeah, but is it going to be harder to get them moving given the same amount of energy coming in from the sun to drive it all? I'd imagine it would be harder to some extent, so wind speeds should generally be slower at ground level than they are here. That said, while they may be slower, they may still actually be close to equally effective since as you say they're more 'powerful'. I wonder if that balances out linearly - would a wind that is half as slow as a wind on Earth but in an atmosphere twice as dense be as effective at eroding/moving stuff, or would it be more/less so?

The warm layer of atmosphere would be thinner.
The wet adiabatic temperature gradient would be closer to dry adiabatic, for the same temperature. The reason would be that starting from the same ocean surface temperature, there would be equal water vapour pressure, but therefore lower percentage of water vapour. As the moist air rises, the condensing water vapour would have to warm more nitrogen, and therefore it would cool faster and rain out faster. Thus, water vapour would be confined to a thinner surface layer.
Since the concentration of greenhouse water vapour would be the same on the surface but be confined to a thinner layer, the greenhouse effect would be weaker. The transparent nitrogen would not trap heat - rather, it would hinder water from trapping heat.

Here I have to admit that my understanding of all this stuff is rather low... is there a website that you can point me to that explains this at all?

Or combination of these approaches.
Likewise a bird could have weaker muscles and heart, or smaller and lighter wings.

Cool, thanks.

Harder to ignite.
At exact same temperature, and same oxygen concentration, reactions would have equal speed at equal temperature. So reactions at low temperature like breathing or rusting would be unaffected.
But in fire, much of the heat goes to heating the nitrogen in air. So increasing the nitrogen concentration means that a given amount of heat will not heat air to the same temperature. But at lower temperatures, the reactions will be slower. So nitrogen will suppress fires.

Now that I wasn't aware of... interesting!

chornedsnorkack
2011-Oct-17, 10:16 AM
I wonder if that balances out linearly - would a wind that is half as slow as a wind on Earth but in an atmosphere twice as dense be as effective at eroding/moving stuff, or would it be more/less so?
Or if it balances out nonlinearly - for example wind pressure, and also kinetic energy per unit volume, is proportional to density times square of speed.

Here I have to admit that my understanding of all this stuff is rather low... is there a website that you can point me to that explains this at all?

For example:
http://en.wikipedia.org/wiki/Lapse_rate
Note that the wet adiabatic lapse rate depends strongly on temperature - and is always between dry lapse rate and dew point lapse rate.

eburacum45
2011-Oct-17, 04:39 PM
Low, thin clouds- that's interesting. I suppose a cumulonimbus might be a couple of kilometers high, rather than 6-10km, assuming they form at all. Most other clouds could be much lower and in thinner layers- that actually makes them easier to render in a 3d program...

Githyanki
2011-Oct-17, 05:46 PM
About 4BYA, Earth did have a thicker-atmosphere. It's hard to say the effects since we don't have any evidence of its effects as none have survived. At that time, the Earth was 99% covered in water with the only land being small volcanic islands poking out of the water.

Tensor
2011-Oct-17, 10:05 PM
EDG, yep, I was wrong. Fortunately, the great thing about this board, chornedsnorkack was there for the correction. Pay attention to him, not to me on this one. Thanks chornedshorkack.

whatif
2012-Jun-28, 06:58 PM
great little forum. I have been thinking about this for sometime. Im not a religious person but remember the great flood story (most religions all have a flood story) and the rainbow was a sign it would not happen again. I wonder if there were no rainbows when the atmosphere was thicker.

and why would a dino have a 40 foot long neck and could reach leaves up to 80 ft high. when most trees now dont grow that large. if most trees were like they are now then those dinos would be looking down on most trees. so Im saying with the thicker atmosphere it supported larger trees since it would allow the water to be pumped farther up the tree then with the current pressure. as of now there is a limit to how large a tree can grow due to the limit of getting water to the top.

bugs. there are fossils of giant bugs. today this is not possible for a bug to grow that large. there is a physical limit to how large a bug can be today and it is probably based on air pressure. with thicker atmosphere you could support large exoskeletons. think about whales. the blue whale is over 100 ft long and weighs over a 100 ton. that animal could not survive on land, there is nothing to support the large mass. where as water is all around the whale supporting it huge body. same goes for bugs and air pressure.

again sorry for bible references. but in the beginning of the bible it talks about ppl living to be 600-900 yrs old. I think there is a scientific reason people living that long. with a thicker atmosphere it would block way more UV rays, cosmic rays, solar flares etc. this would protect our dna more. after the flood life expectancy went from 900 down to 150 and each generation it kept dropping and settled in around 50-90 yrs.

when I get rich I want to try and get an experiment going. create a sealed habitat and increase the pressure 1.5 or 2 times current pressue. put plants, trees, bugs and things like that. then see what happens. will they grow bigger? It would have to be a pretty good size structure, but not as huge as the bio doom.