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How much air Earth could retain?
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| Diakonov | Date: Wednesday, 03.08.2016, 23:39 | Message # 1 |
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| Hi, I'm curious about how much air Earth could retain. It has 1 atm pressure. But what is the maximum atm pressure Earth could retain, related to it's escape velocity? How much air Earth could retain if it was a H/He air, N/O air or CO2 air? Is there a way to calculate this? Could Earth retain an atmosphere of N/O with 100 atm or more for billions of years? As far as I know, Venus has a high atmospheric pressure because of it's heavy air (CO2). If it was like ours, would it be the same?
Could Earth had an atmosphere of 10 atm pressure, assuming it's escape velocity?
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| Watsisname | Date: Thursday, 04.08.2016, 11:58 | Message # 2 |
 Galaxy Architect
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| The surface pressure of a planet isn't actually that strongly or directly determined by the escape speed. It matters, of course, but in the following sense: Escape speed determines how quickly different molecules escape at a given temperature. If molecules move at speeds much less than the escape speed, then they can hang around for a long time. How many there actually are (the mass of the atmosphere) depends on the planet's history (how that gas was introduced vs. how it is taken away).
A simple calculation for surface pressure goes like this:
P=F/A (Pressure is force per area)
F=ma (Force is mass times acceleration -- Newton's 2nd Law)
m=Vρ (mass is volume times density)
V=lwh (volume is length times width times height)
A=lw (area is length times width)
Combining these, we get P=ρah, or ρgh where g is the surface gravity.
However, the density ρ changes with altitude (and so does g to a lesser extent), so we actually have to use calculus to derive the surface pressure of an atmosphere -- this becomes the equation of hydrostatic equilibrium.
Quote Diakonov (  ) As far as I know, Venus has a high atmospheric pressure because of it's heavy air (CO2). If it was like ours, would it be the same?
Actually, the average molecular weight of Venus' air is only about twice as much as Earth's, whereas the surface pressure is about 90 times as much as Earth's. The main reason Venus' surface pressure is so high is because there are more molecules altogether -- about 45 times as many. So the total mass of atmosphere is about 90 times greater. Distribute 90x the mass over about the same surface area with about the same surface gravity, and this explains the ~90x greater pressure. 
Quote Diakonov ( ) How much air Earth could retain if it was a H/He air
Virtually zero. It would escape too quickly.
Quote Diakonov ( ) N/O air or CO2 air
No strict upper limit -- these gases would escape over timescales much longer than the solar system.
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| Diakonov | Date: Thursday, 04.08.2016, 19:20 | Message # 3 |
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| So, maybe if Earth formed more distant from the Sun, say, at 2 or 3 AU, it would be nearer the ice zone, so there would have more volatiles in it's composition, and so there would have more material to form an atmosphere of N/O. But if Earth formed much closer to the Sun, it would have less volatiles to form the atmosphere, so a lesser pressure. And there's also that theory of a planet that collided with Earth that formed our moon. Such collision would have disrupted a lot of the original atmosphere into space...
So, Earth-like planets that form much closer to the sun would have less volatiles, so less atmopshere. And earth-like planets that form much more distant to the sun would have more volatiles, so more air.
Is that right?
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| Watsisname | Date: Friday, 05.08.2016, 04:56 | Message # 4 |
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Galaxy Architect
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| I think it is very hard to predict atmosphere composition or pressure by the planet's mass, composition, or formation distance, so I wouldn't put stock in any simple rule like that. The subsequent evolution is just as important.
Consider Venus. By bulk properties (mass, radius, composition), it's like an Earth twin. It's a bit closer to the Sun, so we might predict it should have a lower surface pressure. But it's the opposite -- enormously so. Venus' atmosphere probably started out a lot like Earth's did from outgassing, but then it underwent a runaway greenhouse. Being closer to the Sun prevented water vapor from condensing, so the steam atmosphere produced a greenhouse and warmed the surface. This brought CO2 out of the rocks, which strengthens the greenhouse effect even further, which brought out more CO2, causing a feedback loop. With no way to sequester or remove the CO2, it simply built up until the surface was completely baked, and the water vapor dissociated and escaped to space, leaving a dry, hostile, pressure cooker of a world.
Earth's atmosphere has also gone through several iterations. It started out with a hydrogen helium envelope from the solar nebula, but this escaped very quickly once the nebula dispersed. A secondary atmosphere followed from outgassing of the interior. And no doubt, the giant impact which formed the moon stripped off all of that atmosphere and started it again from scratch. But the global magma ocean following the impact also allowed plenty of outgassing to bring it back.
But get this -- there was no oxygen! Oxygen didn't begin to show up until about 3 billion years ago due to the rise of photosynthesis, and it took a long time to actually accumulate, because it had to oxidize the iron and other minerals in the rocks first. (You can see the geologic remnants of this process as banded iron formations.) We actually know of no way for a planet to obtain significant amounts of O2 in its atmosphere without photosynthetic life.
Planetary atmospheres are really fascinating things with a lot of complex history and physical and chemical evolution. I think it's really cool to think about them and what kinds of atmospheres can exist out there.
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| Diakonov | Date: Friday, 05.08.2016, 13:31 | Message # 5 |
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| So Earth with it's temperature and mass could potentially have a much higher pressure with nitrogen and oxygen. But it's only not higher because of such drastic events. Maybe if Venus had a similar event, it would have less atmosphere. Got it.
And a planet with the same temperature, but half the mass of Earth, could it have a very pressurized atmosphere too, say, 0,25 Me or 0,5 Me? In fact, if Mars orbited close to a gas giant and had an active interior and magnetic field, could it potentially turn into kind of a mini-venus for billions of years?
Edited by Diakonov - Friday, 05.08.2016, 13:32 |
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| Alek | Date: Friday, 05.08.2016, 18:07 | Message # 6 |
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| Also, look at Mars; if we use the "rule" that atmospheres should be thicker farther out, Mars should have a thicker atmosphere than ours, but maybe just by a little, considering it's smaller size is going to let more gasses escape in general, but it's much thinner due to a loss of magnetic field...this may or may not be predictable considering that Mars could've lasted longer with a magnetic field had some things been different (Say, it had more radioactive elements in it's core to keep the core hot)
Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
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| midtskogen | Date: Friday, 05.08.2016, 18:41 | Message # 7 |
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| Quote Watsisname ( ) Planetary atmospheres are really fascinating things with a lot of complex history and physical and chemical evolution. I think it's really cool to think about them and what kinds of atmospheres can exist out there.
And it doesn't seem implausible that we can detect the composition of those atmospheres of transiting planets with useful accuracy.
NIL DIFFICILE VOLENTI
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| Diakonov | Date: Friday, 05.08.2016, 22:08 | Message # 8 |
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| Obviously a planet like Mars will have a thinner atmosphere, but if it was a moon orbiting very close to a gas giant, say, Jupiter, certainly it would be like Europa or Io, that would keep the interior hot enough to keep a magnetic field (considering if Jupiter was at the same distance from the Earth to the Sun). So maybe a mars-sized world could be more like Earth or Venus for billions of years, but only if it was a moon orbiting close to a gas giant. If that's the case, a mars-sized world could have a very pressurized air.
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| Watsisname | Date: Saturday, 06.08.2016, 22:19 | Message # 9 |
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| Quote Diakonov ( ) And a planet with the same temperature, but half the mass of Earth, could it have a very pressurized atmosphere too, say, 0,25 Me or 0,5 Me? In fact, if Mars orbited close to a gas giant and had an active interior and magnetic field, could it potentially turn into kind of a mini-venus for billions of years?
Yeah, I think this would work. Even at 0.25 Earth masses, if we assume Earth-like density and temperature, then calculations show an Earth-like mixture of gases should be stable against thermal escape for the age of the solar system. Including a magnetic field will also help protect against non-thermal escape.
CO2 in Mars' atmosphere would also be stable against thermal escape, and we might imagine a thick greenhouse atmosphere on Mars if it were protected by magnetic field. However, large impacts can also be responsible for eroding the atmosphere (it's easier to blow off large portions of atmosphere on smaller planets).
And consider Titan. Despite a mass of only ~2.3% of Earth, it can support a nitrogen atmosphere with over 1atm of pressure, because its exobase temperature is so cold (only ~150K).
Quote midtskogen ( ) And it doesn't seem implausible that we can detect the composition of those atmospheres of transiting planets with useful accuracy.
Absolutely; spectroscopy is an amazing tool. The difficult part of this is actually obtaining the spectra, but we're starting to be able to achieve this reliably for certain exoplanets. This will also likely be the method of the first 'unambiguous' detection of life outside the solar system. (Unambiguous if we can show that such composition cannot exist in equilibrium without biology -- e.g. oxygen in Earth's atmosphere.)
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| Diakonov | Date: Sunday, 07.08.2016, 03:17 | Message # 10 |
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| Ok, I'll consider that when creating new planetary systems...
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| Diakonov | Date: Sunday, 07.08.2016, 03:54 | Message # 11 |
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| Could a 0,25 Me or 0,1 Me world support a long-term magnetic field and tectonic activity? Or in this case it would be better if such a world be a moon of a huge gas giant?
Edited by Diakonov - Sunday, 07.08.2016, 03:55 |
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| midtskogen | Date: Sunday, 07.08.2016, 18:37 | Message # 12 |
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| Quote Watsisname ( ) spectroscopy is an amazing tool.
And the fact that we can see the action of electrons at a distance of lightyears is fascinating. :)
Quote Watsisname ( ) This will also likely be the method of the first 'unambiguous' detection of life outside the solar system. (Unambiguous if we can show that such composition cannot exist in equilibrium without biology -- e.g. oxygen in Earth's atmosphere.)
It might be hard to prove such things without assuming a lot about what could exist on other planets.
Titania may produce abiotic oxygen atmospheres on habitable exoplanets.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Sunday, 07.08.2016, 23:27 | Message # 13 |
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| Yeah, we've known of oxygen production by photo-dissociation, but to have enough of it produced to accumulate in the atmosphere despite the vast sinks that exist for oxygen would be really crazy. In this case, it would take a lot of exposed titania -- the paper says 2 orders of mag more than we would expect from chemical abundances. But as proof of concept, it's important to show that the mechanism works if there's enough of it. When finding disequilibrium chemistry in an exoplanet atmosphere, we have to be extremely careful to characterize the processes that are going on there, to be able to convincingly show whether any biomarker is really produced by biology.
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| Diakonov | Date: Tuesday, 09.08.2016, 15:14 | Message # 14 |
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Astronaut
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| Maybe we could have a high pressurized planet with lot of nitrogen but a little oxygen, say, 94% N2, 5% O2 and 1% other gases. And we could have complex and maybe even vertebrate organisms capable of fixing nitrogen, such as some bacteria can do. And for a planet with a lot of oxygen, I think a copper-based blood would be enough.
Edited by Diakonov - Tuesday, 09.08.2016, 15:15 |
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