Calculations for a theoretical planet + a little biology

[Kiwimaster76]

So I'm attempting to write a scientifically accurate story and i need some help with the planets characteristics. I have a few things set up already and need some educated guesses and ideas for how the rest will work. So to start with what i have:
Planet name: Remvale
Radius:~8370 miles
Gravity: 8.73 meters/sec
Atmosphere pressure: 3.7 atmospheres
Atmosphere composition: haven't decided except for around 34% oxygen
Surface: 62% water 38% land

So the questions i have are as follows:
-Density of planet? It has roughly twice the radius of Earth but a lower gravity so it will be much lighter, is the density a realistic mass?
-Is the atmosphere pressure survivable for an entire human lifespan? If so what is the max pressure for normal human life and what adaptions would be needed?
-Could humans adapt to live a normal lifespan in such an oxygen rich environment?(sorry this isn't astrophysics but i heard that increased oxygen levels would shorten lifespan)
-What planetary events could cause a rapid increase in oxygen level? (roughly 20% in almost 1000 years?)
-Would the increased atmosphere pressure cause any optical effects? (ex. venus atmosphere being impossible to see through)
-What gas mixtures would be needed for life and temperatures similar to Earth's to exist? (just basics like nitrogen oxygen and carbon dioxide)
-What basic animal types would survive best on this planet? ( out of reptiles, birds, insects, mammals, amphibians, fish, ect.?)

Sorry, i know this is a lot of questions from a lot of fields. I'm not expecting anyone person to try to answer them all, feel free to do as many or as few as you want. Thanks!

 

[Bystander]

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

 

[Rive]

Atmosphere pressure: 3.7 atmospheres
Atmosphere composition: haven't decided except for around 34% oxygen

If I did the math correctly, the partial pressure of oxygen there would be around six times of on Earth. And yes, Oxygen is toxic.
I would say it would be quite bad within hours. (Six bar partial pressure is not even on this graph below.)

(Graph is from the linked Wiki page)

 

[Bandersnatch]

Radius:~8370 miles
Gravity: 8.73 meters/sec

Those numbers require the average density of 2.3 g/cm3*. Which begs the question: what happened to all the metals during the formation of the planet?

*the relationship is simple: starting with Earth - X times the radius means X times the gravity. Y times the density means Y times the gravity.
Here you have (roughly) twice the radius and the same gravity, so the density must be 1/2.

 

[Bystander]

Anything over 20% oxygen is a bomb. See previous link, https://en.wikipedia.org/wiki/Fire_safety .

 

[Ryan_m_b]

This video, and others on the channel, should help you come up with a realistic planet:



At 3mins 38secs he goes over something that I think is quite relevant: how the relationship between radius and mass determines what kind of planet you get. If the planet is large and low mass it will have formed as a mini-gas planet. If it's too high mass you get a world covered in incredibly deep water/ice. There is a handy chart drawn from a book (which I think is referenced in one of the videos) that shows the range of conditions necessary for a rocky world.

 

[snorkack]

So I'm attempting to write a scientifically accurate story and i need some help with the planets characteristics. I have a few things set up already and need some educated guesses and ideas for how the rest will work. So to start with what i have:
Planet name: Remvale
Radius:~8370 miles
Gravity: 8.73 meters/sec
Atmosphere pressure: 3.7 atmospheres
Atmosphere composition: haven't decided except for around 34% oxygen
Surface: 62% water 38% land

So the questions i have are as follows:
-Density of planet? It has roughly twice the radius of Earth but a lower gravity so it will be much lighter, is the density a realistic mass?

Not really, as other posters explained.

-Is the atmosphere pressure survivable for an entire human lifespan? If so what is the max pressure for normal human life and what adaptions would be needed?

Pressure probably is, but...

-Could humans adapt to live a normal lifespan in such an oxygen rich environment?(sorry this isn't astrophysics but i heard that increased oxygen levels would shorten lifespan)

No. As other posters explained. 1,2 bar partial pressure of oxygen is fatal in a few days.

-What planetary events could cause a rapid increase in oxygen level? (roughly 20% in almost 1000 years?)

Cannot think of any that are this fast.

-Would the increased atmosphere pressure cause any optical effects? (ex. venus atmosphere being impossible to see through)

Increased Rayleigh scattering and refraction. And the larger radius of planet would contribute to both.

-What gas mixtures would be needed for life and temperatures similar to Earth's to exist? (just basics like nitrogen oxygen and carbon dioxide)

Other safe components are Ne and He

-What basic animal types would survive best on this planet? ( out of reptiles, birds, insects, mammals, amphibians, fish, ect.?)

All 6 are classes in Earth Cenozoic, and all save one are vertebrates.
High air density would favour birds.

 

[Tiran]

How would a planet with lower gravity than ear have higher pressure atmosphere? They're related.

A massive die-off of aerobic life could cause plant life to increase oxygen levels greatly.

 

[snorkack]

How would a planet with lower gravity than ear have higher pressure atmosphere? They're related.

Easily. Venus does. So does Titan.

 

[Tiran]

Easily. Venus does. So does Titan.

Of course. By having atmospheres of heavy, poisonous gasses. But the OP is talking about a breathable atmosphere, not one full of methane, CO2, sulphuric acid.

Which is why I asked how the OPs planet manages to have such high pressure. Is it full of argon?

 

[snorkack]

Of course. By having atmospheres of heavy, poisonous gasses. But the OP is talking about a breathable atmosphere, not one full of methane, CO2, sulphuric acid.

Not by this.
Methane is not poisonous, nor heavy. Actually, methane is lighter than neon. And Titan´s atmosphere is just 4,9 % methane - the rest, 1,4 bar, is nitrogen.
Venus is mostly heavy and poisonous CO₂ - but 3,5 % nitrogen in the 93 bar of Venus´ atmosphere is still 3,2 bar.

 

[Tiran]

Not by this.
Methane is not poisonous, nor heavy. Actually, methane is lighter than neon. And Titan´s atmosphere is just 4,9 % methane - the rest, 1,4 bar, is nitrogen.
Venus is mostly heavy and poisonous CO₂ - but 3,5 % nitrogen in the 93 bar of Venus´ atmosphere is still 3,2 bar.

Thank you for the correction. But my basic point is that Venus and Titan are nowhere close to liveable places because of the conditions that make their atmosphere's dense. In the case of Titan the scant sunlight makes less atmosphere bleed into space while keeping temps low enough to keep much of the methane and other compounds in a liquid state so they can act as a recharging reservoir for that gaseous state. Raise Titan's temperature via increased sunlight 300°F and methane is no longer condensing into rain but breaking down in the ultraviolet. But Titan is actually only 40°F above the nitrogen condensing into an ocean.

There may be a way to get a planet in a liveable temperature range with super high atmospheric pressure, but there would need to be a pretty hefty mechanism that either replenishes the atmosphere due to outgassing or have the planet in a state of rapid change were it is in the process of breaking down an enormous quantity of frozen atmospheric components as it warms through the liveable range.

The main point I was trying to make to the OP is that atmospheres are a balance between content, heat and gravity. There would need to be a very good reason a lighter planet has managed to retain more atmosphere than is stable while not being too hot or cold to live in. Atmospheric pressure comes mainly from mass. What keeps that mass in place or replaces it?

 

[Vanadium 50]

In the case of Titan the scant sunlight makes less atmosphere bleed into space while keeping temps low enough to keep much of the methane and other compounds in a liquid state so they can act as a recharging reservoir for that gaseous state.

Huh? Titan's atmosphere is mostly nitrogen, not methane.

 

[Tiran]

Huh? Titan's atmosphere is mostly nitrogen, not methane.

I didn't say it isn't.

So why does Titan have a heavier atmosphere than Earth's? You seem to be implying that methane and other compounds that move easily between liquid and gas at those temps aren't part of the equation.

 

[Vanadium 50]

No, I seem to be implying that discussing the behavior of methane has little to do with an atmosphere that is nearly pure nitrogen.

 

[Tiran]

No, I seem to be implying that discussing the behavior of methane has little to do with an atmosphere that is nearly pure nitrogen.

Does it have little to do with the atmosphere?

 

[Vanadium 50]

Tiran, what the heck are you getting at? You started prattling on about methane being liquid as being an important factor for an atmosphere that is just over 1% methane. I pointed this factr (composition) out. That's all, no more, no less.

 

[Tiran]

Tiran, what the heck are you getting at? You started prattling on about methane being liquid as being an important factor for an atmosphere that is just over 1% methane. I pointed this factr (composition) out. That's all, no more, no less.

And the CO2 level on Earth is only .041%. Why are you suggesting that methane isn't an important factor in why a small planet has an atmosphere that is larger than its gravity suggests it should?

The articles I've read about Titan all make mention of how methane is a factor in Titan's mysteriously dense atmosphere, and seem to indicate the liquid/vapor cycle of methane and compounds with low vapor points are a factor. What is it you are arguing? If you are discounting the effect of everything by N2, what mechanism are you suggesting is at work?

 

[snorkack]

And the CO2 level on Earth is only .041%. Why are you suggesting that methane isn't an important factor in why a small planet has an atmosphere that is larger than its gravity suggests it should?

The articles I've read about Titan all make mention of how methane is a factor in Titan's mysteriously dense atmosphere, and seem to indicate the liquid/vapor cycle of methane and compounds with low vapor points are a factor. What is it you are arguing? If you are discounting the effect of everything by N2, what mechanism are you suggesting is at work?

I´m suggesting that no mechanism is needed. Atmospheric escape, for atmospheres of reasonable size, is small compared to the size of atmosphere, and the effects of smaller or higher gravity are unable to overcome chance effects of initial composition and size. So, within reasonable size, atmospheric size and composition are free parametres not dictated by gravity.

 

[Tiran]

I´m suggesting that no mechanism is needed. Atmospheric escape, for atmospheres of reasonable size, is small compared to the size of atmosphere, and the effects of smaller or higher gravity are unable to overcome chance effects of initial composition and size. So, within reasonable size, atmospheric size and composition are free parametres not dictated by gravity.

So you're saying Earth could have 3 times its current atmosphere and it could also maintain liveable temperatures?

 

[snorkack]

So you're saying Earth could have 3 times its current atmosphere and it could also maintain liveable temperatures?

Yes.
Actually, increasing atmospheric pressure would decrease greenhouse effect.

 

[Tiran]

Yes.
Actually, increasing atmospheric pressure would decrease greenhouse effect.

I didn't say it wouldn't. How much of a decrease in average temperature? Enough to freeze all the water?

 

[snorkack]

I didn't say it wouldn't. How much of a decrease in average temperature? Enough to freeze all the water?

That would take a lot of sensitive computation to actually quantify. Earth average temperature has changed a lot since Ice Age, without much change in atmospheric pressure.
But the tendency would be less greenhouse effect with increase of atmospheric pressure (because of less water).

 

[stefan r]

Yes.
Actually, increasing atmospheric pressure would decrease greenhouse effect.

If you increase atmospheric pressure you increase the boiling point of water. That means water vapor remains a gas while at a higher partial pressure. Vapor pressure is determined by temperature.
The infrared absorption by water should increase when the partial pressure increases. The infrared absorption of water would also increase if the atmosphere containing a fixed partial pressure is thicker (in frequencies where less than 100% is absorbed). A planet with a larger nitrogen atmosphere should have both effects.

 

[snorkack]

If you increase atmospheric pressure you increase the boiling point of water. That means water vapor remains a gas while at a higher partial pressure. Vapor pressure is determined by temperature.

Right. At a given temperature, the partial pressure of water vapour does not depend on atmospheric pressure. And the sea is not boiling hot.

The infrared absorption by water should increase when the partial pressure increases. The infrared absorption of water would also increase if the atmosphere containing a fixed partial pressure is thicker (in frequencies where less than 100% is absorbed). A planet with a larger nitrogen atmosphere should have both effects.

Titicaca is 3812 m above sea, and warms to 14 Celsius in summer.
Great Slave Lake is 156 m above sea, and also warms to 14 Celsius in late summer.
In their respective summers (different hemispheres), water partial pressures over Titicaca and Slave are equal. Air pressures are different.
Dry adiabatic lapse rates above Titicaca and Slave are equal (same gravitational acceleration, same dry heat capacity).
Wet adiabatic lapse rates are different.
Basically, the reason why wet adiabatic lapse rates differ from dry adiabatic lapse rates is that as the wet air cools adiabatically on rising, the water rains out and releases the heat of condensation - slowing down the cooling of air.
Now, over Titicaca and Slave, there is equal amount of water vapour on surface to rain out, but different amount of air for it to warm. Over Titicaca, the water vapour prevents the thin air from cooling, and rains out over thick layer of air. Over Slave, the thick air confines the water vapour to a thinner surface layer.
Since the vapour pressure at the surface of Slave is equal to that of Titicaca, but drops off more steeply with height as the temperature falls above the water surface, the total amount of water vapour between Slave and outer space is less than between Titicaca and space. Thus less greenhouse effect with increased air pressure.