Planetary Atmosphere Question

talktidy

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I am writing a space opera and envisage a world orbiting a Sol-type star. The world is Mars-like, but of a size more commensurate with Earth, approx 400-500 million kilometres from its star (as comparison, Earth is 150 and Mars is an average of 228 million kilometres from our sun).

I am assuming that distance from its star means any water on the surface would not be in liquid form, but what about atmosphere in general? My world has enough gravity to retain one, but I am a bit vague about what that might be – apart from lacking oxygen. I would prefer to avoid Titan-like conditions.

What I would like are conditions that would respond nicely in time to application of a new heat source and seeding the planet with oxygen producing life.

While this is a practice exercise for me, I’d still like to avoid scientific howlers, so any input those with a better scientific grasp can provide would be very much appreciated.
 

King Neptune

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It would depend on the age of the planet to a significant degree. It it is geologically active, then the atmosphere would depend on what kind of gases were being expelled by the volcanoes. If it has stopped being active, then the gases would slowly drift away, and it would be left with heavier gases and a thinner atmosphere.
 

talktidy

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Thanks, King Neptune. I may have to rethink this, but, while I mentioned Mars, I was thinking of keeping the planet comparable to ours in other respects, so roughly about the same age as Earth and geologically active.

I see I shall have to do some googling later, but that far out from its star, would not water precipitate as snow and ice? With comparable Earth gravity, would it still lose atmosphere?
 

King Neptune

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Thanks, King Neptune. I may have to rethink this, but, while I mentioned Mars, I was thinking of keeping the planet comparable to ours in other respects, so roughly about the same age as Earth and geologically active.

I see I shall have to do some googling later, but that far out from its star, would not water precipitate as snow and ice? With comparable Earth gravity, would it still lose atmosphere?

If the planet is about the age of Earth, then the atmosphere would not be depleted, unless it has an oversized moon. That was another factor that I forgot. The Moon strips away the outer atmosphere. If not for the Moon, the Earth would be more like Venus. There are many, many factors.
 

Dennis E. Taylor

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Factors that can affect the atmosphere:

1. Gravity. Lighter gravity, gases escape more easily.
2. Geological activity. More outgassing means more replenishment.
3. Magnetic field. Mars has practically none, and solar activity has stripped a lot of the atmosphere.
4. Distance from sun. See item 3.
5. Gas mixture. Put more CO2 into the atmosphere, and it'll be warmer despite being farther away. Methane, even more so.
6. Presence of life will tend to affect overall atmospheric composition.
7. Rate of spin. A planet with little or no rotation will tend to have water and gases with higher melting points accumulate on the dark side.

Remember that you don't have to be too specific in fiction. Make a nodding reference to some of these points, comment that the combination of factors produced the result you want, and you're done.
 

Mark HJ

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What I would like are conditions that would respond nicely in time to application of a new heat source and seeding the planet with oxygen producing life.

You need to decide where the oxygen is going to come from - if you have photosynthesis in mind, your planet needs a good supply of CO2 for a plant-style process. Otherwise, you need something else with oxygen in it that your life-forms can break down.
 

blacbird

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It would depend on the age of the planet to a significant degree. It it is geologically active, then the atmosphere would depend on what kind of gases were being expelled by the volcanoes. If it has stopped being active, then the gases would slowly drift away, and it would be left with heavier gases and a thinner atmosphere.

An earth-sized planet at that distance from a Sun-equivalent star would be pretty cold, and have enough gravity to hold any but the lightest gases (hydrogen and helium). Water vapor and methane are lighter than diatomic nitrogen and oxygen, but still probably heavy enough to be retained. However, a cold atmosphere would not be able to retain much water vapor, just as on earth in really cold winter days, the air is very dry.

But perhaps more pertinent is the issue of a planetary magnetic field. It is believed that Mars lost much of its atmosphere when it cooled to the point its core could no longer generate a strong magnetic field. That allowed solar wind to simply blast away most of its atmosphere, leaving it with the current tenuous one, mainly of CO2. And even that is slowly being stripped. So your hypothetical planet might need an active magnetic field.

caw
 

DrDoc

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Solar wind can strip away an atmosphere eventually. Our earth is protected from the solar wind by our magnetosphere, which is caused, ultimately, by our magnetic liquid iron core.