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How To 

How Does an Atmosphere Form?



We can thank our atmosphere for quite a lot, particularly our ability to, well, survive. Earth’s atmosphere keeps us from freezing (or boiling), it provides the air we breathe, it stops (some) space debris from clobbering our planet, and it filters out harmful radiation from the Sun. Without the atmosphere, life would be very different or nonexistent. So, how did our atmosphere form in the first place? In general, here’s how atmospheres form:



1. You need the right circumstances during formation.
Way back when the solar system formed, the rocky and outer planets distinguished themselves from one another by the materials they collected. As the name suggests, the inner planets had more rocky material to use during formation, and the outer planets had more gaseous particles. While there wasn’t enough gas for the inner planets to become gas giants, there was plenty to form an initial atmosphere of hydrogen and helium. Gases also became trapped within the rocky planets and would later be released.

2. Stored gases are released through mechanisms like volcanic eruptions.
The hydrogen and helium atmospheres of the rocky planets didn’t last long. Gravity held these gases close to the planets initially, but once the sun began to heat up, they were able to escape. That’s when secondary atmospheres kicked in and began to form. Secondary atmospheres arise from the gases within the planet as well as gases that arrive with impacts from things like comets. CO2, H2O, nitrogen, and maybe even some methane were also released during volcanic eruptions, which sent these gases flying up into the atmosphere where gravity held them in its grip.



3. Atmospheres don’t always stick around.
Of course, sometimes gravity alone isn’t enough to hold onto an atmosphere. Earth’s molten core generates a magnetic field that protects our atmosphere from being blown away by solar winds, which is likely what befell Mars’s atmosphere. In the case of Venus, its atmosphere most definitely did stick around, but it got a bit too carried away and is essentially cooking the planet. Atmospheres are great - when circumstances are right, such as on Earth.
 
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Jem

Feb 18, 2020
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so does mars not have a strong as a magnetic field and what do you mean by don't stick around cause the venus atmosphere is still there but is heating it up like a microwave going on forever
 

rod

Oct 22, 2019
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Jem, Mars has little or no magnetic field today. Mars atmosphere also confronts the Faint Young Sun problem, 4 billion years ago. Mars must have a much denser atmosphere to retain liquid water but if Mars had the atmosphere it is gone today. Space.com also reported Mars is rapidly losing its water, Mars Loses Its Water Even Faster Than Anyone Thought We also have well documented exoplanets that are losing their atmospheres too, some hot jupiters as an example. This How To said "Atmospheres are great - when circumstances are right, such as on Earth." This looks like an important concept :)
 
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Feb 1, 2020
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Jem, Mars has little or no magnetic field today. Mars atmosphere also confronts the Faint Young Sun problem, 4 billion years ago. Mars must have a much denser atmosphere to retain liquid water but if Mars had the atmosphere it is gone today. Space.com also reported Mars is rapidly losing its water, Mars Loses Its Water Even Faster Than Anyone Thought We also have well documented exoplanets that are losing their atmospheres too, some hot jupiters as an example. This How To said "Atmospheres are great - when circumstances are right, such as on Earth." This looks like an important concept :)
Rod, Earth is losing atmosphere too. Just slowly. Come back in three or four billion years, and it will be more like Mars. A half a billion years ago Earth had about one third more atmosphere than it does now. Dinosaurs also breathed a thicker air than we do today. Mars is smaller, so it lost it's atmosphere faster. Our magnetic field helps, but it isn't a total cure.
 
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rod

Oct 22, 2019
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"A half a billion years ago Earth had about one third more atmosphere than it does now. Dinosaurs also breathed a thicker air than we do today."

FYI, there are raindrop fossil studies that do not support this model for Earth's atmosphere. Raindrops In Rock: Clues To A Perplexing Paradox, "The scientists knew that the size of raindrops would tell them how thick the atmosphere was back then. Raindrops fall more slowly through thick air than through thin air. So the thicker the air, the more gently they will fall to earth, and the smaller the craters will be. And the raindrop craters they found were similar to those you get on dusty lava today. So Som and his colleagues conclude that the atmosphere back then was a lot like it is today, maybe a bit thinner, but surely no more than twice as dense."

This report on raindrop fossils was Precambrian strata dated 2.7E+9 years old using radiometric methods. It is not conclusive that Earth's atmosphere was 1.3 bar or more compared to the present 1 bar atmosphere pressure at sea level. The whole issue is wrapped up with the Faint Young Sun problem, Mars too.
 
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