Why did Earth lose its first atmosphere but keep its second?

[curious_ocean]

I'm trying to get a basic understanding of Earth's origins in order to teach an advanced oceanography course to high school students this summer. The course starts with one lecture on the origins of the universe, solar system, the earth, and the ocean. Why did Earth lose its first atmosphere but keep its second?

As I understand, the Earth had a "first atmosphere" that was made up of mostly hydrogen and helium accumulated from the solar nebula. The Earth was too small and too hot to hold on to these gases when our young star began to generate energetic solar winds and the first atmosphere was "blown off". Later a second atmosphere was created by outgassing (volcanic eruptions) and impact degassing (vaporizing asteroids/comets). This atmosphere stayed. Why? Was it because the Earth was now more massive and commanded a stronger gravitational hold of this atmosphere? Was it because this atmosphere was made up of heavier molecules that could not escape to space as easily? Was it because the Earth had cooled, so these molecules did not have the energy to escape? Was it because the sun's solar winds became weaker? Was it because the Earth's magnetic field formed and acted as a shield that protected this second atmosphere?

Thanks in advance for your help!

 

[Drakkith]

I believe it is mostly because hydrogen and helium are so light that the atoms have MUCH higher average velocities than other gases like oxygen and nitrogen when they are at the same temperature.

 

[curious_ocean]

Thanks Drakkith! I had read a lot of different stuff and perhaps this reason is the simplest and therefore best explanation, at least for the present purpose of teaching this class. I appreciate it!

 

[Chronos]

The early Earth had hardly any oxygen. Oxygen is primarily a waste product of biological activity. Lighter gasses, like H and He, were easily lost, as Drakkith noted. The chemistry of the young Earth atmosphere was complex and many reactions occurred. The relatively low gravity of Earth allowed the lighter ones to be lost more easily than heavier ones.

 

[Borek]

I would check what fraction of the molecules of a given mass have speed higher than the escape velocity.

That's not different from what Drak wrote, just less handwavy

 

[AlephZero]

The early Earth had hardly any oxygen.

To be pedantic, it had hardly any oxygen gas. Don't forget that water is almost 90% oxygen by mass, not to mention the oxygen content in rocks - e.g. sand is mainly silicon dioxide.

There was little free oxygen gas because it is highly reactive. As Chronos said, the current high level of free oxygen is maintained by photosynthesis. When photosynthesis first developed, the organisms that used it had a huge advantage in exploiting a "free" energy source (the sun), and the waste product of highly reactive oxygen gas was poisonous to most of the other life on Earth at that time - though of course it also created the opportunity for oxygen-breathing animals to evolve.

 

[Drakkith]

That's not different from what Drak wrote, just less handwavy

I do not wave my hands!

 

[Mordred]

I do not wave my hands!

oh yes you do, usually in exasperation

 

[Drakkith]

oh yes you do, usually in exasperation

No no, that's not exasperation, that's me casting a spell.

 

[DHF]

This is similar to the reason Mars currently has a 95% CO2 atmosphere. In the past it is theorized that it had a very different composition but the lack of a magnetic field and light gravity allowed the solar wind to strip most of the other elements. CO2 remained because it is heavier and harder for the solar wind to strip away.