[Phys-L] atmosphere

[John Sohl <jsohl@weber.edu>]

Anthony asks a LOT of good questions. More than I've got time to cover in
any detail. Sorry.

For starters, any introductory astronomy textbook will describe this
process in detail and will also address the issues of planetary atmospheres
other than Earth.

A few quick comments:
We had a "primary atmosphere" and now have a "secondary atmosphere." The
primary atmosphere was the original one with lots of H and He.
Then the Sun "turned on." When the Sun's energy source shifted from
gravitational heating due to compression of the gasses to a nuclear fusion
energy source it was a whole new ballgame. The enormous change in internal
energy blew out anything that wasn't nailed down. (This is called a T Tauri
Wind: https://en.wikipedia.org/wiki/T_Tauri_wind ) Think of the T Tauri
wind as the solar wind on steroids.
Anything up close (the four rocky planets and their moons) had the primary
atmospheres blown off into the outer solar system where they added a tiny
amount to the gas giant planets' atmospheres. Now our dear planet Earth was
without any atmosphere at all.
Enter phase two: the secondary atmosphere. This is a combination of
volcanoes and comet impacts. Mostly volcanoes. That spit out a lot of CO2
and nitrogen. (By the way, nitrogen continues to slowly seep out of Earth's
crust even today.)
Then, half a billion years later, early life started to form and CO2 was
slowly spit, releasing the oxygen. This did not happen on Venus or Mars in
any significant way. So they kept their CO2.
Also, oceans are a key part of the CO2 cycle. (Which is why the oceans are
becoming more acidic with the extra CO2 we are releasing.) The carbon and
oxygen, once separated, gets used in a lot of processes. As those get
consumed it leaves behind the nitrogen.
Venus still has 3.5% of its atmosphere at Nitrogen. But there is a LOT of
atmosphere on Venus since the CO2 was never removed by oceans and life. So,
by comparison, the amount of nitrogen is "small". But that is only because
of the massive amounts of CO2.

Mars has much less gravity than Earth and Venus. So it is hard to hold on
to what atmosphere it has. Nitrogen on Mars is about 2.7% (based on my
memory, you might want to check that), which is very similar to Venus.
Again, no significant or long lasting oceans or life on Mars means that the
CO2 was not broken down. Make no mistake, both of those planets have
nitrogen in their atmospheres, just like Earth does.

Mercury is both small and close to the solar system's central heat source.
Being small it doesn't have a lot of gravity. Being (on the Sun side
anyway) very hot, the gasses have a lot of kinetic energy and thus
molecular velocity. The result of the two together is that the gases are
close to the escape velocity and leave the planet with (essentially) no
atmosphere. Same game with Earth's Moon. (To get really into the weeds,
both have an "exosphere" but that is so thin as to be effectively no
atmosphere. This exosphere is primarily outgassing from the planet.)

I think I answered most of your questions. As to the questions:
Does anyone really know with any certainty? Is there much research about
the origin/evolution of Earth's atmosphere?"
The answer to both is a resounding "YES!!"

Well, I'm out of time, I hope this gives you a working summary of the
concepts.

John

- - - -
John E. Sohl, Ph.D.
WSU Brady Presidential Distinguished Professor of Physics
Weber State University
1415 Edvalson St., Dept 2508
Ogden, UT 84408-2508

Office: TY 326
Office phone: (801) 626-7907
cell: (801) 476-0589



> From: Anthony Lapinski <alapinski@pds.org>
> To: phys-l@mail.phys-l.org
> Cc:
> Bcc:
> Date: Tue, 17 Nov 2020 20:43:04 -0500
> Subject: [Phys-L] atmosphere
> Any atmospheric scientists on this list? I'm teaching about the Earth in my
> (high school) astronomy class. We're discussing the origin of our
> atmosphere (78% N2, 21% O2). Fascinating! Trying to keep it basic, but I
> realize it is complex. Wasn't Earth's ancient atmosphere just H and He?
> Oxygen came from plant life and photosynthesis (and sunlight breaking down
> water). Fine. But how did we get so much nitrogen? Can volcanoes and comets
> account for all of it? In addition, we discuss the greenhouse effect and
> the amount of CO2 in our atmosphere, how it affects our surface
> temperature, and compare this to Venus and Mars. Yet those planets have no
> nitrogen. Why is Earth's atmosphere so unique? Does anyone really know with
> any certainty? Is there much research about the origin/evolution of Earth's
> atmosphere? I would appreciate any thoughts/references/videos that could
> help.
>
> --