How did the terrestrial planets get their atmospheres?

Two distinctly different processes exist for the formation and development of the atmosphere of a terrestrial planet.  The first is capture of a primitive, or primary, atmosphere.  The second is the outgassing of a secondary atmosphere.

Primitive atmospheres: If these atmospheres ever existed, they would have been the first (or primary) atmospheres for the terrestrial planets.  These atmospheres would have been captured from the gas-rich solar nebula as the protoplanet grew.  With increasing size, the protoplanet's gravity may have become a sink for the concentration of solar nebula gas.  Such a gas would have been dominantly H, H₂, He, Ar, Ne, and perhaps small amounts of H₂O, CH₄, and NH₃.  The lightest of these would have easily escaped (or never been captured by the protoplanet), leaving an atmosphere dominated by the heavier inert gases and hydrogen rich compounds.  Such a hydrogen rich atmosphere is called a reducing atmosphere.

Did the terrestrial planets ever have primitive atmospheres of any significance? For many years, astronomers assumed that Earth's earliest atmosphere was a dense, primitive atmosphere. Hence, the earliest Miller-Urey experiments (making amino acids in a test tube filled with methane, ammonia and water) imitating the supposed primeval soup were done in reducing environments.

We can test this hypothesis and it is found wanting.  How? Neon.

• Neon is heavy and so none can escape from Earth's atmosphere.  (Do the calculation and test this claim!)
• It is inert so it does not bond with rocks or have any other way of escaping into the Earth from the atmosphere.
• It is not produced via radioactive decay, so whatever neon is presently in our atmosphere is a measure of the upper limit for the total amount of neon left over from a primitive atmosphere (note that some neon could have entered the atmosphere via outgassing over Earth's history).
• Given the known ratio of neon to hydrogen and helium and other gases in the sun, we can estimate the total mass of the Earth's primitive atmosphere.
• The result is that the primitive atmosphere could have been no more than 0.9% of Earth's present day atmosphere.
• Therefore, the Earth never had a primitive atmosphere of any significance.

How do secondary atmospheres form?  Gases are vented (outgassed) through volcanic eruptions.  The terrestrial planets must have been warmer and more volcanically active when they were younger.  These gases will accumulate at the surface of the planet and in the atmosphere.  Some can bond to rocks and cycle in and out of the atmosphere; others will dissolve in water, reaching equilibria between atmospheric and oceanic abundances.  Some will remain almost wholly in the atmosphere.

What would an outgassed atmosphere look like? The following gives observed and theoretical estimates of gases added to Earth's atmosphere (as percent composition by weight).  The data is from the William Hartmann's book Moons & Planets (p. 320).
 
 

    a - Hawaiian volcanoes

Let's first compare the dominant constituents of the atmospheres of Venus, Earth and Mars:
 
 

Now, let's look at the water content:
 
 

Note that for the Earth, 0.025% (2.5 ten thousandths) of the total mass of the Earth (1.5 x 10²⁴ gm/5.97 x 10²⁷ gm = 2.5 x 10^-4) is water.

And now the carbon dioxide content:
 
 

*** organic carbon (oil, coal, shale) = 7 x 10²² gm
*** carbonate rocks = 3 x 10²³ gm
*** biosphere (carbohydrates) = 10¹⁹ gm
*** atmosphere = 2.4 x 10¹⁸ gm
*** oceans (bicarbonate ions) = 1.3 x 10²⁰ gm
 
 

The Urey reaction

• on Earth, CO₂ gas dissolves in oceans to form carbonic acid (H₂CO₃):

CO₂   + H₂O -> H₂CO₃
• carbonic acid reacts with silicate rocks (or minerals dissolved in sea water) to form carbonate rocks

enstatite (MgSiO₃) + carbonic acid (H₂CO₃) ->  <- magnesite (MgCO₃) + quartz (SiO₂) + water  (H₂O)
[Note: one could think of this reaction as simply silicate rocks + carbon dioxide yields carbonate rocks and quartz; however this ignores the critical catalytic role played by water. Without the water, this process doesn't go forward.]
• magnesite (and many other carbonates) is not soluble in water so it precipitates and forms rock on the ocean bottom; most CO₂ on Earth is tied up in carbonate rocks and oceans; when these rocks are subducted, the heat drives the reactions backwards and the CO₂ vents from volcanoes.
• high temperature drives reaction to the left (Venus)
• water drives reaction to the right (Earth)
• on Venus, absence of water keeps CO₂ gaseous

what about Mars? Is CO₂ locked into rocks? If so, would the presence of carbonate rocks be evidence for ancient Martian oceans?

And finally, let's compare the nitrogen contents:
 
 

What have we  learned from comparing these three terrestrial atmospheres?

1. Venus and Earth have the same carbon dioxide budgets.
2. Venus has no water; more than one part in ten thousand of the Earth is water.
3. Comets are the most likely source of both water and carbon dioxide.  If you capture a comet, you get both.

• formed in the same part of the solar system from the same general process of accretion
• formed from similar materials
• obtained their volatiles from the same reservoirs of volatile-rich material (outgassing of cometary material collected during early stages of planet formation or late veneer bombardment from comets)

• the same total content of carbon dioxide
• similar amounts of nitrogen (to within a factor of 3)