Why is the Earthian atmosphere so thin?

Question

Venus is somewhat lighter then Earth, yet has a much thicker atmosphere. One would imagine that the following should be true:

1. During the formation phase, all inner planets had captured as much gas as they could hold per gravitational/thermodynamic equilibrium. After all, even puny Mars managed to capture a sizable atmosphere.
2. The rate of atmospheric escape should be much higher for Venus:
   • Venus receives more heat from the sun, thus higher Jeans escape rate
   • Venus has negligible magnetic field, so some of its atmosphere must be lost to direct "blow-off" by solar wind

Yet, it is Earth that seemingly misses a great deal of atmospheric volume. So the question is: what are the current theories regarding the "thinning" of the Earthian atmosphere? When and why the atmospheric gases had left the planet?

Answer 1

The short answer: Atmospheric gases never left Earth, they're in it!

The long answer to this question isn't just about the planets' current states, but rather the processes that led them there. Let's start at the very beginning (a very good place to start).

The very early years

When our solar system began forming 4.6 billion years ago, most of the mass from the collapsed portion of a molecular cloud (see Nebular hypothesis) collected in the center to form the Sun. The mass that didn't collapse into the Sun left a protoplanetary disk—a cloud of dust and gas—surrounding the new star. Gradually, dust particles began to come together through accretion, attracting more and more particles into fledgling planets.

Close to the Sun, where Venus and Earth both reside, it was too hot for many particles to condense, so the planets in this region formed with metals and silicates, which have high melting points. This is why the four planets in the inner Solar System are called the "rocky" or "terrestrial" planets. The earliest atmospheres on these planets began forming with the gradual collection of gases from the solar nebula, mostly hydrogen.

Goldilocks and the two planets

At this point in the evolution of the two planets, they looked pretty similar, but there is one major difference: the distance to the Sun. Earth, it seems, got lucky to be in the "Goldilocks zone", where the temperature is just right to support life. Being in this zone has two major implications: liquid water and, as a consequence, active plate tectonics. (See this article for an in-depth view into why the two are related.)

Carbon sinks

On Earth, there is considerable water contained in liquid oceans. On Venus, this is not the case. It is simply too hot that close to the Sun, so all the water evaporated into the atmosphere. (Venus likely contained liquid water in its early stages, but it all evaporated after a billion years or so.) It is also likely that the young Earth once had a dense, punishing atmosphere like Venus's today. However, surface oceans and plate tectonics both provided ample pathways for gases to be absorbed into the surface of the Earth. Oceans and plate tectonics offer huge amounts of carbonate storage, allowing the transfer and balance of carbon compounds to and from the atmosphere.

So now we have two things reinforcing the differences between Earth's and Venus's atmospheres:

• Evaporation of liquid water: It is too hot on Venus for liquid water to exist. All water has evaporated, resulting in a denser atmosphere. On Earth, water can reside on the surface, lessening the amount in the atmosphere.
• Carbon sinks: Liquid water and plate tectonics allow Earth to absorb considerable amounts of gas, allowing the atmosphere to be thinned of certain compounds like carbon dioxide. There is no such pathway on Venus, forcing all gas to remain in the atmosphere.

With no major mechanisms for gases to be absorbed by the planet, Venus is experiencing a runaway greenhouse effect.

Atmospheric escape

You mention Jeans escape. It is true that this effect is greater at higher temperatures; however, it is much easier for smaller molecules to escape than for larger ones. Hydrogen and helium, being the two smallest elements, are the most greatly effected by this phenomenon. In comparison, carbon dioxide, which makes up the majority of Venus's atmosphere, is not much affected by Jeans escape.

You also mention solar winds. While these play an effect, especially on planets without a magnetic field, this phenomenon is not as powerful as you might think. Ultraviolet light (i.e. photoionizing radiation) causes ionization in the uppermost region of the atmosphere. These charged particles now form a shell (called the ionosphere) that deflect solar winds, much like a magnetic field would. On Venus, the thick atmosphere provides more particles for ionization, resulting in a more powerful deflection. (Compare this to Mars, where solar wind is the primary non-thermal escape mechanism due to the thin atmosphere with few ionized particles.)

The primary atmospheric escape mechanism for Venus is actually a bit more complicated. In the absence of a magnetic field, it is easier for charged particles to escape. In particular, electrons are most susceptible due to their small mass. As electrons escape, the net charge of the ionosphere leans positive, causing the ejection of positive ions, mostly H⁺.

Conclusion

While Earth and Venus formed similarly, Earth got lucky. It has pathways to remove gases from the atmosphere, while Venus does not. Additionally, the two planets do not experience significantly different rates of atmospheric escape. This results in the atmospheric densities where know today: 66 kg/m³ for Venus and just 1.2 kg/m³ for Earth.

Answer 2

I think dpwilson's answer is excellent and I voted him up, but I wanted to post this chart with the old picture is worth a thousand words point of view.

Venus is somewhat lighter then Earth, yet has a much thicker atmosphere. One would imagine that the following should be true:

During the formation phase, all inner planets had captured as much gas as they could hold per gravitational/thermodynamic equilibrium. After all, even puny Mars managed to capture a sizable atmosphere.

Maybe. But during the early solar system, once the sun has formed and starts pumping out light and solar flares, (and the early sun was likely much more active in shooting out solar flares in part due to a faster rotation), a key factor to consider is the Frost Line - which is well past earth.

So, 1 of 2 things can happen in the early formation of the Solar-system. One, the planets form and collect available ices and gas they can before the sun starts melting/pushing away any ice and gas inside the frost line, or 2, the sun forms first and the inner planets have very little gas and water as they form. They do get bombarded by hydrogen ejected from the sun, but mostly the inner planets aren't good at holding onto this hydrogen. In the 2nd scenario, any atmosphere and water they get would have to come from comet impacts.

The early atmosphere of the inner planets were mostly CO2, CH4, NH3 maybe some N2. If Venus was struck by a couple extra comets, that alone would explain it and it's not statistically unreasonable. Now, I'm not saying this is what happened, only that it's possible. Venus retains most of it's CO2, but it could lose over time most of it's H20, CH3, NH3, maybe N2 if it was present, leading to the mostly CO2 atmosphere it has today.

It's also theoretically possible that the giant impact that formed the moon also blew away a lot of Earth's early atmosphere. (not certain of that but the enormous addition of heat and rotation, it's possible).

In the chart above, it suggests Venus won't lose much H20, but other charts have Venus closer to the H20 line. (google gas escape velocity planets for more charts)

The rate of atmospheric escape should be much higher for Venus: Venus receives more heat from the sun, thus higher Jeans escape rate Venus has negligible magnetic field, so some of its atmosphere must be lost to direct "blow-off" by solar wind

This is true. It might well explain why Venus has so little water which is common in the solar-system. But on your last point, Venus has an induced magnetic field - see here. dpwilson explained this in more detail.

Yet, it is Earth that seemingly misses a great deal of atmospheric volume. So the question is: what are the current theories regarding the "thinning" of the Earthian atmosphere? When and why the atmospheric gases had left the planet?

I gather there's still come uncertainty on precisely what Earth's atmosphere was like billions of years ago. It might well have started out with an even denser atmosphere than Venus currently has, but it's hard to know with any certainty (at least, nothing I've read suggests certainty on the subject).

It's worth pointing out that coal, oil and natural gas don't form naturally, but they are the product of dead plants and sea life buried over hundreds of millions of years. Also, many of the rocks we see all around us have Oxygen in them. Granite has Oxygen, for example. (There's no, or at least, very little Granite on Venus). The absorption of atmosphere by life on earth and by Oxygen binding to surface and dissolved ocean minerals likely played an enormous role in thinning out Earth's atmosphere. Life on earth, all by itself, could be enough explain the difference in Earth's to Venus' atmosphere.

Comets:

1) Comets used to be larger. Every pass near the sun, comets shrink. Besides, it's not just comets it's icey moon like objects and asteroids, which, as Jupiter migrated and the late heavy bombardment took place, some of those could have been quite large.

See: Here and Here and Here.

Besides, I didn't say it as definitive, I said it was possible a large part of Venus' atmosphere came from a large comet strike.

Answer 3

It so appears, there exist surprisingly little solid theories as to why Earth and Mars managed to lose the majority of their atmospheric gases, while Venus had managed to retain a most magnificent atmosphere.

One plausible theory was put forward by a prominent chemist Octave Levenspiel et alii, based on the old Soviet research into Earthian crust composition conducted in the 1950s (I could not find any substantial updates to the crust composition model developed since that time).

At a glance, the theory works as following:

1. Earth had formed with an atmosphere similar or denser than Venusian. Its most abundant ingredient should have been CO2 (analogously to Venus and Mars).
2. Earthian atmosphere managed to cool down enough for water to start condensing into liquid phase. The exact mechanism for this is not quite clear to me (Goldilocks zone notwithstanding) because hot and dense CO2/H2O atmosphere should have caused a prominent "green house" effect, preventing the planetary surface from cooling down (unless "green house" effect models are too exaggerated).
3. Atmospheric CO2 started to dissolve in liquid water (this alone would account for about 50% reduction in partial CO2 pressure). Strongly acidic water started to erode calcium from the crust, kick-starting the limestone formation process.
4. Emerging life had accelerated the process, sequestering the remaining atmospheric CO2 into the gigantic limestone and somewhat smaller coal deposits.

The detailed outline of the theory can be seen here: http://pubs.acs.org/subscribe/archive/ci/30/i12/html/12learn.html

I hoped that some answer's here can suggest plausible alternative theories. In particular:

1. "Plate tectonics" have probably nothing to do with the composition and parameters of the present day atmosphere. As far as I know nobody had ever suggested that mantle can re-absorb gases from the atmosphere - on a contrary, gases released from a cooling mantle through a volcanic activity should contribute to a more denser atmosphere (clearly, this process never helped Mars and is not helping Earth that much either). Volcanic gases are mostly composed of CO2 and water vapor (up to 90% by mass), while those two substances are hardly present in the modern day atmosphere (CO2 - ~350ppm, vapor - 0.4%, mostly from evaporation unrelated to volcanic recycling).
2. Comets are relatively lite objects (a decent comet weighs 10000-100000 times less that even a thinnish, present day Earthian atmosphere) with low density. High kinetic energy impact of a comet with a planet will most probably result in escape of the majority of comet contained gases back into space (and impact heating will also add some of the planetary gases to the escaping mix - a process known as "impact erosion"). It is long believed that no substantial matter transfer is possible between comets and planets (http://adsabs.harvard.edu/full/1998ASPC..148..364Z).
3. "Early thin atmosphere" - atmosphere being eroded or lost shortly after Earth (Moon) formation is not plausible for the obvious reason: where does the limestone/coal comes from? If Earthian atmosphere was lost and then replenished through tectonic activity this brings us to back to an original question.