EVOLUTION OF THE ATMOSPHERES OF THE TERRESTRIAL PLANETS

We consider:

I. ATMOSPHERIC EVOLUTION: EARTH, MARS, AND VENUS
II. FREE OXYGEN IN EARTH'S ATMOSPHERE

We first address the atmospheric evolution of the Earth. We then address the question of why the atmospheres of Mars and Venus are so different. We finally look at the important question of the evolution of oxygen in the atmosphere of the Earth.

I. ATMOSPHERIC EVOLUTION: EARTH, MARS, VENUS

Terrestrial planets (the atmosphere ones) are roughly the same sizes and same distances from the Sun and yet, they have grossly different kinds of atmospheres and conditions on their surfaces. Do we have any ideas as to what leads to the huge differences? Surprisingly, there may be simple explanations.
In the beginning, we believe that the material which was outgassed from the interiors or carried in by comets or asteroids onto the Terrestrial planets was similar. That is, the Terrestrial planets started out roughly the same. Originally what they were composed of depends a little on their origins, but they were likely dominated by water and carbon dioxide, with varying amounts of sulfur dioxide, carbon monoxide, nitrogen, and some other molecules and elements mixed in. On each of Venus, Earth and Mars, liquid oceans likely formed initially. On the Earth, oceans formed in the Early Archean period (the time before 2.5 billion years ago perhaps as long ago as 4 billion years). Despite all having started with oceans only the Earth has retained extensive oceans.

What caused the differences in the evolution of the atmospheres of the Terrestrial planets and liquid oceans as shown above?

On Venus, Earth and Mars,

carbon dioxide initially dissolved into the oceans, was rained out of the atmosphere forming carbonic acid which interacted with the surface silicate rocks producing calcium and other ions which were washed into the oceans, or was directly adsorded into the rocks and washed into the oceans. (The first two processes are more efficient at higher temperatures. weathering, the second process, is referred to as silicate weathering and plays a key role in regulating our Greenhouse effect.)

Carbon dioxide deposited into the oceans in this manner, settled and formed sedimentary rocks ===> carbon dioxide was trapped in the crust!

This happened fairly quickly on Earth

After this initial start-up, the evolutionary paths of Venus, Earth, and Mars then diverged.

Earth: Recall that if the atmospheres of the early Terrestrial planets had no Greenhouse effect, which would happen if they lost all of their carbon dioxide, they would not have liquid oceans. In order for the Earth to maintain its Greenhouse effect, there must be a way for the Earth to return the carbon dioxide taken from its atmosphere and trapped in its crust. On the Earth, this happens because we have plate tectonics. Carbon dioxide is liberated from the crust and returned to the atmosphere through the volcanoes that form near subduction zones. Without plate tectonics we would not be able to maintain our temperate climate.
What happens as the Sun continues to get brighter as it ages? This is an interesting question as the Earth has a thermostat which can control its temperature. As the Sun's brightness increases, we expect that the Earth's temperature also rises. This is indeed true, but something interesting happens. As the temperature goes up, the silicate weathering process increases and the biomass increases both tending to remove more carbon dioxide from the atmosphere and so to weaken the Greenhouse effect and keep the Earth temperate. This control works until all of the carbon dioxide is extracted from the atmosphere. At this point, there is nothing to control the temperature, and so it will continue to rise eventually causing the oceans to start to evaporate. The evaporation leads to more water vapor which leads to an enhanced Greenhouse effect which leads to a higher temperature which leads to more evaporation which leads to more water vapor and so on. This positive feedback loop leads to a Runaway Greenhouse Effect which will cause the Earth to lose its oceans. The Runaway Greenhouse Effect will likely occur in around 2 billion years.

Mars: On Mars, a different path was followed. Mars initially had liquid oceans and a mild, Earth-like climate. However, Mars then lost its atmosphere rather quickly, perhaps over a few hundred million years but certainly within a billion years of its formation. There is evidence that the atmosphere of Mars has not changed for the last 3.7 to 3.8 billion years based on the recent Mars missions. This loss of its atmosphere led to the freezing of its oceans and to the nearly atmosphereless, inhospitable state of the current Mars.

There were several thoughts about what drove the atmospheric evolution on Mars: (1) lack of plate tectonics; (2) photodissociation by Solar ultraviolet radiation; (3) large impacts on the surface of Mars; and (4) interactions with the Solar Wind through, for example, sputtering. The Mars Atmosphere and Volatile Evolution Mission appears to have settled the issue.

MAVEN: The Mars Atmosphere and Volatile Evolution mission studied the interaction of the Solar Wind and the upper atmosphere of Mars. MAVEN showed that the Solar Wind, the stream of electrically charged particles continuously blowing outward from the Sun was a significant (if not dominant) contributor to the Mars's atmospheric loss.

In the upper atmosphere, molecules such as CO₂ and H₂O are broken apart by Solar ultraviolet (UV) radiation, the ions here are easy prey for the Solar Wind particles. The Solar Wind particles strip them off (sputtering) as they blow by at speeds of 400 km-s^-1 (900,000 mph). The sputtering process is efficient enough to have stripped as much as 80 to 90 % of the CO₂ from Mars since its birth.
This happened on Mars because, unlike Earth, Mars lost its magnetic field early in its history and so lost the ability to deflect the incoming Solar Wind. Rather, the Solar Wind's charged particles crashed directly into the upper atmosphere of Mars sputtering off the carbon, hydrogen, and oxygen. Mars lost its magnetic field after a few hundred million years, because Mars is small in size and so develops a cold interior rather quickly making it unable to generate a magnetic field over its lifetime.

MAVEN has observed this process in action. Below are pictures of carbon, oxygen, and hydrogen escaping from Mars.

Venus: On Venus, a third path was followed. The evolution started similarly to the Earth and Mars. This is true even though Venus is closer to the Sun than is the Earth and receives twice the amount of Solar energy than does the Earth. With the higher Solar energy input, the temperature on Venus rose high enough to prevent the formation of ice, but it did not rise high enough to prevent the condensation of atmospheric water vapor into oceans. Shallow oceans formed. Venus was in this happy medium zone. So, as on Earth and Mars, the oceans could start to extract CO₂ from the atmosphere and, as on Earth, store some of ita sCO₂ in its crust. Unlike the Earth, however, as the Sun aged and grew brighter, Venus's oceans being a little warmer than on Earth started to evaporate. This happens only because Venus is slightly closer to the Sun than is the Earth. The difference is not large but it is enough. Venus's oceans start to evaporate as the Sun brightens. Again, what happens is that as the amount of water vapor in Venus's atmosphere rises due to evaporation, the heating due to the Greenhouse Effect increasexd, which leads to more evaporation and a stronger Greenhouse Effect and so on. This positive feedback loop again leads to a Runaway Greenhouse effect which vaporizes Venus's oceans. The water vapor then is slowly lost to space as it rises high into the upper atmosphere where Solar UV radiation breaks it apart and the hydrogen and oxygen atoms escape. The CO₂ does not escape but remains in the atmosphere. The CO₂ did not suffer the same fate as the CO₂ on Mars because of Venus's larger mass and stronger gravitational field. The details of Venus's atmospheric evolution is a topic is current research interest, a problem that is now of more than esoteric interest. Models for the evolution of Venus's atmosphere are similar to those used for the future evolution of the Terrestrial atmosphere.

Data obtained by the Venus Express (ESA) studied the upper atmosphere of Venus. Venus Express detected oxygen, hydrogen, and helium escaping from Venus just as MAVEN had detected carbon, oxygen, and hydrogen escaping from Mars. For Mars this meant CO₂ and water H₂O were lost. For Venus, this meant that H₂O and helium were lost but that CO₂ was not. Apparently because of Venus's larger gravity, the heavier CO₂ does not rise high enough in the atmosphere to be broken down and lost through sputtering.

Summarize a few key points:
Earth: The Earth maintains its mild climate (maintains its Greenhouse effect) because a cycle was set up where volcanism returned carbon dioxide to the atmosphere to balance the removal of carbon dioxide by the oceans, plants, silicate weathering. The cycle has run smoothly because of plate tectonics. This happens because the Earth is large enough to have a hot interior which, by the way, also means that it has a magnetic field which shields us from the Solar Wind. The final comment is that we are not as close to the Sun as Venus, if we were, we would likely have also suffered a Runaway Greenhouse Effect.
Mars: Mars lost its atmosphere in a few hundred million years due to sputtering by the Solar Wind. This happened because Mars did not have a strong enough magnetic field to deflect the Solar Wind particles. This situation likely arose because Mars is small and so cannot maintain a hot interior for long times.
Venus: On Venus, as the Sun aged and grew warmer and brighter, Venus became hot enough for it to start to lose its oceans through evaporation triggering the Runaway Greenhouse effect because of the positive feedback loop described above. This happened on Venus and not on Earth, because Venus is a little closer to the Sun than is the Earth. The small difference is enough to make Venus cross the threshold for evaporation early on in its evolution, leading to the vaporization of its oceans.

Alternatively, Is it possible that Venus (and Mars) never had water? , so that we are answering a question that does not need to be answered? This is not likely as indicated by studies of the hydrogen isotope known as deuterium (D).
Deuterium is a hydrogen atom whose nucleus contains a proton and a neutron. A regular hydrogen atom's nucleus conains only a proton. Deuterium is naturally occuring and on Earth we find that a certain fraction of water molecules contains deuterium.
On Venus, the ratio D/H is around 120 times larger than on Earth. This allows us to estimate the amount of water Venus initially contained. Assume Venus loses its water as described earlier. Deuterium, because it is more massive than hydrogen, moves more slowly and thus less likely to escape a planet's pull of gravity. Calculations suggest that the current deuterium-to-hydrogen (D/H) ratio on Venus means that Venus has lost at least 99.9 % of its original water. Venus had water but lost it.
On Mars, the D/H ratio is around 5 times larger than on Earth, which suggests that Mars has lost at least 75 % of its original water. Mars had water but lost it.

II. WHAT ABOUT THE OXYGEN IN THE EARTH'S ATMOSPHERE?

Today, Earth's atmosphere is ~21 % free oxygen. At birth, it had no free oxygen. This is good, however, because chemical reactions thought to produce amino acids are inhibited by oxygen.
Where did the oxygen come from?
(i) Photochemical dissociation - breakup of water molecules by ultraviolet produced free oxygen at ~ 1-2% levels. At these levels, ozone can form to shield Earth surface from ultraviolet (UV) radiation.
(ii) Photosynthesis - carbon dioxide + water + sunlight ===> organic compounds + oxygen molecules. Produced by cyanobacteria (photosynthetic prokareyotes--blue-algae), and eventually higher plants supplied the rest of oxygen to the atmosphere.
In the Archean period (4 billion years to 2.5 billion years ago), there was little or no free oxygen in the atmosphere (< 0.001 % of the current level of oxygen, PAL). Around a billion years before the end of the Archaen period, photosynthesis started. The little oxygen produced by cyanobacteria was probably consumed by the weathering process. Only after rocks at the surface were sufficiently oxidized could free oxygen remain free in the atmosphere.
Interestingly, during the Archaen period, the day may have been as short as 12.3 h with a year lasting 714 days. During this time, the Moon slowed the Earth's rotation rate increasing the length of the day. It was recently suggested that this may have led to the jump in oxygen signaling the end of the Archean period. With a longer day, cyanobacteria had a longer time for uninterrupted photosynthesis which increased oxygen production.
During the Proterozoic era (2.5 to 0.5 billion years ago), the free oxygen rose to 1 % to 40 % of PAL. Most of the oxygen was released by cyanobacteria, which showed a strong increase in abundance (in the fossil record) about 2.45 billion years ago. The present level of free oxygen probably was achieved around ~400 million years ago which coincided with a 3-fold increase in biodiversity on the Earth.