9. Hell, Heaven, and Earth

Part 3 - Earth: The Blue Planet

Evolving Planets

Because the Earth is our home we think of the Earth as being special, and it is. Nevertheless to understand the Earth we need to take the perspective of the Earth as being just another planet and then be objective in applying the laws of physics and chemistry to learn what is going on. When we do this we see why it is logical for Earth to be the terrestrial planet with the thickest atmosphere: Earth is the largest terrestrial planet with the greatest amount of tidal heating pumping volcanic gasses onto its surface and so it should be the terrestrial planet with the thickest atmosphere. Once this is understood we can then move on to the more interesting puzzle of how the Earth transition from having an extremely thick atmosphere to its relatively thin and unique atmosphere that we currently enjoy.

Shrinking planet

The planets and moons of our solar system started evolving almost as soon as they formed, about 4.6 billion years ago. Once the interior of each planet or moon heated up, the lighter elements and compounds within each planet or moon's interior broke free and migrated to the surface where most of these light substances escaped out to space. As these lighter materials left each of these planets or moons, the remaining sphere shrunk in size. Like a toy balloon with a small leak causing it to shrink over time, the planets and moons of our solar system have likewise been shrinking over time. Some planets and moons are still shrinking and evolving, some are not. The Earth’s moon is an example of a heavenly object that is geologically dead, while the Earth is an example of a planet that is still very much geologically alive.

The next step toward understanding Earth is to think about geological time. Even though most of the planets and moons of our solar system are going through changes, our lives are too short for us to notice these changes. Geological times ticks away on a scale of millions of years, sometimes even billions of years. So to see these changes we need to use our imagination. Recognizing that the Earth is evolving is possibly the most important conceptual idea required for understanding the Earth.

Mountains and continents move slowly, sometimes no more than a few centimeters per year, and yet over the vastness of time incredible change take place. The vastness of geological time is difficult to comprehend yet the experience of finding ancient seashells on a mountain trail can be helpful in gaining this understanding. The mind strains to imagine the time required to raise these and other marine fossils from the ocean floor to the highest points on the Earth, and yet there is no means of arguing with the physical evidence.

And yet some people still do! Geological evidence often leads us to mind boggling conclusions that may be difficult to accept. And while nothing is wrong with questioning the interpretation of the evidence, in the end we must accept what the evidence is telling us. We cannot turn our backs to the evidence when it gives us an answer that we do not like. Geologists have done a great job of helping people understand the astonishing geological evidence and yet even they sometimes have trouble seeing the truth. The Earth is billions of years old, hurling through space, and slowly shrinking in size, and yet this is only the beginning of this wild ride.

The Source of the Earth's Water

At geological active locations around the world there are numerous geysers and thermal springs where hot mineral water either shoots out of the ground or bubbles up from the Earth's interior. From all appearances it seems as if this hot mineral water is coming from the depths of the Earth: simply flowing one way from inside the Earth to being released on the surface of the Earth. Yet how can that be? How can water, or any fluid, forever flow from the inside to the outside of a container. It is not possible, and so some geologists explain the phenomena by telling us that this hot mineral water is recycled water that originally came from the surface. However this explanation is wrong. The recycling water belief is a false belief that does not hold up to the evidence. The truth is that the hot spring mineral water is simply making a one way trip from the Earth's interior to its surface, just as it appears. The emerging hot mineral water is water that is finally reaching the surface after being trapped billions of years ago in the Earth's interior when the Earth first formed.

Karst Diagram Karst diagram courtesy of Vancouver Island University.

The now debunked recycling water explanation most likely got its start because of the few similarities between cool water springs and hot mineral springs. However, besides having water gush up from the ground these two types of springs have hardly anything else in common.

Cool water springs show up in karst environments. Karst topography is three dimensional landscapes that are created when water dissolves soluble rocks such as limestone and dolomite to create sinkholes, caverns and cool water springs. In karst environment, water seeps into the ground or more directly just flows into a sinkhole before flowing through underground caverns to later bubble up at a spring at a lower altitude. Geologists have confirmed these facts by releasing special dyes at sinkholes and other suspected entry points and then later detecting those dyes emerging at the cold water springs.

Hot mineral springs, on the other hand, are usually found where the rock layers have been fractured by previous or present geological movement or volcanic activity. As the name implies, water emerging from hot mineral springs can vary in temperature from being mildly warm to scalding and it is typically extremely high in dissolved minerals and chemicals. Also, unlike cool springs, it is just as likely for the hot mineral water to emerge at a relatively high location as it is to flow from a low altitude location. Geologists have tried numerous times to identify surface entry points for the hot mineral springs and they have had no success. Geologists have no evidence to support their claim that the water emerging from hot mineral springs was ever previously on the Earth’s surface.

Hot Spring A Hot Mineral Spring in Yellowstone National Park, Wyoming

Understanding that most of the water and other volcanic fluids emerging at the surface are simply completing a one way trip from the Earth’s interior to the surface is an important conceptual breakthrough. Yet for some people this idea may still seem counterintuitive: how can a fluid be forever flowing out of a vessel? The answer is that it cannot, it is only because of our human prospective of time that it seems that way. The flow of water is actually diminishing over time, yet we fail to recognize this truth because our lives are so short compared to geological time. The illusion of never ending fluid flow is possible because the Earth is a rather large vessel and the amount of volcanic water leaving its interior is rather small in comparison. Furthermore, the Earth is not fixed in size but rather over the pass several billion years it has shrunk considerably as volcanic fluids have escaped from its interior. Nevertheless the reality is that this water and the other volcanic fluids cannot flow forever. Eventually, somewhere between hundreds of millions and billions of years from now, all of these hot mineral springs will run dry. When that happens, without water, the Earth will become just as lifeless as the Moon: so let’s enjoy the good times while we can.

While this case study of the emerging hot mineral spring water is most helpful in clarifying the fact that fluids are continuously flowing out of the Earth's interior, these thermal spring waters are actually only a small portion of the volcanic fluids flowing out of the Earth. The Earth is actually much more geologically active than what most people realize.

Out of Sight Volcanism

Volcano Volcano

Astronomers claim that Io - Jupiter’s nearest moon – is the most geologically active body in our solar system. But is it really? It is a close call, but the most geologically active body in our solar system may be right underneath our feet.

When most people think of volcanoes they think of the volcanoes that they can see, the volcanoes above sea level. Of these volcanoes our attention is quickly drawn to the volcanoes that explode in violent eruptions. But like a magician’s trick, while our focus is on these exciting volcanoes the real action is quietly taking place out of sight in the depths of the oceans.

Broadly speaking, there are two types of volcanoes: those that are expelling new material and those that are recycling old material. At a convergent plate boundary, an oceanic plate will dive under a continental plate. In this process it is believed that low density material is caught between the plates, that then melts and bubbles up through the continental plate. This recycled material is sticky because it is high in silicate. So when it finally breaks through to the surface it tends to produce violent eruptions. The other type of volcanoes are at divergent plate boundaries and hot spots such as the Hawaii Islands. From these outlets pour low silicate magma that easily spreads itself over a large area. Most of these volcanoes are found under the oceans and they release far more volcanic gases to the surface than the exciting volcanoes on the surface. So it is the underwater volcanoes that we are interested in.

Seafloor Global Map Showing the Ocean Ridges

The Mid-Atlantic Ridge, the Indian Ocean Ridge, and the East Pacific Rise extend around the Earth like the stitches on a baseball. From the breaks running down the middle of these ridges molten rock oozes up to form a huge carpet of basalt that spreads out under the world’s oceans.

The extensive volcanic activity of these mid ocean volcanoes is displayed at the one portion of the ocean ridges that lies above the water: Iceland. To get an idea of how much volcanic activity is taking place below the oceans consider that a full one third of the world’s terrestrial volcanic activity takes place on Iceland and yet Iceland is less than one hundredth of the length of these volcanic ocean ridges. The Earth is far more geologically active than what most people realize, and furthermore the Earth was even more geologically active in its past.

Iceland volcanoes - The nothern tip of the Mid Atlantic Ridge Iceland volcanoes are an extension of the Mid-Atlantic Ridge that has risen above sea level.

Of this volcanic activity there are two components, the lava that forms the solid rock material and the gaseous emissions, and for now it is the gaseous volcanic emissions that we are interested in. For the casual observer, the ocean waters and the Earth’s atmosphere may seem unrelated to the foul smelling volcanic gasses emerging from below, yet nevertheless all of the gasses found in the atmosphere are among the volcanic gasses. The volcanic gases are the source that created the atmospheres of the terrestrial planets of Venus, Earth, and Mars. So we would like to first know what are the components that make up these volcanic gases and then figure out how did these volcanic gases transitioned to become the atmospheres of these planets.

In science it often happens that simple questions can be difficult to answer, and this is one of those cases. There are a number of problems associated with our quest to determine the percentages of the various volcanic gases that have been released on the surfaces of Venus, Earth, and Mars over the last several billion years. Ideally we would like to collect volcanic emission samples from each of the three planets over the last several billion years, but without a time machine that obviously is not going to happen. For the most part all we can do is collect volcanic emission samples from present day volcanoes on Earth and even this task is not so easily accomplished. No matter if we are attempting to gather a sample from a volcanic peak or from the black smoker on the ocean floor, volcanic outlets tend to not be the most readily accessible places. Furthermore, because there is so much variation in both the amounts and the percentages of the components at each volcanic outlet, we need to gather a lot of samples before we can apply our statistical methods to determine the average values. In addition, we need to further refine our data to exclude volcanic sources associated with convergent plate boundaries since the gases emerging from these volcanoes will be mostly recycled material rather than virgin volcanic emissions. The upshot of all of this is that we can only roughly estimate the percentage of the world wide volcanic gas composition. From most to least we have roughly 65% water vapor, 25% carbon dioxide, and the remaining 10% of gases including sulfur compounds, hydrogen, carbon monoxide, hydrogen chloride, nitrogen, fluorine, argon and numerous other gases.

Composition of Volcanic gases

So we now have a rough idea of volcanic gases currently being released on the Earth, but how does these values compare to the volcanic gases that have been released over the past four and half billion years of the Earth's existence? Furthermore how do the volcanic gases released from Earth compare to the volcanic gases released on other planets such as Venus and Mars?

One major difference between the volcanic emissions billions of years ago and the present volcanic emissions is that the earlier emissions probably contained a much greater percentage of hydrogen and helium. This is because light elements would be the first to break free and the first to migrate the surface. Nevertheless these light elements had little effect on the forming atmospheres of the terrestrial planets since these lighter hydrogen and helium elements were lost out to space.

Next we need to consider how the volcanic emissions released on Earth should compare to the volcanic emissions on the other planets over the last several billion years. Here is where it is important that we recognize how much these planets have in common rather than be distracted by their difference. If we agree that the planets all formed out of essentially the same material, that was originally swirling around the Sun, then it is logical that they should all have more or less the same gaseous material emerging from their interior. This is especially true for planets that are close to each other on adjacent orbits such as Venus and Earth whereas this idea may not be valid for planets such as Venus and Neptune that are far away from each other.

Yet there is more to becoming a planet's atmosphere then just being released onto its surface, and in fact once a gas molecule is release on the surface there are a few directions that it can go. A gas molecule can either escape out to space, become part of the planet or moon's atmosphere, or chemically bond with minerals on the surface. Which direction a volcanic gas takes is not optional but rather it is determined by either its mass or its chemical bonding properties.

To understand how the Earth and the other planets developed their present atmospheres we need to investigate what determines which pathway the various gas components will take. Let us begin with an investigation of what determines if a planet or moon can hold on to its atmosphere.

Holding on to an Atmosphere

The planets of our solar system Solar System

Explaining Escape Velocity

The saying goes "What goes up must come down". But actually, it is possibly for this expression to not be true. If an object is projected upward with a great enough speed, and air is not in it's way, then it is possible for an object to escape out to space instead of returning to the surface of a planet or moon.

For example: a person can throw a baseball straight up into the air with the expectation that it will come back down so that they can catch it. Likewise a bullet shot up into the air will also come back down because the Earth's gravitational pull will pull it back down. We would get closer to breaking the rule if we shot our bullet from the surface of the Moon where the gravity is only one sixth that of the Earth and there is no air to impede our bullet. Yet even this bullet would still come back down, because the highest muzzle velocity rifles shoot bullets with speeds that are still only about half the escape velocity of the Moon. For all practical purposes, tiny air molecules are the only objects that can travel fast enough to escape from a planet or moon.

Air molecules typically travel in all directions and they can travel faster than the muzzle velocities of rifles and exceed the escape velocity of most moons. Consequently, if there were any air molecules on the Moon it would only be a short time before they would escape out to space. Hence, the Moon does not have an atmosphere

In contrast, most air molecules traveling away from the Earth's surface will not get very far before being blocked by other air molecules. Typically they will travel only about a centimeter before running into other air molecules that are in their way. The only chance they have of escaping the Earth is if they are a light molecule such as hydrogen or helium and they find themselves in the upper atmosphere where the air is extremely thin. In this case, if the light hydrogen or helium molecules have enough speed and they do not run into other molecules, then it is possible for them to leave the Earth and never come back.

The planets and moons of our solar system can be placed in three groups according to their atmospheres or lack of atmospheres: planets or moons that do not have an atmosphere, planets whose atmospheres are so thick that they appear to be entirely made up of gas, and between these extremes are the planets or moons with moderately thick atmospheres containing only the heavier gases.

Mercury and most of the moons make up the first category of objects that for all practical purposes do not have an atmosphere. Because these objects do not have atmospheres they are open to meteorite bombardment. Because of this, Mercury and the moons without atmospheres all have the same similar cratered surface appearance. The second category consist of the outer planets of Jupiter, Saturn, Uranus, and Neptune. These are the huge gaseous outer planets of our solar system. The third category is made up of the terrestrial planets of Venus, Earth, and Mars. We should also note that there are a few large moons, such as Saturn's moon Titan, that have real atmospheres consisting of the heavier gases. These moons with atmospheres actually belong in their own special category.

To determine whether a planet or moon can hold on to an atmosphere, and if so what kind of molecules might be a part of the atmosphere, we make a comparison between the escape speed of a planet or moon and the speed of a gas molecule. The escape speed of a planet or moon is calculated with the equation

ves = √((2 G M) / r)

where G is the gravitational constant 6.67 x 10-11 N-m2/kg2, M is the mass of either the planet or moon in kilograms, and r is the radius of the planet or moon in meters.

The speed of a gas molecule is determine by the equation

vrms = √((3 k T) / m)

Where vrms is the root mean square speed of the gas molecule, k is the Boltzmann's constant equal to 1.38 x 10-23 J / K, T is the temperature given in Kelvin, and m is the mass of the gas molecule given in kilograms.

Additional information is needed to understand these equations.

If we have a container filled with a gas then the gas molecules in that container will be bouncing off the sides of the container as well as each other as they travel with various speeds in every direction. To describe the typical speeds of these gas molecules we can find the values for the average speed, the most probable speed, and the vrms root mean square speed for the gas molecules of a certain gas at a given temperature. Furthermore, while we could use any one of these three values scientists prefer to use the vrms root mean square speed because this is the value derive based on the kinetic theory of gasses.

To solve for the vrms root mean square speeds we need both the molecular weight of the gas molecules and the temperature of the gas. The molecular weight of the gas molecules is easy enough to look up, we will find that the heavier gases travel slower than the lighter gases. Determining what temperature values to use is much more of a problem. While the table in chapter five, The Thick Mesozoic Atmosphere, gives us the average temperature of each planet this really does nothing more than a rough approximate value.

The temperature value that we actually need is the highest temperature that might occur in the upper atmosphere of a planet or moon. This is because for a gas molecule to have a chance of escaping then it needs an unobstructed path out to space, and this is only true in the upper atmosphere where the air is thin. In the lower atmosphere a gas molecule is so crowded by other molecules that any single molecule will travel hardly any distance at all before it ricochets off another gas molecule. For a gas molecule to escape the air needs to initially be so high in the upper atmosphere and the air so thin that even the concept of temperature begins to no longer makes sense. Rather than try to guess at what actual temperatures might exist in the upper atmospheres of the planets the author simply used the actual average values listed in the table.

Solar System Escape Velocity VS Surface Temperature Planets in the upper left corner of the graph can hold on to all of the gasses. Mercury and the moons in the lower right corner cannot hold on to an atmosphere, while the planets and moons between these extremes can hold on to only the heavier gasses.

Another problem is that the application of our equations have led us into confusing logic trap: our goal is to determine if a planet or moon is capable of holding onto an atmosphere and yet to apply one of our test equations we need to first know if a planet or moon has an atmosphere. If want to be rigorous in avoiding this logic trap we could start out by assuming a planet has no atmosphere before testing it to see if it could hold on to the lightest molecules: hydrogen.

A gas molecule's root mean square speed does not need to be greater than the escape speed of a planet, for the planet to lose the gas. Actually the gas molecule's root mean square speed only needs to be only about one sixth of the escape speed for a molecule to escape from a planet. This is because if ever once out of the billions upon billions of collisions a molecule finds itself flying away from the planet or moon with a an exceptionally high speed exceeding the escape speed then that is it, the molecule will fly out to space and never come back. At any given time only an extremely small percentage of gas molecules will be traveling this fast, yet nevertheless over the millions and billions of years of the planet or moon's existence that is all that is needed for gas molecules to steadily slip away from the planet / moon.

When we make these calculations and comparisons we see that our equations come close to matching our observations of the atmospheres in our solar system, yet there are a few discrepancies. The terrestrial planets should be holding on to helium but they are not, Jupiter's major moons should be holding on to the heavier gas molecules but they are not, and Saturn's moon Titan holds on to some gas molecules that according to the calculations it should not. Clearly there is more going on that needs to be investigated, but for our goals at this point we need to call this and move on.

Determining if a Planet or Moon can hold on to an Atmosphere

Planet or Moon Escape Speed
Hydrogen RMS Speed times 6 Helium RMS Speed times 6 Nitrogen RMS Speed times 6 Type of Actual Atmosphere
Mercury 4.2 16.6 8.3 4.46 None
Venus 11.4 18.36 9.18 4.90 No Hydrogen
Earth 11.2 13.2 6.60 3.53 No Hydrogen
Mars 5.0 9.68 4.84 2.59 No Hydrogen
Jupiter 59.5 7.40 3.70 1.98 Gas Planet
Saturn 35.5 6.44 3.22 1.72 Gas Planet
Uranus 21.3 5.30 2.65 1.42 Gas Planet
Neptune 23.5 4.86 2.43 1.30 Gas Planet
Earth's Moon 2.38 13.2 6.60 3.53 None
Jupiter's Io 2.56 7.40 3.70 1.98 None
Jupiter's Europa 2.02 7.40 3.70 1.98 None
J's Ganymede 2.74 7.40 3.70 1.98 None
Jupiter's Callisto 2.44 7.40 3.70 1.98 None
Saturn's Titus 2.64 6.44 3.22 1.72 No Hydrogen

We find that there are three factors that will quickly determine what type of atmosphere we should expect for a planet or moon: the size or more accurately the mass of the planet or moon, the planet or moon’s distance from the Sun, and the mass of the gas molecules that make up the atmosphere.

Atmospheres of the Gas Planets

Planet% Hydrogen % Helium % Other
Jupiter 86.1 13.8 0.1
Saturn 92.4 7.4 0.2
Uranus 84 14 2.0
Neptune 84 14 2.0

The mass of the heavenly body is important because for the most part it is the dominate factor in determining a planet or moon's escape speed. The planet or moon’s distance from the Sun is important because of its relationship to temperature which in turn is an important factor for determining a gas molecule's root mean square speed. Thus with all other factors being equal, it is easier for the planets and moons that are far from the Sun to hold on to their atmospheres than it is for the planets and moons that are close to the Sun. We see how this plays out when we consider the mass of the gas molecule. The planets furthest from the Sun - Neptune, Uranus, Saturn, and Jupiter - are able to hold on to all of their gas molecules including the light gas molecules of hydrogen and helium. The terrestrial planets closer in - Venus, Earth, and Mars - are able to hold on to only the heavier gas molecules, while the planet closes to the Sun - Mercury - is not able to hold on to any atmosphere.

Atmospheres of the Terrestrial Planets

PlanetCarbon Dioxide Nitrogen Oxygen Argon
Venus 96.5 3.5 0 trace
Earth 0.03 78 21 1.0
Mars 95.3 2.7 0 1.6

As explained in the previous chapter, the terrestrial planets all formed out of essentially the same material. The volcanic gasses all came from this same source material and so the volcanic gases released on these planets are essentially the same, and of these gases these planets could only hold on to the heaviest gas molecules. This explains why the terrestrial planets initial had essentially the same atmospheres. Today Venus and Mars still have their carbon dioxide atmosphere while the Earth's atmosphere has gone through further developments to arrive at an atmosphere that is unique among the planets and moons of our solar system. Instead of the Earth having a thick carbon dioxide atmosphere it now has a relatively thin atmosphere that consist of mostly nitrogen and oxygen.

Yes indeed, Earth is an extremely unique planet, and furthermore if not for the Earth being so unique, human beings and most of the other present biological life on this planet would not exist. So with a curious mind, we should be asking ourselves how did the Earth transition from having an extremely thick carbon dioxide atmosphere to the present relatively thin nitrogen and oxygen atmosphere that we so much enjoy.

Venus, Earth, and Mars
Photodissociation: Where is the Water?

Venus, Earth, and Mars Mars, Earth, and Venus

Before we even get to the question of why is the Earth's atmosphere so different from the other terrestrial planets we need to first answer the question of where is all the water that should be covering these planets.

Water and carbon dioxide are the two most abundant volcanic gasses, so it is understandable that there is so much carbon dioxide filling up the atmospheres of Venus and Mars, but what happened to all of the water? With water vapor making up roughly 65% of the volcanic gas, our initial expectation would be that Mars should be covered with a layer of frozen water ice, Earth should have so much water covering its surface that all rock material would be hundreds of miles below the surface, and Venus, because it is so hot, would have an atmosphere consisting mostly of hot water vapor thus creating the ultimate steam bath atmosphere. Where did all of this water go?

The reason these planets lost most if not all of their volcanic water is because ultraviolet radiation was breaking apart the water vapor molecules almost as quickly as they emerged on the surface.

Most of us are aware that we should use sunscreen and wear a hat and sunglasses if we are exposed to long hours of sunlight. This is because a small percentage of the Sun’s rays reaching the Earth’s surface include ultraviolet radiation. This UV radiation can break DNA bonds and in doing so it can cause skin cancer and other mutations. What most people don’t know is that UV radiation can break other chemical bonds as well; of importance to this discussion is that UV radiation can break the bonds of water molecules.

The chemical bonds of the water molecule are broken when the molecule is hit by a photon that has the correct amount of energy for breaking the bond.

On Venus, Mars, and the early Earth the Sun’s UV radiation was separating the water molecules into its components of oxygen and hydrogen almost as quickly as the water was emerging onto the surface. Once separated, the two light and energetic hydrogen atoms were soon lost out to space, while the highly reactive oxygen atom went the other way. The free oxygen atoms quickly oxidize with metallic minerals on the surface.

When oxygen chemically bonds with iron it turns it to a reddish orange color that most people refer to as rust. Besides rusting occurring to unprotected steel parts of a car oxidation will also turn any exposed iron mineral grains to the same reddish orange color and iron is quite abundant on the terrestrial planets. It is because of these oxidized iron minerals, also known as hematite, that the nickname for Mars is the Red Planet. Yet in regards to oxidation we could refer to all three of the terrestrial planets as being red planets. The surface of Venus is red but we do not see Venus’s red surface because our view is blocked by its thick white clouds. Likewise much of Earth’s soil and rock is also red. But because three fourths of the Earth’s surface is covered by water and most of the visual land is covered by vegetation, the view of Earth from space is mostly blue along with green for the vegetation and white due to the clouds. Greater evidence of this oxidation on Earth can be seen in the lopsided abundance of oxygen in the Earth’s crust. Oxygen is the most abundant chemical element in the Earth’s crust: 46.6 % by weight and 93.8 % by volume.

With so much water being destroyed by UV radiation our question has flipped flopped. Instead of wondering why there is no water on Venus or Mars we should wonder how it is that the Earth was able to hold on to some of its water. The answer to that question is life.

Life evolved on Earth. With Earth being larger than Venus and having the Moon’s tidal forces heating its interior, far more water and other volcanic gasses were being exhausted onto the Earth’s surface than that of Venus. Furthermore because Earth is farther from the Sun it receives less UV radiation and so less water was being destroyed than what was occurring on Venus. Even more important, Earth’s distance from the Sun keeps Earth’s surface at the right temperature so that water can exist on Earth in its liquid state. All of these factors combined to allowed enough water to accumulate on the Earth so that simple blue-green algae could evolve. Through the process of photosynthesis the blue-green algae produced diatomic oxygen.

Temperature Profiles of Venus, Earth, and Mars Plot of the atmospheric temperatures of Mars, Earth, and Venus as a function of altitude.

Diatomic oxygen O2 is important since it and ozone O3 effectively shield the lower atmosphere from UV radiation. This is because oxygen is like water vapor in that it is easily broken apart by the UV radiation. First the diatomic oxygen is broken apart by UV radiation at around 240 nm to create two free oxygen atoms that then join intact diatomic molecules to form two ozone molecules. The ozone molecules can then be broken apart by UV radiation at around 290 nm. Later the broken apart ozone molecules can reform with other free oxygen atoms. Because the UV radiation is being absorbed in the upper atmosphere this potentially harmful radiation is blocked from reaching the surface. In addition, the Earth’s upper atmosphere known as the stratosphere is warmed considerably as it absorbs the energy of the UV radiation coming from the Sun. In the plot of the atmospheric temperature profiles of Venus, Earth, and Mars notice how Earth’s plot budges to the right as a result of the ozone absorbing the energy of the UV radiation.

Diatomic oxygen O2 is important since it and ozone O3 effectively shield the lower atmosphere from UV radiation. This is because oxygen is like water vapor in that it is easily broken apart by the UV radiation. Because the UV radiation is being absorbed in the upper atmosphere this potentially harmful radiation is blocked from reaching the surface. In the plot of the atmospheric temperature profiles of Venus, Earth, and Mars notice how Earth’s plot budges to the right as a result of the ozone absorbing the energy of the UV radiation.

Diagram of how the ozone layer blocks UV radiation Once Earth obtained an ozone layer its water was protected from being destroyed by UV radiation. Consequently Earth went on to become the Blue Planet while UV radiation continued to rob Venus and Mars of water and the likelihood of life evolving on these planets.

It is interesting that the photosynthesizing process utilizes the two most abundant volcanic gases.

6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

The photosynthesizing single cell used the energy of sunlight to combine the two most abundant volcanic gasses - water and carbon dioxide - to create the glucose that it desired. Beside the glucose, the photosynthesizing reaction also creates diatomic oxygen as a byproduct. This oxygen had a dramatic effect on the further evolution of life on Earth. One of the most important effects of the diatomic oxygen was the creation of the ozone layer in the Earth's atmosphere and the ozone layer is extremely effective at blocking UV radiation. The blocking of the Sun's UV radiation both greatly reduced the further destruction of water molecules and allowed the first multicellular organisms to evolve.

What happen to the Carbon Dioxide?
Carbonate Rock

In the beginning, Earth was headed in the same direction as Venus, only more so. Earth's carbon dioxide atmosphere was even thicker than Venus' and on Earth water was accumulating on the surface. As explained in previous chapters, Earth was exhausting more volcanic gases because it is larger and - because of the presence of the Moon - it was experiencing much greater tidal heating. So unlike Venus and Mars, on Earth volcanic water was arriving on the surface at a faster rate than it was being destroyed by photodissociation. This allowed water to accumulate on the surface.

This water was the key difference. From water came life and from life came almost everything else: oxygen, ozone layer, and the removal of carbon dioxide from the atmosphere. Yet it still took billions of years for these changes to take effect. At first, whatever oxygen was produce quickly bonded with iron and other minerals and compounds on the Earth's surface. This caused atmospheric oxygen levels to remain low until finally all the exposed iron was oxidized. Once saturation was complete, there was oxygen in the atmosphere and this atmospheric oxygen enabled the ozone layer to form. The ozone layer is extremely important to life on Earth because the ozone layer shields the Earth from the Sun's harmful ultraviolet radiation. With Earth's ozone layer in place life was able to evolve up to multicellular organisms. It was the multicellular organisms that were most effective in facilitating the removal of carbon dioxide from the Earth's atmosphere.

White cliffs of Dover - Carbonated Rock White Cliffs of Dover

Over 20% of all the sedimentary rock on the Earth’s surface is carbonated rock: mostly limestone and dolomite. This is an incredible amount of rock material. The primary ingredient for carbonated rock is carbon dioxide, the carbon dioxide that once filled the Earth's atmosphere.

So how does atmospheric carbon dioxide become locked up in the carbonated rock? While it is possible for limestone to form as an evaporate, the most common way that limestone forms is from the accumulation of shells and corals of marine animals and algal debris. Atmospheric carbon dioxide first dissolved into the water and then one form of marine life or another utilized the carbon dioxide as part of its biological process before leaving it behind as solid sediment. Most often, marine animals construct their shells out of calcium and carbon dioxide and then when the animals dies their seashells collect on the shallow seafloor and eventually solidify as limestone.

The most simplified chemical process showing how carbon dioxide first dissolves into the water and then later solidifies as carbonated rock is shown below:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-1

HCO3-1 ↔ H+ + CO3-2

From here the carbonate will either combine with calcium to form limestone

CO3-2 + Ca+2 ↔ CaCO3

or the carbonate will combine with calcium and magnesium to form dolomite.

2 CO3-2 + Ca+2 + Mg+2 ↔ CaMg(CO3)2

Likewise, life is most likely involved in the creation of dolomite, yet there is still considerable debate among geologists on how dolomite forms. Known as the dolomite problem, dolomite does not form in Earth's present environment and geologists have struggled in their efforts to get dolomite to solidify in the laboratory. Some geologists believe that dolomite is recrystallized limestone while another group of geologists have shown that bacteria can directly facilitate the precipitation of dolomite rock. Unlike limestone, dolomite rarely if ever contain fossils.

Dolomite of the Alps Limestone On the left is a dolomite formation from the Alps while on the right limestone forms a bridge that juts out from a side of the Grand Canyon. Limestone is so abundant that is found almost anywhere there are several layers of sedimentary rock.

The oldest limestone and dolomite deposits are over three billion years old. Thick sedimentary layers of either limestone or dolomite can appear independently, one stacked on top of the other, or even as a series of alternating bands. Dolomite is slightly more abundant among rocks that are over a half a billion years old while limestone is typically much more abundant once the fossil creating multicellular animals evolved.

During all the time that carbon dioxide is being removed from the atmosphere, volcanic activity was adding carbon dioxide. Therefore the change in the thickness of the Earth's carbon dioxide atmosphere depended on the summation of these two competing processes. Not long after the Earth formed it had an extremely thick atmosphere because during this time the Earth was at its peak of volcanic activity while the creation of carbonated rock was still relatively low. But once fossil creating multicellular marine animals evolved the dynamics changed. From this point on, as long as the fossil creating marine animals were thriving, carbon dioxide was removed from the atmosphere faster than it was being replaced by volcanic activity. With the exception of one extreme mass extinction along with a few other minor hiccups, the Earth transitioned from having a thick carbon dioxide and oxygen atmosphere to having its relatively thin nitrogen and oxygen atmosphere that it has today.

In some places we can find impressive displays of carbonate rocks such as the White Cliffs of Dover and the Dolomites of the Alps. Yet most deposits of carbonate rock lie under our feet and out of sight. What lies beneath the ground is mostly hidden to us unless we take a hike in the Grand Canyon or at least take a glance at the sedimentary rock layers as our highway passes through a road cut. Yet geologists know that it is nearly an impossible task to find a set of sedimentary rock layers that do not contain at least one or more thick layers of limestone or dolomite. Massive amounts of limestone and dolomite deposits exist on every continent all over the world.

Earth's Unique Atmosphere

diagram showing change in Earth's atmosphere composition

We look once again at our graphs showing how simplistic it was for the Earth to change from its earlier thick carbon dioxide atmosphere to its present atmosphere and it may appear that our work is done, but not quite! The nitrogen represented by the thin pie slice in the first graph represents about 1/29 of the total amount. In other words, assuming that the amount of nitrogen remained unchanged, then the Earth's previous atmosphere was about twenty nine times thicker than what it is today. While a twenty nine times thicker atmosphere may seem like a lot to some people it still falls far short on several accounts: not enough to explain the amount of carbonated rock, not enough to provide the buoyancy force needed for the evolution of large dinosaurs and pterosaurs, not as thick as the present atmosphere of Venus when Earth's atmosphere should actually be much thicker.

So where is the problem? The problem is with the assumption that nitrogen value should remain unchanged. It is not unchanged because plants utilize nitrogen. Nitrogen is a component of chlorophyll and therefore essential for photosynthesis, so most plants can't live without it. Much of the nitrogen that was once in the air is now locked up in either living or dead organic material. But how much? To account for the evidence listed above plants must have removed roughly 98% of the original nitrogen.

Our last major question about the Earth's atmosphere, at least for now, is why is oxygen never more than the secondary atmospheric gas? It appears that the concentration of oxygen in the atmosphere is limited by the tendency of substances to oxidize more readily at high oxygen concentrations. Many substances will burn if the oxygen level is higher than the usual present day oxygen level of 21%, but they will not burn if the oxygen concentration is lower. For example during the Apollo one tragedy the reason the spacecraft was engulfed in intense flames so quickly was because the cabin was filled with pure oxygen. Conversely researchers have shown that it is not possible to start a wildfire if the oxygen level is less than 15%. It appears that the oxygen concentration level is self regulating such that it can never be more than a secondary atmospheric gas. Presently it is second to nitrogen while throughout most of the Paleozoic era it has been secondary to carbon dioxide.

External Links / References

Geological Time

Hot and Cold Springs

Volcanoes and Plate Tectonics

Volcanic Gases

Comparing the Outer Gas Giants and the Inner Terrestrial Planets

Planets holding on to an Atmosphere

Ultraviolet Radiation and Photodissociation of Water

How the Earth's Ozone Layer Blocks UV Radiation

Rusty Planets

The Reduction of Earth's Atmosphere and Creation of Carbonated Rock