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8. Hell, Heaven and Earth: Part 2 - Heaven
Esker’s Theory on the Evolution of our Solar System

Stonehenge

The wonder of the distant objects in the sky has always been an uplifting spiritual blend of science and religion. Astronomy is the eternal quest to make sense of everything in the sky that lies beyond the clouds.

The most distinguished feature of the heavenly bodies is their regularity. Every morning the Sun comes up, once a month the Moon goes through its phases, and once a year the patterns of stars cycle through their night-time display. As witness by the numerous early observatories that marked the regularity of these events, one could claim that science and indeed civilization began with astronomy. By first recognizing the orderly events in the sky, mankind has slowly come to realized that we exist in a rational universe.

Sunset

Science is based on the belief that we exist in a rational reality. It follows from this that physical evidence is the base that we should use to support all of our beliefs concerning our universe. Science students would reach a greater understanding of science if this founding principle of science was clearly stated at the beginning of all science courses.

In contrast to science, religion is based on faith and so it does not require physical evidence to support its beliefs. Without the requirement of supporting evidence, popularity becomes the most important criteria for a religion to be successful. Religions achieve their popularity by promoting beliefs that make people feel special.

This chapter shows why the Earth should be, and millions of years ago it was, the terrestrial planet with the thickest atmosphere.


Probability Theory and Destiny

Throughout history mankind has often stumbled in seeing the truth because of his belief in his own importance. Centuries ago Galileo’s work was all the more difficult in winning the acceptance of the heliocentric model of the solar system because most people simple assumed that the Earth should be at the center of the universe.

Today science is again being held back because mankind’s belief in his own importance. Even though most astronomers hold secular beliefs, it is difficult for most people to overcome the popular feeling that there is a destiny to our lives that goes beyond the real experiences of our reality. There is no evidence that supports the concept of destiny.

Dice on a backgammon board

All evidence shows that our reality is firmly based on probability theory. This may not make some people happy, but science is not about making people feel good but rather it is about determining the truth concerning our reality. While the truth may at first upset us, once we accept the scientific facts we are better able to understand and successfully interact with our reality. This objectivity is necessary in understanding our solar system.

This difficulty that people have in abandoning the incorrect notion of destiny is seen in the fondness that many religious scientists have for the Einstein quote “God does not play dice”. The implication is that there is a destiny to the workings of the universe guided by a supreme being. But while this may have been a personal religious belief of Einstein, it is not a scientific statement. Probability theory is fundamental to understanding numerous science disciplines such as the second law of thermodynamics, quantum physics, and the Theory of Evolution to mention only a few. From radioactivity, chemical reactions, biological interactions, or even lighting strikes, our whole reality is based on probability theory.

radio telescope

By applying probability theory rather than believing in destiny we empower ourselves to take responsibility and make wise decisions. A person who believes in destiny might drive home while intoxicated on the assumption that they will either arrive home through the help of their guardian angel or they will be in an accident as a result of God’s will. While a wiser, rational, more responsible individual considers the odds of the possible outcomes of his or her actions and makes good choices according to those odds. People make better decisions when they apply probability theory in their decision making process while in contrast people who believe in destiny are excusing themselves from responsibility.

We need to recognize that our reality is a mixture of highly probable events where we can makes choices, and unforeseeable improbable events over which we have little or no control. For most aspects of our lives we can weigh our risk, make choices, and take responsibility for our actions. While for other situation, such as natural catastrophes that may take our lives or the lives of our love ones, we need to make our peace in accepting our meekness in respect to nature.

Gaze up at the stars and attempt to imagine the extent of our universe. Our Milky Way galaxy is huge beyond imagination and yet it is just one galaxy among billions upon billions of galaxies of the universe. Our existence on Earth is insignificant to these cosmic objects. But by coming to terms with our meekness our minds are open to understanding our universe.

There is nothing wrong with thinking of our Earth as being a very special place; the Earth is a very special place. But to understand our Earth and our solar system we need to realize that these objects are not special because we are here, but rather we are here because these objects are special. Improbable yet seemingly ordinary events took place in their formation that created the unique conditions leading to the evolution of the advanced life form known as human beings.


The Formation of our Solar System

Photo evidence of parts of our galaxy would favor the belief that solar systems form out of cosmic dust clouds. This is possible the best place to start in explaining the formation of the solar system rather than trying to go back farther to determining the source of the dust cloud.

heaven

Most ideas on the formation of a solar system start with nebular contraction. Nebular contraction is the result of the small gravitational attraction each dust particle feels towards the center of the dust cloud causing the dust to slowly converge on the central point.

One of the features of spiral galaxies is that they are rotating. Where this rotation comes from at the start is debatable. But whatever slight rotation is present at the beginning becomes a much greater rotation as the cloud contracts. This faster rotation as the cloud contracts is necessary to be in compliance with the conservation of angular momentum.

The dust that is approaching the center along the rotational equator is diverted sideways, while the dust approaching along any path not on the equator is unobstructed in reaching the center. The result of the gravitational forces, conservation of angular momentum, and centripetal forces leads our dust cloud contracting to the shape of a rotating central sphere with a rotating disk of material extending out from its equator.

galaxy

A common problem in astronomy is that we would like to witness events such as this unfolding and yet the time required for most astronomy events is so vast in comparison to our own lifespan that essentially all that we have is a collection of still pictures. However there is a solution to this problem in that if we are able to collect enough still pictures of an object in different stages of development, and then hopefully assemble them in the correct order, we can ‘witness’ these cosmic events taking place.

But while this can work for stars and galaxies there is not enough light coming from distant dust in the process of forming solar systems for us to directly witness the evolution of a solar system. But again there is a solution to this by possibly substituting the evolution of a spiral galaxy for the evolution of a solar system.

galaxy

Our solar system, the satellite systems of the Jovian planets and the form of spiral galaxies all appear to have the same form. That is, for all these entities the vast majority of each system’s mass is located at the center with the remaining mass revolving in a disk around the center’s rotational equator. The common form of these systems would appear to imply that the interplay between the gravitational contraction, the conservation of angular momentum, and the centripetal forces is operating the same. Despite the vast difference in size, the stars of a galaxy are like the dust particles of a forming solar system.

galaxy

But now we come to the next step in understanding the formation of a solar system where our analogy with the spiral galaxies will take us no farther. Unlike our dust particles, stars can only grow so large. Even though stars can be extremely large, some being many magnitudes larger than our own Sun, there is a limit to their size. Based on science principles and calculations, massive stars should have extreme pressures and temperatures at their core, far exceeding that of the smaller stars. These extreme conditions within the massive stars would cause them to burn extremely fast so that they are thought to have a lifespan of only a few million years. In contrast to the largest stars, the indirect evidence implies that the smaller stars have a lifespan of billions or possible even hundreds of billions of years.

We now need to consider how the revolving disk of dust clumps itself together to form the planets, a more perplexing problem than what most people realize. Gravity is only effective when there is a very large mass involved. So while gravity is effective for attracting particles towards the center of the dust cloud it is an ineffective attractor between the tiny dust particles themselves.

galaxy

When we look for other attractive forces we find that the electrostatic and electromagnetic forces are most effective when the particles are the smallest. At the microscopic level a slight imbalance in charge produces a strong attractive force. Likewise tiny metallic particles have a directional magnetic dipole that is effective for attracting other tiny metallic particles. But in both cases the strength of these forces diminishes as the size of the object grows. Both the electrostatic and magnetic forces become ineffective once the particles grow to the size of a grain of sand. This leaves a gap between the small particles held together by the electrostatic and electromagnetic forces and the gravitational force that is effective on objects the size of a moon or larger.

The clue as to what might hold together the nebular dust may come from looking at holds together dust on Earth. On Earth the dusty environments are the dry environments. This is because at the microscopic level, water is often the sticky stuff that holds things together. As it was explained in the Thick Atmosphere chapter, water is an electro dipole molecule. The water molecule dipole is able to produce a local electrostatic force even though it is part of a larger object that has an overall neutral charge. It appears that the growth of ice crystals is the mechanism that allows small nebula dust particles and unattached compounds to clump together to eventually become the much larger moons and planets.

At only a small distance away an electro dipole appears to be electrically neutral; the strength of an electro dipole field decreases with the distance as a function of one over the distance cubed. So while water dipole is effective in connecting particles together it is only mildly effective as an attractor of particles that are not close enough to be bumping up next to it. Thus a forming, compacting solar system would first need to reach a high density of particles bumping into each other before this clumping process would become effective. Yet once there is a high density, the tiny dirty ice clumps they would continue to connect together more material on their way to becoming the size of small moons. As they reach this size, the gravitational forces would then become effective to further accelerate the accretion process. These growing bodies would sweep the rotating disk clear of the other tiny dust material until they, the planets, and their satellite systems would be all that remained.

Some of the evidence supporting this idea is the fact that water ice is the primary component of many of the smaller objects of our solar system: comets, the Kuiper-belt objects, the smaller moons of the outer planets, and even the rings of Saturn.


Applying the Accretion Model to Understand our Solar System

When we look at the exterior features of the planets of our solar system nearly every one of them appear so alien from the rest that it may be difficult to believe that they could have a common ancestry. But by starting from their common beginning as a revolving disk of nebula dust then applying our knowledge of physics and chemistry principles, we can understand how these planets evolved to their present state.

Within the last couple of decades astronomers have discovered planets orbiting nearby stars and in some cases astronomers have found a few planets orbiting a star thus confirming the existence of other solar systems. However, detecting these planets is at the frontier of technology so that the orbital period, radius, and the planet’s mass is about all that is known about these planets. The methods use to detect these planets favors finding the most massive planets that are closes to their star and so several of these large planets have been found. The mass of a planet appears to have little if any relationship to the planet’s orbital radius.

heaven

About 99.8% of the material of our own solar system is contained within the Sun and based on science principles it would appear reasonable that the other solar systems would have about the same uneven distribution of mass. The percentage of material making up the planets is so small compared to the forming star that the material going into the forming disk is effectively just the left-over scrap. Therefore, in the forming of these solar systems, the seemingly insignificant differences between one nebula cloud and another would tend to produce important differences in the final arrangements of each set of planets.

So for example, unlike our own solar system, the third planet from the central star does not need to be the largest of the inner planets just as the nearest jovian planet does not need to be the largest planet overall. Each unique starting nebula produces a unique distribution in regards to how much mass is awarded to each orbiting planet.

Nebula M42

Besides mass, another nearly random variable would be each planet’s rotational tilt. Again it is the unique form of the initial swirling nebular cloud being pulled together that determines these outcomes of each planet’s mass and axial tilt. Furthermore, once the main cloud is taking the sphere/disk shape then the much smaller sphere/disk shapes start forming the satellite systems. Based on our own solar system, the planets along with their satellites would tend to favor a rotation in the same sense as their host star and its revolving disk of material. But while similar rotational axial direction appears to be favored, it is clearly not a requirement. Uranus is tilted 98 degrees on its side while Venus has a 177 degree rotation meaning that it is rotating in the reverse direction of the other planets.

So the first key insight that comes from the nebula contraction model is that the mass and the rotational tilt of each planet are more or less random outcomes derived from the unique shape of the original nebular cloud. In the construction of each solar system it would be as if the mass and axial tilt of each planet were determined by somebody throwing a dice. If you would like to call that imagined somebody God then God most definitely plays dice.

observatory

Of course it is important to us that our Sun has a mass of 2.0 E30 kg, and the third rock from this star is 1.5 E11 m away, has a mass of 6.0 E24 kg, and a rotational tilt of 23.5 degree. If not for these key features and a few others, human beings would have evolved very differently or not at all. But we do not need to rack our minds over why these features are the way they are, because from the perspective of the cosmos our solar system is just one of millions or billions of possible outcomes. Human beings exist in this part of the Milky Way Galaxy, in this part of the universe for no reason other than dumb luck.

The next important insight we gain from our simple model is that despite the exterior appearances of the planets they were initially created out of the same soup of material. Initially when the nebula cloud took the sphere / disk form, the material making up the disk would have been homogenous or at least very nearly so. At the most there may have been some composition differences in the dust material nearest to the center from the dust that is farthest away. But the initial composition of neighboring planets, say Venus, Earth, and Mars would be nearly identical.

The final helpful insight gained from our model comes from the second stage of the solar system formation. For this second stage of formation it was hypothesized that water would be the bonding agent holding together the initial protoplanets. So initially the moons and planets had water ice at their core and once these objects became large and hot the water would then migrate to the surface of each planet.

We are now ready to see how a dirty water-ice snowball becomes a planet.


The Evolution of the Planets of our Solar System

A planet or moon’s evolution is controlled primary by is mass and its orbital position.

Comet

Small objects that are far from the Sun have not evolved much at all. These include the smaller moons of the Jovian planets and the Kuiper belt objects. These objects were born as dirty ice objects and so they remain for all of time.

The one exception is the Kuiper belt objects that are kicked into the path of an elongated ellipse taking them near the Sun at regular intervals. Once this happens then these objects are then comets. Comets lose material on every visit they make around the Sun so that after several passes they disintegrate.

Mass is the most important evolutionary criteria of the objects that remain far from the Sun. The first evolutionary step for a heavenly object is to gain enough mass that it makes the transition from some odd potato shape to that of a sphere. This transition occurs when an object has gathered enough material that the gravitational forces overcome the electrostatic forces holding its original shape to shatter the odd potato shape and reform the core into a spherical form.

Many of the smaller moons of the Jovian planets reach this stage but then go no farther. If we exclude the asteroids, all of these small moons have an overall density of around 1.1 g/cm3. If water ice along with the relatively small amount of dust are the main ingredients of the small moons and KOB objects, then a density of 1.1 g/cm3 would appear to make sense. Ice has a density of 0.917 g/cm 3 and so adding a small percentage of high density metallic dust to the ice would raise the average density of these moons to 1.1 g/cm3.

It would be logical for all of them to have a density of 1.1 g/cm3 if not for moons farthest away picking up additional metallic dust. The moons of Uranus have densities ranging 1.4 and 1.7 g/cm3. Not only are the most distant moons picking up metallic dust, but the most distant planets are gathering up heavy metal as well. After Saturn having the lowest density of all the planets, Uranus has a density of 1.27 g/cm 3 and the planet farthest away, Neptune, has a density of 1.64 g/cm3. Apparently the outermost planets and moons are collecting stray metallic dust that is attracted to our solar system.

Another way for a moon to reach a high density is to grow through accretion to become one of the larger moons. The three largest moons of the Jovian planets, Saturn’s Titan and Jupiter’s Ganymede and Callisto, all have a density of 1.9 g/cm3. The greater material produces greater gravitational forces producing heat and pressure that melts some of the moon’s interior ice that then migrates to the surface. The result in an increase in the density of the moon’s interior.

The remaining heavenly bodies that have obtained higher densities than this have done so through the application of tidal gravitational forces.


Tidal Heating of Planets and Moons

The planets of our solar system revolve around the Sun because they each feel a gravitational attraction towards the Sun. The Sun in turn feels an equal force of attraction towards each of the planets. The gravitational attraction between any two bodies is computed as:

F = G M1 M2 / R2

where F is the force, G is the universal gravity constant equal to 6.67 E-11 N m2/kg2, M1 and M2 are the mass of object one and object two, and R is the distance between the center of the mass of the two objects.

While the attractive forces on each of the two objects are equal in magnitude, the magnitude of the gravitational field surrounding each object can be dramatically different. The magnitude of the Sun gravitational field is much larger than the Earth’s gravitational field. The gravitational field is strongest when we are near a massive object. The strength of the gravitational field g is given as:

g = G M / R2

A rotating planet in a strong gravitational field evolves to greater density because of the tidal forces that generate internal heat. The strength of these tidal forces is proportional to the strength of the gravitational gradient. The strength of the gravitational gradient is computed as

Gravitational gradient equation

where delta g over delta R is the gravitational gradient, G is the universal gravity constant, M is the mass of the central massive object such as the Sun or one of the Jovian planets, and R is the distance away from that object. The negative sign just means that the strength of the massive object’s gravitational gradient becomes weaker as we move away from the massive object generating the field. Because the gravitational gradient is a function of one over the radius cubed the strength of the gravitational gradient is more dependent on the distance from the massive object than the mass of the large object. For this reason the moons of the Jovian planets experience a stronger gravitational gradient than the planets.

Jupiter’s
moons
Δg/ΔR
(s-2 E-15 )
Rotation
Io 3370000 Locked
Europa 839000 Locked
Ganymede 207000 Locked
Callisto 38100 Locked
Earth’s moon
Moon 14100 Locked
Planets
Mercury 1375 Slow
Venus 211 Slow
Earth 258* Fast
Mars 23 Fast
Jupiter <1 Fast
Saturn <1 Fast
Uranus <1 Fast
Neptune <1 Fast

Because the Jovian moons exist in extremely strong gravitation gradients nearly all of these moons are locked in synchronous rotation. This means that they rotate at the same rate that they revolve so that the same side of the moon always faces towards the host planet. A consequent of a heavenly body being locked in synchronous rotation is that these moons do not generate heat as a result of their interaction with their host planet.

Even though Jupiter’s moon Io is the most noted in regards to tidal heating, the tidal heating of Io is actually a special case that will be discussed in a moment. In the more fundamental type of tidal heating the most significant factors are the strength of the gravitational gradient and whether or not the planet or moon is rotating with respect to its host planet or the Sun. All of the terrestrial planets fulfill these two requirements in that they are rotating within a moderately strong gravitational gradient.

Among the terrestrial planets Earth is a fast rotating body within a moderately strong gravitational field and so we are not surprised that the Earth is both geologically active and it has the highest density of any object in the solar system. Venus has a gravitational gradient almost as strong as the Earth but because it rotates much slower it has a much diminished level of volcanic activity. Mars rotates fast but its gravitational gradient is so low that the heat it generates is no longer adequate for producing volcanic activity. Mercury’s high gravitational gradient and high density suggest that Mercury must have been extremely active when the solar system was first formed but now, billions of years later, it is dormant since it has completed its evolutionary process.

We can gain further understanding of the importance of rotation in regards to tidal heating by taking a closer look at our own Earth – Moon system.

The Earth is about eighty times more massive than the Moon and so the gravitational gradient that the Earth applies to the Moon is about eighty times greater than the gravitational gradient that the Moon applies to the Earth. From this information alone we might expect that more heat would be produce within the Moon than within Earth when actually the opposite is true. The difference is because the Earth is rotating in respect to the Moon’s view while the Moon is not rotating with respect to the Earth’s view.

The Moon produces two tidal bulges on the Earth. One high tide bulges faces the Moon while the other high tide bulge faces away from the Moon. Each day, because the Earth is rotating, points on or near the Earth’s surface within the lower and middle latitudes experience a change in the strength of the Moon’s gravitational gradient. As a result, these lower and middle latitude locations on the surface of the Earth elevate up and down as they move through the Earth’s tidal bulges.

Diagram of Earth's Tidal Bulge

In contrast to the Earth, the surface of the Moon does not go through these daily oscillations. Like the Earth the Moon’s otherwise spherical shape is stretch along the line drawn between the two objects. But unlike the Earth the Moon is frozen in this shape. Without the internal flexing, there is almost no internal frictional heat being generated. Without interior heating the Moon has cooled to being completely solidified so that now it is geologically dead.

This then explains why the terrestrial planets all have high densities, a feature that indicates a history of strong tidal heating. All of these objects are near enough to the Sun to be within its strong gravitational gradient and yet they are all still rotating in respect to the view from the Sun. As they rotate their shape is flexing in response to the gravitational gradient. This inelastic flexing causes the tidal bulges to be a few degrees off from the direct line of pull from the Sun. The result is that 1) internal thermal energy is being produced within the planet, 2) the planet’s rotational speed is slowing down and 3) the terrestrial planets are slowly moving away from the Sun. The closes planets, the ones rotating through the strongest gravitational gradients will eventually lock in synchronous rotation with the Sun. In time, at least in this respect, these planets of our solar system will then be similar to the Jovian moons.

It would appear that these concepts are matched well with the evidence until we come to the exceptions of Jupiter’s two closes Galilean moons: Io and Europa. These moons are locked in synchronous rotation so we might expect them to be geologically dead like our Moon. Yet both of these moons have high densities and they are geologically active: Io has a density of 3.5 g/cm3 and Europa has a density of 3.0 g/cm3. The reason Io and Europa have high geological activity and high density is not because of their interaction with Jupiter but rather it is because of their interaction with each other. They are in fact further affirmation of how tidal forces heat the heavenly bodies of our solar system.

If Io were Jupiter’s only large moon then it would not have a high density and it would be just as geologically dead as the Earth’s moon. But instead of being alone, Io has Europa nearby. Every 3.55 days Io passes by Europa on the inside orbital. When these two large moons pass close to each other they feel the tidal tugs from each other. That is, they are massive enough to create their own strong gravitational gradient, and when they pass close enough to each other they feel the effect of each other’s gravitational gradient. As a result, the moons rotate and their shapes distort slightly each time they pass each other. This repeated flexing of the shape of these moons is similar to the flexing that the terrestrial planets experience as they rotate through the Sun’s gravitational gradient. So likewise there is internal friction and corresponding internal heating to produce their geological activity.

The Galilean moons that are farther out, Ganymede and Callisto, also experience this effect but to a much lesser degree. These remaining moons pass each other less often and more important when they pass each other they are farther apart from each other. The fact that they are farther apart from each other is the more critical factor since the strength of the gravitational gradient is a function of one over the distance cubed.

A planet or moon’s overall density is a key feature telling us the extent of that heavenly body’s evolution.


The Creation of a Terrestrial Planet’s Atmosphere

As heat is added to a substance its temperature will either increase or it will go through a change in phase. A phase change is usually a solid changing to a liquid or a liquid changing to a gas. If enough heat is added to the substance it will both increase in temperature and go through phase changes.

The various compounds have different melting and boiling temperatures based on their chemical bonding. In general the lighter, simpler compounds will melt or boil at lower temperatures than the larger compounds or heavy metals.

Diagram Showing Relationship between Heat, Temperature, and the Phase of a Substance


diagram showing ice-water phase change

As heat is generated within a terrestrial planet its internal temperature increases. When the temperature reaches the melting point of water-ice and other light compounds these compounds complete their phase change from solid to liquid. With further increase in temperature even the rock turns soft and starts to melt. With the melting of the rock the lower density compounds will then migrate up to the surface. It is this tidal heating that allows a planet to differentiate into layers according to the material’s density. The dense metals sink to the center, the medium density rock fill the mantle and crust, while the lightest compounds escape as a gas onto the surface.

The extent of a planet’s differentiation is primary a function of the temperature within the planet. The planets that generate the most internal heat and then retain that heat will be the planets that obtain the highest internal temperatures. While tidal forces are the primary means of generating heat, it is the size of the planet that helps it retain that heat. As explained in Scaling Properties larger objects take longer to dissipate heat because they have a lower surface area to volume ratio than the similar smaller objects, thus larger planets are better able to hold on to their thermal energy. To summarize, a large fast-spinning terrestrial planet in a strong gravitational gradient will receive and retain more heat and go through greater differentiation than a small slow-spinning planet in a weak gravitational gradient.

The more heating, the more differentiation of the planet, the more gas expelled on a planet’s surface. This process is extended over a long time since once the low density compounds break free it may take hundreds of millions of years for them to migrate up to the surface. These light compounds can either be liquids or soft toothpaste like solids, but on reaching the surface the release of pressure will cause many of these light liquid compounds to become volatile as they transition into gas. Most of this gas, the very lightest gas such as the hydrogen, is then lost into space. With the lost of the lightest compounds the remaining planet becomes denser. Thus the planets that experience the greatest heating, the greatest differentiation, are the planets that have the greatest overall density.

Terrestrial planets do not have strong enough gravitational fields to hold on to the lightest gases such as hydrogen and helium and so these light gases are lost to space. The remaining heavier gases have the option of either become part of the planet’s atmosphere or to react with other compounds on the surface. Which path the gas takes depends on its chemical properties. The most reactive gases such as monatomic oxygen will react with iron and other compounds on the surface. While the highly inert gases such as nitrogen or argon can stay in the atmosphere for almost indefinitely.

Factors that Determine the Evolutionary Development of a Terrestrial Planet

PlanetDistance from Sun
(A.U)
Mass
(Earth Mass)
Δg/ΔR
(s-2 E-15 )
Solar Rotation
(E-6 rad/s)
Density
(g/cm3)
Mercury 0.39 0.055 1375 0.41 5.4
Venus 0.72 0.82 211 -0.62 5.2
Earth 1.00 1.00 258* 73 5.5
Mars 1.52 0.11 23 71 3.9

*For the Earth the gravitational gradient due to the Moon and that of the Sun are added together. I doubt that this is the best way to handle this data and yet I do not know what else to do. I am open to suggestions.

Mars

The thickness of a terrestrial planet’s atmosphere is primarily depended on the size of the planet and how much internal heating has ‘cooked’ the planet so as to release its light compounds onto its surface. If a planet or moon is too small, such as the case with Mercury, then that heavenly body’s gravitation field is too weak to hold on to an atmosphere, so for Mercury it does not matter if gases are being released on its surface or not. But for the remaining terrestrial planets we are most interested in how large it is, how fast it is spinning, and the strength of the gravitational gradient as a means of modeling predictions concerning the thickness of the planet’s atmosphere.

After Mercury, Mars is the next largest planet. It is no surprises that Mars has such a thin atmosphere. Its mass is only 11% of that of the Earth and even though it is a fast spinning planet it is the terrestrial planet that is farthest from the Sun. Being so far from the Sun the gravitational gradient is too weak to produce much heat in the interior of Mars and being so small it loses that heat quicker than the larger planets. In the earlier stages Mars was able to produce enough gas to precipitate an ocean. Yet without enough heat supplying new gas out of its volcanoes its ocean has long since dissipated. As Mars grew denser over time the little tidal heat being produced internally became insufficient in allowing the lighter molten material to reach the Martian surface. With the remaining light material trapped within the interior, Mars now appears to be geologically dead.

The Moon and Venus

The next larger planet is Venus. It is easy to understand why Venus has a much thicker atmosphere than Mars. Venus does not spin fast but it scores high on the other criteria that are more important. Venus is a large terrestrial planet in that it is 82 % as much mass as the Earth and it is even closer to the Sun than Earth so it is experiencing strong tidal forces. Because it is spinning in a strong gravitational gradient and it is large the temperature within Venus would be much greater than that of Mars. This logic is backed by a comparison of the densities of these two planets. The mildly heated, under develop Mars has a planet density of 3.9 g/cm3 while the hotter, more evolved Venus has a planet density of 5.2 g/cm3. The atmosphere of Venus is over ten thousand times thicker than the atmosphere of Mars.

An amazing often overlooked fact is that the chemical compositions of the atmospheres of these two terrestrial planets are for all practical purposes identical. In addition if we also include the evidence that we have about the Earth’s early atmosphere then all three of these terrestrial planets once had the same chemical composition for their atmospheres. The matching compositions of these planet’s atmospheres shows that these planets formed out of the same cloud of dust and compounds. This is outstanding evidence in support of our model of the formation of our solar system.

The atmosphere of Mars is 95.3% carbon dioxide, 2.7% nitrogen, and 1.6% argon while the atmosphere of Venus is 96.5% carbon dioxide, 3.5% nitrogen, and trace amounts of argon. The close matching between these two atmospheres is even more astonishing when we consider that the average surface temperature of Mars is 210 Kelvin while the surface temperature of Venus is 730 Kelvin: a difference of 520 degrees Celsius. It is surprising that this large difference in temperature did not generate different chemical reactions on each planet so as to produce a significant difference in the chemical compositions of the atmospheres of these planets.

The Earth has the greatest density of all the planets and it is likewise the most fully evolved planet within our solar system. It has everything going for it in regards to generating heat within its interior. It is the largest terrestrial planet and it is fast spinning planet within a strong gravitation gradient. In fact, even though the Earth is farther away from the Sun than Venus it experiences a stronger gravitation gradient because of the strong tidal forces applied by the Moon. In regards to generating internal heat the Earth scores higher than Venus in every category.

Once we think in terms of science principles we recognize that the Earth’s present atmosphere is not the norm. It is clearly more logical for Earth to be the terrestrial planet with the thickest atmosphere and throughout most of the Earth’s 4.6 billion years of existence the Earth has been the terrestrial planet with the thickest atmosphere. As best as it can be determined from the geological evidence, the only times that the Earth has not had the thickest atmosphere was during the late Paleozoic era and the present.

When we take a more thoughtful inspection of the composition of Earth’s atmosphere we are able to recognize our connection with our neighboring planets. We start with the standard atmospheric chemical composition of a terrestrial planet: about 96% carbon dioxide, 3% nitrogen, and trace amounts of argon. We add oxygen to account for the life on Earth producing diatomic oxygen through the process of photosynthesis. Then we remove nearly all of the carbon dioxide to account for the incredible amount of the carbonated rock that is found all over the Earth. The end result is the Earth’s present day atmosphere of 78% nitrogen, 21% oxygen, 1% argon, and only 0.03% carbon dioxide.

diagram showing change in Earth's atmosphere composition
dolphin jumping out of the water

To summarize, the terrestrial planets of Venus, Earth, and Mar were all created out of the same soup of material that was once a disk of dust and whatever revolving around what was to become the Sun. Through accretion that started with the forming of water-ice clumps this disk of material became the planets. Tidal forces then heated these terrestrial planets allowing the lighter compounds to migrate to the surface of each of these planets. The amount of light compounds reaching the surface to become the planet’s atmosphere was dramatically different because the size of each planet was different and the amount of internal heating for each planet was different. Mars generated the least amount of atmosphere, Venus produced ten thousand times more atmosphere than Mars, and initially the Earth created far more atmosphere than Venus. Initially these atmospheres all had the same chemical composition. But then Earth’s atmosphere went through further developments that changed its overall chemical composition and greatly reduced its thickness.

But what is it about the Earth that made it special so that its atmosphere went through further developments? Earth is the right distance from the Sun so that liquid water can exist on its surface. Venus is too close to the Sun and so it is too hot, Mars is too far away and so it is too cold, yet on Earth the surface temperature is correct for the existence of liquid water. The oceans of water that cover the Earth are responsible for changing the Earth’s atmosphere to what it is today. Water: the universal solvent, the giver of life. Water is what makes Earth the blue planet.



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