8. Hell, Heaven and Earth: Part 2 - Heaven: Evolution of our Solar System

Introduction to New Ideas in Astronomy

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.

Science is based on the belief that our reality is rational, and following this belief has proved to be a remarkably successful strategy. This is not to say that everything we currently know about reality makes sense to us, but rather that the belief that our reality should make sense is an approach that can be extremely helpful in our efforts to discover the workings of the universe. In the case of astronomy, this is the difference between simply recording observations about stars and planets and attempting to figure out the logical reasons explaining these objects.

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.

For mankind to thrive as rational beings, a strong distinction must be made between science and religion. Scientists should align themselves with beliefs that are best supported by evidence, rather than those that gain popularity simply because they make us feel good. In science education, lessons should include practicing the application of evidence-based reasoning for the purpose of drawing logical conclusions. Whenever a new scientific belief is introduced, teachers should lead a discussion of the evidence supporting it; merely stating that a belief is the consensus of experts is completely unacceptable. Science distinguishes itself from religion by encouraging people to develop their reasoning skills and focusing on evidence, rather than accepting beliefs as a matter of faith.

While the vast majority of scientific beliefs are true, history shows that leading scientists often initially cling to incorrect beliefs before eventually accepting the truth. Therefore, until there is considerable evidence supporting a belief - such as in the case of the Theory of Evolution - caution is needed in determining what is actually true. Pay attention to the strength of evidence-based arguments rather than how often a claim is repeated. A major red flag indicating that a popular belief is likely wrong is when scientists argue that their belief must be true simply because it is their consensus that it is true; in real science, the popularity of a belief cannot be used as a substitute for the presentation of clear evidence-based arguments in support of a belief.

Astronomers face a greater challenge than most other scientific disciplines in sorting through false beliefs on their way to discovering the truth. This is because, unlike other fields, in astronomy it is rarely possible to run experiments to test hypotheses. While leading scientists are usually extremely stubborn about abandoning incorrect beliefs, this inability to test hypotheses makes it even more difficult to discard bad ideas.

In the previous chapter, we learned that tidal forces - caused by the stretching of the Earth - explains why Earth’s interior is so warm. This new insight raises the question: does tidal heating also apply to other major objects in our solar system? Indeed, there is universal agreement among planetary scientists that tidal heating is the mechanism warming the interior of Jupiter’s moon Io. Furthermore, besides Io, astronomers have identified several other moons of Jupiter and Saturn where tidal heating plays a significant role in heating their interiors. Yet, astronomers still cling to the incorrect belief that radioactivity explains the internal heating of most planets and moons, including Earth. This is just another example of stubborn scientists refusing to yield to the evidence indicating that their belief is wrong.

This chapter will demonstrate how tidal heating can explain the evolutionary development of all the planets and moons in our solar system. In contrast to the radioactivity heating hypothesis, tidal heating has universal predictive power that matches observations throughout the solar system. It is the key component of the Theory of Planetary Evolution - a theory that greatly advances our understanding of the solar system

The Theory of Planetary Evolution, along with the laws of nature, gives us guidance towards understanding how the planets of our solar system - despite having a common collective beginning - could have arrived at their present remarkably unique states.

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.

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.

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.

It’s About Time: The Age of the Universe

In the 1950s and 1960s, based on radioactive dating of meteorites and terrestrial rocks, geologists and planetary scientists established that Earth - and along with it, the solar system - is about 4.6 billion years old. Thus, more than 4.6 billion years ago, a dusty molecular cloud existed that condensed due to gravitational attraction, eventually forming our solar system. This molecular cloud marks the beginning of our story of the formation of the solar system.

Yet it is natural to wonder: What came before this molecular cloud? Going further, it is natural to wonder where our Milky Way galaxy came from or why our reality should even exist.

Starting in the late 1960s and 1970s, cosmologists began promoting the Big Bang Theory as the dominant explanation for the origin of the universe. According to this hypothesis, about 13.8 billion years ago, the universe began as an extremely hot and dense singularity and has been expanding and cooling ever since. From this time forward, the Big Bang Theory has been presented to the public as the authoritative answer to the question of how the universe came into existence. Since alternative hypotheses are no longer given serious consideration, the public is led to believe that cosmologists’ Big Bang Theory - and their claim that the universe is 13.8 billion years old - is unquestionably true. However, in reality, cosmologists have been downplaying the fact that there is considerable conflicting evidence indicating that not only is this debate far from over, but that nearly all of their major beliefs may be incorrect.

The primary problem is that, in terms of cosmic events, 13.8 billion years is actually a ridiculously short amount of time.

While our models of reality are limited by imagination, nature has no such constraints. The universe is so vast it defies comprehension: there are more stars in the cosmos than grains of sand on every beach and desert on Earth. We accept the enormity of space because we observe it, but we struggle with the enormity of time because it demands imagination.

When cosmologists first considered the age of the universe, they likely saw 13.8 billion years as an inconceivably long time - but it is not. The supposed age of the universe is only three times the age of our solar system.

To consider the feasibility of a 13.8 billion years age for the universe, we should use our solar system’s 4.6-billion-year age as a reference. Is it realistic that entire spiral galaxies could form in only slightly more time than it took Earth to form? To give another example, our sun, like all surrounding stars, formed from the remnants of earlier exploded stars, which themselves formed from even earlier explosions, and so on. How could all these cycles occur in little more than the lifespan of our Sun?

Beyond these logical inconsistencies, direct evidence indicates that the universe is older than 13.8 billion years: several stars, galaxies, and globular clusters have been found that are older than 14 billion years. Particularly baffling is cosmologists’ claim that there is no conflict between the supposed age of 13.8 billion years and the observable universe’s radius of approximately 46.5 billion light-years. While they redefine the definition of speed to explain how light could have traveled more than three times the speed of light, this still does not explain why distant images show mature galaxies when the light supposedly left its source not long after the universe began. As we peer farther into space, the uniformity of the universe’s appearance leads to the seemingly impossible conclusion that the universe may be both endless and ageless.

So why do cosmologists continue to claim the universe is only 13.8 billion years old?

The 13.8-billion-year estimate stems from astronomers’ efforts to explain the redshifting of light from distant objects. They observed that distant objects exhibit redshift, with the farthest ones being the most redshifted. Based on the belief that the redshifting was due to the objects moving away from us, cosmologists concluded that all these objects must have emerged from a single point in space: as a result of the Big Bang. Based on the degree of redshift, they calculated the speed of these objects and along with this the supposed age of the universe as being 13.8 billion years.

Since evidence now indicates the universe is older than 13.8 billion years, something must be wrong with their calculations - or worse, their assumption that redshifting is due to the objects’ movement may be incorrect. Understandably, cosmologists are reluctant to accept this conclusion, as doing so could mean that many of their long-held beliefs, including the Big Bang Theory, are wrong.

So, how old is the universe and how did it come into existence? Honestly, scientists do not know.

While it is intriguing to wonder about the universe’s origins, given the current uncertainty, we must acknowledge that such discussions remain largely speculative rather than scientific. Therefore, if we limit ourselves to evidence-based science, the formation of our solar system is about as far back in time as we can reliably explore.

Our Solar System: From Observation to Understanding

Astronomy is steeped in the tradition of making observations. While early astronomers carefully tracked the movements of planets and moons, today astronomers take millions of images of the surfaces of planets and moons. It seems a bit excessive: like keeping a car in first gear while driving down the highway. While there is nothing wrong with making observations - indeed, it is the first step of science - science does not actually advance until we start making sense of these observations.

Making sense of observations involves thinking in terms of the laws of nature. We need to constantly ask ourselves - based on the laws of nature - why things are the way they are. Once we start recognizing patterns or rules and receive confirmation that our hypotheses work, we are able to make accurate predictions of future or yet-unseen events. Science does not always give us the answers we want to hear, but it is comforting to know that the natural world does make sense.

Another problem that astronomy has, one that is really the same problem all scientific disciplines have: is an inability to look at evidence with a fresh perspective. Science disciplines often get stuck in a rut. This happens because the first scientists who try to make sense of a newly discovered phenomenon typically do not have enough evidence to reach the correct conclusions. Yet these early researchers will still draw their conclusions anyway. This sets up a paradigm of acceptable thoughts regarding the observation, and this paradigm remains firmly in place despite newer evidence casting doubt on the initial conclusions. This groupthink, which limits the questioning of beliefs, can hold back science for years, decades, or even centuries. To move science forward, a fresh look at all available observations, along with imaginative thinking unburdened by earlier beliefs, is often needed to break the stagnation.

It is time to apply these ideas for the purpose of achieving a better understanding of our solar system.

Our first casual observation of the planets gives us the impression that most of these planets have little in common with each other. Mercury has a cratered surface, Venus is obscured by thick white clouds, Earth is the blue planet since it is mostly covered with water, and Mars is the barren red planet. The large gaseous planets that are further away from the Sun have even less in common with these terrestrial planets. However, appearances can be deceiving. The differences between these planets that we see today are the result of more than 4.6 billion years of planetary evolution.

Planet	Axial Tilt	Notes
Mercury	0.034o	No tilt
Venus	177.4o	Rotates retrograde
Earth	23.44o	Causes our seasons
Mars	25.19o	Similar to Earth
Jupiter	3.13o	No tilt
Saturn	26.73o	Similar to Earth
Uranus	97.77o	Rolls on its side
Neptune	28.32o	Similar to Earth

In our quest to understand our solar system, we should consider how the laws of nature would cause the planets to evolve in different ways. For example, the laws of nature explain why the terrestrial planets close to the Sun have very little hydrogen or helium. The planets close to the Sun cannot hold on to hydrogen or helium because the warmth from the Sun gives these molecules so much high-speed kinetic energy that they soon escape the gravitational pull of these planets. Conversely, further away, the planets are much cooler and so have no difficulty holding on to whatever hydrogen or helium comes their way. The point is that when we apply our understanding of the natural laws, we typically find that nature makes sense.

Yet, while most planetary features can be understood, we need to accept the fact that there are some features of planets that likely more a matter of chance. For example, Earth experiences seasons because its rotational axis is tilted at 23.5 degrees—a fact that has major consequences for life on our planet. However, a planet’s axial tilt is more of a chance outcome, a statistical result of the many chaotic collisions that occurred during the solar system’s formation. A survey of the planets shows that while the most likely outcome of these chance collisions is for a planet to have a slight tilt or no tilt, Uranus rolls on its side, and Venus is flipped such that its rotation is retrograde. There is no point in dwelling on these by-chance outcomes, but instead our focus should be on making sense of the features that are deterministically created.

Beyond these two characteristics of planets, there are many more: a planet’s mass, its density, material composition, atmospheric thickness, average surface temperature, magnetic field strength, orbital distance, rotational speed, whether it has moons, whether life exists on the planet, and so on. 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.

The Formation of our Solar System
Angular Momentum and Sphere / Ring Objects

Within our own Milky Way galaxy, astronomers observe regions they believe are stellar nurseries, where new stars form. Billions of years ago, our solar system likely began its existence in a similar environment. This is possible the best place to start in explaining the formation of the solar system.

The most widely accepted hypothesis for the origin of the solar system begins with the gravitational collapse of an interstellar cloud of gas and dust. While the presence of solar systems within these dust clouds strongly suggests that they are indeed stellar nurseries, it remains a mystery as to why a nebula cloud would begin to collapse on itself. Although gravity is thought to play a primary role, gravity only works effectively when large masses are involved rather than tiny dust particles or molecules.

Regardless of how the collapse begins, once the molecules and dust particles start converging toward a central point, the entire cloud begins to rotate – although where this rotation came from is also not entirely clear. Nevertheless, even if the rotation is unnoticeable at first, as the cloud collapses, the rotational speed increases dramatically in accordance with the conservation of angular momentum.

While most of the material is drawn toward the center to form a spherical mass, the high speed of rotation causes some of the dust to either be thrown off from the equator or simply left behind as it rotates around the sphere. This material becomes the disk or ring portion of the forming solar system.

There are examples of these sphere-disk-shaped objects at vastly different scales: spiral galaxies, solar systems, and planet-satellite systems. The consistent emergence of this structure across cosmic scales suggests that the same physical laws - gravity, angular momentum conservation, and centrifugal effects - shape them all.

This dynamic process results in a distinctive structure: a dense, central sphere surrounded by a thin, equatorial disk, with both components rotating in the same direction. While this might suggest a balanced two-part system, the mass distribution is typically lopsided, heavily dominated by the central body. For example, in our solar system, the Sun contains about 99.8% of the system’s total mass, while the planets hold about 98% of the angular momentum.

All the material that went into creating our solar system, only 0.2% went toward creating the planets and moons, and only a small portion of this already small amount went toward creating the Earth. Yet the most important takeaway of this model is that all the planets and moons came from the same dust material that was originally revolving around the Sun.

While it would be nice to witness the formation of these sphere-disk systems, the time required for these processes may as well be infinite compared to our short lives. In fact, the incredibly long timescales required for most cosmic events have always been a challenge for astronomy. Although we realize that distant objects like galaxies are moving at incredibly high speeds, to us they will always appear as still images. For some cosmic objects, astronomers address this problem by gathering pictures of similar objects at different stages of development and, hopefully, assembling them in the correct order so that they can "witness" these cosmic events taking place.

Despite the vast difference in size, we can think of the stars that come together to form a spiral galaxy as being analogous to the tiny dust particles that come together to form a single solar system. And yet, while this visual comparison is helpful, we can only take this comparison so far.

The formation of a solar system and the formation of a planet-satellite system make for a better comparison of sphere-ring systems. With the exception of size, and the fact that some point the Sun began its nuclear fusion reaction, the Sun and the four planet-satellite systems of our solar system - Jupiter, Saturn, Uranus, and Neptune – have much in common.

Sticking Together

According to astronomers, at some point during the formation of our solar system, the molecules and tiny particles swirling around the Sun began clumping together into ever-larger particles. These larger particles then clumped together to form even larger particles or small chunks of mass, which themselves clumped together to form proto-planets and eventually the planets we see today. It makes for a good story - until we ask: How did this stuff stick together?

When it comes to getting objects to stick together, there is a significant gap between what works for small objects and what works for large ones. At the microscopic level a slight imbalance in electric 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. Electrostatic forces and magnetic attraction are effective for holding together atoms, molecules, and small particles - up to about the size of a grain of sand - while larger objects must be at least the size of a mountain or small moon before gravity becomes the dominant force in attracting matter together. So, this begs the question: How did the small particles of a nebula cloud grow to the size of small moons?

Perhaps it is difficult to recognize why cosmic dust particles are so restricted in size, after all, here on Earth, we are surrounded by countless non-microscopic size household items. Yet take note that these objects are typically either life byproduct objects - such as wood or paper - or natural or manmade homogeneous objects such as plastics, metals, ceramics, or glass. Even rocks are made up of homogeneous crystals (minerals) that are either interlocked or fused together, or bonded together by silicate or calcite cement. The bonding strength of these materials goes up as a function of how much contaminates are removed from the desired pure compounds.

Dirt is considered a contaminate and it does not help the bonding of materials (this statement may be on the back of every tube of glue ever created!). Space dust is essentially cosmic dirt, and so it does not stick together well nor does it feel an attraction to other nearby particles. Usually, the only way to get dirt to stick together is to add water.

At the microscopic level, water is often the sticky substance that holds things together. As first explained in the Thick Atmosphere chapter, water is an electric dipole molecule. The water molecule’s dipole is able to produce a local electrostatic force, even though it is part of a larger object that has an overall neutral charge. What this means is that while most dusty space particles have no chance of growing larger, particles that are covered with either liquid or frozen water can clump together with other particles, and as long as there is some water in their surface they can continue clumping together until they reach the size of a moon. Once again, water is the miracle compound: this time, it is the special substance that enabled small cosmic dust particles to grow to the size of moons.

With the exception of Earth, water is typically a scarce commodity on the surface of most planets and moons. Yet water is far more abundant throughout our solar system than most people realize. It is primarily because radiation from the Sun breaks apart water molecules – photodissociation - that it is difficult to find water on the surface of most planets or moons. However, beneath the surface of these planets or moons, or farther from the Sun, water (either liquid or frozen ice) tends to be almost everywhere. Comets and distant Kuiper Belt objects are typically 50 to 70 percent frozen water, most of the moons of the outer planets are composed more of frozen ice than rock, and the inner rings of Saturn are nearly 100 percent frozen water.

Saturn's Rings: The Key to Understanding Our Solar System

The classical explanation of Saturn’s rings is an excellent example of how initial scientific investigators often draw the wrong conclusions about a phenomenon, and later scientists become stuck in the dead-end paradigm that the earlier scientists created.

In 1610, Galileo was the first to observe the rings of Saturn, but he did not understand what he was seeing because his telescope was too weak to create an unambiguous image. It was not until 1655 that Christiaan Huygens - using a more powerful telescope - recognized that Saturn was surrounded by a thin, flat ring that did not touch the planet. After this, it was not until the late 1800s before astronomers felt they had a reasonable explanation for why Saturn has rings. Yet, while this ‘torn apart moon’ explanation seemed to make sense at the time, more recent evidence shows that this classic explanation is wrong.

To explain the origin of Saturn’s rings, astronomers proposed that one or more of Saturn’s moons drifted too close to Saturn and consequently it was torn apart by tidal forces. A key part of this hypothesis was the introduction of the Roche limit - the calculated minimum distance at which a weakly held together satellite can orbit a planet without being torn apart by tidal forces.

Let’s go over the primary evidence clarifying the fact that Saturn’s rings are not the remnants of a torn-apart moon or moons.

Roche Limit is Misleading – Because gravity attracts objects together, it might seem like common sense that orbiting moons would slowly drift toward their host planet. However, this is not the case. Saturn’s moons, like Earth’s moon and the vast majority of moons, have a prograde orbit, meaning they orbit in the same direction as the planet rotates. The gravitational pull of these moons distorts the host planet, which in turn produces a slight tug on the moon directed away from the planet. In summary, contrary to what is implied by the Roche Limit explanation, moons generally do not drift toward their host planet. Instead, as these moons continuously orbit a planet, they are also very slowly drifting away.

The Water Ice of Saturn’s Rings Is Far Too Clean – If Saturn’s rings came from a shattered moon, we should see significant amounts of rock and dust in the composition of Saturn’s rings. But Saturn’s rings - especially the innermost visual ring, the B ring—are far purer than its moons. Instead of the rings’ composition matching that of the moons, there is actually a distinct gradient in purity starting from the inner ring to the larger outer moons. Saturn’s inner B ring is about 99.9% pure water ice, the next ring out is about 99% pure, the small moons that are farther out are 90 to 95% pure, while the larger moons that are still further out are 50 to 90% pure.

Saturn’s Rings Are Extremely Young – Similar to how our bookshelves get dusty if they have not been cleaned for a while, objects out in space also get dusty over time, and we can get a sense of how old these objects are by how much dust has accumulated. The fact that Saturn’s nearest ring is 99.9% pure water ice indicates that these rings are very young indeed - possibly no more than ten million years. Furthermore, the progression of these objects becoming increasingly dirtier the farther we move outward indicates that the objects farthest away from Saturn are the oldest.

The Source of Saturn’s Rings Has Been Identified – To create the nearly pure water ice chunks that make up Saturn’s B ring requires a source of nearly pure water. In 2017, the Cassini spacecraft passed through the gap between Saturn and its rings and, in doing so, detected charged water-group ions (H₂O⁺, HO⁺, and O⁺). These charged ions, either by themselves or combined with hydrogen atoms, form the water that creates the rings. Furthermore, the fact that these are all positively charged ions means that even an extremely modest electrostatic field gives them the ability to overcome Saturn’s gravitational field: an electric field is lifting these ions up to Saturn’s rings.

This evidence indicates that while the scientists of the late 1800s thought they had figured out the origin of Saturn’s rings, they had actually got the process backward: instead of Saturn’s moons drifting inward and then being ripped apart to form the rings, water isotopes are being lifted up from Saturn’s equator before solidifying into a ring of countless tiny ice chunks. From this moment on they have been growing steadily larger as they bump into and stick to other ice clumps, while at the same time cosmic dust has been collecting on surfaces. Eventually these dirty ice chucks become so large that they are identified as moons, the smaller moons having odd potato shapes while the larger moons take on the typical spherical shape - the effect of greater gravitational pressure and internal heating.

Despite all the new evidence indicating time’s direction - that the ice chunks of Saturn’s rings are evolving into moons - the astronomy community is so deeply stuck in the past that it is unable to see the truth. Every astronomer today learned during childhood that Saturn’s rings are the remnants of a torn-apart moon, and so now they use pretzel logic to try to explain away the evidence indicating that their lifelong-held belief is wrong. While astronomy is a science, their unwillingness to adjust their beliefs in response to new evidence gives the astronomy community the feel of being more like a cult.

Moving on, with Saturn’s ring–moon system we are able to witness in real time the creation of moons, and this gives us insight into how all moons were created, and indeed into the formation of the planets themselves.

While our solar system was forming - before the fusion process began that lit our Sun - this region of the galaxy must have been an extremely cold place. With temperatures close to absolute zero, the contracting solar system would have been throwing off water ions that would quickly freeze. Considering the extremely cold conditions that must have existed before the Sun’s fusion process began, there were possibly other icy compounds forming as well as water ice. It was water ice and other icy substances that enabled rocky material to stick together and eventually grow into planets, and billions of years later many of these dirty blocks of ice still exist in the outer reaches of our solar system.

Having the correct explanation of how the moons and planets formed is much more than a history lesson; it is what gives us a true understanding of our solar system.

Is Saturn Shrinking?

As stated earlier, there is a progression in the amount of dust on the objects orbiting Saturn: Saturn’s inner B ring is about 99.9% pure water ice, the next ring out is about 99% pure, the small moons farther out are 90 to 95% pure, while the larger moons still farther out are 50 to 90% pure. Furthermore, since the amount of dust that accumulates correlates with the age of these objects, this means that while the inner rings may be only a few million years old, the outer moons may be more than a billion years old. This progression of age correlating with these objects’ distance from Saturn implies that, for the last billion years or so, once ice formed near Saturn’s surface, it has been steadily drifting away from Saturn.

Or so it appears.

Earlier it was explained that - contrary to what is implied by Roche Limit, and against what must seem like common sense - orbiting moons do not normally drift towards their host planet. Yet at the same time, when there are numerous moons orbiting a planet, these moons are not all that motivated to move away from the planet either.

The Earth’s moon is slowly drifting away from Earth at a rate of 3.8 cm per year. The reason this is occurring is that its gravitational pull on the Earth produces a tidal bulge, and because the Earth rotates faster than the moon revolves around it, the bulge pulls the moon forward and into a slightly higher orbit. While this is the effect of one moon, this effect is diminished as we increase the number of satellites orbiting a planet.

Consider the effect of the countless number of ice chunks that form the rings of Saturn. Since the ice in Saturn’s rings is evenly distributed, the gravitational pull of this mass of rings cannot create a tidal bulge on Saturn, and so there is no bulge pulling on these individual chunks of ice. Consequently, there is no reason to believe that the rings are slowly drifting away from Saturn.

When there are several moons orbiting a planet, the situation varies between times when most of them are all pulling in the same direction and thus creating a tidal bulge on the planet, and most other times when they are pulling from an assortment of directions and mostly canceling out each other’s effects. The end result is that when there are several moons, these moons will slowly drift away from the planet, yet this drifting rate will be so small that it is hardly noticeable or worth mentioning.

But if Saturn’s rings are not really moving away from Saturn, then how is it that there is this progression of younger to older orbiting objects as we travel away from Saturn? The answer is that movement is relative. It is not so much that these objects are moving away from Saturn as it is that the surface of Saturn is moving away from them: Saturn is shrinking.

Over billions of years, as Saturn collapsed in on itself, its rotational speed increased due to the conservation of angular momentum. Yet with greater rotational speed comes greater centrifugal force attempting to throw material off at its equator. Consequently, as Saturn is shrinking, it is also throwing off material at its equator - the latitude where the centrifugal force is the greatest. Furthermore, the material that it is throwing off is the positive water ions, because in an electric field the positive charge of these ions gives them an extra lifting force over the neutral charged material. For all the time that Saturn has been shrinking, it has been leaving behind rings of ice crystals that, over time, have clumped together while collecting dust and eventually evolving into the moons of Saturn.

Finally, Saturn is not an outlier among the gaseous planets but rather is simply the last one in completing this stage of its evolution. Not only do all of the other large gaseous planets have satellite systems - moons consisting of a mixture of ice and rock - but all of these planets have faint icy rings.

Still, we can expand this idea even further to consider how the solar system itself formed. While our solar system was forming, before the fusion process began that lit our Sun, this region of the galaxy must have been an extremely cold place. With it being close to absolute zero temperature, the contracting solar system would be throwing off water ions that would quickly freeze.Considering the extremely cold temperatures that must have existed before Sun’s fusion process began, there were possibly other icy compounds forming as well as the icy water. Similar to the process of how Saturn’s moons formed, these ice crystals soon gathered dust, clumped together, and eventually formed the planets.

Orbital Spacing of Moons and Planets

Because Saturn’s rings are an even distribution of ice chunks, from Earth, the rings look like a continuous sheet of material. However, further out, beyond the rings, we no longer see this sheet of material because it has all been gathered up by the orbiting moons. The reason for this change is that once some of these ice chunks become fairly large - the size of small moons - gravity then becomes effective in attracting and gathering up all the remaining small ice chunks. Besides removing the smaller ice chunks, the gravitational force also brings together many of these smaller moons.

Gravity is the attractive force between the center of mass of one object and the center of mass of another object. If these objects are isolated in space, such as when a small object approaches a much larger moon or planet, it is fairly easy to calculate the acceleration and movement of the smaller object as it is attracted to the much larger moon or planet. However, when these two objects are closer in size and are orbiting a much larger third object - such as the case for two moons orbiting a planet - determining the movements of these objects can be surprisingly complicated and even difficult to accurately predict. In astrophysics and rocket science, this is known as the three-body problem.

The speed of a moon orbiting a planet is determined by the radius of its orbit: the greater the radius, the slower its speed. Hence, if there are two moons orbiting a planet, the moon in the lower orbit will travel faster and eventually pass the slower moon at the higher orbit. Furthermore, if the orbits of these two moons are close to each other, the two moons will be strongly attracted to each other as one moon passes the other. The paths of these moons may change as a result of this gravitational attraction.

During this close encounter, there is an exchange of energy. As the fast moon in the lower orbit is approaching, it gains energy because the other moon is pulling it forward. After it passes, it loses energy because the pull from the other moon is slowing it down. Yet, out of the whole exchange, the fast moon in the lower orbit will gain more energy than it loses. The effect of this gain in energy is that the moon in the lower orbit is slowly moving into a higher orbit.

Because all forces are equal and opposite, a similar yet opposite result occurs with the moon in the higher orbit. Each time it is being passed, it first feels a pull slowing it down, and then, after the faster moon goes by, the moon in the higher orbit feels a pull speeding it up. Yet the total effect of these pulls is that the moon in the higher orbit is losing energy, and so it is slowly falling into a lower orbit. Consequently, with every orbital pass, these two moons are slowly coming ever closer to being in the same orbit.

Once these moons come really close to each other, depending on how they approach and the differences in their mass, a variety of outcomes are possible. If the two moons are nearly the same mass, it is possible that they will start doing a little "dance" where they switch orbitals every time they come near each other. Another possibility is that the pull from the larger moon will send the smaller moon off in an odd direction. And of course, there is also the possibility that one moon will simply collide with the other to create a single larger moon. The upshot of this chaotic evolution is that the surviving moons often end up in a dynamically stable configuration, with orbital radii that may exhibit roughly geometric spacing and/or orbital resonances.

Moon	Orbital Period (days)	Ratio to Io
Io	1.769	1
Europa	3.551	2.007
Ganymede	7.155	4.043
Callisto	16.689	9.43

The classic example of this orbital resonance is seen in the major moons of Jupiter. Their orbital spacing is based on the resonance of their orbital periods. The orbital periods of Io, Europa, and Ganymede are in a 1:2:4 resonance, meaning that as Io - Jupiter’s closest large moon - completes four orbits, Europa completes two, and Ganymede completes one. Besides these first three moons, there is also Callisto, which - being on moon that is furthest away - is not caught up in this resonance pattern. This resonance pattern - the doubling of the periods - makes for the most stable orbital positions when the moons are fairly large.

Because Saturn’s moon system is still a "work in progress," the resonance pattern is not so obvious for these younger, smaller moons. But look closer: Tethys’ orbital period is twice that of Mimas, and likewise, Dione’s orbital period is twice that of Enceladus. These four moons are intertwined even more since these two resonance patterns are overlapping. For these moons - Mimas, Enceladus, Tethys, and Dione - the next moon further out has a period that is about 1.43 times greater. Unlike Jupiter’s moons, these intermediate positions are currently still stable because these much smaller moons of Saturn have weaker gravitational fields, and so the closer spacing is tolerable.

Moon	Orbital Period (days)	Ratio to Mimas
Mimas	0.942	1
Enceladus	1.370	1.45
Tethys	1.888	2.00
Dione	2.737	2.91

While it is possible to make some sense out of the spacing of these moon systems, that is not to say that there is rhyme or reason to the spacing of all the moons of our solar system.

Beyond Dione, there isn’t really much of a clear resonance pattern for Saturn’s more distant moons: Rhea, Titan, Hyperion, and Iapetus. The same goes for the moon systems of Uranus and Neptune; for either of these moon systems, there is no clear resonance pattern similar to Jupiter’s moons.

This brings us to the Titius-Bode Law, which gives the spacing of the planetary orbits in our solar system.

In 1766, Johann Daniel Titius published his discovery of a simple numerical pattern that matched the orbital spacing of the planets. This mathematical rule was later popularized by Johann Elert Bode and is now known as the Titius-Bode Law.

The rule begins with the sequence 0, 3, 6, 12, 24, 48, 96, 192, 384. Adding four to each of these numbers gives 4, 7, 10, 16, 28, 52, 100, 196, 388, and then dividing each result by 10 yields 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, and 38.8. These numbers correspond to the orbital radii of the planets in astronomical units (AU), with 1 AU being the orbital radius of Earth.

Planet	n value	Predicted (AU)	Actual (AU)
Mercury	–∞	0.4	0.39
Venus	0	0.7	0.72
Earth	1	1.0	1.00
Mars	2	1.6	1.52
Asteroid belt	3	2.8	2.77
Jupiter	4	5.2	5.20
Saturn	5	10.0	9.58
Uranus	6	19.6	19.18
Neptune	7	38.8	30.07

These numbers can also be generated by using the formula:

A = 0.4 + 0.3×2ⁿ

Where:

• n = −∞ for Mercury, and then
• n = 0, 1, 2, 3, … for Venus, Earth, Mars, etc.

With the exception of the last planet, Neptune, the Titius-Bode Law is surprisingly good at predicting the actual planetary distances. However, this is somewhat problematic for the astronomy community because they still have not come up with a reasonable explanation for why it works.

In modern times, the discovery of numerous exoplanet systems has led astronomers to wonder if the planets of these exoplanet systems might also follow the Titius-Bode Law. Currently, the answer to this question could be either yes or no, depending on one’s interpretation of the data. Just as the moon systems in our own solar system do not all show resonance patterns, some of the exoplanet systems show logarithmic spacing of their planets that is similar to the Titius-Bode Law, while some do not.

What is interesting about our solar system’s orbital spacing pattern - the Titius-Bode Law - is why there should be a 0.4 added to all the values, and how is it that the spacing does not appear to be based on the resonance of the planets’ periods?

The Evolution of Moons and Planets

Our solar system consist of objects that have hardly evolved at all since the time they were created, those planets and moons that are currently evolving, and the celestial objects have finished their evolution and so they are now geologically dead.

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.



Comet

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

There are two ways that planets or moons can be stretched: either it is because other large objects come close to them because they on traveling in a nearby orbit, or it is because they are rotating in the presence of a strong gravitational field. Of course the significance of this stretching is that these tidal forces warm the interior of the moon or planet causing significant geological activity as the ice and other materials melt and then migrate to the surface.

Initially most celestial objects rotate faster on their axis than they revolve around their host planet. However if they are close to the planet such that they are in a strong gravitational gradient they will experience strong tidal forces that stretches them and slows down their rotation until they tidally lock with their host planet. In this tidally locked position the moon will slowly rotate at the same speed as they revolve around the planet so that the same side always faces their host planet. For example, the Earth’s moon is tidally locked with the Earth and so when we look at the Moon we are always looking at the same features on this near side of the Moon.

Because the Moon is tidally locked with the Earth, the Moon is no longer experiencing strong tidal forces attempting to stretch it. Furthermore, because it is not being flexed it is no longer generating heat within its interior. Consequently the Earth’s moon is geologically dead.

In contrast the Earth is very much geologically alive. Because the Earth in rotating within the gravitational fields of the Moon and the Sun, what happens to the Earth is very different from what happens to the Moon. Because of the tidal pull from the Moon, Earth’s surface – along with the ocean waters – rises and falls twice a day. This movement generates frictional heating within the Earth, resulting in high temperatures within the Earth, earthquakes and continental movement, and volcanic activity. How tidal forces heat the Earth was the subject of the previous chapter.

Despite our greater familiarity with the Earth and Moon each of these celestial objects are actually special cases. Whereas Earth just has the one moon while the four outer planets all have several moons in orbit around them. The difference between there being one moon or several is that when there are several moons orbiting the same planet these moons will be pulling on each other each time they lap each other as they orbit the planet. The result is that even though they are tidally locked with the host planet, they are still experiencing strong tidal forces each time these moons are passes near each other. It is this interplay between these moons producing the tidal heating in their interiors.

Earth is a special case in regards to tidal heating because the Earth’s moon is so large. Whereas the moons of the gas planets are typically thousands of times less massive than their host planet, Earth’s moon is about one eightieth (1/80) the mass of Earth. It is because the mass of the Moon is comparable to the mass of the Earth, and because it is relatively close to the Earth, that it is capable of applying such strong tidal forces to the Earth. In return the Earth applies even greater tidal forces to the Moon and yet these forces to not generate heat within the Moon because the Moon is not rotating in respect to the Earth – it is tidally locked.

In the previous chapter we learned that tidal heating is what warms the interior of the Earth. Tidal heating is also the mechanics for warming Jupiter’s moon Io. But why stop there? Physics processes are general meaning that what works in one part of our universe should also work everywhere else in our universe where the conditions are the same. Tidal heating is the primary means of heating the interior of the moons and planets of our solar system – it is the primary way that moons and planets evolve.

There are two ways that planets or moons can be stretched: either it is because other large objects come close to them because they on traveling in a nearby orbit, or it is because they are rotating in the presence of a strong gravitational field. Of course the significance of this stretching is that these tidal forces warm the interior of the moon or planet causing significant geological activity as the ice and other materials melt and then migrate to the surface.

Most celestial objects rotate or at least initially they rotated. Whether they still rotate today mostly depends on the strength of gravitational gradient. A moon in a strong gravitational gradient will experience strong tidal forces until it becomes tidally locked such that one of its sides always faces the host planet. Even after becoming tidally locked it still possible for tidal heating to occur if there are any other moons nearby – such is the case of Jupiter’s moon Io.

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



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 daily flexing of Earth from the pull of the Moon and Sun heats its interior, increasing its density.

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.

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.

*****




Comparative Size of the Planets of Our Solar System




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.

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



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.



The daily flexing of Earth from the pull of the Moon and Sun heats its interior, increasing its density.

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

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

Planet	Distance 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



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.



Venus and Moon

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.



This is the Thick Atmosphere Theory explained in one simple diagram. If not for the evolution of life on Earth, Earth's present atmosphere would be an extremely thick carbon dioxide atmosphere: a composition similar to the other terrestrial planets of Venus and Mars.



From water springs life. This is what makes Earth a unique planet.

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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External Links / References

Ancient Observatories

• The Role of Astronomy in Ancient Monuments - The Archaeologist
• Ancient Observatories around the World - NETGELVIN
• Ancient Observatories - STANFORD SOLAR CENTER
• Ancient Observatories - Teach Astronomy

Probability Theory

• Probability Theory - Geeks for Geeks
• Probability Theory - Cuemath
• Basic Probability

Destiny

• Is There Such A Thing As “Destiny”? - Living in Wellbeing
• Destiny: Is it a reality or a superstition? - Psychology Spot
• Destiny - Free Dictionary

Formation of Solar System

• The Condensation Theory - Chaisson
• An Overview of the Solar System - Bill Arnett
• Cosmic Dust - David Darling

Spiral Galaxies

• What Is a Spiral Galaxy? - Space
• Galaxies: Spiral Galaxies - NASA
• Galaxies - SEDS

Magnetic Attraction

• How Do Magnets Work? - Science News Today
• Magnetism - Geeks for Geeks
• Ferromagnetism - Hyperphysics
• Attraction Of Magnets - Magnet Manila

Electric Static Attraction

• Electrostatic Force - Science Facts
• Electrostatics - Examples
• Fundamentals of Electrostatics - EE Power



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