CHAPTER 4. ECOLOGY OF OUR GALAXY, SOLAR SYSTEM, AND EARTH
Chapter one discussed the origin of the universe some fifteen billion years ago. This chapter discusses the creation of our galaxy, solar system, and earth. This particular development created an ecological setting that was fortunate indeed. The Earth exists in an ecological niche in the universe that permitted the appearance of life. If it were not for the peculiar ecological distribution of our solar system, human beings would not exist. Humans exist because the earth was the planet that fell into an orbit not too close to and not too far from the sun.
This chapter shows that the development of the earth was a natural process, following basic physical laws. (From here on in, instead of referring to time as marching forward from the time of the Big Bang, as done in the previous chapter, time will be measured in relation to the present time.)
Humans have a tendency to think that because the origins of life and humans in particular are so complicated that these events must have been directed by some force beyond natural laws. However, it is only in hindsight that the origin of life and the development of man seem highly ordered. The evolutionary trip to humans took billions of years and was the result of the survival and death of millions of species. The end result only appears highly directed because humans have a hard time understanding billions of years of time. While it is possible that a superior being set the whole universe in process, the actual origins of the earth and life are explainable in terms of basic natural laws.
Origins of Our Solar System
A clump of different solar systems constitutes a galaxy and there are perhaps as many as 100,000 million galaxies in the universe. Our particular solar system is in the galaxy known as the Milky Way. This galaxy stretches 100,000 light-years from one end to the other and contains roughly 100 billion stars.
The Earth is a part of our solar system, which consists of the sun and nine planets. The Sun itself is a stationary star (i.e., a self-luminous sphere of gas), while the planets are not self- luminous and orbit the Sun.
The origins of our solar system can be traced to about 4.6 billion years ago when a "cloud," called a solar nebula, consisting mainly of hydrogen, formed. This cloud began to swirl, its rotation the result of slight imbalances among the strong attractive forces. As it swirled, the solar nebula collapsed upon itself, its temperature increasing by its own gravitational energy.
The amount of spinning motion that an object has is called angular momentum. The name derives from the fact that a twirling object has to rotate through the 360 degrees of a circle. Different points on a spinning object move at different speeds. The outer point of a disk travels much farther in one second than any inner point. Since the outer part of the disk is moving faster, it generates more momentum than the inner part of the disk. Therefore the angular momentum of a spinning object depends on its mass, its rate of rotation, and its radius. The larger the mass or the faster the velocity, the more momentum a spinning object has. Due to the conservation of angular momentum, the rotation speed of the solar nebula increased. As the disk collapsed, the cloud spun faster to maintain the same momentum. The rapid spin of the gases caused the solar nebula to evolve into a disk shape with most of the material confined to a thin sheet. At the center of the disc, the continuing collapse ultimately led to self-sustaining nuclear reactions, thereby creating the Sun.
At the end of the collapse phase, with no more gravitational energy to heat it, the nebula began to cool. In the center, however, the newly formed Sun kept the temperatures high. This created a temperature gradient, with the temperature varying with the distance from the sun. As the nebula cooled further, the gases interacted to produce chemical compounds, and eventually these compounds condensed into liquid droplets or solid grains.
The solid grains became sorted chemically by distance from the sun. This process is known as the chemical condensation sequence. An examination of the composition of the planets and the Sun shows a clear progression from the metal-rich planets close to the sun, through predominantly rocky materials, out to domination by ice in the planets farthest from the sun.
Just how the planets formed is not known. There are two competing theories. According to accretion theory, the planets formed when the nebula condensed into lumps at various places. It is believed that these lumps formed rather quickly into larger and larger aggregates, until most of the solid material was in the form of planetesimals (bodies smaller than the planets themselves). Some planetesimals were large enough to attract their neighbors gravitationally and thus to grow by accretion. The second theory stresses the role of gravity. This proto-planet theory holds that nebular dust accumulated around a number of centers of gravity and built up the planets at those places.
There could have been a third scenario that combined the two theories: accretion for the inner solar system and proto-planets for the outer system. In the inner solar system, four large centers of accretion developed, with perhaps several dozen more objects of at least the size of our Moon still whizzing about among them. Measured in units of distance (with the distance from Earth to the Sun being a unit of one), the distances from the Sun for the innermost or inner planets are: Mercury 0.39, Venus 0.72, Earth 1.00, and Mars 1.52. Venus, Earth and Mars are rocky planets composed largely of silicate rocks and metals.
In the outer solar system, where the building blocks included ices as well as silicates, much larger bodies grew, with masses of ten to twenty times the mass of the Earth. The distances from the Sun for the outer five planets are: Jupiter 5.20; Saturn 9.60; Uranus 19.00; Neptune 30.00; and Pluto 40.00. The first four outer planets are large or Jovian planets, while Pluto is much smaller. The Jovian planets lack solid surfaces; their composition is primarily light elements (hydrogen, helium, argon, carbon, oxygen, and nitrogen) in gaseous or liquid form.
The Earth itself formed about 4.5 billion years ago. About four billion years ago, a section that became the moon split away from the Earth after being hit by a large planetesimal about the size of Mars. Much of what remained of the interloper fused to the Earth and became part of the overall developing planet.
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