Planet Earth the Origin of Land and Life

PLANET EARTH THE ORIGIN OF LAND AND LIFE This chapter records the formation of the Earth and the creation of life. Earth was born during an unusual time of star formation. Only a fraction of the stars in the Milky Way galaxy are single, medium-sized stars like the Sun, and even fewer have planetary systems orbiting. Therefore, the Sun with its nine planets and satellites is considered rare among the stars. Earth is unique among planets, with roving continents and evolving life forms. It is the only planet with an ocean of water and an atmosphere of oxygen. It also has a rather large moon compared to its parent planet, a pair that still defies explanation. The atmosphere and oceans evolved during turbulent periods of crustal formation, volcanic outgassing, and comet degassing. Many giant meteorites hit the Earth, adding unique ingredients to the boiling cauldron. The raging storm brought flood after flood and an unimaginable display of electricity. From this chaos comes life. SOLAR Way back in the very hazy past, about 12 billion years ago, the universe originated with such force that the farthest galaxy flung away from our Milky Way was an elliptical galaxy, with five spiral arms flaking off a central bulge. The new stars come from dense regions of interstellar gas and dust called giant molecular clouds. Several times a century, a giant star more than 100 times more massive than the Sun explodes, producing a supernova a billion times brighter than a regular star. When a star reaches the supernova stage, after an extremely hot existence for several hundred million years, the nuclear reactions in its core become extremely explosive. While the star sheds its outer covering, the core compresses into a very dense and hot object called a neutron star, similar to Earth's condensation to the size of a golf ball. The stellar material that expanded from the supernova formed a nebula composed mostly of hydrogen and helium along with particles composed of all other known elements. About a million years later, the solar nebula collapsed into a star. This process begins when shock waves from a nearby supernova compress parts of the nebula, causing the nebula's material to collapse on itself by the force of gravity into a protostar. When the solar nebula collapsed, it rotated faster and faster, and the spiral arms released the rapidly rotating nebula to form a protoplanetary disk. Meanwhile, the heat of compression initiates thermonuclear reactions in the core, and a star is born. A new star forms in the Milky Way galaxy every few years or so. About 4.6 billion years ago, the Sun, a regular main sequence star, lit up in one of the galaxy's dusty spiral arms about 30,000 light-years from the center. Medium-sized single stars like the Sun are rare. Because of their unique evolution, these stars seem to be the only ones with planets. So, of the myriad stars overhead, only a handful may have planetary systems orbiting, and even fewer may contain life. When the Sun first lit up, the strong solar wind blew away the lighter components of the solar nebula and deposited them into the outer regions of the solar system. The remaining matter in the inner solar system consists mostly of rocky and metallic fragments, ranging from fine grains of sand to boulders. In the outer solar system, where temperatures are much cooler, rocky material along with dense chunks of water ice, frozen carbon dioxide, and crystalline methane and ammonia condense. The outer planets are believed to have rocky cores the size of Earth, surrounded by a mantle composed of water ice and frozen methane, and thick atmospheres, composed mostly of hydrogen and helium. The sun was extremely unstable during the first billion years of its existence. The solar output is only about 70 percent of its current intensity. The Sun only provided warmth on Earth as it does today on Mars. The early sun quickly rotated on its axis, completing a single rotation in just a few days compared to 27 days today. Periodically, the Sun expands to one-third larger than its current size, and enormous solar flares leap millions of miles into space. This intense activity makes the Sun emit more heat, allowing it to cool and return to normal size. During the early developmental stages of the Sun, it was surrounded by a protoplanetary disk consisting of several bands of coarse particles, called planetesimals that accreted from dust grains thrown by supernovae. About 100 trillion planetesimals orbited the Sun during the early stages of the solar system's development (Figure 2). As they continue to grow, tiny rocky chunks swing around the baby Sun in highly elliptical orbits along the same plane called the ecliptic. Constant collisions among planetesimals build larger bodies, some of which grow to more than 50 miles in width. However, most of the planet's mass is still in small planetesimals. The presence of large amounts of gas in the solar nebula slows down planetesimals, allowing them to coalesce into planets. Planetesimals in orbit between Mars and Jupiter cannot merge into planets due to Jupiter's strong gravitational attraction and instead form asteroid belts, many of which are several hundred miles wide. An incredible amount of water, one of the simplest molecules, resides in the solar system. As the Sun emerges from the gas and dust, small pieces of ice and rock debris begin to gather in the cold, flat planetesimal disk surrounding the baby star. Temperatures in some parts of the disk may have been warm enough for liquid water to exist in the first solid bodies in the solar system. In addition, water vapor in the primordial atmospheres of the inner terrestrial planets may have been eroded by the bombardment of planetesimals and blown out of Mars by the strong solar wind of the baby Sun. Once planted at the end of the solar system, water coalesces to create icy bodies streaked by the Sun as comets. The solar system is quite large, consisting of nine known planets and their moons (Figure 3), although controversy persists as to whether Pluto is actually a planet or another type of body. The image of the original solar disk can be traced by observing the motion of the planets. All of them revolve around the Sun in the same direction as they rotate, and all but one, Pluto, orbit within 3 degrees of the ecliptic. Since Pluto's orbit is inclined 17 degrees to the ecliptic, it may be a captured planet or perhaps a moon of Uranus that was thrown out of orbit due to a collision with another object or a comet from the Kuiper belt. About 7 billion miles from the Sun lies the heliopause, which marks the boundary between the Sun's domain and interstellar space. About 20 billion miles from the Sun is a region of gas and dust, possibly remnants of the original solar nebula. A ring of comets that make up the Kuiper belt is located on the ecliptic in this region. A few trillion miles from the sun is a comet shell called the Oort cloud that formed from the remnant gas and ice of the original solar nebula.