Summary of the Birth of the Earth-Moon System

Scientists have concluded that the solar system (the Sun and the nine planets that orbit it) formed from a vast cloud of gas floating in space. This cloud is referred to as a nebula, contained hydrogen and helium which is the left over from the big bang (the cataclysmic explosion considered to mark the origin of the Universe) as well as other elements that had been formed by fusion reactions in stars and supernovae. Due to the force of gravity, the gases in a nebula gradually pulled together to form a rotating disc of gas with a bulbous center. The ball at the center of this disc eventually became dense and hot enough for fusion reactions to begin, and at that instant it became the glowing Sun. Rings of residual gas surrounded the Sun. Within these rings, dust particles (composed of minerals and metals) gradually condensed. Gravity caused these particles to attract one another so that they combined to form solid planetesimals. Eventually, planetesimals collided and combined to form larger bodies called protoplanets. Once a protoplanet became large enough, it became hot and soft inside, and reformed into a sphere. The Earth formed in this way. Soon after Earth formed, it collided with a large protoplanet. The collision blasted off debris that then coalesced to form the Moon. With time, the atmosphere and oceans formed on the Earth, and tectonic processes yielded the continents.

The Nebular Hypothesis

The nebular hypothesis for the origin of the Solar System can be summarized as follows:
(1) The solar nebula takes form.
(2) It begins to rotate, then flattens and contracts.
(3) The nebula is now distinctly discoidal and has a central concentration of matter that will become the sun.
(4) Thermonuclear reactions begin in the primordial sun as material in the thinner area of the disk condenses and accretes to form protoplanets.
(5) In this late stage in the evolution of the Solar System planets are fully formed, and materials not incorporated into planets are largely swept away by solar wind and radiation.

As the sun was contracting from the nebular cloud, the flattened plane of the cloud began to dissociate itself into its own matter lumps. Small bodies moving through the dust and gas of the cloud began to collide and accumulate. The largest of these bodies probably had diameters no greater than a few tens of kilometers, and they have been appropriately called planetesimals.

Even though the orbiting planetesimals traveled rapidly about the newly forming sun, their speed relative to one another was not great, and they were attracted by gravity into larger and larger masses. These larger bodies, called protoplanets, were bombarded by planetesimals as they swept through their orbital paths. Earth grows, therefore, from the accretion of planetesimalms. This accretion began about 4.6 billion years ago, and it most likely ended with a completed Earth by 3.8 billion years ago. The end of the planetesimal bombardment can be deduced from the 3.9-billion-year age for the oldest rocks found on our planet. Any older rock material was probably destroyed by the high-speed impacts of these gravitationally attracted bodies. Because the planetesimals accreted in the nebular cloud from which the sun also formed, this explanation for planetary formation is known as the nebular hypothesis.
 
Our sun's system of planets and other bodies does not appear to be unique. With the help of the Hubble Space Telescope, some satellites, and ground-based radio and optical telescopes, astronomers have now identified increasing numbers of suns enveloped in nebular clouds. The picture of the Orion Nebula <http://www.noao.edu/image_gallery/html/im0349.html>, a stellar nursery, reveals the common occurrence of disks from which planets can form <http://www.noao.edu/outreach/aop/observers/m43.html>. Typically, the disks have larger diameters than our Solar System, with the largest observed disk having a diameter of almost 84 billion kilometers (53 billion mi). One of the young Orion stars and its attendant disk are shown  <http://www.noao.edu/outreach/aop/observers/m42.html>  Another view of the Orion Nebula shows a disk heated by the star Theta C Orion is, revealing lumps of matter that may be evolving into protoplanets <http://www.noao.edu/outreach/press/pr03/pr0301.html>

As the protoplanets of our Solar System accreted from the nebular cloud, the four closest to the sun formed from an abundance of silicon and iron, becoming largely rocky bodies. These planets Mercury, Venus, Earth, and Mars-are very dense spheres, and because they are relatively small and Earth-like, are known as the terrestrial planets (from terra, Latin for "earth"). By contrast, four of the five outer planets are very large. Called the Jovian planets (after the Roman god Jove, or Jupiter), Saturn, Uranus, Neptune, and Jupiter are large, gaseous spheres with unknown interiors. The cloud covers on these planets are too thick for cameras and telescopes to penetrate.

Regardless of their clouds and their distance from us, we know much about the planets and other celestial bodies, such as comets and Earth's moon, because of observations made with the aid of telescopes and spacecraft. We have sent spacecraft to land on both Mars and Venus, and all but Pluto have been visited by instrument packages, such as the Voyager craft. In December 1995, the Galileo spacecraft sent a probe into the upper atmosphere of Jupiter. What follows is a brief summary of what we have learned.

The Planets: Planetary Motions

Planets exhibit two types of motion. The first, rotation, is a spinning motion centered on an axis. The ends of the axis are the poles, labeled north and south. As planets rotate, they turn their spheroid shapes simultaneously toward and away from the sun. The side opposite the sun experiences night, with little incoming radiation (none from the sun). The side oriented toward the sun experiences day, and the sun's electromagnetic radiation warms the surface. One rotation is a day, but each planet's period of rotation (thus, its day) is different.

Earth's rotation has been divided into 24 units, or hours, which represent ISO of rotation ("Time Zones"). Based on Earth's rotation, other planets have either longer or shorter days. Mercury, for example, requires 59 Earth days for a single rotation; Neptune, by contrast, rotates in only 22 hours. Because the planets differ in size, each spins about its axis at a different rate. Although Neptune's day is only 2 hours shorter than Earth's, its spinning must be much faster to turn a spheroid about four times larger. Jupiter, the largest planet, turns its 448,620-kilometer (280,934-mi) equatorial circumference in just 9 hours and 50 minutes.

The second type of planetary motion is revolution, an almost circular motion around the sun in a path called an orbit. Astronomers designate Earth's revolution as a year. Because a period of revolution is dependent on distance from the sun, planets farther than 1 AU have longer years than Earth: Saturn's year is equivalent to 29.46 Earth years; Pluto requires 247.7 Earth years for one revolution. The orbital speed of the planets slows with increasing distance from the sun. Earth travels around the Sun at 24.1 km per second (14.94 mph), whereas in its orbit, Pluto orbits the sun at 4.7 kilometers per second (2.96 mph).

Planetary Compositions and Structures

The planets differ in composition because they formed in different parts of the nebular cloud. The terrestrial planets seem to be dominantly formed from oxygen, silicon, iron and aluminum, whereas the Jovian planets are gaseous balls of hydrogen and helium. Other elements and many compounds, including those that make up rocks, are also present in the Jovian realm. Water (and thus, oxygen) is an important component, but it exists mostly as ice, such as the particles that makes up the rings of Saturn or its ice-ball moon Enceladus. At least five of Uranus's moons are ice.

The inner planets all appear to have dense cores dominated by iron. These cores vary in size, and each seems to be overlain by a sphere of rocky material called a mantle. The outermost structure, the surface, is a crust that has been shaped by volcanic activity, chemical reactions, physical processes, planetesimal or meteorite bombardment, and internal forces. The structure of the outer planets is difficult to determine. The Jovian planets' thick veils of gas prevent observation of their interiors. An attempt to penetrate the atmosphere of Jupiter was part of the Galileo spacecraft's mission, and its probe entered the planet's atmosphere in July 1995 (see chapter 4)

The Solar System: the Sun and the group of objects in orbit around it
The Earth is one of nine planets in our solar system, the Sun and the group of objects in orbit around it. In addition to the Sun and the planets, the solar system also includes 64 known moons, a vast number if asteroids, millions of comets and innumerable fragments of rock and dust called meteoroids.

All the objects in our solar system move through space in smooth, regular orbits, held in place by gravitational attraction. The planets, asteroids, comets, and meteoroids orbit the Sun, and the moons orbit the planets.

The planets can be separated into two groups on the basis of their physical characteristics and distances from the Sun. The innermost planets Mercury, Venus, Earth, and Mars-are small, rocky, and relatively dense. They are similar in size and chemical composition, and are called terrestrial planets because they resemble Terra ("Earth" in Latin). With the exception of Pluto, the outer planets are much larger than the terrestrial planets, yet much less dense. These jovian planets-Jupiter, Saturn, Uranus, and Neptune-take their name from Jove, an alternate name for Jupiter. The jovian planets probably have small solid centers that may resemble terrestrial planets, but much of their planetary mass is contained in thick atmospheres of hydrogen, helium, and other gases. The atmospheres are what we actually see when we observe these planets. Pluto-the smallest of the nine principal planets-doesn't fit into either of these planetary groups: it is much smaller than the jovian planets but much less dense than the terrestrial planets. In many respects, Pluto is more like a large comet than a planet.

Terrestrial planets: The inner plantges of the solar system: Mercury, Venus, Eafrth and Mars. Jovian planets: the “gas gigant” outer planets: Jupiter, Saturn, Uranus, and Neptune.

The Origin of the Solar System

How did the solar system form? We may never know the precise answer to this question, but we can discern the outlines of the process from evidence obtained by astronomers and Earth scientists, and from the laws of physics and chemistry. The process began in a part of space that was not entirely empty because earlier stars had exploded, scattering matter across vast distances of space. Most of the matter in this cloud of interstellar gas and dust was composed of the element hydrogen, but small percentages of all the other chemical elements were present too. Everything in the solar system was eventually constructed from such matter. In a sense, the Earth and everything on it is made of star dust.

The swirling cloud of dust and gas began to thicken over a very long period, as the atoms in it gravitated to each other. As the atoms moved closer together, the rotating gas cloud flattened into a disk (Fig. 1). Near the center of the disk, pressure and temperature were extremely high. There, in the newly forming Sun, hydrogen atoms were subjected to such high pressures and temperatures that they began to undergo nuclear fusion. The fusion of hydrogen atoms to form heavier helium atoms is still going on inside the Sun; this is the source of the Sun's radiant energy. However, nuclear fusion within the Sun does not produce many elements heavier than helium. Almost all of the heavier elements in our solar system were inherited from the explosion of earlier generations of stars that were much more massive than our Sun.

Eventually, the outer portions of the cosmic gas and dust cloud cooled enough to allow solid particles to condense, in the same way that snowflakes condense from water vapor. The materials that condensed from the cloud eventually formed the planets, moons, and other solid objects of the solar system, including the Earth (Fig. 1.6D). The gas and dust cloud is called the solar nebula, and this story of the birth of the solar system and its component parts is called the nebular theory.
Condensation of the gas cloud is only the first part of the planetary birth story. Condensation formed innumerable small, dust-sized particles, but the particles still had to be joined together to form a planet. This happened as a result of impacts between fragments drawn together by random collisions and gravitational attraction. The largest masses slowly swept up more and more of the condensed dust particles, growing into even larger planetesimals and eventually becoming the planets. The largest of these bodies held huge gaseous envelopes primordial atmospheres-in their gravitational grasp.
This growth process, the gradual gathering of more and more bits of solid matter from surrounding space, is called planetary accretion. Ancient rocky fragments that are left over from that long-ago process still exist in space and still fall to Earth from time to time; we call them meteorites. Meteorites and the scars of ancient impacts provide evidence of the way the terrestrial planets grew to their present sizes. The formation of the Earth and other planetary bodies through the processes of condensation and planetary accretion was essentially complete 4.56 billion years ago.

The Family of Planets

The nine planets and other planetary bodies (meteoroids, asteroids, moons, and  comets) are like siblings; all were born of the same processes that gave rise to the whole solar system family. Some of the planets are more alike than others. The Earth and its close neighbor Venus, for example, are so much alike in size, density, and chemical composition that they are almost "twins." As a group, the terrestrial planets have many things in common beyond their small sizes and rocky compositions. They have all been hot and, indeed, were partially molten at some time early in their histories. During the period of partial melting, all of the terrestrial planets separated into three layers of differing chemical composition: a relatively thin, low-density, rocky crust, the outermost layer; a rocky, intermediate-density mantle; and a metallic, high-density core. This separation process is called planetary differentiation.

There are other similarities as well, three of which are very important. First, all of the terrestrial planets have experienced volcanic activity. The volcanism is dominated by the formation of a special kind of volcanic rock called basalt. The second similarity is that all of the terrestrial planets passed through a period of intense meteorite impacts and surface modification by cratering processes that continue today, though fortunately at a much slower pace. The third similarity arises from the size and closeness of the terrestrial planets to the Sun. Unlike the jovian planets, the terrestrial planets were too small to hold on to their original envelopes of gas, which were swept away by intense eruptions of the Sun early in the history of the solar system. The three terrestrial planets that ended up with atmospheres (Earth, Mars, and Venus) evolved new gaseous envelopes from material that slowly leaked out from their interiors via volcanoes.

To summarize, when we look at the solar system we see a group of planets and other objects that are related by the way they were formed and by their association with the Sun. Within this system is a smaller group, the terrestrial planets, which are related even more closely by their similar origins and planetary characteristics. The Earth is the way it is because of all the things that have happened during its long history. This history is different enough from those of the other terrestrial planets to make the Earth habitable, while the others are not.