The Solar System 7: Touring Our Solar System

The Atmospheres of the Planets 2

A comparatively warm body with a small surface gravity, such as our moon, cannot hold even heavy gases, like carbon dioxide and radon. Thus, the moon lacks an atmosphere. The more massive terrestrial planets of Earth, Venus, and Mars retain some heavy gases. Still, their atmospheres make up only a very small portion of their total mass. In contrast, the Jovian planets have much greater surface gravities. This gives them escape velocities of 21 to 60 kilometers per second—much higher than the terrestrial planets. Consequently, it is more difficult for gases to escape from their gravitational pulls. Also, because the molecular motion of a gas depends on temperature, at the low temperatures of the Jovian planets, even the lightest gases are unlikely to acquire the speed needed to escape.

Formation of the Solar System

Between stars is "the vacuum of space." However, it is not a pure vacuum because it is populated with regions of dispersed dust and gases. A cloud of dust and gas in space is called a nebula (nebula = cloud; plural: nebulae). A nebula, like the one shown in the figure, often consists of 92 percent hydrogen, 7 percent helium, and less than 1 percent of the remaining heavier elements. For some reason not yet fully understood, these thin gaseous clouds begin to rotate slowly and contract gravitationally. As the clouds contract, they spin faster. For an analogy, think of ice skaters—their speed increases as they bring their arms near their bodies.

Planetesimals were the predecessors of the planets. How did planetesimals form?

Bits of matter collided and clumped together.

The Planets: An Overview 3

Density, chemical makeup, and rate of rotation are other ways in which the two groups of planets differ. The densities of the terrestrial planets average about five times the density of water. The Jovian planets, however, have densities that average only 1.5 times the density of water. One of the outer planets, Saturn, has a density only 0.7 times that of water, which means that Saturn would float if placed in a large enough water tank. The different chemical compositions of the planets are largely responsible for these density differences.

Earth planetesimal history

Earth's early history was dominated by a multitude of high-energy collisions. The kinetic energy from these impacts created a great deal of heat. Gravitational compression contributed to the protoplanet's rising temperature, too. Over time, the temperature of the Earth's interior rose to the melting point of iron. Molten iron moved toward the planet's core due to gravity. In exchange, silicate minerals and other lighter materials migrated toward the crust and mantle. Today, Earth's high internal temperature is believed to come from a combination of heat leftover from planet accretion and that produced through the radioactive decay of heavy minerals and elements in Earth's interior.

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Planetesimals 3

In the inner solar system, close to the sun, temperatures were so high that only metals and silicate minerals could form solid grains. It was too hot for ices of water, carbon dioxide, and methane to form. The inner planets grew mainly from substances with high melting points. In the frigid outer reaches of the solar system, on the other hand, it was cold enough for ices of water and other substances to form. Consequently, the Jovian planets grew not only from accumulations of solid bits of material but also from large quantities of ices. Eventually, the Jovian planets became large enough to gravitationally capture even the lightest gases, such as hydrogen and helium. This enabled them to grow into giants.

Planetesimals 2

Planetesimals acquired enough mass to exert a gravitational pull on surrounding objects. Eventually, the objects cleared the area of their orbits of small objects and debris. More mass also exerts enough force to pull the planetesimals into a sphere, unlike small bodies such as asteroids, which may retain an irregular shape. In addition, the additional mass increases the temperature of the planetesimals. This increase in temperature also led to the differentiation of the materials in the object. Differentiation occurs as fluid materials move based on density. Denser materials, such as heavy metals move to the core of a planetesimal and lighter materials such as silicates rise towards the crust. In this way, they added still more mass and grew into true planets.

Nebular Theory

Scientific studies of nebulae have led to a theory concerning the origin of our solar system. According to the nebular theory, the sun and planets formed from a rotating disk of dust and gases. As the speed of rotation increased, the center of the disk began to flatten out, as shown in the figure. The matter became more concentrated in this center, where the sun eventually formed.

The Planets: An Overview 2

Size is the most obvious difference between the terrestrial and the Jovian planets. The diameter of the largest terrestrial planet, Earth, is only one-quarter the diameter of the smallest Jovian planet, Neptune. Also, Earth's mass is only 1/17 as great as Neptune's. Hence, the Jovian planets are often called giants. Because of their distant locations from the sun, the four Jovian planets are also called the outer planets. The terrestrial planets are closer to the sun and are called the inner planets. As we shall see, there appears to be a correlation between the positions of these planets and their sizes.

The Atmospheres of the Planets

The Jovian planets have very thick atmospheres of hydrogen, helium, methane, and ammonia. By contrast, the terrestrial planets, including Earth, have meager atmospheres at best. A planet's ability to retain an atmosphere depends on its mass and temperature, which accounts for the difference between Jovian and terrestrial planets. Simply stated, a gas molecule can escape from a planet if it reaches a speed known as the escape velocity. For Earth, this velocity is 11 kilometers per second. Any material, including a rocket, must reach this speed before it can escape Earth's gravity and go into space.

What is the basis of the classification of substances that make up the planets?

The classification of the substances that make up the planets is on the basis of their melting points.

Planetesimals

The growth of planets began as solid bits of matter began to collide and clump together through a process known as accretion. Accretion occurs as small objects crash into each other. Some of the kinetic energy is converted into thermal energy on impact. Sometimes these objects collide with enough energy to slightly melt and stick together. The colliding matter formed small, irregularly shaped bodies called planetesimals. As the collisions continued, the planetesimals grew larger. As the planetesimals grew in mass, gravity caused the bodies to compress. As the planetesimals compress, the internal energy of the object increases, and the temperature of the object will increase. Radioactive materials will also increase the temperature as the materials decay.

How do the terrestrial planets differ from the Jovian planets?

The most obvious difference between the terrestrial planets and the Jovian planets is size. The terrestrial planets are relatively small and rocky. The Jovian planets are gas giants.

The Planets: An Overview

The planets fall quite nicely into two groups. The terrestrial planets—Mercury, Venus, Earth, and Mars—are relatively small and rocky. (Terrestrial = Earth-like.) The Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are huge gas giants. (Jovian = Jupiter-like.)

The Interiors of the Planets

The substances that make up the planets are divided into three groups: gases, rocks, and ices. The classification of these substances is based on their melting points. 1. The gases—hydrogen and helium—are those with melting points near absolute zero (-273°C or 0 kelvin). 2. The rocks are mainly silicate minerals and metallic iron, which have melting points above 700°C. 3. The ices include ammonia (NH3), methane (CH4), carbon dioxide (CO2), and water (H2O). They have intermediate melting points. For example, H2O has a melting point of 0°C.

Introduction

The sun is the hub of a huge rotating system of planets, their satellites, and numerous smaller bodies. An estimated 99.85 percent of the mass of our solar system is contained within the sun. The planets collectively make up most of the remaining 0.15 percent. As the figure shows, the planets, traveling outward from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Guided by the sun's gravitational force, each planet moves in an elliptical orbit, and all travel in the same direction. The nearest planet to the sun, Mercury, has the fastest orbital motion at 48 kilometers per second, and it has the shortest period of revolution. By contrast, the most distant planet, Neptune, has an orbital speed of 5 kilometers per second, and it requires 165 Earth-years to complete one revolution. Imagine a planet's orbit drawn on a flat sheet of paper. The paper represents the planet's orbital plane. The orbital planes of seven planets lie within 3 degrees of the plane of the sun's equator. Mercury's orbit is inclined by 7 degrees.

The Interiors of the Planets 2

The terrestrial planets are dense, consisting mostly of rocky and metallic substances, and only minor amounts of gases and ices. The Jovian planets, on the other hand, contain large amounts of gases (hydrogen and helium) and ices (mostly water, ammonia, and methane). This accounts for their low densities. The outer planets also contain substantial amounts of rocky and metallic materials, which are concentrated in their cores.

The four terrestrial planets are alike because they all _____.

are relatively small and rocky

Among the eight planets, which have the largest size and least density?

the four Jovian planets

According to the nebular theory, what formed the sun and planets of the solar system?

the gravitational contraction of a huge, rotating disk of dust and gases