Introduction to the Solar System

Introduction to the Solar System AST 2010: Chapter 6

Some Basics Facts • The solar system consists of • The Sun • Nine planets • More than a hundred (100) satellites of the planets • A large number of smaller bodies • Asteroids and comets • Cosmic dust • Countless grains of broken rock AST 2010: Chapter 6

Planetary Orbits in the Solar System AST 2010: Chapter 6

Inner Solar System • The Sun and the four planets closest to it belong to what is called the inner solar system • Thus the inner planets are • Mercury • Venus • Earth • Mars • These 4 inner planets are also called rocky or terrestrial planets • The Earth and the other 3 are similar in that they are all composed primarily of rocks (mostly silicates, made of silicon and oxygen) and metals (mostly iron) AST 2010: Chapter 6

Terrestrial or Rocky Planets • The Earth and the other 3 terrestrial planets • have solid surfaces that bear the record of their geological history in the forms of craters, mountains, and volcanoes • are said to have oxidized chemistry because they are largely composed of oxygen compounds • have relatively high densities • have relatively slow rotation • have no rings and few satellites • Venus has an unusual direction of rotation about its axis, compared to the other planets • It spins on its axis backwards, in a retrograde direction AST 2010: Chapter 6

Outer Solar System • The part of the solar system outside the inner solar system is called the outer solar system • Thus, the planets in the outer solar system are • Jupiter • Saturn • Uranus • Neptune • Pluto • Jupiter, Saturn, Uranus, and Neptune are several times larger than the Earth and hence are called the giant planets • They are also called thejovian planets AST 2010: Chapter 6

Jovian Planets • Jupiter, Saturn, Uranus, and Neptune • are composed primarily of hydrogen and helium • have deep atmospheres • are said to have reduced chemistry because they are largely composed of hydrogen and its compounds • have relatively low densities • have relatively rapid rotation • have extensive ring system and many satellites AST 2010: Chapter 6

Mass of Members of the Solar System AST 2010: Chapter 6

The Nine Planets (1) The planets’ sizes are not to scale AST 2010: Chapter 6

The Nine Planets (2) Outer planets The planets’ sizes are to scale Inner planets

Main Facts about the Planets

(Natural) Satellites • OnlyMercury and Venus do not have a moon or (natural) satellite • There are more than 100 satellites known today • More are being discovered on a regular basis • The largest of the satellites are as big as a small planet, including • our Moon • the four largest moons of Jupiter • called the Galilean satellites after their discoverer • the largest moons of Saturn and Neptune • respectively called Titan and Triton AST 2010: Chapter 6

Rings (1) • Each of the jovian planets has rings made up of countless small bodies • They range in size from grains of dust to terrestrial mountains • Each ring system orbits its planet at its equator • The rings of Saturn are the best known, the widest, and by far the easiest to see AST 2010: Chapter 6

Rings (2) • Recent missions near the jovian planets showed that the rings • have complex shapes • are influenced by the pull of the planets' satellites Jupiter’s rings Uranus’ rings Neptune’s rings

Asteroids • These are • rocky and metallic objects, quite modest in size • important members of the solar system • found in great number between the orbits of Mars and Jupiter • believed to be remnants of the initial population of the solar system • Some of the smallest satellites of the planets are thought to be asteroids captured by the gravitational pull of the planets • The two moons of Mars are nowadays believed to have such an origin Asteroids Mathilde, Gaspra, and Ida AST 2010: Chapter 6

Comets • These are another class of small bodies • composed in part of ice, made of frozen gases such as water, carbon dioxide, carbon monoxide, and methyl alcohol • Comets are also believed to be remnants from the formation of the solar system • With rare exceptions, comets orbit the Sun in distant, cooler regions • Their orbits are large and very eccentric AST 2010: Chapter 6

Recent Comet Visits

Cosmic Dust • The solar system contains countless grains of broken rock which one refers to simply as cosmic dust • Comic-dust particles constantly collide with the planets and become trapped by the planets’ gravitational pull • Millions of these hit the Earth's atmosphere every day • When these particles enter the atmosphere, they burn up, producing brief flashes of light that we see as shooting stars or meteors • Occasionally some of the larger chunks survive the passage through the atmosphere and land on Earth to become meteorites AST 2010: Chapter 6

Trends in Temperature • The planets and satellites • do not generate their own heat as the Sun does • are heated by the radiant energy of the Sun • Mathematically, the kelvin temperatures decrease approximately in proportion to the square root of the distance from the Sun • Thus, the farther a planet or satellite is from the Sun, the cooler it is • Mercury has a surface temperature of 500 K • Pluto’s temperature is only about 50 K, which is colder than water’s freezing point (273 K) AST 2010: Chapter 6

Earth’s Uniqueness for Life Support • Because of the strong dependence of temperature on distance from the Sun, the Earth is the only planet where • the surface temperatures lie between the freezing and boiling points of water • water can be liquid on the planet’s surface • Therefore, the Earth is the only known planet that can support life — at least life as we know it AST 2010: Chapter 6

Physical Appearance • The external appearance of a planet is determined by • its surface composition • external bombardments on it • geological activity on the planet AST 2010: Chapter 6

Planets under Attack!! • Externally, planets are bombarded with a slow rain of projectiles from space • The impact of the projectiles on the surface of the planet leaves pockmarked by impact craters of all sizes • The amount of craters a planet has on its surface may provide clues about its history AST 2010: Chapter 6

Shoemaker-Levy 9 • A dramatic example: impacts of large pieces of Comet Shoemaker-Levy 9 with Jupiter in summer 1994 Sequence of images taken by Hubble Space Telescope AST 2010: Chapter 6

Geological Activity (1) • The geological activity on the terrestrial planets involves the internal forces that constantly reshape their surfaces by • buckling and twisting their crusts • building up mountain ranges • creating and erupting volcanoes • Geological activity results from a hot interior • Mountains and volcanoes arise from the buildup and release of heat escaping from a planet’s core AST 2010: Chapter 6

Geological Activity (2) • Each of the planets and satellites is believed to have been heated at the time of its birth • This primordial heat initially powered extensive volcanic activity • However, smaller bodies, such as our Moon, soon cooled off • The larger the body, the more likely it is to retain its internal heat • Thus, it is more likely to see surface evidence of geological activity on the larger planets/satellites • The terrestrial planets appear to conform to this expectation • Mercury and our Moon are geologically dead • Earth and Venus are still geologically active • Mars represents an intermediate case AST 2010: Chapter 6

The Dating Game • Two popular methods for estimating the age of a planet or satellite or its surface • Crater counting • Radioactive dating • The crater counting method is based on the assumptions that • the rate at which asteroids or comets bombard a planet’s surface is roughly constant for a long time • the planet’s surface has not been reshaped, and the craters have never been erased, since it experienced a major change • If these assumptions are satisfied, then the number of craters will be proportional to the length of time the surface has been exposed AST 2010: Chapter 6

Counting Craters • The number of impact craters on the surface of a planet, satellite, or even asteroid may provide clues about the length of time the surface has undergone cosmic bombardment since the surface solidified or geological activity stopped The Moon Mercury

Radioactive Decays (1) • Radioactivity is a natural phenomenon that was discovered at the beginning of the 20th century • Some atomic nuclei are not stable, but can spontaneously split apart, or decay, into smaller nuclei • Such nuclei are said to be radioactive • For any one radioactive nucleus, the decay process happens randomly • It is not possible to predict when the decay will occur • Radioactive decays involve the emission of particles, such as electrons, or of radiation in the form of gamma rays AST 2010: Chapter 6

Radioactive Decays (2) • The rate at which nuclei decay is most easily expressed in terms of their half-life • The half-life is a specific time period during which the chances are fifty-fifty that decay will occur for any one of a large number of radioactive nuclei • For example, if you had 1 gram of pure radioactive nuclei of one type whose half-life is 10 years, then after 10 years you would have ½ gram, after 20 years ¼ gram, after 30 years 1/8 gram, and so on • After many half-lives, the original radioactive nuclei do not disappear, but instead are replaced by their decay products • The original nuclei are also called parents and the decay products are called daughters AST 2010: Chapter 6

Radioactive Decays (3) • The number of radioactive nuclei in a sample decreases exponentially AST 2010: Chapter 6

Radioactive Elements as Clocks • Radioactive elements can serve as natural clocks if one can determine how many of the initial radioactive parents have been replaced with their daughters • By comparing how much of a radioactive parent material is left in a rock with how much of its daughter material has accumulated, one can learn how long the decay process has been going on and hence how long ago the rock formed • This method involves the assumption that the rock being studied has been isolated since its formation • That is, none of the parent and daughter nuclei in the rock have leaked out of it or been polluted by outside contaminants AST 2010: Chapter 6

Radioactive Decay Reactions Often Used to Date Rocks AST 2010: Chapter 6

Ages of the Moon and Earth • Astronauts that visited the Moon brought back lunar rock for radioactive age dating • Counting craters had given only relative ages for different parts of the Moon’s surface • Samples brought back enabled a better dating of the lunar surface which suggested that the Moon is an ancient, geologically dead body • Radioactive dating of rocks brought back by the Apollo mission and of rocks here on Earth indicated that the two bodies have similar ages • According to this dating method, both the Earth and the Moon were formed some 4.5 billion years ago AST 2010: Chapter 6

Origin of the Solar System • An observation of the Sun and the nine planets of reveals some patterns • The planets all revolve around the Sun in the same direction • They orbit in approximately the same, flat plane • The Sun spins in the same direction about its own axis • Astronomers regard these facts as evidence that the Sun and the planets formed together as a spinning system of gas and dust, referred to as the solar nebula AST 2010: Chapter 6

Origin and Composition (1) • The composition of planets provides another clue about the origin of the solar system • Spectroscopic analysis allows a determination of what chemical elements are present in the Sun and the planets • Such an analysis shows that the Sun has the same hydrogen-dominated composition as Jupiter and Saturn • This suggests that these three bodies were formed from the same reservoir of material AST 2010: Chapter 6

Origin and Composition (2) • By contrast, spectroscopic analysis shows that the terrestrial planets and satellites • are relatively deficient in the light gases and the various ices composed of the common elements oxygen, carbon, and nitrogen • contain mostly heavy elements like iron and silicon — these are rare on the Sun and the giant planets • This pattern suggests that the processes which led to the formation of the inner planets must have somehow rejected much of the lighter materials • These appear to have escaped, leaving behind just a residue of heavy stuff AST 2010: Chapter 6

Other Planetary Systems (1) • An additional approach to understanding the origin of the solar system is to look outside the solar system for evidence • Many stars in space are much younger than the Sun, and planet formation might be occurring in some of these star systems that could be accessible to direct observation • Astronomers have observed other “solar nebulas” or circumstellar disks — flattened spinning clouds of gas and dust surrounding young stars • These are believed to be examples of what our solar system may have looked like when it started to form AST 2010: Chapter 6

Other Planetary Systems (2) • Unfortunately, because of the enormous distance involved, and the fact that planets forming are not likely to reflect light very efficiently, astronomers have yet to observe the actual formation of planets in distant solar nebulas • Astronomers have now developed tools that enable the observation of planets orbiting distant stars • The technique is still limited to the detection of giant planets only • Observations nonetheless indicate clearly that other solar systems have configurations that vastly differ from the configuration of our solar system • Some have planets with very elliptical orbits • In other cases, giant planets are very near their stars AST 2010: Chapter 6

Introduction to the Solar System