THE SOLAR SYSTEM Tillery, Chapter 17 Science 330 Summer 2007 This view of the rising Earth was seen by the Apollo 11 astronauts after they entered orbit around the Moon. Earth is just above the lunar horizon in this photograph. Ideas about the Solar System The Geocentric Model This model saw the solar system as perfect spheres with attached celestial bodies rotating around a fixed Earth. The planets rotated around the Earth in perfect circles. This model grew out of the ideas that: Humans were at the center of a perfect universe created just for them. Since Heaven was a perfect place, then everything that existed in the Heavens would be perfect. This naturally meant that the solar system must be a place of perfect epicycles moving on perfect spheres, which were moving in perfect circles. The paths of Venus and the Sun at equal time intervals according to the Ptolemaic system. The combination of epicycle and Sun movement explains retrograde motion with a stationary Earth. (A)The Ptolemaic system explanation of Venus as a morning star and evening star. (B) The heliocentric system explanation of Venus as a morning star and evening star. 1
The Heliocentric Model Nicolas Copernicus in 1543 suggested that the Earth revolves around the Sun. In this model each planet moved around the Sun in perfect circles at different distances and at faster speeds the closer to the Sun a planet was. Tycho Brahe Made precise measurements of the Sun, Moon, planets, and the stars. Johannes Kepler Found that the planets did not move in perfect circles. Planets move in the path of a ellipse. The heliocentric system explanation of retrograde motion. Kepler s Laws of Planetary Motion Kepler s First Law. Each planet moves in an orbit that has the shape of a ellipse, with the Sun located at one focus. Kepler s Second Law An imaginary line between the Sun and a planet moves over equal areas of the ellipse during equal time intervals. The velocity of a planet would therefore vary as it is not always the same distance from the focus. The point where the planet comes closet to the Sun is the perihelion and the point at which it is farthest from the Sun is the aphelion. Kepler's first law describes the shape of planetary orbit as an ellipse, which is exaggerated in this figure. The Sun is located at one focus of the ellipse. Kepler's second law. A line from the Sun to a planet at point A sweeps over a certain area as the planet moves to point B in a given time interval. A line from the Sun to a planet at point C will sweep over the same area as the planet moves to point D during the same time interval. The time required to move from point A to point B is the same as the time required to move from point C to point D, so the planet moves faster in its orbit at perihelion. Kepler s Third Law. The square of the period of a planet s orbit is proportional to the cube of that planet s semimajor axis t 2 d 3 2
Origin of the Solar System Kepler's third law describes a relationship between the time required for a planet to move around the Sun and its average distance from the Sun. The relationship is that the time squared is proportional to the distance cubed. Heliocentric model of the solar system Commonly the most accepted model of the solar system Protoplanet nebular model most accepted theory of the origin of the solar system. Formation of the solar system according to the protoplanet nebular model, not drawn to scale. (A) The process starts with a nebula of gas, dust, and chemical elements from previously existing stars. (B) The nebula is pulled together by gravity, collapsing into the protosun and protoplanets. 3
Stage A All the elements that made up the current solar system were derived from stars that disappeared billions of year ago, even before our Sun was born. Hydrogen fusion in the core of large stars results in the formation of elements through iron. Elements that are heavier that iron are formed in rare supernova explosions of dying massive stars. (C) As the planets form, they revolve around the Sun in orbits that are described by Kepler's laws of planetary motion. Stage B All of the elements from Stage A began to form large, slowly rotating nebula Under the influence of gravity, the size of this nebula begin to decrease, which increased its rate of spin Eventually this spinning nebula formed an accretion disk which is an enormous, bulging disk of gases and elements formed as nebula condense. Eventually this accretion disk became compressed into protoplanets and a protosun Stage C An initial flare-up of the Sun from the warming and condensing established the protosun as a star and it became our Sun. Between the orbits of Mars and Jupiter there is an Asteroid belt that some think was formed by the breakup of a larger planet. The Planets Introduction The solar system consists of a middle aged main sequence G type star called the Sun with nine planets, nearly fifty moons, thousands of asteroids, and many comets revolving around it. This is all held together by the force of gravitational attraction Terrestrial planets are those which have a composition very similar to the Earth s composition these planets are composed mostly of rocky material with iron These planets have a density of 4 5.5 g/cm 3. 4
The Jovian (Giant) planets are those that are composed mostly of hydrogen, helium, and methane with a density of 2g/cm 3. The probably contain a core of material much like the makeup of the terrestrial planets which is surrounded by a thick layer of gases. The order of the planets out from the Sun. The orbits and the planet sizes are not drawn to scale. Earth's rotation around its axis results in day and night. One year is required for one revolution. (A)The early secondary atmosphere of Venus lost hydrogen to space and oxygen became combined with rocks as ultraviolet radiation decomposed water molecules. (B) On Earth, water formed the oceans, carbon dioxide was removed, and plants released oxygen, which in turn formed an ozone layer in the atmosphere, protecting the water and life below. Mercury Innermost planet Period of revolution is 88 days. Rotation once every 59 days. High kinetic energy of gas molecules and a low gravitational pull Surface temperature 427 O C (800 O F)in the sunlight to 180 O C (-350 O F)in the dark Surface covered with craters Presence of magnetic fields and high density so must have a high iron content with at least a partial molten core Mercury is close to the Sun and visible only briefly before or after sunrise or sunset, showing phases. Mercury actually appears much smaller in an orbit that is not tilted as much as shown in this figure. 5
A photomosaic of Mercury made from pictures taken by the Mariner 10 spacecraft. The surface of Mercury is heavily cratered, looking much like the surface of Earth's Moon. All the interior planets and the Moon were bombarded early in the life of the solar system. Venus Very bright in morning and evening sky. revolves around the Sun once every 225 days. Rotates on its axis once every 243 days. Exhibits retrograde rotation where the rotation of the planet is opposite its direct of revolution around the Sun and opposite most other planets. Average surface temperature is 480 O C (900 O F) Atmospheric pressure approximately 100 times that experienced on Earth. Clouds and rain consisting of sulfuric acid. No satellites and no magnetic field. This is an image of an 8 km (5 mile) high volcano on the surface of Venus. The image was created by a computer using Magellan radar data, simulating a viewpoint elevation of 1.7 km (1 mile) above the surface. The lava flows extend for hundreds of km across the fractured plains shown in the foreground. The simulated colors are based on color images recorded by the Soviet Venera 13 and 14 spacecraft. Mars Unique, bright reddish color which exhibits a swift retrograde motion against the background of stars. Revolves around the Sun in 687 days. Rotates on its axis once every 24 hours, 37 minutes Has an atmosphere A geologically active past and is divided into 4 provinces: Volcanic regions Systems of canyons Terraced plateaus near the poles Flat regions pitted with impact craters. average temperature is 53 O C (-63 O F) Atmosphere is 95% CO Atmospheric pressure 0.6 percent of Earth s atmospheric pressure Two satellites Deimos 13 km across Phobos 22 km across Both are thought to be captured asteroids Surface picture taken by the Viking 1 lander found reddish, fine-grained material, rocks coated with a reddish stain, and groups of blue-black volcanic rocks. 6
A view of the surface of Mars taken by the Viking Orbiter 1 cameras. The scene shows three volcanoes that rise an average of 17 km (about 11 mile) above the top of a 10 km (about 6 mile) high ridge. Clouds can be seen in the upper portion of the photograph, and haze is present in the valleys at the lower right. Jupiter Largest of all planets Twice as massive as all other planets combined and about 318 times as massive as the Earth. Radiates twice as much energy as it gets from the sun due to slow gravitational compression Average density of 1.3 g/cm 3 Made mostly of hydrogen and helium with some rocky substances A solid core with a radius of about 14,000 km (8,500 miles) The interior structure of Jupiter. Photos of Jupiter taken by Voyager 1. (A) From a distance of about 36 million km (about 22 million mi). (B) A closer view, from the Great Red Spot to the South Pole, showing organized cloud patterns. In general, dark features are warmer, and light features are colder. The Great Red spot soars about 25 km (about 15 mi) above the surrounding clouds and is the coldest place on the planet. Above this core is about 35,000 km (22,00 mi) of liquid hydrogen, which is compressed so tightly by millions of atmospheres of pressure that it is able to conduct electricity and is termed metallic hydrogen. Above this is 20,000 km (12,000 mi) of liquid hydrogen under much less pressure. The outer layer is about 500 km (300 mi) of hydrogen, helium, ammonia gas, and crystalline compounds with a mixture of ice and water. Sixteen satellites four are called Galilean moons as they were discovered by Galileo in 1610 Io Europa Ganymede Callisto The four Galilean moons pictured by Voyager 1. Clockwise from upper left, Io, Europa, Ganymede, and Callisto. Io and Europa are about the size of Earth's Moon; Ganymede and Callisto are larger than Mercury. 7
(A)This image, made by the Hubble Space Telescope, clearly shows the large impact site made by fragment G of former Comet Shoemaker-Levy 9 when it collided with Jupiter. (B) This is a picture of Comet Shoemaker-Levy 9 after it broke into twenty-two pieces, lined up in this row, then proceeded to plummet into Jupiter during July, 1994. The picture was made by Hubble Space Telescope. Saturn Slightly smaller and less massive than Jupiter with a system of rings The rings are made up of particles, some which are meters across and some that are dust sized particles. 10 moons Janus Mimas Enceladus Tth Tethys Dione Rhea Titan Hyperion Iapetus Phoebe Titan is the only moon in the solar system with an atmosphere and is larger than the planet Mercury A part of Saturn's system of rings, pictured by Voyager 2 from a distance of about 3 million km (about 2 million mi). More than sixty bright and dark ringlets are seen here; different colors indicate different surface compositions. Uranus, Neptune, and Pluto Uranus revolves around the Sun once about every 84 years. Both Uranus and Neptune have a core of rocky material surrounded by water and ice. Uranus and Neptune have an atmosphere of hydrogen and helium Average temperature on Uranus is-210 O C (-350 O F) Average temperature on Neptune is 235 O C (-391 O F) Uranus tilts 82 O on its axis, which is different from the less that 30 O for other planets. 15 Known satellites around Uranus Uranus has 10 narrow rings and a number of dusty bands. Neptune has 8 satellites Neptune also has a series of rings Pluto is the smallest planet in the solar system. Pluto has a density of 2 g/cm 3 Pluto s atmosphere is probably mostly nitrogen, with some methane and CO 2 Pluto orbits around the Sun once every 248 years. 8
This is a photo image of Neptune taken by Voyager. Neptune has a turbulent atmosphere over a very cold surface of frozen hydrogen and helium. The interior structure of Uranus and Neptune. Smaller bodies of the Solar System The orbital planes of the planets. Notice the exceptional tilt of the orbit of Pluto. Comets Comets Small relative to other bodies in the solar system Made of frozen CO 2, NH 3, CH 4 and particles of dust and rock mixed in. Originate approximately 30 A.U. from the Sun Oort Cloud A spherical cloud of objects that occur out past the orbit of Pluto made of water ice, frozen methane, frozen ammonia, dry ice, and dust. Occurs about 30,00000000 A.U. out to a light year or more from the Sun 9
Kuiper Belt A disk shaped region of smaller icy bodies from 30 to 100 A.U from the Sun. This is the source of short period comets. Centaurs Kuiper Belt Objects that orbit between Jupiter and Neptune Comets have two types of tails Ionized gases. Dust The comets tail always points away from the Sun, so it follows the comet as it approaches the Sun and leads the comet as if moves away from the Sun As a comet nears the Sun it grows brighter, with the tail always pointing away from the Sun. Comets originate from the Oort cloud, a large spherical cloud of icy aggregates, or from the Kuiper Belt, a disk-shaped region of icy bodies. The Kuiper Belt is illustrated here, extending from 30 to 100 A.U. from the Sun. The Oort cloud is a spherical "cloud" that ranges from about 30,000 A.U. to a light-year or more from the Sun. Asteroids The asteroid belt lies between Mars and Jupiter The asteroids in this belt are between 1 km up to the largest (Ceres) which is 1,000 km (600 mi) The asteroids in the inside of the belt are made of a stony material and the asteroids on the outside of the belt are made of a carbon material Some other asteroids are metallic, made of iron and nickel Most of the asteroids in the asteroid belt are about halfway between the Sun and Jupiter 10
Meteors and Meteorites Meteroids Remnants of asteroids and comets after the heat of the Sun and collisions. Meteor The streak of light and smoke left in the sky by a meteoroid is called a meteor. These are the shooting stars that we see in the sky Meteor shower Occurs when the Earth passes through a stream of particles that are left by a comet. There is an intense meteor shower the third week in October as the Earth crosses the path of Halley s Comet. Meteorite A meteorite is what is left of a meteoroid if it survives the flight through the Earth s atmosphere. Classified according to their makeup Iron meteorites made up mostly of iron and nickel material Stony meteorites composed mostly of rocky material much like that found on Earth chondrites have a structure of small spherical lumps of silicate materials. Achondrites do not contain this silicate clumps Stony-iron meteorites Made up of a conglomerate of both types of materials. (A)A stony meteorite. The smooth, black surface was melted by friction with the atmosphere. (B) An iron meteorite that has been cut, polished, and etched with acid. The pattern indicates that the original material cooled from a molten material over millions of years. 11