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UNIT 4The Solar System
Models of the Solar System
Section 1
▪ 2,000 years ago, many people developed ideas about the universe based on what they saw !
▪ Aristotle promoted an earth-‐centered (geocentric) model of solar system !
▪ Sun, stars, planets revolved around earth
▪ Did not explain why some planets appear to reverse direction sometimes
▪ Backward motion à retrograde motion ▪ About 500 years after Aristotle, another Greek astronomer, Claudius Ptolemy, put forth model
▪ Each planet had two motions ▪ One motion was revolution around earth ▪ Other motion in small circles like links of chain ▪ Epicycles à small circular movements
▪ According to Ptolemy, motion of planet in epicycle would make it appear to move backward sometimes
Copernicus’s Model
▪ In the 1500s Nicolaus Copernicus challeneged ideas of Aristotle and Ptolemy
▪ Proposed a heliocentric (sun-‐centered) model of solar system
▪ Earth and other planets revolve around sun ▪ Also proposed all planets orbit in same direction but each moves at different speeds and distances from sun
▪ Faster planets would pass slower planets
▪ In early 1600s Galileo Galilei was able to observe motions of planets with telescope (just invented)
▪ He collected evidence ▪ Proved heliocentric model was correct
Kepler’s Laws
▪ Before the invention of the telescope, Tycho Brahe, Danish astronomer, made detailed observations of positions of stars and planets
▪ He hired German astronomer, Johannes Kepler, as an assistant
▪ Kepler explained Brahe’s observations in mathematical terms
▪ Developed 3 laws that explained most aspects of planetary motion
1. Law of Ellipses
▪ Kepler’s first law ▪ Each planet orbits the sun in a path called an ellipse
▪ Ellipse à oval whose shape is determined by 2 points within the figure
▪ Each point is called a focus ▪ The sun is at one focus of the orbit of a planet ▪ If you draw a line from any point on ellipse to each of two foci, the total length of lines will always be the same
▪ Some look like circles ▪ Circle – ellipse where 2 foci are at same point
▪ b/c orbits of planets are ellipses, planet is not always same distance from sun
▪ Point where orbit is closest to sun -‐ perihelion ▪ Point where orbit is farthest -‐ aphelion
▪ Distance of planet from sun defined as average of distances from sun at perihelion and aphelion
▪ Ex. Aphelion of Earth’s orbit is 152 million km ▪ Perihelion is 147 million km ▪ Average = 149.5 million km ▪ This distance is known as one astronomical unit (AU)
▪ Distances of other planets measured in AU
2. Law of Equal Areas
▪ Describes speed at which planets travel at different points in their orbit
▪ Kepler found that orbit of earth was a nearly perfect circle, with sun off-‐center
▪ Found that earth moves fastest when it’s closest to sun
▪ Calculated a line from center of sun to center of planet sweeps through equal areas in equal periods of time
▪ Imagine a line that connects the center of the sun to the center of a planet
▪ When planet near sun, the line is short ▪ Planet moves quickly and after, say, 10 days the line sweeps through a short, wide triangular section
▪ When planet is farther from sun, line is longer ▪ But planet moves more slowly ▪ Line sweeps through long, thin triangular section in 10 days
▪ Kepler’s second law à the area of the long, thin section is the same as the area of the short, wider section
3. Law of Periods
▪ Describes relationship between average distance of a planet from the sun and the orbit period of the planet !
▪ Orbit period à time required for the planet to make one revolution around the sun
▪ The cube of the average distance of a planet from the sun (r) is always proportional to the square of the period (p)
▪ Math formula is K x r³ = p² ▪ K is mathematical constant ▪ When distance measure in Aus and period is in earth-‐years, K = 1 and r³ = p²
Example
▪ Radius of earth’s orbit (distance from sun) is 1 AU
▪ Period is 1 year
▪ 1 x 1³ = 1² ▪ 1 = 1
▪ Jupiter is 5.2 AU from sun ▪ Period is 11.9 years
▪ K x 5.2³ = 11.9² ▪ K x 140.6 = 141.6 ▪ Nearly equal ▪ Errors caused by rounding periods and Aus ▪ When precise, always 1
Newton’s Application of Kepler’s Laws
▪ Kepler’s laws explained how planets orbit sun ▪ Isaac Newton asked why planets moved this way
▪ Explanation described the motion of objects on earth and motion of planets in space
▪ Hypothesized that moving body will change motion only if outside force causes it to do so
▪ Inertia à tendency of a moving body to move in a straight line at a constant speed until an outside force acts on it
▪ Newton compared planet to rolling ball ▪ b/c planet doesn’t follow straight path, outside force must cause it to curve
▪ Gravity ▪ Gravitational pull of sun causes planets to move around it
Section 2
The Inner Planets
▪ 4 planets closest to sun are inner planets ▪ Mercury, Venus, Earth, Mars ▪ Also called terrestrial planets b/c they’re similar to earth
▪ Formed close to sun so heat caused materials with low boiling points to go away
▪ Inner planets don’t have rings ▪ Number of moons: 0-‐2 ▪ Have bowl-‐shaped depressions on surface called impact craters
▪ From collisions with objects in space (later stages of solar system formation)
Mercury
▪ Mercury is closest to sun ▪ Has shorter orbit period than any other planet – 88 days
▪ Romans named planet Mercurius after fast messenger of gods
▪ Rotates slowly on axis – once every 59 days ▪ So close to sun the sunlight usually blocks a clear view
▪ Has no moons
▪ In 1974 and 1979 Mariner 10 visited Mercury ▪ Transmitted photos to earth that showed surface was heavily cratered
▪ Suggests Mercury hasn’t changed much since formation of solar system
▪ Some craters look filled with hardened lava ▪ If so, Mercury was volcanic
▪ Photos also showed line of cliffs hundreds of km long
▪ May be wrinkles in crust that developed when molten core cooled and shrank
▪ Has thin atmosphere for 2 reasons ▪ 1. closeness to sun ▪ Solar heat causes gas molecules near surface to move fast
▪ 2. size ▪ Gravitational pull too weak to hold them to form an atmosphere
▪ Absence of dense atmosphere and slow rotation causes huge daily temperature range !
▪ Daytime: 427℃ !
▪ Nighttime: -‐173℃
▪ Before Mariner 10 expedition, astronomers thought Mercury too small and rotated too slowly to have magnetic field !
▪ Instruments on Mariner 10 detected weak magnetic field !
▪ Core may still be molten
Venus
▪ 2nd planet from sun ▪ Orbit period = 225 days ▪ Rotates once very 243 days ▪ Rotates in opposite direction than other planets
▪ Sun rises in west and sets in east ▪ No moons
▪ Some ways Venus is earth’s twin ▪ Same size, mass, density ▪ Venus is much hotter b/c it’s closer to sun and atmosphere is dense and has insulating effect
▪ Average surface temp is 464℃ ▪ Atmospheric pressure is 90 times more than earth
▪ High temperature and dense atmosphere are related
▪ When Venus was formed, temp was lower and atmosphere was less dense
▪ Volcanic activity happened ▪ Oceans may have formed ▪ As sun became hotter, oceans evaporated ▪ No water to combine with CO2 released by volcanoes, level of CO2 increased
▪ Result: atmospheric pressure increased ▪ CO2 is 96% of atmosphere ▪ Allows orange wavelengths of solar energy to get through but blocks escape of heat
▪ Planet too hot to support life ▪ Above surface, temp decreases ▪ Very little water vapor ▪ Instead, clouds made of sulfuric acid
▪ In 1970s, Soviet Union sent six Venera probes to explore surface
▪ Survived heat and pressure long enough to transmit surface images
▪ Showed smooth plain, with some mountains and valleys
▪ Other instruments showed surface made of basalt and granite
▪ These commonly found on earth
▪ Between 1990 and 1992, Magellan orbiter made radar images of most of Venusian surface
▪ Domes, mountains, volcanoes and lava plains are most common
Earth
▪ 3rd planet from sun ▪ Orbit period = 365.24 days ▪ 1 rotation = 23 hours 56 minutes ▪ 5th largest planet ▪ 1 moon
▪ Very active geologic history ▪ Over last 250 million years, continents separated from single landmass and drifted to present positions
▪ Weathering and erosion changed and continue to change surface
▪ Life possible b/c of distance from sun ▪ Temp warm enough for water to exist as liquid ▪ Mercury and Venus don’t have liquid water ▪ Mars and outer planets too far away (ice) ▪ Only planet in solar system known to have oceans of liquid water
▪ Geologists think that as oceans formed on earth, water combined with CO2 in atmosphere
▪ Since CO2 didn’t build up in atmosphere, solar heat able to escape
▪ Life was able to form
Mars
▪ 4th planet from sun ▪ Orbit period = 687 days ▪ Rotation = 24 hours 37 minutes ▪ Has seasons like earth’s b/c of tilt of axis ▪ Has 2 moons
▪ Large surface areas covered with lava ▪ Volcanoes on Mars are largest in solar system ▪ Largest – Olympus Mons ▪ 3 times higher than Mt. Everest
▪ Deep canyons also cover surface ▪ Largest – Valles Marineris ▪ As long as US – 4,500 km ▪ Fault zones may have formed them
▪ Atmospheric pressure and temperature too low for water to exist as liquid
▪ Spacecraft Viking 1 and Viking 2 found evidence of erosion by water
▪ Also recent Pathfinder mission showed images of large flood plain
▪ Assume Mars used to have warmer and wetter climate
▪ Temp near equator about 20℃ during summer
▪ Near poles about -‐130℃ ▪ A little water remains trapped in polar ice caps
Section 3
The Outer Planets
▪ Outer planets – Jupiter, Saturn, Uranus, Neptune, Pluto
▪ 1st 4 are called gas giants – largest planets in solar system
▪ Also called Jovian planets b/c they are similar to Jupiter
▪ Less dense than inner planets ▪ Mostly hydrogen and helium atmosphere ▪ Rock, metals and water at core
Jupiter
▪ 5th planet from sun ▪ Largest planet in solar system ▪ Mass is twice all 8 other planets combined ▪ Orbit period = 12 years ▪ Rotation every 9 hours 50 minutes ▪ Has at least 17 moons and 4 rings
▪ May or may not have solid, rocky core (don’t know)
▪ Large mass causes pressure and temp in interior much more than earth
▪ Temps at interior as high as 30,000℃ ▪ Changed much of interior to sea of liquid metallic hydrogen
▪ Electric currents may be source of huge magnetic field
▪ Made mainly of H and He – 92% ▪ Makes composition of Jupiter similar to sun ▪ Surface not solid – mixture of hot gases and liquids
▪ Formed about 4.6 billion years ago ▪ Not enough mass to start nuclear fusion ▪ So it never became a star
▪ Surface unique b/c of alternating light and dark cloud bands
▪ Orange, gray, blue and white bands spread out parallel to equator
▪ Colors suggest presence of ammonia, methane, and water vapor
▪ Scientists think quick rotation causes gases to swirl around planet – forms bands
▪ Average temp of atmospheric layers = -‐160℃ ▪ Lower layers about 20℃ ▪ Also lightning and thunderstorms
The Great Red Spot
▪ Distinguishing feature of Jupiter ▪ Astronomers think heated material rising to surface from interior causes it
▪ Giant rotating storm (like hurricane) ▪ Been for several hundred years or more ▪ Galileo spacecraft data (1995) showed Jupiter has wind speed up to 540 km/hr
Saturn
▪ 6th planet from sun ▪ Over ½ billion km farther from sun than Jupiter
▪ 2nd largest planet in solar system
▪ Average temp = -‐176℃ ▪ Has at least 18 moons and several rings
▪ Spins rapidly ▪ Rotates every 10 hours 30 minutes ▪ Rotation of Saturn and Jupiter causes it to bulge at equator and flatten at poles
▪ Also has bands of colored clouds parallel to equator
▪ Same interior as Jupiter
▪ Saturn differs from Jupiter in 3 ways 1. Saturn much less dense than Jupiter
1. Least dense planet in solar system 2. Density less than water
2. Orbit period = 29.5 years 1. Almost 20 years longer than Jupiter
3. Has much more complex system of rings
Uranus
▪ 7th planet from sun ▪ 3rd largest ▪ Discovered in 1781 ▪ 1st planet to be discovered since ancient times ▪ Nearly 3 billion km from sun ▪ Hard to study ▪ Has at least 20 moons ▪ At least 11 small rings ▪ Orbit period = 84 years
▪ Most unique feature – rotates like rolling ball !
▪ Axis almost horizontal to plane of orbit !
▪ Rotation rate not discovered until 1986 !
▪ Voyager 2 passed by !
▪ Rotates once every 17 hours
▪ Greenish color indicates atmosphere contains methane
▪ Atmosphere contains mainly H and He
▪ Average cloud-‐top temp = -‐214℃ ▪ Astronomers think temp much higher below clouds
▪ Core might be rock and metals with temp about 7000℃
Neptune
▪ 8th planet from sun ▪ Similar to Uranus in size and mass ▪ Orbit period = 164 years ▪ Rotates every 16 hours ▪ Has 8 moons ▪ Possibly 4 rings
▪ Existence of Neptune predicted before it was discovered
▪ Astronomers noticed variations of Uranus’s orbit from expected orbit
▪ Suspected gravity of unknown planet behind Uranus might be responsible
▪ Mid 1800s scientists calculated possibility of planet beyond Uranus
▪ 3 years later Johann Galle discovered bluish-‐green disk where one scientist predicted – 4.5 billion km from sun
▪ Named Neptune after Roman god of the sea
▪ Data from Voyager 2 showed atmosphere mainly H, He and methane
▪ Upper atmosphere has white clouds of frozen methane
▪ Images from Voyager also show Neptune has active weather system
▪ Solar system’s strongest winds – 1000 km/hr ▪ Has earth-‐sized storm – The Great Dark Spot ▪ Average surface temp about -‐225℃
Pluto
▪ No longer a planet ▪ Now called a dwarf planet – more than 40 of them in solar system
▪ A planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity
▪ A planet has to dominate the neighborhood around its orbit
▪ Pluto has been demoted because it does not dominate its neighborhood !
▪ Charon, its large "moon," is only about half the size of Pluto, while all the true planets are far larger than their moons.
Section 4
Asteroids, Comets and Meteoroids
▪ Solar system includes millions of smaller bodies of matter than planets
▪ Some are just bits of dust or ice floating in space
▪ Others as big as moons ▪ Left over from nebula that formed solar system
Asteroids
▪ Largest of smaller bodies in solar system ▪ Fragments of rock that orbit sun ▪ Astronomers have seen more than 50,000 ▪ Millions may exist in solar system ▪ Orbits are mostly ellipses (like planets) ▪ Largest known asteroid is Ceres (1,000 km diameter)
▪ Most exist in area between orbits of Mars and Jupiter
▪ Asteroid belt ▪ Begins 100 million km beyond orbit of Mars ▪ Stretches for about 150 million km toward Jupiter
▪ Asteroids usually classified into 3 types depending on composition
1. Mostly carbon materials – dark appearance 2. Mostly iron/nickel – metallic appearance 3. Mostly silicate materials – look like earth
rocks (most common)
▪ Many astronomers think asteroids in asteroid belt are remains of planetesimals not able to form planet b/c of gravity from Jupiter
▪ Composition of asteroids is same materials as planets
▪ Ex. Iron common in cores, silicate minerals common in crusts
▪ Some asteroids – Trojan asteroids and earth-‐grazers – orbit sun
▪ Not in asteroid belt ▪ Trojan asteroids concentrated in groups just ahead and behind planet Jupiter
▪ Earth-‐grazers have elongated elliptical orbits ▪ Sometimes bring them very close to sun and earth
▪ Do sometimes collide with earth
Comets▪ Orbit sun in long ellipses ▪ Body of rock, dust, methane, ammonia, ice ▪ Core (nucleus) made of rock, metals and ice ▪ Usually 1 km – 100 km diameter ▪ Coma à spherical cloud of gas and dust surrounding the nucleus
▪ Tail is gas and dust that streams from head (nucleus and coma)
▪ Forms as sunlight and solar wind pushes gas and dust away from head
▪ Whatever direction comet travels, tail always points away from sun
▪ Some larger comets have tails more than 80 million km long
▪ Astronomers think most comets start in Oort cloud – spherical cloud of dust and ice that contains nuclei of as many as a trillion comets
▪ Bodies in cloud circle sun at 140 m/s ▪ Takes a few million years to complete 1 orbit
▪ Cloud surrounds solar system ▪ Starts 1 light year from sun ▪ Reaches halfway to nearest star ▪ Matter may have been left over from formation of solar system
▪ Gravity of star passing near solar system may cause comet in cloud to fall into long elliptical orbit
▪ Orbit stretches from Oort cloud to sun and back to cloud
▪ Some long-‐period comets have periods of several thousand or several million years !
▪ Short-‐period comets have periods up to 100 years
▪ Gravity of outer planets can affect comets as they orbit sun
Meteoroids
▪ Smaller bits of rock or metal ▪ Most less than 1 mm diameter ▪ Pieces of matter detached from passing comets
▪ Larger meteoroids – 1 cm – made by collisions between asteroids
▪ If meteoroid passes through earth’s atmosphere, air pressure heats it and slows it down
▪ Most burn up in atmosphere before reaching surface
▪ Make bright streak of light à meteor ▪ “shooting star”
▪ Meteoroids sometimes vaporize quickly in brilliant flash of light -‐ fireball
▪ Sometimes large number of small meteoroids enter in short period of time
▪ Meteor shower à lots of meteoroids burning up
▪ Occur at same time each year when earth intersects orbits of comets that left trail of meteoroids
▪ Astronomers estimate 1 million kg matter from meteoroids falls to earth every day
▪ Millions of meteoroids enter earth’s atmosphere each day
▪ Few do not burn up entirely and fall to earth ▪ Meteorite à meteoroid or any part left after it hits earth
▪ Most are small (mass less than 1 kg)
▪ Meteor Crater in Arizona ▪ Created by meteoroid that hit earth about 20,000 years ago
▪ Meteor was 50 m in diameter ▪ Weight 500,000 tons ▪ Left crater 1.3 km diameter, 180 m deep
▪ Meteorites can be classified into 3 basic types 1. Stony 2. Iron 3. Stony-‐iron
Stony Meteorites
▪ Similar in composition to rocks on earth !
▪ Contain carbon substances similar to materials in living things
Iron Meteorites
▪ Easier to find ▪ Stony meteorites look like earth rocks ▪ Often not noticed ▪ Iron meteorites have distinct metallic appearance
Stony-‐Iron Meteorites
▪ Contain both iron and stone ▪ Very rare
▪ Almost all meteorites probably come from collisions between asteroids
▪ Oldest meteoroids are about 100 million years older than earth or moon
▪ Meteorites can give information about makeup of solar nebula that existed before earth and moon formed