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Moons and Other Solar System Objects The Moon • July 20, 1969 – humans first landed on moon • What was the first word transmitted from the surface of the moon? • “Houston …” • Retroreflector – placed on moon and designed to reflect a laser beam from earth – Rates of Continental Drift on earth, change in earth’s tilt, distance to moon, Gravitational Constant (G) • Average distance = 384,000 km (240,000 mi) Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 2 The Moon • Second brightest object in the sky • “moon” – unknown origin of the word • Many primitive and modern societies base their religious ceremonies on the cycles of the moon (e.g., new and full moons). • Our month is based on moon’s cycle. • Human ovarian cycle is also synchronized to the 29.5 day lunar cycle. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 3 Composition and History of the Moon • Since the first manned lunar landing on July 20, 1969, there have 5 other lunar landings – Apollo 12, 14, 15, 16, & 17. • 379 kg (over 800 lbs) of lunar material have been brought back to earth. • Using radiometric dating techniques – Mountain rocks – 4.4 to 3.9 billion years old – Plain rocks – 3.9 to 3.1 billion years old • No rocks older than 4.4 or younger than 3.2 billion years old have been found. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 4 General Features of the Moon • Largest moon of any of the terrestrial planets (Mercury, Venus, Earth, and Mars) • One complete revolution = 29.5 days (approx.) • The moon rotates on its axis at the same rate, and therefore we only see one side of the moon. • Since the moon rotates every 29.5 days, the sun would appear to rise/set every 29.5 days. • The surface features of the moon formed millions of years ago without erosion to erase them. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 5 Characteristics of the Moon • Nearly spherical, with a diameter of 3476 km (2160 mi) – approx. ¼ the earth’s diameter • Mass of the moon = 1/81 of the earth • Average density of 3.3 g/cm3 (earth is 5.5) • Surface gravity of the moon is only one-sixth of Earth’s. • Therefore one’s weight on the moon would only be one-sixth of that on Earth. • Average reflectance (albedo) = only 7% (only 7% of the light received from the sun is reflected) Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 6 Composition of the Moon • • • • Core, mantle, crust All thought to be solid Crust, thicker on one side Presently does not have a magnetic field • Interestingly though, the rocks brought back show some magnetism, indicating that the moon had a slight magnetic field at the time of rock crystallization. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 7 Prominent Features of the Moon • Other than the very obvious phases of the moon, the moon’s most prominent features include craters, basins, plains, rays, rills, mountains, and faults. • Crater – Greek for “bowl-shaped” • Lunar craters are large to small and believed to have been formed by meteorite impacts. • For individual craters, the volume of the rim and the crater are about equal. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 8 Craters and Basins • Craters are the best known features of the moon’s surface. • About 30,000 can be seen using an earth-based telescope. • Range in size – microscopic to 100’s km • The shapes of the craters change with diameter. – 1 km – smooth bowl-shaped interiors – >1 km – flat interiors – >>1 km – flat floor with central peak Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 9 Craters of the Moon • Most of the craters were formed between 4.4 and 3.9 billion year ago, due to a time of intense meteorite bombardment. • After 3.1 billion years ago, the moon cooled down enough that molten rock could no longer get to the surface. • The moon appears to have been geologically quiet for the past 3.1 billion years or so. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 10 Lunar Plains • Also call maria (Latin for ‘seas’) • Lunar plains - large, dark, flat areas on the moon believed to be craters formed by meteorite impact that then filled with volcanic lava • Dark in appearance because these flat plain areas reflect even less light than the average surface area of the moon • Surface of the moon covered by a layer of loose debris called regolith • Rock samples examined - similar to volcanic rock on earth Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 11 Plains of the Moon • Plains were produced by volcanic eruptions and the resulting enormous lava flows. • Apparently the moon’s interior was hot enough to cause these major eruptions from 3.9-3.1 billion years ago • Plains are more common on the near side of the moon, probably due to the thinner crust (where it is easier for the molten material to reach the surface). Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 12 Craters on the Moon Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 13 Rays • Rays - streaks that extend outward from some of the craters • Thought to be pulverized rock that was thrown out when the crater formed • These rays are brighter (reflect more light) than the crater – (powdered rock reflects more light than regular-sized rock) • Occasionally the ray systems are also marked by secondary craters formed by flying debris thrown from the primary crater. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 14 Rills • Rills - long, narrow trenches or valleys • Some are straight and some are curved. • Thought to represent a separation (or crack) caused by moonquakes • Similar features are known to form because of earthquakes. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 15 The Straight Wall • A unique steep slope on the eastern side of Mare Nubium • The wall is 113 km long by 244 m in height. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 16 Mountain Ranges • Mountain ranges on the moon can be as high as 6100 m (>19,000 ft) • All mountain ranges appear to be formed in a circular pattern, bordering the lunar plains. • Therefore, they were probably not formed by the same processes as mountain ranges on earth. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 17 Faults • Fault - a break or fracture in the surface of the moon, along which movement has occurred • Results from the movement of the moon’s crust along a break • Since there is little/no erosion on the surface of the moon, the difference in elevation caused by these faults remain for millions of years. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 18 Origin of the Moon?? • Must take into account these facts: • Lunar rocks are similar to the earth’s mantle. • Oxygen isotope ratios indicate that the earth and the moon were formed at a similar distance from the sun. • There is no water in lunar rocks. • There is a deficiency of volatile elements (which were driven off by heat). • Relative to earth, moon has less iron. • 3.3 g/cm3 = moon; 5.5 g/cm3 = earth • The oldest rocks on earth and moon are similar. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 19 Origin of the Moon • Most widely accepted theory is the great impact theory. • A planet-sized object (size of Mars) struck the earth with a glancing blow 4.4 billion years ago, resulting in the ejection of matter into orbit to form the moon. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.1 17 | 20 Lunar Motions • The lunar orbital plane does not coincide with earth’s orbital plane. – Approximately 5o with respect to earth’s orbital plane. • Due to this 5o tilt, it is possible for the moon to be directly overhead at any latitude between 28.5oN and 28.5oS. • Both the rotation of the earth on its axis and the moon’s revolution around the earth are counterclockwise (from a N pole perspective). Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 21 Relative Motions of the Moon and Earth Section 17.2 Two Different Lunar Months • Sidereal Month – 27.33 days, lunar cycle with respect to a star other than the sun • Synodic Month – 29.5 days, lunar cycle with respect to the sun – From one’s perspective on earth the synodic month is one complete month of lunar phases. – In actuality the moon revolves more than 360o during a synodic month. • From earth it appears that every day the moon rises in the east and sets in the west. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 23 Phases of the Moon • The moon’s periodic change in appearance is its most outstanding visual feature. • One-half of the moon is always reflecting light from the sun, but only once during that cycle can an observer on earth see the entire illuminated half, called a “full moon.” • The starting point for the moon’s synodic month is arbitrarily taken a the “new moon” position. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 24 New Phase or New Moon • Occurs when earth, sun, and moon are all in the same plane, with the moon positioned between the Sun and Earth • At this position, the dark side of the moon is fully toward the Earth (“dark of the moon”). Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 25 Lunar Phase Vocabulary • Waxing phase - the illuminated portion is getting larger • Waning phase – the illuminated portion is getting smaller • Crescent moon – less than ½ of the visible portion of the moon is illuminated • Gibbous moon – more than ½ of the visible portion of the moon is illuminated Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 26 Phases of the Moon As observed from any latitude north of 28.5oN The “observer” is looking south; therefore east is on the left. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 27 Waxing Phases of the Moon • Waxing crescent phase – appears as a crescent moon, less than 90o east of sun (7.375 days) • First-quarter phase – when the moon is exactly 90o east of sun • Waxing gibbous phase – appears larger than ½ illuminated, but < a full moon (7.375 days) • Full Moon – 180o Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 28 Waning Phases of the Moon • Waning gibbous phase – appears < a full moon, but larger than ½ moon (7.375 days) • Last-quarter phase – when the moon is exactly 270o east of sun • Waning crescent phase – appears smaller than ½ illuminated (7.375 days) • New Moon – 360o Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 29 Phases of the Moon Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 30 Altitude of the Full Moon • A full moon is always on the opposite side of the earth from the sun. • Thus when the sun is at its lowest position (winter solstice), the moon will be at its highest. • Due to the 5o difference in the earth’s and moon’s orbital planes, the moon will be situated directly over the 28.5o N latitude line on the winter solstice. Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 31 Eclipses • The sun provides the light for our solar system • Planets and moons within the solar system cast shadows that extend away from the sun – The size and shape of the shadow cast depends on the object’s size, shape, and distance from the sun – The Earth and the Moon cast conical shadows, as viewed from space • Eclipse – the darkening of the light of one celestial body by another Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 32 Eclipses • The shadows cast by the Earth and the Moon cast two different degrees of darkness • Umbra – the darkest and smallest region – A total eclipse occurs within the umbra region • Penumbra – the semidark region – A partial eclipse occurs within the penumbra region Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 33 Solar Eclipse • Solar eclipse – occurs when the moon blocks some or all of the sun’s rays from an observer on Earth • A solar eclipse occurs when the moon is at or near new phase and is in or near the ecliptic plane • When the moon lies between the sun and Earth in nearly a straight line, the moon’s shadow will fall on the Earth Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 34 Positions of the Sun, Moon, and Earth During a Total Solar Eclipse The umbra and penumbra are, respectively, the dark and semidark shadows cast on the Earth by the Moon Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 35 Types of Solar Eclipses • The length of the moon’s shadow varies with the moon’s distance from the sun – In some cases the moon’s umbra does not reach all the way to Earth • If the umbra does not reach Earth, an observer on Earth would see the moon’s disk projected against the sun and a bright ring or annulus outside the dark moon – this is called an annular eclipse • The maximum diameter of the umbra on Earth is about 270 km Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 36 Solar Eclipses Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 37 Lunar Eclipse • Lunar eclipse - occurs when the Earth blocks some or all of the sun’s rays to the moon • A lunar eclipse occurs when the moon is at or near full phase and is in or near the ecliptic plane • When the Earth lies between the sun and moon in nearly a straight line, the Earth’s shadow will conceal the face of the moon – A total lunar eclipse can last for more than 1.5 hours, and partial eclipses may last for over 3 hours Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 38 A Lunar Eclipse Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 39 Spring Tide • Spring tide occurs when the sun, moon, and Earth are all positioned in nearly a straight line • In this situation, the gravitational forces of the sun and moon combine to produce higher high tides and lower low tides • The variations between high and low tides are the greatest during a spring tide • Spring tides occur twice during each lunar month; at new and full moons Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 40 Neap Tide • Neap tide occurs when the sun and the moon are at angles of 90o with respect to the Earth • In this situation, the gravitational forces of the sun and moon tend to cancel and produce lower high tides and higher low tides • The variations between high and low tides are at a minimum during the neap tide • Neap tides occur twice during each lunar month; at first and last quarter Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 41 Spring and Neap Tides Copyright © Houghton Mifflin Company. All rights reserved. Section 17.2 17 | 42 Moons on the Terrestrial Planet • • • • Mercury: No moons Venus: No moons Earth: 1 moon Mars: 2 known moons Copyright © Houghton Mifflin Company. All rights reserved. Section 17.3 17 | 43 Moons of the Jovian Planets and Pluto • Jupiter: 62 known moons Copyright © Houghton Mifflin Company. All rights reserved. Section 17.4 17 | 44 The Galilean Moons of Jupiter Copyright © Houghton Mifflin Company. All rights reserved. Section 17.4 17 | 45 The Moons of Jupiter Copyright © Houghton Mifflin Company. All rights reserved. Section 17.4 17 | 46 Saturn: 35 known moons—Titan, the largest Copyright © Houghton Mifflin Company. All rights reserved. Section 17.4 17 | 47 The Moons of Uranus Copyright © Houghton Mifflin Company. All rights reserved. Section 17.4 17 | 48 Other Solar System Objects: • • • • Asteroids Meteoroids Comets Interplanetary Dust Copyright © Houghton Mifflin Company. All rights reserved. Section 17.5 17 | 49 Asteroids or Minor Planets Copyright © Houghton Mifflin Company. All rights reserved. Section 17.5 17 | 50 Meteroids • Interplanetary metallic and stony objects that range in size from a fraction of a millometer to a few hundred meters Copyright © Houghton Mifflin Company. All rights reserved. Section 17.5 17 | 51