Ch17 Moon and small solar system bodies.docx

MOONS AND SMALL SOLAR SYSTEM BODIES General Features The moon is the second brightest object in the sky. It reflects sunlight and produces no light of its own. It is so bright because it is only 384,000 km (240,000 mi) from the Earth. It is the only other celestial body where humans have walked. "That's one small step for man, one giant leap for mankind" Neil Armstrong (1969) The Moon revolves around the Earth in 29.5 solar days and rotates at the same rate. Because of this, the same side of the Moon always faces the Earth. The Moon is nearly spherical with a diameter of 3476 km, about one fourth of the Earth's diameter. The mass of the Moon is only 1/81 of the Earth's mass and its average density is about .6 that of the Earth. The surface gravity on the Moon is about 1/6 that of the Earth. The Moon is thought to be geologically inactive with a small, solid, iron rich core, a solid mantle and a crust about 64 to 125 km thick. The Moon has no magnetic field, but there is evidence in Moon rocks that at one time it did. The surface features of the Moon include craters, basins, plains, rays, rills, mountain ranges, and faults. Craters are small and large diameter depressions surrounded by raised areas. They are believed to have been formed by meteoroid collisions with the Moon. Since the Moon has no atmosphere, meteoroids do not burn up before they strike the surface. Over the years there have been many impacts. Also, since there is no atmosphere, once a crater forms, it remains relatively intact unless it is destroyed or covered up by another meteoroid strike. Plains were originally called Maria (meaning seas) by Galileo. They are thought to be areas filled with lava flows after meteoriod impacts. They are darker than the surrounding material and are visible to the naked eye during a full moon. Rays are streaks that extend outward from some craters. They are believed to be pulverized rock thrown out when the crater was formed. The pulverized rock appears lighter than the undisturbed rock. Rills are long, narrow trenches or valleys. They vary from a few meters to 5 kilometers in width and extend hundreds of kilometers in length. They have very steep walls and fairly flat bottoms. They are thought to have been caused by Moonquakes. Mountain ranges on the Moon have peaks as high as 6100 meters. They probably were not formed by the same processes that form mountain ranges on the Earth. A fault is a break or fracture in the surface along which vertical or horizontal movement has occurred. The Straight Wall is a vertical fault 113 km long and 244 m high. Composition and History of the Moon Astronauts have collected and brought back 379 kg of lunar material. Rocks from the highlands were formed between 4.4 and 3.9 billion years ago. Rocks from the plains have ages of 3.1 to 3.9 billion years. No rocks with ages outside of this range have been found. Most of the craters on the Moon's surface were probably formed between 4.4 and 3.9 billion years ago when there was more space debris from the formation of the new solar system. Lunar Motions The moon revolves around the Earth from west to east. The Moon completes a sidereal month once every 27.33 days. This is the time for the moon to revolve through 360 degrees. The Moon completes a synodic month once every 29.5 days. This is a month of phases and returns the Moon to the same place in the sky relative to the Sun. This path covers more than 360 degrees. On the Earth, the Moon appears to rise in the east and set in the west. This apparent motion of the Moon is due to the rotation of the Earth from west to east. The Moon rises about 50 minutes later each day. This happens because the Moon moves in its orbit as the Earth rotates. At the end of 24 hours, the Moon has moved far enough that the Earth must rotate for another 50 minutes to catch up. Phases of the Moon One half of the surface of the Moon is always illuminated. The changing relative positions of the Earth, Moon and Sun cause only a part of the illuminated half to be visible from the Earth most of the time. The new moon occurs when the moon is on the side of the Earth facing the Sun. Since the illuminated side always faces the Sun, we could only see the dark side if it were visible. Solar eclipses sometimes occur during this phase. It is highest in the sky at 12:00 noon. A waxing crescent phase occurs when we can see a crescent shaped slice of the Moon on the western side that appears to be increasing in size. Less than ¼ of the Moon's surface appears to be illuminated as seen from the Earth. It is highest in the sky around 3:00 PM. The first quarter phase occurs when the Earth, Moon and Sun form a 90 degree angle with the Moon east of the Sun. The western one fourth of the Moon appears to be illuminated as seen from the Earth. It is highest in the sky at 6:00 PM. A waxing gibbous phase occurs when more than one fourth of the Moon is illuminated on its western side as seen from the Earth. It is highest in the sky at about 9:00 PM. A full moon phase occurs when the Moon is on the opposite side of the Earth from the Sun. The entire half of the Moon facing the Earth is illuminated. The full moon is highest in the sky at 12:00 midnight. The waning gibbous phase is next. It appears the same as the waxing gibbous phase except that the eastern side of the Moon is illuminated. It is highest in the sky at 3:00 AM. The last quarter phase is similar to the first quarter phase but the 90 degree angle with the Earth and Sun is now west of the Sun and the eastern side of the Moon is illuminated. This phase is highest in the sky at 6:00 AM. A waning crescent phase is similar in appearance to a waxing crescent but the eastern side of the Moon is illuminated. This phase is highest in the sky about 9:00 AM. Did you know that a full moon can happen twice in one month? When this happens, the second full moon of the month is called a Blue Moon! Eclipses There are two types of eclipses involving the Moon. They are solar and lunar eclipses. During a solar eclipse, the Moon passes between the Earth and the Sun during daytime hours so that the Sun is completely or partially blocked out by the Moon. The Sun is much larger in diameter than the Moon, but the Moon is much closer to the Earth. This coincidence of distance and diameter makes the apparent size of the Moon almost identical to that of the Sun. When the Moon is closer to the Earth in its orbit, it blocks out the disk of the Sun completely, allowing us to view the solar corona(hot gasses surrounding the Sun). When the Moon is farther from the Earth in its orbit, a ring of the Sun may still be visible around the Moon. This is called an annular(ringed) eclipse. The darkest part of the shadow cast by the Moon on the surface of the Earth is called the umbra. In this region direct light from the Sun is completely blocked out and we experience a total eclipse. Outside of the umbra, a larger circular region of partial darkness called the penumbra, exists. In this region part of the Sun's disk is still visible and we call this experience a partial eclipse. In order for a solar eclipse to happen, the Moon must be in the New phase and it must be on or near the ecliptic plane. We don't have an eclipse every new moon because the Moon's orbit takes it slightly above and below the ecliptic plane. The average total solar eclipse lasts only 3 to 4 minutes since the Moon's shadow moves at about 1600 km/hr and its diameter averages less than 270 km. A lunar eclipse occurs when the Sun, Earth, and Moon are aligned so that the Earth blocks the Sun's light from reaching the Moon. Since the Moon is only visible because it reflects light from the Sun, the Moon becomes dark during a lunar eclipse. Lunar eclipses occur when the Moon is at or near full phase. This is the time when it is possible for the Earth to come directly between the Sun and the Moon. When the Moon enters the Earth's penumbra, it loses some of its brightness and may even take on a reddish color. When it enters the Earth's umbra it is very difficult if not impossible to see. Since the Moon's orbit is angled at about 5 degrees with respect to the ecliptic plane, it is normally above or below the ecliptic plane. Only two times a month is it on the ecliptic plane. When it is rising above the ecliptic, this point is called an ascending node. When, in its orbit, it is sinking below the ecliptic, this is called a descending node. In order for an eclipse to occur, one of these nodes must correspomd to a new moon or a full moon. If it corresponds to a new moon, we have a solar eclipse. If it corresponds to a full moon we have a lunar eclipse. Ocean Tides There are two high tides and two low tides each day. The two high tides are 12 hours and 25 minutes apart as are the low tides. This is one half of the Moon's synodic period and is an indication that high and low tides are caused by the Moon's gravitational attraction for the oceans on the Earth. The water on the side of the Earth nearest the Moon is attracted strongly to form a tidal bulge. On the opposite side of the Earth, the Earth is actually pulled away from the water causing another tidal bulge. These bulges correspond to high tides and the resulting areas of depression correspond to low tides. Although the Moon plays the strongest role in the formation of tides, the Sun's gravitational attraction is significant. When the Sun, Earth, and Moon are in a straight line, the Sun's gravitational attraction is added to that of the Moon causing the highest high tides and the lowest low tides. These are called spring tides and occur during full or new moon phases. When the Sun, Earth, and Moon form a 90 degree angle, the Sun's gravity pulls water upward in the areas of depression. This causes the low tides to be higher than normal and the high tides to be lower than normal. These are called neap tides and occur during first and last quarter phases of the Moon. The interaction between the Earth and Moon through tides causes the Earth's rate of rotation to slow by .002 seconds per century. Since angular momentum of the system must be conserved, the Moon moves about 1.3 cm farther from the Earth each year. Other Solar System Objects Other objects in the solar system include asteroids, meteoroids, comets and interplanetary dust. Asteroids are small (< 1 meter) to very large(940 km) rocky objects orbiting the sun between the orbits of Mars and Jupiter. They occupy a location where a planet should have formed based on the distance ratios of all the other planetary orbits. The largest is called Ceres. The two other largest are Pallas and Vesta. Asteroids are believed to be early solar system material that never collected into a single planet. Meteoroids Meteoroids are metallic and stony objects found between the planets. They can be as small as a grain of sand or as large as a few hundred meters in diameter. When they enter the Earth's atmosphere, friction with the air heats them up so that they give off light. Then they are called meteors or shooting stars. If one survives its trip through the atmosphere and its collision with the Earth, it is called a meteorite. When a meteorite strikes the ground, it leaves a crater. One of the largest known is the Barringer Meteorite Crater near Winslow, Arizona. Comets The name comet comes from the Latin words meaning long haired stars. A comet is composed of dust and ice and revolves around the sun in a highly elliptical orbit. When it is far away from the Sun it is a dirty ice ball. As it approaches the Sun, heating from the Sun causes vapors and dust to stream out from the comet in a direction away from the sun. A comet consists of four parts(1) the nucleus, a small rocky core consisting of stony or metallic material as well as solid water, ammonia, methane and carbon dioxide, (2) the head or coma which surrounds the nucleus in a spherical fashion, (3) the tail which is composed of ionized molecules, dust, or a combination which can be millions of kilometers in length, and (4) a spherical cloud of hydrogen surrounding the coma. Halley's Comet is one of the most famous. It orbits the Sun once every 76 years. We last saw it in 1986. Two regions in the solar system are thought to be the source of comets. The Oort Cloud consists of matter located about 50,000 AU's from the Sun. The Kuiper Belt is located between Pluto's orbit(39 AU) and the Oort Cloud. Interplanetary Dust Interplanetary dust consists of very small, solid particles called micrometeoroids. A faint glow called zodiacal light can be seen along the zodiac (ecliptic) just after sunset on a very dark night. This faint glow is due to sunlight being reflected by interplanetary dust.