The Moon Arny, 3 rd Edition, Chapter 6

The Moon2 Introduction  Introducing the Moon Earth’s nearest neighbor in space Earth’s nearest neighbor in space

The Moon3 Once the frontier of direct human exploration Once the frontier of direct human exploration

The Moon4 Born in a cataclysmic event into an original molten state, the Moon is now a dead world – no plate tectonic or volcanic activity and no air Born in a cataclysmic event into an original molten state, the Moon is now a dead world – no plate tectonic or volcanic activity and no air

The Moon5 SSSSuffered early impact barrage

6 Plays major role in eclipses and tides Oct High tides caused major flooding in Looe

The Moon7 Description of the Moon  General Features Moon is ¼ the Earth’s diameter Moon is ¼ the Earth’s diameter

The Moon8 AAAA place of “magnificent desolation” – shapes of gray without color BUZZ ALDREN

9 Surface Features Surface divided into two major regions Surface divided into two major regions

The Moon10  Highlands – Bright rugged areas composed mainly of anorthosite (a rock rich in calcium and aluminum silicates) and pitted with craters

The Moon11 Maria – Large, smooth, dark areas generally surrounded by highlands and composed primarily of basalt (a congealed lava rich in iron, magnesium, and titanium) which is more dense than anorthosite

The Moon12

The Moon13 Other surface features Other surface features Craters - circular features with a raised rim and range in size from less than a centimeter to a few hundred kilometers – some of the larger crater have mountain peaks at their center Craters - circular features with a raised rim and range in size from less than a centimeter to a few hundred kilometers – some of the larger crater have mountain peaks at their center

The Moon14

The Moon15 Rays – Long, light streaks of pulverized rock radiating away from many craters and best seen during full Moon Rays – Long, light streaks of pulverized rock radiating away from many craters and best seen during full Moon

The Moon16 Rilles – Lunar canyons carved either by ancient lava flows or crustal cracking Rilles – Lunar canyons carved either by ancient lava flows or crustal cracking

The Moon17 Description of the Moon  Origin of Lunar Surface Features Nearly all lunar features (craters, maria, rays) are the result of impacts by solid bodies early in the Moon’s history Nearly all lunar features (craters, maria, rays) are the result of impacts by solid bodies early in the Moon’s history

The Moon18 A circular crater forms when a high-velocity projectile disintegrates upon impact in a cloud of vaporized rock and fragments which blast a hole in the surface

The Moon19 The highlands are the result of the very intense bombardment by solar system bodies soon after the Moon formed and created a solid surface The highlands are the result of the very intense bombardment by solar system bodies soon after the Moon formed and created a solid surface

The Moon20 A mare forms when early in the Moon’s history, a few large bodies (over 100 kms) strike the Moon, blasting a huge crater, pushing up mountain chains, and cracking the Moon’s crust to the point that still-molten interior material floods the newly-formed lunar depression and eventually congeals

The Moon21 Structure of the Moon IIIIntroduction The Moon lacks the folded mountain ranges and variety of volcanic peaks seen on Earth Lack of activity due to Moon cooling off much faster than Earth Moon’s higher surface-to-volume ratio (relative to Earth) allows heat to escape from it faster Being much less massive than the Earth, the Moon also has a smaller source of radioactive material to supply heat

The Moon22  Crust and Interior Interior (including crust) studied by seismic detectors set up on Moon by astronauts – essentially found to be inactive and has simpler structure than Earth’s Interior (including crust) studied by seismic detectors set up on Moon by astronauts – essentially found to be inactive and has simpler structure than Earth’s

The Moon23 Structure of the Moon Features of Moon’s surface layer Features of Moon’s surface layer Surface layer is shattered rock chunks and powder (from repeated impacts) forming a regolith tens of meters thick Surface layer is shattered rock chunks and powder (from repeated impacts) forming a regolith tens of meters thick Regolith is basaltic in maria and anorthostic in highlands Regolith is basaltic in maria and anorthostic in highlands Regolith may extend to several hundred meters in some places Regolith may extend to several hundred meters in some places

The Moon24 Below regolith is Moon’s crust Average thickness of 100 km, although crust is thinner on side that faces Earth – reason for asymmetry is not clear, but may be related to the difference in the Earth’s gravitational force across the Moon Very few maria exist on side of Moon away from Earth Crust is composed of silicate rocks rich in aluminum and poor in iron

The Moon25 The Moon’s core The Moon’s core The Moon’s low average density (3.3 g/cm3) tells us interior contains little iron The Moon’s low average density (3.3 g/cm3) tells us interior contains little iron Some molten material may be below mantle, but core is smaller and contains less iron and nickel than Earth’s Some molten material may be below mantle, but core is smaller and contains less iron and nickel than Earth’s The relatively cold Moon interior, low iron/nickel content, and slow rotation imply no lunar magnetic field – found to be the case by the Apollo astronauts The relatively cold Moon interior, low iron/nickel content, and slow rotation imply no lunar magnetic field – found to be the case by the Apollo astronauts

The Moon26 Structure of the Moon  Crust and Interior (continued) Beneath the crust is the mantle Beneath the crust is the mantle Relatively thick extending 1000 km down Relatively thick extending 1000 km down Probably rich in olivine Probably rich in olivine Appears too cold and rigid to be stirred by the Moon’s feeble heat Appears too cold and rigid to be stirred by the Moon’s feeble heat

The Moon27 Structure of the Moon  The Absence of a Lunar Atmosphere Moon’s surface is never hidden by lunar clouds or haze, nor does reflected spectrum show any signs of gas and hence no winds Moon’s surface is never hidden by lunar clouds or haze, nor does reflected spectrum show any signs of gas and hence no winds Lack of an atmosphere means extreme changes in lunar surface temperature from night to day Lack of an atmosphere means extreme changes in lunar surface temperature from night to day Mean surface temperature (day)107°C Mean surface temperature (night)-153°C Maximum surface temperature123°C Minimum surface temperature-233°C

The Moon28 No atmosphere for two reasons Lack of volcanic activity to supply source of gas Moon’s gravitational force not strong enough to retain gases even if there was a source Lack of atmosphere and plate tectonics implies that the Moon has been relatively unchanged for billions of years and will continue to be so into the foreseeable future

The Moon29 Orbit and Motions of the Moon IIIIntroduction The Moon’s orbit around the Earth is elliptical with an average distance of 380,000 km and a period of 27.3 days relative to the stars

Light and Atoms The Moon's orbit is tipped 5° with respect to the Earth's. The angle is exaggerated for clarity. Back

Light and Atoms The Moon rotates once each time it orbits the Earth, as can be seen from the changing position of the exaggerated lunar mountain. Notice that at (A) the lunar peak is to the right, while at (B) it is to the left. Thus, from the Earth, we always see the same side of the Moon even though it turns on its axis. To help see that the Moon rotates even though it keeps the same face toward the Earth, put a coin on the figure of the Moon and move it around the Earth so that the same edge of the coin always faces the Earth. Back

The Moon32 Determining the Moon’s distance can be done with high precision by bouncing a radar pulse or laser beam off the Moon Determining the Moon’s distance can be done with high precision by bouncing a radar pulse or laser beam off the Moon

The Moon33  The Moon’s Rotation The Moon keeps the same face toward the Earth as it orbits The Moon keeps the same face toward the Earth as it orbits The fact that the Moon rotates at the same rate as it orbits the Earth is called Synchronous Rotation The fact that the Moon rotates at the same rate as it orbits the Earth is called Synchronous Rotation

The Moon34 Orbit and Motions of the Moon  Oddities of the Moon’s Orbit The Moon’s orbit is tilted about 5 with respect to the ecliptic plane The Moon’s orbit is tilted about 5° with respect to the ecliptic plane It is also tilted with respect to the Earth’s equator – very unlike most of the moons in the solar system which lie almost exactly in their respective central planet’s equatorial plane It is also tilted with respect to the Earth’s equator – very unlike most of the moons in the solar system which lie almost exactly in their respective central planet’s equatorial plane

The Moon35 The Moon is also very large relative to its central planet – again unlike most of the other moons in the solar system The Moon is also very large relative to its central planet – again unlike most of the other moons in the solar system These oddities indicate that the Moon formed differently from the other solar system moons These oddities indicate that the Moon formed differently from the other solar system moons

The Moon36

The Moon37

The Moon38

The Moon39 Origin and History of the Moon  Before Apollo missions, three hypotheses of the Moon’s origin: Moon originally a small planet orbiting the Sun and was subsequently captured by Earth’s gravity during a close approach (capture theory) Moon originally a small planet orbiting the Sun and was subsequently captured by Earth’s gravity during a close approach (capture theory) Earth and Moon were twins, forming side by side from a common cloud of gas and dust (twin formation theory) Earth and Moon were twins, forming side by side from a common cloud of gas and dust (twin formation theory) The Moon spun out of a very fast rotating Earth in the early day of the Solar System (fission theory) The Moon spun out of a very fast rotating Earth in the early day of the Solar System (fission theory)

The Moon40 Origin and History of the Moon  Each of these hypotheses gave different predictions about Moon’s composition: In capture theory, the Moon and Earth would be very different in composition, while twin theory would require they have the same composition In capture theory, the Moon and Earth would be very different in composition, while twin theory would require they have the same composition In fission theory, the Moon’s composition should be close to the Earth’s crust In fission theory, the Moon’s composition should be close to the Earth’s crust  Moon rock samples proved surprising For some elements, the composition was the same, but for others, it was very different For some elements, the composition was the same, but for others, it was very different None of the three hypotheses could explain these observations None of the three hypotheses could explain these observations

The Moon41 Origin and History of the Moon TTTThe new Moon formation hypothesis: Moon formed from debris blasted out of the Earth by the impact of a Mars-sized body Age of lunar rocks and lack of impact site on Earth suggests collision occurred at least 4.5 billion years ago as the Earth was forming

Light and Atoms The birth of the Moon. Back

The Moon43

The Moon44  This “large impact” idea explains: The impact would vaporize low-melting- point materials (e.g., water) and disperse them explaining their lack in the Moon The impact would vaporize low-melting- point materials (e.g., water) and disperse them explaining their lack in the Moon Only surface rock blasted out of Earth leaving Earth’s core intact and little iron in the Moon Only surface rock blasted out of Earth leaving Earth’s core intact and little iron in the Moon Easily explains composition difference with Earth Easily explains composition difference with Earth

The Moon45 Origin and History of the Moon  This “large impact” idea explains (continued) : The splashed-out rocks that would make the Moon would more naturally lie near the ecliptic than the Earth’s equatorial plane The splashed-out rocks that would make the Moon would more naturally lie near the ecliptic than the Earth’s equatorial plane Earth’s tilted rotation axis is explained Earth’s tilted rotation axis is explained As Moon’s surface solidified, stray fragments from original collision created craters that blanket highlands As Moon’s surface solidified, stray fragments from original collision created craters that blanket highlands A few of the larger fragments created the large basins for the maria to form A few of the larger fragments created the large basins for the maria to form By the time the Maria filled with molten material and solidified, little material was left for further lunar bombardment – thus the smooth nature of the maria By the time the Maria filled with molten material and solidified, little material was left for further lunar bombardment – thus the smooth nature of the maria

The Moon46 Eclipses IIIIntroduction An eclipse occurs when one astronomical body casts its shadow on another For observers on Earth, two types of eclipse: Lunar eclipse – Earth’s shadow falls on Moon Solar eclipse – Moon’s shadow falls on Earth Some past and upcoming solar and lunar eclipses

Light and Atoms Location of some recent and upcoming total solar eclipses. (From Kuhn, Karl F. Astronomy,p. 204, St. Paul, Minn., 1989 West Publishing.) Back

The Moon48  Rarity of Eclipses Because of the Moon’s tilt relative to the ecliptic, eclipses will not necessarily occur at new and full Moon – the shadows will fall either above or below their target Because of the Moon’s tilt relative to the ecliptic, eclipses will not necessarily occur at new and full Moon – the shadows will fall either above or below their target

The Moon49 Eclipses Since the Moon’s orbit tilts nearly in the same direction through the year, twice a year the Moon’s orbit will pass through the Sun giving the possibility of an eclipse – these times are called eclipse seasons When a solar occurs at new Moon, conditions are right for a lunar eclipse to occur at the full Moon either before or after the solar eclipse Since the Moon’s orbit actually precesses once every 18.6 years, the time between eclipse seasons is less than 6 months Lunar eclipses can be seen from anywhere on Earth as long as the Moon is above the horizon, while an observer must be in the path of the Moon’s small shadow to see a solar eclipse

Light and Atoms The Moon's orbit keeps approximately the same orientation as the Earth orbits the Sun. Because of its orbital tilt, the Moon generally is either above or below the Earth's orbit. Thus the Moon's shadow rarely hits the Earth, and the Earth's shadow rarely hits the Moon, as you can see in (A) and (C). Eclipse seasons occur when the Moon's orbital plane “points” at the Sun. A solar eclipse will then occur at new moon and a lunar eclipse at full moon, as you can see in (B) and (D). Back

The Moon51 Eclipses  Appearance of Eclipses Lunar eclipse Lunar eclipse In a total lunar eclipse, the Earth’s shadow takes about an hour to cover the Moon In a total lunar eclipse, the Earth’s shadow takes about an hour to cover the Moon At totality, the Moon generally appears a deep ruddy color At totality, the Moon generally appears a deep ruddy color The color of the eclipsed Moon is caused by Earth’s atmosphere scattering out most of the blue in sunlight and bending the remaining reddish light at the Moon The color of the eclipsed Moon is caused by Earth’s atmosphere scattering out most of the blue in sunlight and bending the remaining reddish light at the Moon Solar Eclipse Solar Eclipse Hardly noticeable at first, at totality, a solar eclipse will give the appearance of nightfall Hardly noticeable at first, at totality, a solar eclipse will give the appearance of nightfall Solar corona is also evident at totality Solar corona is also evident at totality

Light and Atoms (A) Photograph of a total lunar eclipse. (Photo courtesy Dennis di Cicco.) (B) As sunlight falls on the Earth, some passes through the Earth's atmosphere and is slightly bent so that it ends up in the Earth's shadow. In its passages through our atmosphere, most of the blue light is removed, leaving only the red. That red light then falls on the Moon, giving it its ruddy color at totality. Back

Light and Atoms (A) Photograph of a total solar eclipse. The bright halo of light is the Sun's corona, its outer atmosphere. (Photo courtesy Dennis di Cicco.) (B) The landscape is eerily lit during a total solar eclipse. The dark color of the sky is the Moon's shadow. (Courtesy Richard Wainscoat.) Back

The Moon54 Tides  Introduction The regular change in height of the ocean surface is called the tides The regular change in height of the ocean surface is called the tides Tides are mainly caused by the Moon Tides are mainly caused by the Moon  Cause of Tides The Moon exerts a gravitational force on the Earth that is stronger on the side closest to the Moon and weakest on the far side The Moon exerts a gravitational force on the Earth that is stronger on the side closest to the Moon and weakest on the far side This difference in force from one side of an object to the other is called a differential gravitational force This difference in force from one side of an object to the other is called a differential gravitational force This differential force draws water in the ocean into a tidal bulge on the sides facing and opposite the Moon This differential force draws water in the ocean into a tidal bulge on the sides facing and opposite the Moon Earth’s rotation leads to 2 high/low tides per day Earth’s rotation leads to 2 high/low tides per day

Light and Atoms Tides are caused by the Moon's gravity creating tidal bulges. Back

Light and Atoms (A) Arrows schematically show Moon's gravitation force at different points on the Earth. (B) Tidal forces from the point of view of an observer on the Earth. These arrows represent the difference between the Moon's gravitational force at a given point and its force at the Earth's center. Back

The Moon57 Tides  Solar Tides The Sun creates tides as well, although not as large in variation The Sun creates tides as well, although not as large in variation When the Sun and Moon create identically oriented tidal bulges (new and full Moon), the abnormally large “spring” tides occur When the Sun and Moon create identically oriented tidal bulges (new and full Moon), the abnormally large “spring” tides occur With the Moon at first or third quarter, the so- called neap tides occur with tides not as extreme as normal tides With the Moon at first or third quarter, the so- called neap tides occur with tides not as extreme as normal tides  Tidal Braking Tides create forces that slow the Earth’s rotation and move the Moon farther away - tidal braking Tides create forces that slow the Earth’s rotation and move the Moon farther away - tidal braking Tidal braking caused the Moon’s synchronous rotation Tidal braking caused the Moon’s synchronous rotation

Light and Atoms As the Earth rotates, it carries points along the coast through the tidal bulges. Because there are two bulges where the water is high and two regions where the water is low, we get two high tides and two low tides each day. Back

The Moon59 Moon Lore  Folklore filled with stories concerning the powers of the Moon over humans Claims that the Moon triggers social behavior – hence the word “lunatic” Claims that the Moon triggers social behavior – hence the word “lunatic” Claims the full Moon responsible for accidents, murders, etc. Claims the full Moon responsible for accidents, murders, etc. No scientific backing for these claims No scientific backing for these claims  Some “Moonisms” have a touch of truth “Once in a blue Moon”, meaning a rare event, may be related to an unusual atmospheric effect in which the Moon appears blue “Once in a blue Moon”, meaning a rare event, may be related to an unusual atmospheric effect in which the Moon appears blue “Harvest Moon”, the full Moon nearest in time to the autumn equinox, rises in the east at sunset giving farmers additional light for tending to crops “Harvest Moon”, the full Moon nearest in time to the autumn equinox, rises in the east at sunset giving farmers additional light for tending to crops

The Moon60  January Wolf Moon  February Snow Moon  March Worm Moon  April Pink Moon  May Flower Moon  June Strawberry Moon  July Buck Moon  August Sturgeon Moon  September Harvest Moon  October Hunter's Moon  November Beaver Moon  December Cold Moon

Light and Atoms The Moon. (Courtesy Stephen E. Strom.) Back

Light and Atoms Photograph showing the different appearance of the lunar highlands and maria. The highlands are heavily cratered and rough. The maria are smooth and have few craters. (© UCO/ Lick Obs image.) Back

Light and Atoms (A) Overlapping craters in the Moon's highlands. (B) Isolated craters in the smooth Mare. (©UC Regents; UCO/Lick Obs image.) Back

Light and Atoms The long, narrow, white streaks radiating away from the crater at the bottom are lunar rays. (©UC Regents; UCO/ Lick Obs image.) Back

Light and Atoms Photograph of a lunar rille. (Courtesy NASA.) Back

Light and Atoms An artist's impression of the Moon's interior. Notice the thinner near-side crust and the displacement (exaggerated for clarity) of the core toward the Earth. Back

Light and Atoms Finding the distance from the Earth to Moon by triangulation and radar and laser ranging. Back

Light and Atoms A rain of debris creates craters on the young Moon. Large impacts late in the process form the maria basins. Lava floods the basins to make the maria. Back

Light and Atoms (A) Precession of the Moon's orbit. Notice its similarity to twisting a tilted book that has one edge resting on a table. (B) Precession of the Moon's orbit causes eclipses to come at different dates in successive years. Back

Light and Atoms The Sun's gravity creates tides too, though its effect is only about one-third that of the Moon. (A) The Sun and Moon each create tidal bulges on the Earth. When the Sun and Moon are in line, their tidal forces add together to make larger than normal tides. (B) When the Sun and Moon are at 90° as seen from Earth, their tidal bulges are at right angles and partially nullify each other, creating smaller than normal tidal changes. Back

Light and Atoms Tidal braking slows the Earth's rotation and speeds up the Moon's motion in its orbit. Back