Agenda • Announce: – Project Part II due Tue – No class next Thursday...Tgiving break! Chapter 7 – No class 12/14 (last day) – Spectral Lines Lab due • Pass Back Test 2 The Moon • Discuss grades • NYT article on gamma ray bubbles • My talk • The Moon 1 • Lab 2

The Earth’s Moon The Moon

– Moon is 1/4 the • Earth’s nearest neighbor is space Earth’s diameter • Once the frontier of direct human exploration – Gravity is 1/6 as • Born in a cataclysmic event into an original strong molten state, the Moon is now a dead world – no – A place of plate tectonic or volcanic activity and no air “magnificent • Suffered early impact barrage desolation” – • Plays major role in eclipses and tides shapes of gray without color

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“Magnificent Desolation” Surface Features • Surface divided into two major regions – Highlands – Bright rugged areas composed mainly of anorthosite (a rock rich in calcium and aluminum silicates) and pitted with craters – Maria – Large, smooth, dark areas surrounded by highlands and composed primarily of basalt (a congealed lava rich in iron, magnesium, and titanium), which is more dense than 5 anorthosite 6

Rays Craters • Long, light • Craters – circular streaks of features with a raised pulverized rim and range in size from less than a rock radiating centimeter to a few away from hundred kilometers – many craters some of the larger and best seen craters have mountain during full peaks at their center Moon

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Rilles 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 – A circular crater forms when a high-velocity projectile disintegrates upon impact in a cloud of vaporized rock and • Lunar canyons carved either by ancient lava fragments that blast a flows or crustal cracking hole in the surface 9 10

Origin of Lunar Surface Features Formation of Maria

• The highlands are the result of the very intense bombardment by solar system bodies soon after the Moon • A mare forms when early in formed and created a the Moon’s history, a few solid surface large bodies (over 100 km) strike the Moon. Molten material floods the newly formed lunar depression and 11 eventually congeals 12

Structure of the Moon The Interior of the Moon • The Moon lacks the folded mountain ranges and variety of volcanic peaks seen on Earth • Interior (including • Lack of activity due to Moon crust) studied by cooling off much faster than seismic detectors set Earth up on Moon by – Moon’s higher surface-to-volume astronauts – ratio (relative to Earth) allows heat essentially found to to escape from it faster be inactive and has – Being much less massive than the simpler structure Earth, the Moon also has a smaller source of radioactive material to than Earth’s supply heat 13 14

The Surface of the Moon - Regolith The Interior of the Moon - Crust

• Surface layer is • Average thickness of shattered rock 100 km, although chunks and crust is thinner on side that faces Earth powder (from • Reason for repeated asymmetry is not impacts) clear, but may be forming a related to the regolith tens of difference in the meters thick • Regolith is basaltic in maria and Earth’s gravitational • Very few maria exist on side of anorthostic in highlands force across the Moon away from Earth Moon • Regolith may extend to several • Crust is composed of silicate rocks hundred meters in some places rich in aluminum and poor in iron 15 16

The Interior of the Moon - Mantle The Interior of the Moon - Core

• The Moon’s low average density (3.3 g/cm 3) tells us interior contains little iron • Some molten material may be below mantle, but • The relatively cold Moon interior, • Relatively thick, extending 1000 km down core is smaller and contains less low iron/nickel content, and slow • Probably rich in olivine iron and nickel rotation imply no lunar magnetic • Appears too cold and rigid to be stirred by than Earth’s field – found to be the case by the Apollo astronauts the Moon’s feeble heat 17 18

Lunar Atmosphere Lunar Atmosphere • No atmosphere for • Moon’s surface is never two reasons hidden by lunar clouds – Lack of volcanic or haze, nor does activity to supply reflected spectrum show source of gas any signs of gas and – Moon’s hence no winds gravitational force • Lack of an atmosphere not strong enough means extreme changes to retain gases even in lunar surface if there was a • Lack of atmosphere and plate tectonics temperature from night source implies that the Moon has been relatively unchanged for billions of to day years and will continue to be so into the 19 foreseeable future 20

Orbit and Motion of the Moon Synchronous Rotation

• The Moon’s orbit around the Earth is elliptical • The Moon keeps the with an average distance same face toward of 380,000 km and a period of 27.3 days the Earth as it orbits relative to the stars • The fact that the • Determining the Moon’s Moon rotates at the distance can be done same rate as it orbits with high precision by the Earth is called bouncing a radar pulse synchronous or laser beam off the rotation Moon 21 22

The Moon’s Orbit Origin and History of the Moon

• The Moon’s orbit is tilted • It is also tilted with • The Moon is also very large relative to its central planet about 5° with respect to respect to the Earth’s – again unlike most of the other moons in the solar system the ecliptic plane equator – very unlike most of the moons in the • These oddities indicate that the Moon formed differently solar system from the other solar system moons! 23 24

Lunar Formation Hypotheses Lunar Formation Hypotheses • Each of these hypotheses gave different • Before Apollo missions, three hypotheses of the predictions about Moon’s composition: Moon’s origin: – In capture theory, the Moon and Earth would be very – Moon originally a small planet orbiting the Sun and different in composition, while twin theory would require they have the same composition was subsequently captured by Earth’s gravity during a close approach ( capture theory ) – In fission theory, the Moon’s composition should be close to the Earth’s crust – Earth and Moon were twins, forming side by side from a common cloud of gas and dust ( twin • Moon rock samples proved surprising formation theory ) – For some elements, the composition was the same, but for others, it was very different – The Moon spun out of a very fast rotating Earth in – None of the three hypotheses could explain these the early day of the Solar System ( fission theory ) observations 25 26

The Large Impact Hypothesis The Large Impact Solution • The new Moon • formation This “large impact” idea explains: hypothesis: – The impact would vaporize low-melting-point – Moon formed from materials (e.g., water) and disperse them debris blasted out of explaining their lack in the Moon the Earth by the – Only surface rock blasted out of Earth leaving impact of a Mars- Earth’s core intact and little iron in the Moon sized body – Age of lunar rocks – Easily explains composition difference with Earth and lack of impact – The splashed-out rocks that would make the site on Earth Moon would more naturally lie near the ecliptic suggests collision than the Earth’s equatorial plane occurred at least 4.5 billion years ago – Earth’s tilted rotation axis is explained 27 28

Tides Tides

• The Moon exerts a • This differential gravitational force on the Earth that is force draws water stronger on the side in the ocean into closest to the Moon a tidal bulge on and weakest on the far the sides facing side and opposite the • This difference in force from one side of an Moon object to the other is called a differential gravitational force 29 30

Tides Solar Contributions to Tides

• Earth’s rotation leads to two high/low tides per day

• When the Sun and Moon • With the Moon at first line up (new and full or third quarter, the so- Moon), abnormally large called neap tides occur, spring tides occur with tides not as extreme as normal tides 31 32

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 33