1. 8 February 2005AST 2010: Chapter 8 1 The Moon & Mercury

2. 8 February 2005AST 2010: Chapter 82 Cratered Worlds The Moon is our nearest cosmic neighbor the only other world humans have ever visited Mercury is in many ways similar to the Moon Both are relatively small lacking in atmosphere deficient in geological activity dominated by the effects of impact cratering

3. 8 February 2005AST 2010: Chapter 83 Properties of the Moon and Mercury MoonMercury Mass (Earth=1)1/801/18 Diameter (km)3,4764,878 Diameter (Earth=1)1/41/3 Density (g/cm 2 )3.35.4 Surface gravity (Earth=1)0.17 ~ 1/60.38 ~ 1/3 Escape velocity (km/s)2.44.3 Rotation period (days)27.358.65 Surface area (Earth=1)0.270.38

4. 8 February 2005AST 2010: Chapter 84 General Properties of the Moon The Moon has 1/80 of the mass of the Earth 1/6 of the surface gravity of the Earth The surface gravity of the Moon is not strong enough for it to hold on to gases to form an atmosphere Without an atmosphere, the Moon’s surface is not altered by weather erosion The Moon is geologically dead Craters on the Moon are a record of impacts over its history

5. 8 February 2005AST 2010: Chapter 85 Unmanned Exploration of the Moon 1959: Soviet spacecraft Luna 3 flew to the Moon and photographed its far side 1962: President Kennedy set the goal of landing men on the Moon 1966: Luna 9 landed on the Moon and transmitted pictures to Earth 2000: Lunar Prospector spacecraft detected frozen water on the Moon

6. 8 February 2005AST 2010: Chapter 86 Manned Exploration of the Moon 9 Moon flights and 6 landings between 1968 and 1972 Apollo 8 to 17 Apollo 11 Neil Armstrong and Buzz Aldrin stepped onto the Moon Astronauts performed experiments and brought back samples of rock and soil

7. 8 February 2005AST 2010: Chapter 87 The Moon’s Composition and Structure The Moon’s composition is not the same as that of the Earth The Moon’s average density is 3.3 g/cm 3 compared to 5.5 g/cm 3 for Earth The Moon’s material is like that of the Earth’s mantle and crust The Moon is depleted in iron and other metals, and it lacks a large metal core Water ice has been found in craters near the Moon’s poles

8. 8 February 2005AST 2010: Chapter 88 Lunar Surface Two main features of the lunar surface: Dark, large flat “seas” or maria Heavily cratered highlands The maria (singular of mare, Latin for sea) are areas of ancient lava flows Lunar mountains are all the result of impacts The mountains are impact debris accumulated around the lips of craters

9. 8 February 2005AST 2010: Chapter 89 Lunar History Radioactive dating of lunar samples yields ages of 3.3 to 4.4 billion years This is consistent with the theory that the Earth and Moon formed about 4.5 billion years ago The highlands are believed to be the older surface areas, formed early in lunar history The maria are thought to be younger surfaces Lava flowed from volcanoes present shortly after the Moon formed, when it still had molten material

10. 8 February 2005AST 2010: Chapter 810 Evidence of Volcanic Activity The gas bubbles are characteristic of rock formed from lava Mare Orientale

11. 8 February 2005AST 2010: Chapter 811 On the Lunar Surface The surface of the Moon is covered with fine powdery material, a few inches thick This “dust” is the product of impacts Astronaut’s bootprint in the lunar soil

12. 8 February 2005AST 2010: Chapter 812 Impact Craters Craters on the surface of the Moon are a record of its history The craters were created by impacts, NOT volcanic activity (the maria were produced by lava flows, not violent eruptions) It is important to understand the craters on the Moon and apply the results to other planets and moons

13. 8 February 2005AST 2010: Chapter 813 Volcanic Versus Impact Origin of Craters Volcanoes and impact craters have different shapes

14. 8 February 2005AST 2010: Chapter 814 The Cratering Process

15. 8 February 2005AST 2010: Chapter 815

16. 8 February 2005AST 2010: Chapter 816

17. 8 February 2005AST 2010: Chapter 817 Using Crater Counts The maria are believed to exhibit a slow rate of cratering over the last few billion years or so To fit the cratering of the highlands with the age of the surface, we must assume that the rate of cratering was higher before then

18. 8 February 2005AST 2010: Chapter 818 The Origin of the Moon Hypotheses for the origin: 1. Fission theory 2. Sister theory 3. Capture theory 4. Giant impact theory

19. 8 February 2005AST 2010: Chapter 819 Impact Computer Models

20. 8 February 2005AST 2010: Chapter 820 Origin of the Moon The theory must explain: Why the Moon’s composition is similar to the Earth’s mantle and crust (sister theory) Why the Moon and Earth are nearly the same age (capture theory) How the Moon came to be Earth’s satellite (fission and capture theory) Similarities and differences in chemistry of rocks Similarities in isotopic abundances of oxygen

21. 8 February 2005AST 2010: Chapter 821 Mercury Nearest planet to the Sun Named for the messenger god of Roman mythology Outwardly similar to our Moon in size and appearance Heavily cratered No mountains or valleys Except for Pluto, Mercury has The largest eccentricity Largest angle to the ecliptic Smallest size

22. 8 February 2005AST 2010: Chapter 822 Composition and Structure Mercury’s density is high for a planet with no atmosphere Most likely model predicts that Mercury has a large metallic core surrounded by a thin (compared to the Earth) mantle

23. 8 February 2005AST 2010: Chapter 823 Mercury’s Strange Rotation Difficult to determine rotation from surface markings Mercury’s rotation measured with doppler radar

24. 8 February 2005AST 2010: Chapter 824 Doppler Radar One side of the planet is rotating toward Earth, while the other side is rotating away Part of the signal is reflected with higher frequency and part with lower frequency The amount of frequency spreading tells us the amount of rotation

25. 8 February 2005AST 2010: Chapter 825 Mercury’s Rotation Mercury rotates with respect to the stars in 59 Earth-days This is Mercury’s sidereal day Mercury orbits the Sun in 88 Earth-days It’s sidereal day is 2/3 of its orbital period, a situation astronomers predict is stable for a planet A solar day on Mercury is the length of 2 orbits, or 176 days!

26. 8 February 2005AST 2010: Chapter 826 Surface of Mercury

27. 8 February 2005AST 2010: Chapter 827 Origin of Mercury How to explain the large fraction of metal in Mercury? The giant impact hypothesis: A giant impact during the early period of the solar system could have ripped much of Mercury’s original mantle free The mantle and impactor then disappeared, perhaps into the Sun