The Moon & Mercury

1. The Moon & Mercury AST 2010: Chapter 8

2. Cratered Worlds • The Moon is • our nearest cosmic neighbor • the only other world humans have ever visited • Mercuryis 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 AST 2010: Chapter 8

3. Properties of the Moon and Mercury AST 2010: Chapter 8

4. 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 AST 2010: Chapter 8

5. 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 AST 2010: Chapter 8

6. 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 AST 2010: Chapter 8

7. 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/cm3 compared to 5.5 g/cm3 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 AST 2010: Chapter 8

8. Lunar Surface • Two main features of the lunar surface: • Dark, large flat “seas” ormaria • 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 AST 2010: Chapter 8

9. 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 AST 2010: Chapter 8

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

11. 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 AST 2010: Chapter 8

12. 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 AST 2010: Chapter 8

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

14. The Cratering Process AST 2010: Chapter 8

15. AST 2010: Chapter 8

16. AST 2010: Chapter 8

17. 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 AST 2010: Chapter 8

18. The Origin of the Moon Hypotheses for the origin: • Fission theory • Sister theory • Capture theory • Giant impact theory AST 2010: Chapter 8

19. Impact Computer Models AST 2010: Chapter 8

20. 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 AST 2010: Chapter 8

21. 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 AST 2010: Chapter 8

22. 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 AST 2010: Chapter 8

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

24. 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 AST 2010: Chapter 8

25. 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! AST 2010: Chapter 8

26. Surface of Mercury AST 2010: Chapter 8

27. 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 AST 2010: Chapter 8