13 July 2005AST 2010: Chapter 8 1 The Moon & Mercury

13 July 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

13 July 2005AST 2010: Chapter 83 Basic Properties of the Moon & Mercury

13 July 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

13 July 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

13 July 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

13 July 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/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 This raises the possibility of future human habitations near the poles

13 July 2005AST 2010: Chapter 88 Interiors of the Moon and the Earth

13 July 2005AST 2010: Chapter 89 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 The lunar mountains, the highlands, are the results of impacts The mountains are impact debris accumulated around the lips of craters

13 July 2005AST 2010: Chapter 810 Lunar History Radioactive-dating techniques applied to lunar samples yielded 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

13 July 2005AST 2010: Chapter 811 Evidence of Volcanic Activity The gas bubbles are characteristic of rock formed from lava Mare Orientale

13 July 2005AST 2010: Chapter 812 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

13 July 2005AST 2010: Chapter 813 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 July 2005AST 2010: Chapter 814 Volcanic Versus Impact Origin of Craters Volcanoes and impact craters have different shapes

13 July 2005AST 2010: Chapter 815 Stages in the Formation of Impact Crater

13 July 2005AST 2010: Chapter 816 Animation: the Cratering Process

13 July 2005AST 2010: Chapter 817 The Origin of the Moon Hypotheses for the origin: Fission theory Sister theory Capture theory Giant impact theory

13 July 2005AST 2010: Chapter 818 Impact Computer Models

13 July 2005AST 2010: Chapter 819 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

13 July 2005AST 2010: Chapter 820 How would Things be Different If Our Earth Had No Moon?

Mercury It is the planet nearest to the Sun and has the shortest period of revolution about the Sun the highest average orbital-speed Outwardly similar to our Moon in size and appearance, Mercury is heavily cratered has no atmosphere has no mountains or valleys Next to Pluto, Mercury has the largest eccentricity largest angle to the ecliptic smallest size

13 July 2005AST 2010: Chapter 822 Composition and Structure Mercury’s density is high for a planet with no atmosphere The most likely model for its interior predicts that Mercury has an unusually large metallic core surrounded by a thin (compared to the Earth) mantle

13 July 2005AST 2010: Chapter 823 Interiors of Earth, the Moon, & Mercury

13 July 2005AST 2010: Chapter 824 Mercury’s Strange Rotation (1) Visual studies of Mercury’s surface did not yield the correct information about its rotation (spinning) Not until the mid-1960s was conclusive information about Mercury’s rotation obtained with 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

13 July 2005AST 2010: Chapter 825 Doppler-Radar Measurement The Doppler-radar measurements yielded Mercury’s rotational speed The amount of frequency spreading tells us the speed of rotation

13 July 2005AST 2010: Chapter 826 Mercury’s Strange Rotation (2) Mercury rotates (spins) with respect to the stars with a period of 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!

13 July 2005AST 2010: Chapter 827 Surface of Mercury It is heavily cratered

13 July 2005AST 2010: Chapter 828 Origin of Mercury How to explain the large fraction of metal in Mercury? This question is the reverse of the problem posed by the composition of the Moon The giant-impact hypothesis: One or more giant impacts during the early period of the solar system may have torn away much of Mercury’s original mantle and crust leaving the planet dominated by its iron core The mantle and impactor may have then disappeared, perhaps into the Sun