April 4, 2006Astronomy 20101 Chapter 8 Cratered Worlds: The Moon and Mercury The Moon is an object of lore and superstition. The Moon is our nearest neighbor,

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April 4, 2006Astronomy 20101 Chapter 8 Cratered Worlds: The Moon and Mercury The Moon is an object of lore and superstition. The Moon is our nearest neighbor, and the only place beyond Earth to be visited by humans. What have we learned about the Moon? Superficially, Mercury is similar to the Moon. What differences lie beneath the surface?

April 4, 2006Astronomy 20102 Properties of the Moon and Mercury MoonMercury Mass1/80 x M earth 1/18 x M earth Diameter (km)34764878 Diameter¼ x D earth 1/3 x D earth Density (g/cm 2 )3.35.4 Surface gravity (Earth=1)0.17 = 1/60.38 = 1/5 Escape velocity (km/s)2.44.3 Rotation period (days)27.358.65 Surface area (Earth=1)0.270.38 Both are small, lack an atmosphere, and are heavily cratered.

April 4, 2006Astronomy 20103 8.1 General Properties of the Moon The Moon has 1/6 the surface gravity of the Earth. This is not enough for the Moon to hold onto gases for an atmosphere. Without an atmosphere, the surface is not altered by erosion. The Moon has been geologically dead for 3 to 4 billion years. Craters of the Moon are a record of impacts over billions of years.

April 4, 2006Astronomy 20104 8.1.1 Exploration of the Moon 1959: Soviet spacecraft Luna 3 flies to the Moon and photographs the far side. 1962: President Kennedy sets goal of landing men on the Moon. 1966: Luna 9 landed on the Moon and transmitted pictures to Earth. 2000: Lunar Prospector spacecraft detects water on the Moon. 2005: Hubble telescope detects water- containing minerals on the Moon.

April 4, 2006Astronomy 20105 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 step onto the Moon. Astronauts performed experiments and returned samples of rock and soil.

April 4, 2006Astronomy 20106 8.1.2 Composition and Structure of the Moon Not the same as the Earth. The density of the Moon is 3.3g/cm 2 compared to 5.5g/cm 2 for Earth. The Moon’s material is like the Earth’s mantle and crust. The Moon lacks a large metal core. Ice has been found in craters near the poles.

April 4, 2006Astronomy 20107 8.2 The Lunar Surface There are two main features of the lunar surface: Large flat “seas” or maria Heavy cratered highlands Green cheese (ha ha ) The maria (singular is mare) are areas of ancient lava flows. The Moon has no mountains, rising features are the lips of craters. More recent large craters show starburst patterns of ejecta.

April 4, 2006Astronomy 20108 8.2.2 Lunar History Radioactive dating of lunar samples yields ages of 3.3 to 4.4 billion years. Consistent with the theory that the Earth and Moon formed 4.5 billion years ago. The highlands are the older surface areas. The maria are younger surfaces, lava flows from volcanoes present shortly after the Moon formed when it still had molten material (3.3 to 3.8 billion years ago).

April 4, 2006Astronomy 20109 Evidence of Volcanic Activity Moon rock that resembles lava rocks on Earth. Note the voids from gas bubbles.

April 4, 2006Astronomy 201010 8.2.3 On the Lunar Surface The surface of the Moon is covered with fine powdery material, several cm thick. This “dust” is the result of impacts.

April 4, 2006Astronomy 201011 8.3 Impact Craters Craters on the surface are a record of the history of the Moon. The craters are due to impacts, NOT volcanoes (the mare were produced by lava flows, not violent eruptions). It is important to understand the origin and nature of the craters on the Moon and apply the results to other planets and moons.

April 4, 2006Astronomy 201012 8.3.1 Volcanic Versus Impact Origin of Craters Volcano and impact craters have different shapes.

April 4, 2006Astronomy 201013 8.3.2 The Cratering Process

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April 4, 2006Astronomy 201015 8.3.3 Using Crater Counts The maria exhibit a slow rate of cratering over the last 3 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.

April 4, 2006Astronomy 201016 8.4 The Origin of the Moon Hypotheses for the origin: 1.Fission theory 2.Sister theory 3.Capture theory 4.Giant impact theory

April 4, 2006Astronomy 201017 Impact Computer Models

April 4, 2006Astronomy 201018 8.4 The Origin of the Moon The theory must explain: Why the Moon’s composition is similar to the Earth’s mantle and crust. (sister) Why the Moon and Earth are nearly the same age. (capture) How the Moon came to be Earth’s satellite. (fission and capture) Similarities and differences in chemistry of rocks. Similarities in isotopic abundances of oxygen.

April 4, 2006Astronomy 201019 8.5 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

April 4, 2006Astronomy 201020 8.5.2 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.

April 4, 2006Astronomy 201021 8.5.3 Mercury’s Strange Rotation Difficult to determine rotation from surface markings. Mercury’s rotation measured with doppler radar.

April 4, 2006Astronomy 201022 Doppler Radar One side of planet is rotating toward Earth while the other side is rotating away. Part of signal reflected with higher frequency and part with lower frequency. The amount of each frequency tells us the amount of rotation.

April 4, 2006Astronomy 201023 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 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!

April 4, 2006Astronomy 201024 8.5.4 The Surface of Mercury The Caloris Basin on Mercury, a large impact basin resembling the Orientale basin on the Moon.

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April 4, 2006Astronomy 201026 Surface (cont’d) Discovery Scarp in Mercury is 1km high and 100km long. It runs through several craters  must have formed after the craters.

April 4, 2006Astronomy 201027 8.5.5 The 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.

April 4, 2006Astronomy 201028 Summary The Moon and Mercury are outwardly similar: No (very little) atmosphere Not geologically active Heavily cratered They differ greatly internally: the Moon is similar to Earth’s crust and mantle Mercury has a large metal core But both are explained by “large impacts” early in the formation of the solar system.

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