Lecture 11 Earth s Moon January 6d, 2014

Transcription

1 1 Lecture 11 Earth s Moon January 6d, 2014

2 2 Moon and Earth to Scale Distance: a = 385,000 km ~ 60R Eccentricity: e = Galileo Spacecraft Dec. 1992

3 3 [Review question] Eclipses do not occur each month because A. the Moon s orbit is so elliptical. B. the Moon s orbit is tilted with respect to the Earth s orbit around the Sun. C. the Moon takes 6 months to complete its orbit around the Earth. D. the Earth s rotation axis is tilted. E. the statement is false. Eclipses do occur each month, but they are usually visible only in very remote parts of the Earth.

4 4 [Review question] Eclipses do not occur each month because A. the Moon s orbit is so elliptical. B. the Moon s orbit is tilted with respect to the Earth s orbit around the Sun. C. the Moon takes 6 months to complete its orbit around the Earth. D. the Earth s rotation axis is tilted. E. the statement is false. Eclipses do occur each month, but they are usually visible only in very remote parts of the Earth.

5 5 Tidal Forces Earth & Moon Gravity of Moon pulls on Earth Force on near side stronger than far side Gravitational Force Gm1m 2 r 2

6 6 Tidal Forces Net result is to pull object apart. Difference in forces on far and near side is the tidal force Causes bulge in water on Earth. Earth also slightly distorted

7 7 The photographs below were taken at high tide and the next low tide. About how much time elapsed between the pictures? A. 3 hours B. 6 hours C. 12 hours D. 24 hours E. 1 month Figure 6.24, Arny and Schneider, 5 th ed. Explorations, 2008 The McGraw-Hill Companies, Inc.

8 8 The photographs below were taken at high tide and the next low tide. About how much time elapsed between the pictures? A. 3 hours B. 6 hours C. 12 hours D. 24 hours E. 1 month Figure 6.24, Arny and Schneider, 5 th ed. Explorations, 2008 The McGraw-Hill Companies, Inc.

9 9 Surface of the Moon No atmosphere Temperature too high to hold onto gases Frozen water detected in craters at poles Detected by Clementine and Lunar Prospector Missions (radar imaging) NASA LCROSS impact 10/09/2009 Sun does not reach bottoms of craters at poles Water not evaporated and lost to space Ice deposited by comets would remain

10 10 South Pole of Moon Clementine Mission: U.S. Air Force

11 11 Surface Features on the Moon Two types of Surfaces Highlands More heavily cratered surface is older. Lighter colored. Predominantly anorthosites Maria ( Seas ) Less heavily cratered surface is younger. Darker colored Basalts Predominantly found on near side of the Moon

12 12 Moon -- Part of Far Side Mare Orientale impact crater

13 13 Craters on the Moon Craters due to bombardment by debris in space. Impact creates shock wave creating circular crater Material is pushed away from crater forming ejecta blanket Rock beneath crater is shattered, some is melted by impact

14 14 Lunar Highlands Geology Image width = 250 km

15 15 Apollo Mission

16 16 Interior Structure Mass ~ 1/80 of Earth s mass Size ~ 1/4 of Earth Density: = 3300 kg/m 3 Made mainly of rock Interior mainly solid Why? Moon smaller cools more rapidly No global magnetic field Little metal or metal is not molten.

17 17 Interior Structure Moon is differentiated Outer Crust Mantle Possible ironrich core Structure is known from study of moonquakes Caused by tidal forces from the Earth

18 18 Origin of the Moon Giant Impact Theory 4.6 byr ago -- Moon formed, possibly due to collision of Earth and a large, Mars-sized object. If Earth already differentiated, most material expelled would be mantle material, not iron core

19 19 Giant Impact Theory Rock at impact heated, losing volatile elements Some debris falls back to Earth, some forms Moon. Earth and Moon each pulled back into spherical shape by gravity.

20 20 Continuing Lunar History byr ago Era of Accretion ( heavy bombardment ) Moon differentiates, denser material moves inward. Lighter material moves to surface, forms anorthositic crust ~60 km thick (Lunar Highlands) Heavy cratering

21 21 Continuing Lunar History ~ 4-3 byr ago Heavy bombardment ends Interior molten due to radioactive decay. Basalts fill in low areas (large impact basins) to form maria. Old craters in the maria are erased. Light cratering occurs after maria have formed

22 22 Continuing Lunar History ~ 3 byr ago -- Moon cools, light cratering continues. ~ 3 byr ago - Present -- dead Moon Light cratering continues. No evidence for geologic activity

23 23 Dating the Surface of the Moon Apollo missions collected lunar rocks and then applied radiometric dating techniques They also recorded the density of craters at those sites (number of craters/area) We can date other areas of the Moon by looking at the density of craters in different regions.

24 24 The large number of craters on the lunar highlands compared to those on the maria is evidence that A. the surface of the maria is liquid and craters quickly disappear there. B. the material composing the highlands is very soft and easily cratered. C. the bodies that struck the Moon and made the craters always missed hitting the maria. D. the maria are much younger than the highlands. E. the maria are much older than the highlands.

25 25 The large number of craters on the lunar highlands compared to those on the maria is evidence that A. the surface of the maria is liquid and craters quickly disappear there. B. the material composing the highlands is very soft and easily cratered. C. the bodies that struck the Moon and made the craters always missed hitting the maria. D. the maria are much younger than the highlands. E. the maria are much older than the highlands.