Chapter 7BasicsSurface FeaturesCratersRaysRillesHighlands and MariaOrigin of Lunar Surface FeaturesStructure of the MoonWhich formed first?Crust and InteriorDifferences between the sides of the MoonThe Moon’s Lack of AtmosphereOrbit and Motions of the MoonOrbit and Motions of the MoonWhich of the following is NOT true about the Moon’s motion?Hypotheses of Moon Formation (pre Apollo)So what do we actually see?New Idea: The Violent-Birth HypothesisSlide 20Slide 21Slide 22The TidesWhy are there 2 tidal bulges?Solar TidesTidal BrakingThe tides are largest during the…Which of the following is NOT the result of tidal forces?Some properties of the MoonFor any conspiracy theoristsWhy is the Earth’s mantle liquid, but the Moon’s is solid?Did we actually land on the Moon?Supermoon Lunar EclipseChapter 7The MoonBasics•The moon has a diameter roughly 1/4 of Earth’s.•The moon has a gravity roughly 1/6 of Earth’s.•No colors, just shades of grey.•A world of “magnificent desolation” -Buzz AldrinSurface Features•Craters•Rays•Highlands•Maria•RillesCraters•Formed by impacts, range in size from centimeters to 240 km.•Impact pushes material outward, causing a raised rim•Arrangement of craters can tell us in what order they formed•Similar process created craters on Earth, but plate tectonics has erased most of the record.Rays•Long light streaks extending from craters•Composed of rock pulverized in the impact that caused the crater.Rilles•Lunar canyons•Potentially carved by ancient lava flows or wind away from craters.•Straight rilles likely the result of crustal cracking.Highlands and Maria•Maria: •Large, dark, smooth areas. •Few craters. •Composed of basalt (solidified lava with lots of iron, magnesium, and titanium silicates)•Highlands: •Brighter and less dense. •More craters.•Composed mainly of anorthosite (lots of calcium and aluminum silicates)Origin of Lunar Surface Features•Highlands formed first. Large asteroid impacts create basins that then filled with molten rock and cooled to form the maria.Structure of the Moon•Because the moon has a small volume relative to its surface area, it cooled more quickly than the Earth.•No tectonic activity led to the Moon being covered with regolith (“blanket of rock”) 10s of meters deep.•The regolith is made of chunks and powder from the same type of material as whatever is underneath (maria or highlands).Which formed first?A. Lunar highlandsB. Lunar mariaC. RaysCrust and Interior•Moon is low in iron relative to the Earth. We can tell this from its low density (3.3 g/cm3 vs 5.5 for Earth)•Seismic wave analysis shows the Moon has a crust, a solid mantle, and a partially-molten core.•The crust is thinner on the side facing the Earth, possibly a result of Earth’s gravity moving the core off-center.•Very few maria on the far side of the moon, likely because the crust is thicker on that side.Differences between the sides of the MoonThe Moon’s Lack of Atmosphere•Two main reasons•1) Lack of volcanic activity because the Moon cooled quicker than the Earth•2) Surface gravity is weak on the Moon, so even if an atmosphere formed, it wouldn’t be retained for long.•Lack of atmosphere means wild temperature swings: very hot during the day and very cold at night.The far side of the moonOrbit and Motions of the Moon•The Moon’s orbit is elliptical, making the moon appear larger (angular size) at some times and smaller at others.•Apogee (furthest distance, smallest moon) vs perigee (closest distance, biggest moon)•The Moon rotates at the same rate it orbits the Earth. This means the same side of the moon is always facing the Earth.Orbit and Motions of the Moon•The Moon’s orbit is tilted by 5º from the ecliptic, meaning it can be tilted anywhere from 18º to 29º from Earth’s equator.•This is unusual, as most other moons in the solar system lie nearly exactly in their planet’s equatorial plane.•The moon is also bigger relative to Earth than any other moon relative to it’s planet.•This all suggests the Moon formed very differently from other moons.Which of the following is NOT true about the Moon’s motion?A. The same side of the Moon always faces EarthB. The moon orbits in Earth’s equatorial planeC. The Moon’s orbit is ellipticHypotheses of Moon Formation (pre Apollo)•Capture theory: The Moon was originally a small planet that approached the Earth and was captured by the Earth’s gravity.•Twin formation theory: The Moon and Earth formed side by side from a common source of dust and gas.•Fission theory: The Earth used to spin much faster, forming a bulge that ripped away from Earth.•Moon should be made from entirely different materials from Earth.•Moon should be made from the same materials as Earth.•Moon should be mostly made of materials similar to Earth’s crust.So what do we actually see?•For some elements the compositions were the same, for others it was very different.•Relatively high abundance of high-melting-point materials like titanium.•Near complete lack of low-melting-point materials like water.•Much less iron than the Earth.Reality didn’t agree with the theoriesThrow out the theoriesNew Idea: The Violent-Birth Hypothesis•Explains the Moon’s composition (part similar to Earth, part different)•Explains why there is so little water and low-melting-point materials (vaporized during the impact)•Explains the moons tilted orbit (orbit based on the collision rather than the Earth’s rotation)•Only surface rock blasted out from Earth, coalescing into the moon.•The cores of the planets combined, leaving the Moon with only a small core.•As more debris fell onto the moon, it created the many craters in the highlands.•Further impacts before the moon fully solidified created the maria. By this time there was little debris left to create craters on the maria.•Recent computer models show that there may have been two moons at first.If the Moon had been composed of the same material as Earth’s crust, which formation theory would have been supported?A. Twin formationB. CaptureC. FissionD. Violent-birthWhat kind of impact formed the Moon according to the Violent-birth hypothesis?A. Direct hitB. Glancing blowC. Slow spiral inwardD. No actual impact, just gravitational interactionThe Tides•Caused by a differential gravitational force from the Moon on Earth’s oceans.Why are there 2 tidal bulges?•Recall