ASTR 151 Chapter 8 Part 1
ASTR 151 Chapter 8 Part 1 ASTR 151 001
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This 5 page Class Notes was uploaded by Wesley Fowler on Tuesday April 5, 2016. The Class Notes belongs to ASTR 151 001 at a university taught by Dr. Sean Lindsay in Spring 2016. Since its upload, it has received 18 views.
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Date Created: 04/05/16
Wesley Fowler ASTR Chapter 8 The Moon Distance from Earth: 384,000 km Radius: 1,738 km (0.27 Earth radii) Mass: 7.3 x 10 22kg (~1/8 Earth Masses) Orbital Period: 27.3 days Rotational Period 27.3 days - The moon has no atmosphere and no global magnetic field The side of the moon that can be seen from Earth is called the Lunar near side, while the face that cannot be seen is the Lunar far side. We can see about 59% of the moon due to its slightly elliptical orbit, which causes a lunar libration. - Lunar libration: The oscillation of the moon, or its rocking back and forth. The moon’s rotational period is equal to its orbital period around Earth, both being 27.3 days. This is why only the lunar near side is visible from earth. The moon is in synchronous orbit. Tidal locking: When an orbiting moon is in synchronous orbit with its associated planet. - Most moons in the solar system are tidally locked. The Moon’s Surface Two dominate surface features: 1. The Lunar Maria: The younger, smoother surface that has fewer craters. Darker in color due to high amount of iron in rock. 2. The Cratered Highlands: Elevated above maria by several kilometers. Older surface with much more craters, a lighter color. The region is filled with mountains. The Lunar near side is mostly maria with highlands at the southern tip, while the far side is mostly highlands with some scattered maria. Impact craters: Ring-like impressions on the moon created from meteor impacts. Simple craters: Clear and singular bowl shape. Simple physics. Complex craters: Large craters that have a central peak. Complex physics. Ejecta: The material ejected from the moon due to a meteor strike. - Ejecta blanket: White surrounding material around crater. - Ejecta rays: White streaks from craters that stretch across the moon. The older surfaces of the moon have more craters than younger surfaces. Superposition: What is on top is younger. If there is a crater in a mare or a larger crater, the crater is obviously younger. Radioactive dating of lunar samples is a more accurate method than crater observation. • End of Late Heavy Bombardment: 3.9 billion years ago • Formation of Maria: 4.1 – 3.9 billion years ago • Cratered Highlands: 4.4 billion years ago Regolith: The fine dust that covers the moon, caused by the breakup of the moon’s surface. - Can be over 100m deep, this is why the astronauts left footprints on the moon. - The steady buildup of regolith creates an erosion rate of 5 meters per billion years. Permanently shadowed craters provide evidence of once-existing water ice at the poles of the moon. These regions, that have never seen sunlight, contain hydrogen and water ice molecules. - Less water in these regions than the driest place on Earth. Lunar Volcanism Crater-chains: Craters that were not created by impact, but by the collapsed of lava domes Volcanic Rilles: Long ditches where lava once flowed. These cover the surface of the moon. Wesley Fowler ASTR Chapter 8 The Moon’s Interior 3 Average Density: 3300kg/m - Lower than Earth’s, but higher than the Moon’s surface. Thus an iron core. Total Radius: 1,738 km Layers: Solid iron inner core: 240km (small) Fluid outer core: 330Km Molten inner mantle Large, rocky outer mantle: Out to 1600Km Thin rocky crust No plate tectonics No electromagnetic field Maria is believed to be made of lunar mantle material mixed with the crust Origins of the Moon Earth’s mantle and crust are very compositionally similar to the Moon. They are all made up of similar rocky and metallic materials. - The Moon lacks a substantial metal core Co-formation (sister) Theory: Earth and Moon formed separately, but at the same time, out of the same material. - The moon’s deficiency of iron refutes this theory. Capture Theory: Earth and Moon formed separately apart from each other, with the Moon being captured by the Earth’s gravitational force, causing it to orbit. - The physics required for the Earth to capture the Moon in its orbit are not possible. Fission (Daughter) Theory: Earth and Moon form as single object, and the Earth and moon split up due to a very high rotation rate. - The physics required for this split are not possible Giant Impact Theory: When the Earth was mostly molten and young, a grazing collision happened with a “Mars-sized” protoplanet caused a chunk to separate from Earth. - This physics required are possible, and the theory explains compositional similarities Evolution of the Moon Oldest rocks dated: 4.4 billion years ago (age of highlands) - Crust must have formed by this time Shortly after formation, the Moon’s surface was entirely molten, it was covered by what is called the lunar magma ocean. Lunar Magma Ocean Sequence: 1) Formation result of hot accretion 2) Light material (anorthosite) floats -> Crust, such that thinner crust formed on near-side 3) Dense materials sink 4) Large impacts form huge basins during Late Heavy Bombardment – Precursors to maria 5) Volcanism over next several hundred million years (3.9 – 3.2 Gya) filled impact basins to form maria with mantle compositions 6) No significant geologic activity after 3.2 Gya; Surface evolution dominated by the steady of impacts (generating new craters and regolith) Near-side crust is thinner than the far-side, and thus can flood with lava.