Section 3 Earths Moon Introduction Section Objectives Describe

Section 3: Earth’s Moon Introduction • Section Objectives: • Describe how the physical features of the lunar surface were created • Explain the history of the moon

6. The Lunar Surface • Through his telescope, Galileo saw two different types of lunar landscape • Dark lowlands and bright highlands • Because the dark regions resembled seas on Earth, they were later named maria, which is Latin for sea • We know that there is no water on the surface of the moon and virtually no atmosphere • So the lunar surface has not been eroded in the same way that Earth’s surface is • Instead, the moon’s surface is eroded by impact by meteoroids • Solid particles that travel through space, that range in size from microscopic to a km or more in diameter • Continual bombardment over billions of years

The Lunar Surface (cont. ) • Craters • The most obvious features of the lunar surface are craters • Round depressions in the surface of the moon v. Most craters were produced by the impact of rapidly moving debris or meteoroids • There are many billions of craters, ranging in size from microscopic to very large • The largest craters are about 250 km in diameter, about the width of Indiana

The Lunar Surface (cont. ) • Earth has fewer easily recognized impact craters • Friction with Earth’s atmosphere burns up small debris before it reaches the ground • Evidence for craters has been destroyed by erosion and other forces that change Earth’s surface • Upon impact, the colliding object compresses the material it strikes • In larger craters, a central peak forms as a result of the impact

The Lunar Surface (cont. ) • Most of the ejected material lands near the crater, building a rim around it • Heat generated by the impact is enough to melt rock • Astronomers brought back to Earth samples of lunar glass • A meteoroid only 3 m in diameter can blast out a 150 m wide crater • A few large craters, such as those named Kepler and Copernicus, formed from the impact of bodies 1 km or more in diameter • These craters are thought to be relatively young because the bright rays (elongated streaks that radiate outward for hundreds of kms) are still visible

• Highlands The Lunar • Most of the lunar surface is made up light-colored, Surface (cont. ) mountainous areas known as highlands • Cover most of the surface of the far side of the moon • Within the highlands regions are mountain ranges and large concentrations of impact craters • The highest lunar peaks reach elevations of almost 8 km • Only 1 km lower than Mount Everest • The origin of lunar mountains is thought to be a different process that the origin of Earth mountains • Astronomers think lunar mountains are formed when asteroids struck the surface

The Lunar • Maria dark, relatively smooth areas on the moon’s Surface (cont. ) • The surface are called maria (singular: mare) v. Maria, ancient beds of basaltic lava, originated when asteroids punctured the lunar surface, letting magma “bleed” out • The craters were flooded with layer upon layer of very fluid basaltic lava • The lava flows are often over 30 m thick • The total thickness that fills the maria could reach thousands of meters • Long channels called rilles are associated with maria • Rilles look similar to river valleys • May be remnants of ancient rivers of lava

The Lunar Surface (cont. ) • Regolith • All lunar terrains are covered with a layer of gray debris that came from billions of years of bombardment from space debris • This uppermost layer, called lunar regolith, is composed of igneous rocks, beads, and fine lunar dust. • It is thought that the regolith could be more than 20 m thick in some areas

7. Lunar History v. The most widely accepted model for the origin of the moon is that when the solar system was forming, a body the size of Mars impacted Earth • The impact would’ve liquefied Earth’s surface and ejected huge quantities of crustal and mantle rock from an infant Earth • A portion of this ejected debris would’ve entered an orbit around Earth where it combined to form the moon • The giant-impact hypothesis is consistent with other facts known about the moon • The ejected material would’ve been mostly iron-poor mantle and crustal rocks. • These would account for the lack of a sizable iron core on the moon • The ejected material would’ve remained in orbit long enough to have lost the water that the moon lacks

Lunar History (cont. ) • One of the methods space geologists have used to work out the basic details of the moon’s more recent history is to observe variations in crater density • The number of craters per unit area • The greater crater density, the older the surface must be • From such evidence, scientists concluded that the moon evolved in three phases: • The original crust (highlands), followed by maria basins, and finally rayed craters

Lunar History • During its early history, the moon was continually impacted as it swept up debris (cont. ) • This continuous bombardment, combined with radioactive decay, generated enough heat to melt the moon’s outer shell • Remnants of this original crust occupy the cratered highlands • These highlands are about the same age as Earth, 4. 5 byo • The next phase of the moon’s history was the formation of maria basins • Analysis of rock samples from lunar maria has shown the maria to be between 3. 2 - 3. 8 byo • The maria formed about a billion years after the moon formed • In places, these lava flows overlap the highlands, which is further evidence that the maria are younger than the highlands

Lunar History (cont. ) • The most recent prominent features that formed are the rayed craters • Material ejected from these relatively young depressions can be clearly seen covering the lunar surface and many older craters • The rays can be seen on top of other surface features, which is evidence that the rays are younger than those other features • If this crater had formed on Earth, erosional forces would have erased it long ago • Compared with Earth, the lunar surface changes very slowly