Section 1 Earth S Moon- the Lunar Surface
Section 1 Earth’s Moon- The Lunar Surface
Craters, Rilles, and Ridges
- crater a bowl-shaped depression that forms on the surface of an object when a falling body strikes the object’s surface or when an explosion occurs
•The surface of the moon is covered with craters, rilles, and ridges. Most of the craters formed when debris struck the moon about 4 billion years ago.
•Rilles are long, deep channels that run through the maria. Rilles are thought to be leftover lava channels from the formation of the maria.
•The moon’s surface also has several ridges, which are long, narrow elevations of rock that rise out of the surface and criss-cross the maria.
Regolith
•The moon’s surface is very susceptible to meteorite hits because the moon has no atmosphere for protection.
•Over billions of years, these meteorites crushed much of the rock on the moon’s surface into a layer of dust and small fragments called regolith.
•The depth of regolith on the moon varies from 1 m to 6 m.
The Interior of the Moon
•The interior of the moon is less dense than the interior of earth.
•Scientists have determined that the moon’s interior is layered by studying seismic information collected during moonquakes.
•Like, Earth, the moon has three compositional layers: the crust, the mantle, and the core.
The Moon’s Crust
•The side of the moon that faces Earth is called the near side, while the side of the moon that faces away from Earth is called the far side.
•The moon’s crust is thicker on side than the other.
•The crust on the near side is about 60 km thick. The crust on the far side is up to 100 km thick.
•The difference in thickness was caused by the pull of Earth’s gravity during the formation of the moon.
The Moon’s Mantle and Core
•Scientists think that the moon has a small iron core that has a radius of less than 700 km.
•Studies of the core have shown that the moon’s rotation is not uniform and that the core is neither completely solid nor completely liquid.
The Formation of the Moon- The Giant Impact Hypothesis
•Most scientists agree that the moon formed in three stages.
•The giant impact hypothesis states that the formation of the moon began when a large object collided with Earth more than 4 billion years ago.
•The collision ejected chunks of Earth’s mantle into orbit around Earth. The debris eventually clumped together to form the moon.
•This hypothesis explains when moon rocks share many of the chemical characteristics of Earth’s mantle.
Differentiation of the Lunar Interior
•Early in its history, the lunar surface was covered by an ocean of molten rock.
•Over time, the densest materials moved toward the center of the moon and formed a small core.
•The least dense materials formed an outer crust.
•The other materials settled between the core and the outer layer to form the moon’s mantle.
Meteorite Bombardment
•When the moon first formed, it was bombarded with meteorites, creating craters and regolith on the moon’s surface.
•About 3 billion years ago, less material struck the lunar surface, and few new craters formed.
•Craters that have rays formed during the most recent meteor impacts. During this stage, virtually all geologic activity stopped on the moon.
Lava Flows on the Moon
•After impacts on the moon’s surface formed deep basins, lava flowed out of cracks, or fissures, in the lunar crust.
•This lava flooded the crater basins to form maria.
Lava Flows on the Moon
•Because of the uneven thickness of the moon’s crust, more maria formed on the near side of the moon than on the far side.
•Scientist do not yet know how magma formed in the lunar interior or how magma reached the surface because there is no evidence of plate tectonics on the moon.
•Some scientists think the magma was formed by the large amount of energy that was produced by the long period of intense meteorite bombardment. Otherscientists think radioactive decay of materials may have caused magma to form.
The Earth-Moon System
Lunar Rotation
•The moon’s revolution around Earth and its rotation on its axis take the same amount of time.
•Because the rotation and revolution take the same amount of time, observers on Earth always see the same side of the moon.
•The part of the moon illuminated by . . sunlight changes as the moon orbits Earth.
Tides on Earth
•Bulges in Earth’s oceans, called tidal bulges, form because the moon’s gravitational pull on Earth decreases with distance from the moon.
•As a result, the ocean on Earth’s near side is pulled toward the moon with the greatest force.
•The solid Earth experiences a lesser force.
•These differences cause Earth’s tidal bulges. Because Earth rotates, tides occur in a regular rhythm at any given point on Earth’s surface each day.
Asteroids
•asteroid a small, rocky object that orbits the sun; most asteroids are located in a band between the orbits of Mars and Jupiter
•In addition to the planets and their moons, our solar system includes millions of smaller bodies, such as asteroids, comets, and meteoroids.
•The largest of these smaller bodies are asteroids, which are fragments of rock that orbit the sun.
Composition of Asteroids
•The composition of asteroids is similar to that of the inner planets.
Comets
•comets a small body of rock, ice, and cosmic dust that follows an elliptical orbit around the sun and that gives off gas and dust in the form of a tail as it passes close to the sun
Meteoroids
- meteoroids a relatively small, rocky body that travels through space
•Most meteoroids have a diameter of less than 1 mm.
•Scientists think that most meteoroids are piece of matter that become detached from passing comets.
•Large meteoroids, which are more than 1 cm in diameter, are probably the result of collisions between asteroids.
Meteors
•meteor a bright streak of light that results when a meteoroid burns up in the Earth’s atmosphere
•When a meteoroid enters Earth’s atmosphere, friction between the object and the air molecules heats the meteoroid’s surface.
•As a result of the friction and heat, most meteoroids burn up in Earth’s atmosphere.
•As the meteoroid burns up, it produces a bright streak of light called a meteor. Meteors are often called shooting stars.
Meteorites
•Meteoroids that do not burn up, but fall to Earth’s surface, are called meteorites.
•Most meteorites are small and don’t cause much damage, but occasionally large meteorites strike Earth’s surface with the force of a large bomb.
•There are three types of meteorites: stony, iron, and stony-iron.