9.3 Impact Craters
3 min read•june 12, 2024
Lunar tell a story of cosmic collisions. These circular depressions, formed by asteroids and comets slamming into the Moon, reveal clues about the Moon's history and age. The involves contact, excavation, and modification stages, creating unique features.
Counting craters helps scientists estimate the age of lunar surfaces. Older areas have more craters, while younger regions show fewer impacts. By comparing distributions with radiometrically dated samples, researchers can determine absolute ages and piece together the Moon's geological timeline.
Impact Craters on Lunar Surfaces
Formation of lunar impact craters
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- Asteroids or comets collide with the lunar surface at high velocity creating circular depressions with raised rims and ejecta blankets ()
- Crater size and shape depend on the 's size, speed, and angle of impact (larger, faster, and more perpendicular impacts create larger craters)
- Crater formation involves three main stages:
- Contact and compression stage
- Impactor makes contact causing shock waves to propagate through both objects
- High pressure vaporizes the impactor and melts or fractures the target rock ( and )
- Excavation stage
- Expanding shock wave pushes target rock outward creating a
- Ejecta is thrown out and deposited around the rim (continuous and discontinuous ejecta rays)
- Modification stage
- Transient crater collapses under gravity causing walls to slump inward and floor to rebound ( or )
- Final crater shape determined by strength and cohesion of target rock (simple bowl-shaped craters in loose , complex craters with terraced walls and central peaks in solid bedrock)
- Contact and compression stage
- The cratering process affects the final , including features like central peaks, terraced walls, and ejecta patterns
Crater counts for age estimation
- estimates relative age of lunar surfaces based on number and size distribution of impact craters
- Older surfaces have more craters accumulated over longer exposure times (heavily cratered highlands)
- Younger surfaces have fewer craters due to shorter exposure times (lightly cratered maria)
- Crater () quantifies the number of craters of different sizes per unit area
- CSFD plotted on log-log graph with crater diameter on x-axis and cumulative number of craters per unit area on y-axis
- Slope of CSFD curve indicates age and resurfacing history (steeper slopes for older surfaces, shallower slopes for younger surfaces)
- Absolute age estimates obtained by comparing CSFD of lunar surface with CSFD of surfaces with known ages from Apollo samples
- Radiometric dating of returned samples calibrates the crater counting method (3.9-3.2 billion years for maria, 4.5-4.1 billion years for highlands)
- Extrapolating crater production rate over time estimates absolute age of surface (assuming constant impact rate)
- dating techniques can be used to determine the age of lunar rocks and refine crater-based age estimates
Theories of lunar crater origins
- Impact theory
- Most widely accepted theory supported by crater morphology, ejecta blankets, and (, high-pressure minerals)
- Most lunar craters formed by asteroid and comet impacts during ~4.1 to 3.8 billion years ago
- Explains random distribution of craters across lunar surface and presence of impact melt and breccias (jumbled rock fragments)
- Volcanic theory
- Early hypothesis suggested lunar craters formed by volcanic eruptions like calderas on Earth (Crater Lake)
- Supported by circular shape of craters and some crater chains and clusters (Marius Hills)
- Fails to explain high-pressure shock features, impact melt, and ejecta blankets
- Electrical discharge theory
- Fringe theory proposes lunar craters formed by electrical discharges between Moon and charged body like plasma cloud or comet
- Claims to explain circular shape, raised rims, some glass, and magnetic anomalies
- Lacks supporting evidence and fails to account for most crater features and distribution on lunar surface
Key Terms to Review (25)
Breccia: Breccia is a type of rock composed of angular, broken fragments of minerals or rocks cemented together by a fine-grained matrix. It is typically formed through processes such as tectonic activity, weathering, or impact events, and is commonly associated with the formation of impact craters on planetary surfaces.
Central Peak: A central peak is a prominent feature found at the center of some impact craters on planetary bodies, formed by the rebound of the crater floor following the initial impact event. This distinctive geological structure provides insights into the mechanics and dynamics of crater formation.
Crater: A crater is a circular depression on a planetary surface typically formed by the impact of a meteorite. Craters can also result from volcanic activity or explosions but are most commonly associated with impacts in an astronomical context.
Crater Counting: Crater counting is the process of analyzing the number and distribution of impact craters on the surface of a planetary body to determine its age. It is a fundamental technique used in the field of planetary geology to study the geological history and evolution of planets, moons, and other celestial objects.
Crater counts: Crater counts involve measuring the number and size of impact craters on a planetary surface to determine its age. This method helps scientists understand the history and evolution of celestial bodies.
Crater Morphology: Crater morphology refers to the physical characteristics and features of impact craters formed on planetary surfaces. It encompasses the various shapes, sizes, and structures observed in craters resulting from the impact of meteoroids, asteroids, or comets with a planet or moon.
Crater Rim: The crater rim refers to the elevated circular edge that surrounds the depression or cavity formed by the impact of a meteorite, comet, or other celestial object on the surface of a planet, moon, or other solid body. The crater rim is a distinctive feature of impact craters and plays a crucial role in understanding the formation and characteristics of these geological features.
Cratering Process: The cratering process refers to the formation of impact craters on the surfaces of planetary bodies, such as moons, asteroids, and terrestrial planets, through the collision and penetration of meteoroids, asteroids, or other extraterrestrial objects. This process is a fundamental mechanism for shaping the surfaces of many celestial bodies in the Solar System.
CSFD: CSFD stands for Crater Scaling and Formation Diameter, which is a crucial concept in the study of impact craters. It describes the relationship between the size of an impactor and the resulting crater size, as well as the factors that influence crater formation and dimensions.
Ejecta Blanket: An ejecta blanket is the layer of material that is ejected and deposited around an impact crater during a meteorite or asteroid impact event. It is a key feature of impact craters and provides important information about the dynamics and mechanics of the impact process.
Gilbert: Gilbert is a prominent lunar impact crater located on the Moon's near side. Named after American geologist Grove Karl Gilbert, it provides insights into the geological history of the Moon's surface.
Impact Crater: An impact crater is a depression or hole in the surface of a planet, moon, or other celestial body, formed by the high-speed impact of a meteoroid, asteroid, comet, or other cosmic projectile. These craters are a common geological feature across the solar system and provide valuable insights into the history and evolution of planetary bodies.
Impact craters: Impact craters are depressions on the surface of planets, moons, or other celestial bodies caused by the collision of a meteoroid, asteroid, or comet. These craters often have distinctive features such as raised rims and central peaks.
Impact Melt: Impact melt refers to the molten or partially molten rock material that is generated during a meteorite or asteroid impact event. This material is produced by the intense heat and pressure associated with the impact and can have significant implications for the formation and modification of impact craters.
Impactor: An impactor is a celestial body, such as an asteroid or comet, that collides with a larger body, such as a planet or moon, creating an impact crater on the surface. Impactors play a crucial role in the formation and evolution of planetary bodies, as well as the distribution of materials within the solar system.
Isochron: An isochron is a line on a graph that represents points of equal age or time. In the context of impact craters, an isochron is a useful tool for determining the age of a crater by analyzing the distribution and characteristics of the ejecta material surrounding the crater.
Late Heavy Bombardment: The Late Heavy Bombardment (LHB) refers to a period in the early history of the Solar System, approximately 4.1 to 3.8 billion years ago, when the inner planets experienced an intense bombardment by a large number of asteroids, comets, and other planetesimals. This event had significant implications for the composition, structure, and evolution of the planets, as well as the development of life on Earth.
Meteor Crater: Meteor Crater, also known as Barringer Crater, is a well-preserved impact crater located in Arizona, USA. It was formed approximately 50,000 years ago by the impact of a nickel-iron meteorite about 50 meters in diameter.
Peak Ring: A peak ring is a distinctive geological feature found in large impact craters on planetary bodies, where a circular ring of elevated terrain surrounds the central crater floor. This structure forms as a result of the complex dynamics involved in the impact cratering process.
Rays: Rays are the straight lines or paths that light or other forms of energy travel along. In the context of impact craters, rays refer to the bright, radial patterns that extend outward from the crater's center, formed by the ejection of material during the impact event.
Regolith: Regolith is the layer of loose, unconsolidated rock and dust that covers the surface of a planetary body, such as the Moon, Mercury, or Mars. It is the result of the continuous breakdown and weathering of the underlying bedrock through various geological processes.
Shatter Cones: Shatter cones are distinctive rock formations that form during the initial stages of a meteorite impact event. They are characterized by a radiating, cone-shaped pattern of grooves and striations on the surface of rocks, indicating the immense pressures and shock waves generated by the impact.
Shock Metamorphism: Shock metamorphism refers to the dramatic physical and chemical changes that occur in rocks and minerals when they are subjected to the intense pressure and heat generated by the impact of a meteorite or asteroid. This process can transform the target materials in various ways, creating distinctive features that are characteristic of impact events.
Size-Frequency Distribution: The size-frequency distribution is a statistical representation of the relative abundance of different-sized objects within a population or sample. It is commonly used in the context of impact craters to understand the distribution and frequency of crater sizes on planetary surfaces.
Transient Crater: A transient crater is a temporary, unstable crater formed during the initial stages of an impact event, before it collapses and forms the final, stable impact crater. This is a crucial concept in understanding the formation and evolution of impact craters on planetary surfaces.