Lunar impact craters 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 cratering process 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 crater 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

• Asteroids or comets collide with the lunar surface at high velocity creating circular depressions with raised rims and ejecta blankets (rays)
• Crater size and shape depend on the impactor's size, speed, and angle of impact (larger, faster, and more perpendicular impacts create larger craters)
• Crater formation involves three main stages:
  1. 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 (impact melt and breccia)
  2. Excavation stage
     • Expanding shock wave pushes target rock outward creating a transient crater
     • Ejecta is thrown out and deposited around the rim (continuous ejecta blanket and discontinuous ejecta rays)
  3. Modification stage
     • Transient crater collapses under gravity causing walls to slump inward and floor to rebound (central peak or peak ring)
     • Final crater shape determined by strength and cohesion of target rock (simple bowl-shaped craters in loose regolith, complex craters with terraced walls and central peaks in solid bedrock)
• The cratering process affects the final crater morphology, including features like central peaks, terraced walls, and ejecta patterns

Crater counts for age estimation

• Crater counting 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 size-frequency distribution (CSFD) 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)
• Isochron 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 shock metamorphism (shatter cones, high-pressure minerals)
  • Most lunar craters formed by asteroid and comet impacts during Late Heavy Bombardment ~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