9.3 Impact Craters

Impact Craters on Lunar Surfaces

Lunar impact craters are circular depressions formed when asteroids and comets slam into the Moon's surface at tremendous speeds. Studying these craters reveals the Moon's history: how old different surfaces are, what hit them, and when the bombardment was heaviest. The three key topics here are how craters form, how scientists use crater counts to estimate surface ages, and the competing theories for crater origins.

Formation of lunar impact craters

When an asteroid or comet strikes the Moon, it's traveling at speeds of 20 km/s or more. That collision creates a circular depression with a raised rim and an ejecta blanket, the debris thrown outward from the impact site. Bright streaks called rays can extend hundreds of kilometers from fresh craters.

Crater size and shape depend on the impactor's size, speed, and angle of impact. Larger, faster, and more perpendicular impacts produce larger craters.

The formation process unfolds in three stages:

1. Contact and compression stage

   • The impactor strikes the surface, generating shock waves that propagate through both the impactor and the target rock.
   • Pressures are so extreme that the impactor vaporizes while the surrounding rock melts or shatters, producing impact melt and breccia (rock made of jumbled fragments cemented together).

2. Excavation stage

   • The expanding shock wave pushes target rock outward and upward, carving out a bowl called the transient crater.
   • Material ejected from the crater lands around the rim, forming a continuous ejecta blanket close in and discontinuous ejecta rays farther out.

3. Modification stage

   • The transient crater is unstable. Under gravity, the walls slump inward and the floor can rebound upward, sometimes forming a central peak or peak ring.
   • The final shape depends on the target rock's strength. Smaller craters in loose regolith (surface dust and rubble) tend to be simple, bowl-shaped depressions. Larger craters in solid bedrock develop complex features like terraced walls and central peaks.

These three stages together determine the crater's final morphology, including all the features you see in images: terraces, peaks, melt pools, and ejecta patterns.

Crater counts for age estimation

The basic idea is straightforward: a surface that has been exposed longer accumulates more craters. Scientists use this principle to estimate the relative and absolute ages of lunar surfaces.

• Older surfaces have more craters from longer exposure. The lunar highlands are heavily cratered, dating to roughly 4.5–4.1 billion years ago.
• Younger surfaces have fewer craters. The maria (dark, flat plains formed by lava flows) are lightly cratered, dating to about 3.9–3.2 billion years ago.

To make this quantitative, scientists use the crater size-frequency distribution (CSFD). This measures the number of craters of different diameters per unit area on a given surface.

• CSFD data is plotted on a log-log graph, with crater diameter on the x-axis and cumulative number of craters per unit area on the y-axis.
• The slope of the resulting curve reflects the surface's age and resurfacing history. Steeper slopes correspond to older surfaces; shallower slopes suggest younger or resurfaced terrain.

Turning relative ages into absolute ages requires calibration. That's where the Apollo samples come in:

1. Astronauts brought back rock samples from specific locations on the Moon.
2. Those samples were radiometrically dated in labs on Earth, giving precise absolute ages.
3. Scientists matched those ages to the CSFD measured at the same locations.
4. By comparing the CSFD of an undated surface to these calibrated curves, researchers can estimate its absolute age.

This method assumes a roughly constant impact rate over time, which is a reasonable approximation for the last ~3 billion years but less certain for earlier periods.

Theories of lunar crater origins

Impact theory is the overwhelmingly accepted explanation for lunar craters. The evidence is strong:

• Crater morphology matches what you'd expect from high-speed impacts: circular shapes, raised rims, and ejecta blankets.
• Rocks around craters show shock metamorphism, including shatter cones and high-pressure minerals like coesite and stishovite, which only form under extreme pressures.
• Impact melt and breccias are found in and around craters.
• Craters are distributed randomly across the surface, consistent with incoming projectiles from all directions.
• Most lunar craters formed during the Late Heavy Bombardment, roughly 4.1 to 3.8 billion years ago, when the inner solar system experienced a spike in impacts.

Two alternative hypotheses have been proposed but are not supported by the evidence: