Lesson 3 of 6 · Count craters without inventing a clock
Count craters without inventing a clock
10 minute lesson · 3 checks along the way
Your goal for this lessonUse covering relationships and comparable crater densities while recognizing resurfacing and sampling limits.
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An impact changes more than a circle
A high-speed impact excavates rock, ejects material, fractures the surroundings, and can generate melt. Larger craters can develop terraced walls and central peaks as the shocked ground moves and collapses. The Lunar Orbiter 2 photograph of Copernicus gives a striking oblique view of this relief. Shadows emphasize shape, but their lengths alone cannot yield heights without knowing illumination and viewing geometry.
The recovered historical image also contains striping associated with its imaging and reconstruction history. Those lines are not parallel geological faults. Separate the landform from the way it was recorded before making a map. Crater appearance varies with size, target material, preservation, and illumination; a dramatic central peak is not by itself an exact age indicator or proof of a volcanic origin.
Lunar Orbiter 2 photographed Copernicus on November 24, 1966. This recovered historical view emphasizes relief through oblique geometry and shadows. Striping from the image’s recording and recovery history should not be interpreted as a set of lunar faults.
Look closely
Why would equal rectangles drawn near the front and back of this photograph be a poor basis for comparing crater density?
See what to notice
Perspective makes equal image areas correspond to different ground areas. A density comparison needs mapped ground area, a consistent detectable size range, and comparable preservation, rather than a count of arbitrary image rectangles.
Covering and cutting put events in order
If crater lie on top of a lava flow, the impact came after that flow was emplaced. If lava floods part of an existing crater, that filling came later than the crater. These relationships establish relative ages without requiring a date in years. They are especially valuable when numerical age estimates are unavailable or depend on uncertain regional calibration.
A region can therefore contain several clocks at once. The rock beneath a plain may be ancient, the covering flow younger, and a crater in that flow younger still. A recent impact can expose old rock. Describing the exposed rock as young simply because its excavation was recent confuses two events. Always specify whether you are ordering formation, deposition, excavation, or exposure.
Check 1 of 3
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Work through a decision
Work the evidence: compare equal quantities
Two invented mapped units were imaged well enough to detect the same declared crater-size range. Unit A has 80 craters across 400 square kilometers. Unit B has 30 across 100 square kilometers. Their densities are 80/400 = 0.20 and 30/100 = 0.30 craters per square kilometer. B has fewer craters in total, yet 1.5 times the density.
If preservation, crater production, and counting methods are sufficiently comparable, the greater density supports a longer exposed history for B. It does not automatically mean B is exactly 1.5 times as old. The impact rate has changed through time, and the counted population may have other complications. The calculation corrects the area comparison; the geological interpretation still needs evidence for the assumptions connecting density to age.
Crater counts and mapped area
Original instructional comparison using invented counts and equal detection criteria. Density bars share the same zero and scale; they are not bars of surface age.
Open full-size diagram in a new tab: Crater counts and mapped areaCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
A preserved record is not a complete record
Lava can bury craters, later impacts can erase earlier ones, and ejecta can form secondary craters that are not independent new arrivals from space. Resolution controls which small craters are visible. Very heavily cratered surfaces can approach saturation, where making new craters also destroys old ones. These effects limit a simple rule that more craters always means proportionally more elapsed time.
Returned samples help calibrate chronology by connecting laboratory ages to mapped geological units. Basalts returned by Chang'e 5 provide an approximately two-billion-year anchor that improves coverage between other dated surfaces. The Moon retains far more ancient surface evidence than geologically active Earth, but that evidence has still been rearranged and partially lost. Combining mapped relationships, sample ages, and carefully selected crater populations produces a stronger history than any one method alone.
Check 3 of 3
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Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: LPI: impact-crater geologyEjecta, secondary craters, and geological relationships.
- Read: LPI: crater structure and relative agesImpact morphology and crater-density interpretation.
- Read: Chang’e-5 calibration of lunar chronologyPrimary research connecting dated basalt with crater-count chronology.
- Read: NASA: restored Lunar Orbiter 2 Copernicus imageThe 1966 photograph and its recovery history.