Theory — Impact Cratering and the Terrestrial Worlds
The terrestrial worlds — Mercury, Venus, Earth, the Moon, and Mars — are rocky bodies with solid surfaces. Those surfaces record their histories through the craters they carry.
1. How craters form
A meteoroid strikes at tens of kilometres per second. The impact releases so much energy that it excavates a round crater many times larger than the impactor, throwing out ejecta and often leaving a raised rim and a central peak. Crater size depends on the impactor’s energy, not its shape or direction, which is why craters are almost always circular.
2. Crater counting as a clock
Impacts happen at a roughly steady average rate, so the number of craters per unit area measures how long a surface has been exposed. The rule is simple:
A young, resurfaced region has few craters; an ancient region is saturated with them.
Crater density lets us order surfaces by age even without knowing the exact rate, and with a calibrated rate it gives absolute ages.
3. Resurfacing
Craters can be erased. Volcanism floods a region with lava (the dark lunar maria), tectonics and erosion by wind and water wear craters away, and thick atmospheres burn up small impactors before they land. A fresh-looking surface tells you one of these processes has been at work recently.
4. Comparing the terrestrial worlds
The Moon and Mercury have no significant atmosphere and little geological activity, so they are heavily cratered and ancient. Mars is intermediate, with old cratered highlands and younger volcanic plains. Venus has been resurfaced by volcanism and has few craters. Earth’s craters are mostly erased by plate tectonics, weather, and life. The number of craters is a direct read-out of how active a world has been.
Apparatus
Studying surfaces uses tools for imaging, measuring, and counting craters. In the simulation these are modelled, but the readings match what each instrument would give.
Instructions
Work through both tabs. For the counting task, count carefully first, then press the button to compare.
Part A — Crater counting and relative age
- Two surface patches are shown with a counting frame of known area.
- Count the craters in each frame and compute the crater density (craters per unit area) for each.
- Enter the density of the left patch and press Check. Then decide which surface is older.
Part B — Comparative planetology
- Select a world and read its typical crater density.
- Predict the age ranking (young or old) and whether it has been resurfaced, then press Check.
Simulation
Team Questions
Example Report
Worked example: comparing two lunar surfaces
In a frame of the lunar highlands you count 42 craters; in an equal frame of a mare you count 6.
The densities are 42 and 6 craters per frame. Because impacts accumulate at a roughly steady rate, the highland surface, with seven times the crater density, is much older. The mare is young because lava flooded and erased its earlier craters.
Ordering the surfaces by counting craters, without needing the exact impact rate, is the core skill of the lab.