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:

Relative-age rule More craters per unit area  =  older surface
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.

Crater cross-section
A profile of an impact crater showing the raised rim, bowl, and central peak.
Impactor
A meteoroid whose kinetic energy excavates a crater far larger than itself.
Planetary globe
An imaged globe of a terrestrial world used to map its cratered and smooth regions.
Counting grid
A frame of known area laid over an image so crater density can be measured.
Ruler
A scale bar that converts a crater’s image size into its true diameter.
Orbiter camera
A spacecraft camera that photographs a surface at high resolution for crater studies.

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

Part B — Comparative planetology

Simulation

Crater Counting BenchCount first, then check against the simulation.

Team Questions

1. Of two surfaces, the one with more craters per unit area is:
2. Craters are almost always circular because crater size depends on:
3. Earth has few visible impact craters mainly because of:

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.

Practice Questions

1. The dark lunar maria have few craters because they were:
2. Which world is the most heavily cratered and ancient?
3. A thick atmosphere reduces small craters because it: