High-speed impacts don't just leave dents — they explode, excavating a crater 10 to 15 times wider than the object that made it, throwing debris outward into an ejecta blanket, and sometimes pushing up a central peak where the ground rebounds after a big impact.
Here's what makes craters useful as history: impacts have struck the solar system at a roughly steady rate for the past few billion years. So on a world with little erosion or internal activity to erase them, crater count is roughly proportional to how long the surface has sat there — more craters means more time exposed, fewer craters means something younger, or something that got reset.
The Moon gives the clearest textbook example of crater counting in action. The bright highlands — rugged, ancient terrain covering most of the Moon — are absolutely riddled with craters. The dark maria (Latin for "seas," though they're actually solidified lava plains) are dramatically smoother, with roughly 10 times fewer craters per equal area.
Click a region to see its relative crater density — more craters means less time since the surface was last disturbed.
The maria formed later, when huge lava flows flooded low-lying basins and buried the older, heavily cratered ground beneath them — resetting the crater clock to zero for that region only.
Here's where the story gets genuinely strange. If today's impact rate had held steady for the Moon's entire history, having 10 times more craters would mean the highlands are roughly 10 times older than the maria. But when Apollo astronauts brought back actual rock samples for radioactive dating, the highlands turned out to be only slightly older than the maria — about 4.2 billion years, versus 3.3 to 3.8 billion years.
A surface has 10 times more craters than another. Naively assuming a constant impact rate, how old should it be if the less-cratered surface is 4 billion years old — and why is that number a red flag?
The Moon's maria aren't a one-off — the same resetting process shows up across the terrestrial planets. Much of Venus's surface is broadly volcanic, giving it a relatively young, lightly cratered look almost everywhere, hidden beneath its permanent cloud cover and mapped mostly by radar. On Mars, the Tharsis region — including Olympus Mons, the largest known volcano in the solar system — shows far fewer craters than the ancient southern highlands nearby, because repeated lava flows kept resetting the clock there long after the rest of the planet had gone quiet.
Put crater density and volcanic terrain together, and a planet's surface stops being scenery — it becomes a timeline. Heavily cratered regions are old and undisturbed. Lightly cratered regions were resurfaced, whether by lava, water, or plate tectonics, and the size of the resurfaced patch tells you roughly how active that world has been, and for how long.
| World | What the surface tells you |
|---|---|
| Mercury & the Moon | Small, cold, geologically dead — old terrain survives everywhere |
| Venus | Broadly resurfaced by volcanism — young-looking almost everywhere |
| Mars | A mix — ancient cratered highlands next to young volcanic regions like Tharsis |
| Earth | Almost no ancient terrain survives at all — constant resurfacing by tectonics, water, and erosion |