Astronomy 2e · Other Worlds: An Introduction to the Solar System
Dating Planetary Surfaces
On this page 9 sections
In 30 seconds
When planetary scientists study the Moon, Mars, or Mercury, one of the first questions they ask is not "what is this made of?" but "how old is this surface?" A heavily cratered highland and a smooth lava plain record different chapters of history. Dating planetary surfaces means determining when a surface formed — more precisely, when it was last modified — through two complementary methods. Relative dating orders events from oldest to youngest, usually by counting impact craters. Absolute dating attaches real ages in years using radioactive decay inside rock samples. The two were fused by the Apollo missions: lunar samples dated in laboratories were matched against craters counted on the same regions, creating a calibration curve now applied across the solar system.
Why this matters
Surface ages reveal which worlds are geologically alive and which died billions of years ago: a young, smooth surface means something erased the craters — volcanism, tectonics, or flowing material — and that activity signals internal heat, which governs magnetic fields and habitability. Dating surfaces also dates the solar system, since the oldest rocks and meteorites cluster near 4.5–4.6 billion years (commonly taught reference values). And when mission planners choose a landing site, they are choosing which chapter of planetary history to sample. Every later chapter of this book leans on these techniques.
The college version
Core Concepts
Crater counting is relative dating
A fresh surface starts with zero craters, and impacts accumulate over time, so the more craters a surface has, the older it is. Counting craters above a chosen size gives a crater density that ranks surfaces from oldest to youngest. The rule has two limits: the surface must not have been resurfaced (lava, wind, water, or ice can erase craters and restart the clock), and once craters are so crowded that new impacts overlap old ones, the surface reaches Saturation New craters overlapping old ones Full entry → — counting can no longer distinguish 3.5 from 4.0 billion years.
The impact flux problem
Craters do not accumulate at a steady rate. The inner solar system suffered a much heavier bombardment during its first few hundred million years, including an episode often called the late heavy bombardment, commonly placed roughly 4.1–3.8 billion years ago (details are still debated). A 4-billion-year-old surface acquired most of its craters early and has added few since; a 100-million-year-old lunar surface would look nearly pristine. Crater counts therefore become ages only through a calibration curve of impact rate versus time, anchored by dated samples.
Radiometric dating gives absolute ages
Certain isotopes are radioactive: they convert into daughter isotopes at a fixed statistical rate. Each isotope has a Half-life Time for half of a parent isotope to decay Full entry → — the time for half of the parent atoms to decay. Measuring the parent-to-daughter ratio in a mineral and knowing the half-life reveals when that mineral crystallized. Common systems include uranium–lead (long half-lives, ideal for the oldest rocks and meteorites) and potassium–argon (argon escapes while lava is molten, so the clock resets at eruption). A key subtlety: radiometric dates record when a rock solidified, not necessarily when its surface was exposed.
Calibrating craters with lunar samples
Apollo astronauts returned rocks from both the ancient lunar highlands and the younger dark maria (vast lava plains). Lab dating showed the highlands to be about 4.4–4.5 billion years old and the maria about 3.1–3.9 billion years old (commonly taught reference values). Counting craters on those same regions converted crater density into ages — the calibration curve. That lunar curve is then applied to Mercury, Mars, and other cratered bodies by analogy, with corrections for differences in impact rate and gravity.
Resurfacing and limits
A surface age is the time since the surface was last modified, not the age of the planet. On Earth, plate tectonics and erosion recycle the crust so thoroughly that almost no oceanic crust survives past about 200 million years (a commonly cited reference value). Radiometric dating Dating rocks by measuring decay products Full entry → requires samples, which so far exist only for Earth, the Moon, and meteorites (including meteorites blasted off Mars), so Crater counting Measuring crater density to rank surface ages Full entry → remains the main tool for most worlds — and distant surface ages carry real uncertainty, often reported as ranges.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Relative age | Absolute age | Relative is only an order (older/younger); absolute is a number of years |
| Crater counting | An exact dating method | It ranks surfaces; it gives real ages only after radiometric calibration |
| An old surface | An old planet | Planets can be ancient with young surfaces; Earth is ~4.5 billion years old with oceanic crust under 200 million years old |
| Radiometric date | Age of surface exposure | The date records crystallization; a surface can be exposed much later |
| Half-life | Lifetime of an atom | Individual atoms decay unpredictably; half-life is a statistical property of a population |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Leave a cookie on a sidewalk and the longer it sits, the more ants find it — counting ants tells you how long it's been there, but only if you know how busy the ants are. Scientists count craters the same way, and they use real moon rocks dated in labs to figure out how fast the "crater ants" were working billions of years ago.
Worked example
A student is asked how old a 500-km-wide Martian region is. She counts craters larger than 10 km across and finds a density that, on the lunar calibration curve, corresponds to roughly 3.7 billion years — assuming Mars's cratering history resembles the Moon's after corrections for position and gravity, and reporting a range because the Martian impact rate is less certain. But she also notices a lava flow covering part of the region with almost no craters, implying it erupted only a few hundred million years ago. Her conclusion: the region has two distinct surfaces — an ancient cratered terrain resurfaced in places by much younger lava. The example shows the whole toolkit: relative ordering, calibrated crater ages, recognition of resurfacing, and honest uncertainty.
Key takeaways
- Crater counting is relative dating: more craters generally means an older surface — until saturation sets in.
- Radiometric dating is absolute dating: half-lives turn parent-to-daughter ratios into ages in years.
- A half-life is the time for half of the parent atoms to decay; it describes populations, not individual atoms.
- The Moon is the calibration key: dated Apollo samples were matched to crater counts, producing the curve used for other worlds.
- The early impact rate was much higher, so crater density is not linear with time.
- Surface age ≠ planet age: resurfacing resets the crater record; absolute ages require samples.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why can crater counting rank surface ages without using any dates?
Show answer
Because craters accumulate over time on an undisturbed surface: the longer the surface has existed, the more impacts it has collected.
What is the difference between relative and absolute dating, and why is the Moon essential for connecting them?
Show answer
Relative dating gives order only (crater density); absolute dating gives years (radioactive decay). The Moon connects them because Apollo samples were both dated in labs and counted for craters, producing the calibration curve.
What does Resurfacing Any process that erases or buries old surface features Full entry → do to a crater record?
Show answer
Resurfacing (lava, tectonics, erosion, ice) erases or buries craters and restarts the accumulation clock, so the measured age is the time since the last modification — not the age of the planet.
What does a half-life tell you?
Show answer
A half-life is the time for half of the parent atoms to decay into daughters; the parent/daughter ratio, with the half-life, gives the age.
Why can't we radiometrically date the surface of Mars directly?
Show answer
Radiometric dating requires physical rock samples; Mars has not yet been sampled by sample-return missions, and Martian meteorites date the rock, not the surface region.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Relative age
- Age as "older/younger than," no year attached
- Absolute age
- Age in years, from radiometric dating
- Crater counting
- Measuring crater density to rank surface ages
- Impact flux
- The rate impacts arrive over time
- Saturation
- New craters overlapping old ones
- Half-life
- Time for half of a parent isotope to decay
- Radiometric dating
- Dating rocks by measuring decay products
- Resurfacing
- Any process that erases or buries old surface features
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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