Impact cratering is a fundamental process that has shaped every solid body in the solar system, providing a universal record of cosmic history. Planetary scientists use the distribution of these impact scars as a primary chronological tool to understand how and when a surface formed or was last modified. By analyzing the number and size of craters in a given area, they determine the relative age of different geological terrains. This technique relies on the principle that a surface accumulates more craters over time, meaning a heavily pockmarked area is inherently older than a smooth one.
The Observable Difference in Crater Density
The most striking difference between older and newer planetary surfaces is the sheer number and arrangement of impact craters. Ancient surfaces, such as the lunar highlands, are described as being “saturated” with craters, meaning new impacts often overlap or obliterate previous ones, creating a densely packed, multi-layered terrain. These old regions host a high proportion of very large, degraded impact basins. In contrast, newer surfaces, like the Moon’s volcanic plains, display a dramatically lower crater count. The key metric for comparison is crater density, the quantifiable number of craters of a specific size per unit area, which provides a direct measure of a surface’s exposure time to impactors.
Factors Driving the Historical Impact Rate
The profound difference in crater density results from the non-uniform impact rate throughout the solar system’s history. For the first 600 million years, the rate of collisions was vastly higher than it is today, largely attributed to the Late Heavy Bombardment (LHB) between 4.1 and 3.8 billion years ago. Surfaces exposed before the LHB ended, such as the Moon’s highlands, rapidly accumulated their dense, overlapping crater record. Following the LHB, the population of available impactors declined steeply, establishing the stable, low impact rate observed over the last three billion years, meaning any surface formed or resurfaced after this period accumulated craters much slower.
Geological Processes That Erase Impact Evidence
Crater density provides a measure of age, but the absence of craters does not automatically mean a surface is young, as active geological processes can effectively reset the surface clock. Volcanism is a powerful resurfacing mechanism, as seen in the Moon’s maria, where massive lava flows buried older, heavily cratered terrain, and on Venus, where a catastrophic volcanic event may have smoothed the entire surface. On Earth, plate tectonics and mantle convection constantly recycle crustal material, rapidly destroying impact evidence; the oldest known impact structures are only about two billion years old, highlighting the efficiency of terrestrial geological activity. Furthermore, processes like erosion, caused by wind, water, or ice, actively smooth out crater topography, eventually erasing them completely.
How Scientists Determine Relative Surface Age
Planetary scientists employ a systematic technique called crater counting to translate visual evidence into a chronological framework. This involves mapping and measuring the diameter and spatial distribution of all primary impact craters within a defined area, resulting in the crater size-frequency distribution. By comparing the crater density of one region to another on the same body, scientists reliably determine the relative age; a higher density indicates an older surface. To assign an absolute age, the crater count must be calibrated against impact flux models anchored by the radiometric dating of lunar rocks from the Apollo missions. This correlation allows scientists to extrapolate absolute ages to other planetary surfaces, such as those on Mars or Mercury, where physical samples have not been collected.