Impacts, Craters and Planetary History
Craters as a Geological Tool
Every solid body in the Solar System that has a hard surface bears the scars of meteorite impacts. On the Moon and Mars, ancient craters survive for billions of years because those bodies lack the erosion and crustal recycling that erase them on Earth. On Earth, impacts have occurred throughout geological history — but most craters have been destroyed by weathering, burial, or subduction. Studying impact craters across the Solar System, and the rare well-preserved examples on Earth, reveals both the bombardment history of the inner Solar System and the profound effects that large impacts have had on the evolution of life.
Structure of an Impact Crater
When a meteorite or comet strikes a planetary surface at high velocity, it releases enormous energy in a fraction of a second. The impactor vaporises and the target rock melts, fractures and is thrown outward in all directions, forming a bowl-shaped impact crater. The key features of a typical impact crater, from largest to smallest, are:
- The rim — a raised ring of uplifted and overturned rock surrounding the depression.
- The ejecta blanket — a sheet of material thrown outward from the impact, covering the surrounding terrain to a depth that decreases with distance from the rim.
- The crater floor — which may be filled with impact melt rock (solidified from the initially melted target rock) or, over time, with sediment.
- A central peak (in larger craters) — a rebound structure formed as the initially depressed floor rebounded upward after the impact.
The four main features of an impact crater are: (1) the raised rim of uplifted rock; (2) the ejecta blanket of material thrown outward; (3) the crater floor, which may contain impact melt or sediment; and (4) a central peak (in large craters) formed by the elastic rebound of the crater floor. These features allow geologists to identify ancient craters even where erosion has partially degraded them.
Moon Craters and the Bombardment Record
The Moon’s surface records the full history of meteorite bombardment in the inner Solar System because it has no atmosphere, no liquid water and no plate tectonics to erase craters over time. The lunar highlands — the pale, heavily cratered regions — preserve impact craters up to about 4 billion years old. The dark basaltic plains (the maria) have fewer craters because they were formed by volcanic eruptions between about 3.9 and 3.0 billion years ago, resurfacing and burying older craters.
By counting the number of craters of different sizes on a surface and applying crater counting, geologists can estimate the relative ages of different terrain units. Densely cratered regions are older; sparsely cratered regions are younger. On the Moon, the youngest recognisable features are small, fresh-looking craters with sharp rims and bright ejecta rays — features that fade over time as the surface is gardened by micrometeorites and space weathering.
Crater counting applies the principle of superposition at a planetary scale. If a small crater sits on top of an older, larger crater, the smaller one is younger. If a crater is itself heavily cratered and its rim is degraded, it is older than a nearby fresh crater with a sharp rim. This method allows geologists to build a relative time sequence for planetary surfaces without any radiometric dates — though Apollo lunar samples, dated using radiometric methods, have calibrated the crater count chronology for the Moon, giving numerical ages to different crater-count densities.
Mars Craters and the Jezero Crater
Mars preserves impact craters ranging from tiny bowl-shaped depressions a few metres across to ancient multi-ring basins hundreds of kilometres in diameter. The most scientifically significant crater currently being studied is Jezero Crater, which is approximately 49 km wide. Orbital imagery reveals fan-shaped sedimentary deposits at the crater rim that are interpreted as river delta deposits — evidence that liquid water once flowed into the crater from outside, bringing sediment and potentially organic material. The Perseverance rover has been drilling rock cores from these deposits since 2021, searching for signs of past microbial life.
Jezero Crater illustrates how impact craters can function as natural traps for sediment, water and any materials carried in by rivers. On Earth, ancient impact craters that have been filled with sediment have sometimes produced economically important mineral deposits — the Sudbury Basin in Canada, for example, is a 1.85-billion-year-old impact structure that hosts one of the world’s largest nickel-copper ore deposits, thought to have formed when the impact melted and redistributed metal-rich rocks.
Jezero Crater on Mars is about 49 km across and contains sedimentary delta deposits at its rim, interpreted as the remains of an ancient river that flowed into the crater. NASA’s Perseverance rover landed there in February 2021 to drill cores from these deposits, which could preserve signs of ancient microbial life in the ancient lake sediments.
Impacts and the Evolution of Life on Earth
Large impacts have profoundly shaped Earth’s biosphere. The most famous example is the Chicxulub impact, which struck the Yucatan Peninsula (Mexico) about 66 million years ago. The impactor was roughly 10 km in diameter and released energy equivalent to billions of nuclear weapons. The immediate effects included wildfires, a global “impact winter” (dust and soot blocking sunlight for months to years), and the disruption of photosynthesis. The geological evidence for this event is a thin layer of iridium-enriched clay found at the Cretaceous–Palaeogene boundary in rocks worldwide. Iridium is far more abundant in meteorites than in Earth’s crust, so its global spike at this boundary is interpreted as fallout from the Chicxulub impactor.
The Chicxulub event caused the end-Cretaceous mass extinction, eliminating approximately 75% of all species, including all non-avian dinosaurs. Paradoxically, it also opened ecological opportunities for the surviving mammal lineages, allowing their diversification into the forms that eventually led to primates and humans. Large impacts, while catastrophic in the short term, are therefore part of the mechanism that shaped the trajectory of biological evolution on Earth.
Earth has been struck by at least as many meteorites as the Moon, but the geological record preserves very few confirmed impact structures compared to the lunar surface. This is because Earth has: (1) active weathering and erosion by water and wind, which gradually remove crater rims and infill depressions; (2) plate tectonics, which subducts old ocean floor (where many oceanic impacts would have occurred) and drives mountain building that deforms ancient terrains; (3) abundant vegetation that covers and masks topographic features. Only about 190 confirmed impact structures are known on Earth. The Moon, with no such processes, has hundreds of thousands of craters visible to telescopes. This contrast dramatically illustrates how active geological processes on Earth erase evidence of past events that are permanently readable on quieter planetary bodies.
Reading Planetary Surfaces: Connecting the Unit
Impact craters, like all geological features, are records of events in time. The youngest craters on any planetary surface are sharp-rimmed and show unmodified ejecta; the oldest are heavily degraded or buried. By reading these features, geologists construct relative timelines for planetary bodies — the same fundamental approach used in stratigraphy on Earth. Together, the lessons in this unit have traced Earth’s climate and Solar System history through rock evidence, ice cores, sea level change, atmospheric chemistry and planetary geology, demonstrating that the geological record — on Earth and beyond — is a rich archive waiting to be decoded.