How Craters Are Made
Impact craters are bowl-shaped depressions on a planetary surface, formed when a celestial object, such as an asteroid or comet, collides at high speed. The impact instantly vaporizes and melts rock at the point of impact. The force of the collision excavates a significant amount of surface material, throwing it outwards to create a central depression and an elevated rim.
Shockwaves propagate through the target body, further contributing to the crater’s structure and potentially altering the underlying geology. The size and appearance of the resulting crater depend on several factors, including the impactor’s mass, velocity, and angle of entry. The composition and strength of the target surface also play a role, influencing how easily material is excavated and how stable the crater’s structure remains.
Worlds with Many Craters
Many celestial bodies across the solar system bear the scars of countless impacts, showcasing a long history of bombardment. Mercury, the innermost planet, presents a heavily cratered surface remarkably similar to Earth’s Moon. Its lack of a substantial atmosphere means there is no protection from incoming space rocks, nor any weather to erode existing features.
Earth’s Moon offers a prime example of an airless body preserving its impact history, with vast plains and highlands covered in craters. Without atmospheric erosion or significant geological activity, these ancient scars remain undisturbed. The density of craters on its surface provides a record extending back billions of years.
Mars also displays numerous craters, particularly in its southern highlands, which are among the oldest regions on the planet. While Mars possesses a thin atmosphere that can cause some incoming objects to burn up, and evidence of past water and volcanic activity has reshaped parts of its surface, many impact features endure. Other bodies, such as the dwarf planet Ceres and moons like Callisto and Rhea, also exhibit heavily cratered terrains. This indicates that bodies lacking significant atmospheric or geological processes retain most of their impact scars.
Worlds with Few Craters
Some planets exhibit few visible impact features. Earth, for instance, has a relatively small number of easily recognizable craters compared to its Moon. This scarcity is due to dynamic geological processes like plate tectonics, volcanism, and erosion by wind and water, which constantly reshape and resurface the planet. Many ancient impact craters have been subducted, buried by lava flows, or worn away over millions of years.
Venus, with its dense atmosphere and extensive volcanic activity, also shows a relatively pristine surface with few craters. Radar mapping has revealed that the entire planet appears to have undergone a global resurfacing event 300 to 800 million years ago, erasing most older impact features. Its thick atmosphere also incinerates smaller incoming objects before they can reach the surface.
The gas giants—Jupiter, Saturn, Uranus, and Neptune—do not possess solid surfaces for craters to form. Incoming objects plunge into their thick, turbulent atmospheres, burning up due to atmospheric friction and immense pressure. While these giants do not display craters themselves, some of their icy moons, like Europa and Enceladus, show limited cratering, suggesting active geological or cryovolcanic processes that resurface their exteriors. Titan, Saturn’s largest moon, also has relatively few visible craters due to its dense atmosphere and cryovolcanic activity.
What Craters Tell Us
Impact craters provide insights into the history and evolution of the solar system. Craters act as geological “time capsules,” preserving evidence of past bombardment events that shaped planetary surfaces. By analyzing the density and degradation of craters on a surface, scientists can estimate its age, with more heavily cratered regions indicating older terrains.
This analysis helps reconstruct the intensity of early solar system bombardment and track changes in impact rates. Craters also reveal details about the composition and geological processes of celestial bodies. Their unique structures can expose subsurface materials, and their distribution can indicate areas of past volcanic activity, tectonic resurfacing, or atmospheric erosion.