Mars is covered in craters, which are a defining feature of the planet’s landscape. The Martian surface hosts an extraordinary record of impacts, with over 43,000 craters larger than five kilometers in diameter documented by scientists. They are the primary tools planetary scientists use to understand the Red Planet’s four-billion-year history. By studying their form and distribution, researchers gain insights into the planet’s subsurface composition, ancient climate, and the chronological order of major geological events.
The Mechanics of Impact and Preservation
An impact crater forms when a meteoroid, asteroid, or comet collides with a planetary surface at high velocity, typically tens of kilometers per second. The immense kinetic energy creates a shockwave that compresses and fractures the target rock, followed by an excavation phase that ejects material and forms the characteristic bowl shape. The size and morphology of the resulting crater depend on the impactor’s mass, collision speed, and the physical properties of the target surface.
Mars has preserved this impact history far better than Earth due to key geological differences. Unlike Earth, Mars lacks plate tectonics, which constantly recycles and erases surface features. Earth’s thick atmosphere and abundant liquid water also rapidly erode and fill craters through weathering and sedimentation.
The Martian atmosphere is extremely thin, meaning it does not provide enough weathering to quickly degrade large impact features. The rate of wind erosion, or aeolian processes, on Mars is estimated to be incredibly slow, filling in craters at a rate of only about 0.0001 centimeters per year. This low global degradation rate since the end of the Late Heavy Bombardment approximately 3.8 billion years ago has resulted in exceptionally well-preserved crater morphologies.
Unique Features of Martian Craters
Martian craters often display distinct morphologies that differ significantly from those found on the Moon or Mercury, primarily due to the presence of subsurface water ice or other volatiles. The most common of these unique features is the rampart crater, characterized by a fluidized, lobate ejecta blanket that looks like a solidified mudflow radiating outward. This splash-like pattern suggests that impact heat melted or vaporized underground ice, causing the ejected debris to behave like a fluid before settling.
Rampart craters are further categorized by the number of ejecta layers they possess. The structure of these layers provides clues about the depth and thickness of the volatile-rich layer that the impactor penetrated, indicating whether the impactor only reached the icy layer or penetrated through it into a rockier substrate below.
Another unusual feature is the pedestal crater, where the crater and its ejecta blanket sit on a raised platform above the surrounding terrain. Pedestal craters form because the ejected material creates a layer that is more resistant to wind erosion than the surrounding ground. Over time, the unprotected land erodes away, leaving the crater and its ejecta elevated on a pedestal-like mesa. The presence of both rampart and pedestal craters strongly implies that near-surface ice layers are widespread across the planet.
Reading Mars’s History Through Crater Density
The most profound scientific application of Martian craters is their use in relative dating through a technique known as crater counting. The principle is straightforward: a surface exposed for a longer period accumulates more impacts, resulting in a higher density of craters. By counting the number of craters within a defined area, scientists can determine the relative age of that region compared to others.
This method has revealed a dramatic geological dichotomy on Mars. The Southern Hemisphere, known as the Southern Highlands, is densely packed with ancient, heavily degraded craters, affirming it as the oldest surface on the planet, dating back roughly four billion years. In contrast, the Northern Lowlands are far smoother and have a much lower crater density.
The younger Northern Lowlands suggest that this region experienced significant geological resurfacing events, such as vast lava flows or sedimentary deposition, which wiped clean older impact scars. By comparing the crater densities of these two regions, scientists establish the chronological order of major events like ancient volcanism and water flow. This difference provides a quantifiable record of Mars’s long-term evolution.