The surface of Mars presents a landscape of extremes, shaped by ancient geological forces and persistent atmospheric processes. While defined by its signature reddish hue from a distance, a closer examination reveals a diverse topography of colossal mountains, immense canyons, and evidence of a dramatically different past. These features speak to a history of volcanism, water flow, and continuous wind erosion that has sculpted a unique planet.
The Signature Red Hue and Texture
Mars earns its nickname, the Red Planet, from a ubiquitous layer of fine-grained dust that covers most of its surface. This distinctive color is caused by the oxidation, or rusting, of iron minerals within the Martian dust, resulting in iron(III) oxide. The dust is primarily composed of this iron-rich material, which is finely ground and easily lofted by winds.
The actual surface material is called regolith, loose, rocky material that is mostly basaltic rock, similar to Earth’s volcanic rock. While the planet appears uniformly red from space, up close, the surface exhibits a range of colors, including golden, brown, tan, and even greenish hues, depending on the underlying rock composition. The Martian atmosphere is perpetually hazy with suspended dust, causing the daytime sky to appear yellowish-brown rather than the blue seen on Earth.
Defining Global Geographical Features
The Martian surface is fundamentally divided by a striking contrast known as the Martian dichotomy, which separates the hemispheres into two distinct provinces. The northern third of the planet consists of vast, low-lying plains, which are comparatively smooth and sparsely cratered, suggesting a geologically younger surface. Conversely, the southern two-thirds are heavily cratered highlands, elevated several kilometers above the northern plains and representing the planet’s older crust.
Within this topography lie two of the solar system’s most massive geographical features. Olympus Mons, a shield volcano, is the tallest mountain on Mars and the largest known volcano in the solar system, towering over 21 kilometers above the surrounding plains. Its base is approximately 600 kilometers across, a size roughly comparable to the state of Arizona or the country of Italy. This immense size is attributed to the absence of plate tectonics, allowing lava to accumulate over a single hotspot for billions of years.
Stretching for over 4,000 kilometers near the equator is Valles Marineris, a canyon system so vast it spans nearly a quarter of the planet’s circumference. This colossal rift is up to 200 kilometers wide and reaches depths of 7 to 10 kilometers, making it the largest canyon system in the solar system. For comparison, the Grand Canyon on Earth is only about 800 kilometers long, 30 kilometers wide, and 1.8 kilometers deep. Valles Marineris is thought to have formed primarily as a tectonic crack as the crust in the nearby Tharsis region swelled, later widened by erosion.
Traces of Ancient Water and Ice
The Martian surface retains numerous features that serve as geological records of a wetter past. Extensive networks of dry river valleys, outflow channels, and ancient lakebeds suggest that liquid water once flowed freely across the surface billions of years ago. Evidence of past standing water, such as wave-formed ripples in sedimentary rock, has been found in places like Gale Crater. Chemical analysis of surface rocks also points to this watery history, with minerals like sulfates and salt deposits found in equatorial regions.
While liquid water is unstable on the surface today due to the thin atmosphere and cold temperatures, vast amounts of water exist as ice. The permanent polar caps are multi-kilometer thick layers composed primarily of water ice, with a seasonal covering of frozen carbon dioxide (dry ice). Significant stores of water ice are also buried beneath the surface, particularly in the mid-latitudes and even near the equator, where radar has detected massive subsurface deposits within the Medusae Fossae Formation. If melted, the ice within this equatorial deposit alone could cover the entire planet in a layer of water between 1.5 and 2.7 meters deep.
The Dynamic Role of Wind and Dust
Today, the surface of Mars is actively shaped by aeolian, or wind-driven, processes. The thin Martian atmosphere, primarily carbon dioxide, generates winds strong enough to lift the fine, pervasive iron-oxide dust. These winds create vast, shifting dune fields common in craters and polar regions, continuously redistributing surface material. Wind erosion also sculpts the surface, creating features like yardangs—elongated ridges carved from soft rock—and dark streaks where dust has been removed.
Wind speeds, measured up to 160 kilometers per hour, are more energetic than previously modeled, allowing for effective mobilization of the regolith. This intense wind activity is also responsible for the spectacular, sometimes planet-encircling dust storms that periodically engulf Mars. These global storms can last for weeks or months, significantly obscuring the surface and demonstrating the power of atmospheric interaction on the Martian landscape.