The planets closest to the Sun—Mars, Earth, Venus, and Mercury—are made mostly of rock. The rocky planets all formed in our inner solar system. Their geological history is preserved on their surfaces. Their landscapes reveal the processes that shaped them: impacts, crustal movements, volcanic activity, and erosion. Gravity, temperature, air, and water all play leading roles in their geological stories.

Planets and moons across our solar system bear the scars of collisions. Impact craters form on their surfaces when a dust particle, rock, asteroid, or comet smashes into them. Impact craters come in all sizes and shapes, depending on the impacting object size, impact angle, and surface into which the object crashes.

A recent impact crater on Mars shows stunning “rays.” These were formed when material from the impact was ejected across the surface.

Rocky worlds can also reshape themselves from internal forces that push and pull at their crustal materials, a process called tectonics. Compressional forces shove crustal material together to create ridges and mountains. Extensional forces stretch and pull the crust apart to form fault scarps, canyons, and valleys. While impacts are sudden, tectonic forces operate over long periods of time. Large rift valleys can result when crustal rock is pulled apart by extensional forces. Valles Marineris on Mars, seen here, is the largest such valley in our solar system—hundreds of miles wide, several thousand miles long, and up to six miles (10 kilometers) deep.

Like Earth, volcanism also plays a role on Mars. Mars has a hot interior core surrounded by hot mantles. One way these rocky worlds release interior heat is through volcanic activity. This can involve molten rock, or magma, being forced into the crust. As the interior cools, it shrinks, causing the crust to wrinkle like the skin of an apple as the core dries and shrivels over time. Volcanism creates a variety of landforms, not just volcanoes, depending on the properties of the lava (such as viscosity and composition) and on the planetary environment (like gravity and presence of an atmosphere). 

Some lava flows erupt from long cracks, called fissures. The volcanic eruption seen here produced an 8-mile-wide, smooth, dark deposit surrounding a 20-mile-long volcanic fissure in the Cerberus Fossae system on Mars. 

Erosion wears down landforms, but it also creates new ones. Most of the craters in the martian highlands provide evidence for extensive erosion early in martian history. In fact, the erosion was so intense that any crater smaller than ~5 kilometers in diameter did not survive. Today, wind is the primary geologic process eroding the surface, creating extensive dune fields and even eroding boulders seen by the martian landers. Occasionally, the entire planet becomes enveloped in a global dust storm that is driven by strong winds during southern hemisphere summer.

This image illustrates both wind deposition and erosion. Large sand ripples have formed perpendicular to the wind-carved yardang ridges, which trend from the upper left to lower right.

Mars has a thin, cold, carbon dioxide atmosphere. Its air pressure is less than 1% that of Earth’s. However, Mars has thick ice caps at both poles, similar to Earth.

The surface of Mars is a cold, dry desert. But during its early history, it had rivers and lakes—and perhaps even a northern ocean. Small streams on Mars joined to form larger ones, creating tree-like networks of tributary valleys. During a wetter period on Mars over 3.5 billion years ago, some of this water also flowed into craters to form lakes.

Eberswalde crater on Mars contains an ancient river delta of meandering stream channels that once emptied into a body of water (like a lake).