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.

Hokusai crater on Mercury is a typical complex impact crater.  The center of a complex crater often rebounds after impact, creating a central peak. In an impact basin, the central peak sometimes collapses and forms a central ring.

Erosion wears down landforms, but it also creates new ones. Planets with and without atmospheres experience erosion, but by different geologic processes. On Mercury, a world without wind or liquid water, impact cratering, caused by tiny, micrometeorite bombardment of Mercury's surface erodes the landscape. These micrometeorites are generated by the dust particles released from comets.

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. The landforms they create can take millions of years to form.

Although it is the smallest planet, Mercury has some of the largest fault scarps in our solar system. The shrinking of its interior has forced its single-planet surface to contract, creating fault scarps all over the planet. The largest is 620 miles (1,000 kilometers) long.

When forces pull equally in all directions, fractures and graben develop in circular or polygonal patterns. Shown here are circular and polygonal graben in lava plains that buried an impact crater on Mercury. The circular graben outline the rim of the buried “ghost crater.”

Like Earth, volcanism also plays a role on Mercury. Mercury 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). 

The mosaic was assembled from individual images taken by the MESSENGER spacecraft as it approached the planet. Visible in the mosaic are many lava-flooded craters and large expanses of smooth volcanic plains, which appear similar in texture to volcanically emplaced mare deposits on the Moon.

Mercury has concentrations of ice around its north and south poles. On Mercury some impact crater floors near the poles are always in shadow. They stay very cold and have accumulated water ice. Areas on Mercury that might have ice deposits are shown in yellow.