When studying rocky planets—Mercury, Venus, Earth, and Mars—you can see their history, literally. 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. Join us in taking a closer look at how these planets took shape.  

Impact 

Planets and moons across our solar system bear the scars of collisions. Impact craters form on their surfaces when another object, such as a dust particle, rock, asteroid, or comet smashes into them. Scientists often use the number of impact craters on a planet’s surface as a proxy for the relative age of that surface (more craters = older). Impact craters come in all sizes and shapes, depending on the impacting object size, impact angle, and surface into which the object crashes. Craters range in size from microscopic—smaller than the width of a human hair (100 microns, or .0001 meter)—to as wide as the continental United States.

A recent impact crater on Mars shows stunning “rays.” These formed when material ejected by the impact disturbed the surface. Image courtesy of NASA.

Craters from impact take different shapes—some simple, some complex.  

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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. Materials pushed out and thrown outside of a crater create a raised rim. Hokusai crater on Mercury, seen here, is a typical complex impact crater.

 

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Simple craters have smooth, steep walls. Complex craters have shallower walls that often slump or collapse into terraces. Ejecta are materials thrown from the impact crater. Some are melted by the impact and called impact melt. 

Simple craters are bowl-shaped depressions surrounded by a raised rim and a blanket of ejecta. They are deeper relative to their diameter. They lack features like wall terraces and central peaks.  

This is a meteor crater in Arizona. Known as Barringer Crater, it is one of the best preserved craters on Earth. It formed about 50,000 years ago when an ironnickel meteorite about 98 feet (30 meters) in size slammed into the desert. The impact left a crater three-fourths of a mile (1.2 kilometers) across. Image courtesy of NASA. 

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The size and shape of a crater are affected by many factors. The bigger and faster moving the object is, the larger the crater it will make. The more gravity a planetary body has, the faster an object will be moving when it hits. Gravity, the presence of an atmosphere, the angle of impact, and the nature of the surface material also affect crater shape. For instance, the atmosphere of Venus is so dense that some objects break apart and form clustered impact craters, like this one. Image courtesy of NASA.

    Tectonics 

    Rocky worlds can also reshape themselves from internal forces that push and pull at their crustal materials, a process called tectonics. Contractional forces shove crustal material together to create cliff-like fault scarps, ridges, and mountains. Extensional forces stretch and pull the crust apart to form trough sand rift valleys. While impacts are sudden events, tectonic forces operate over long periods of time. The landforms they create can take millions of years to form.  

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    Illustration of lobate scarps that were formed when the lunar crust was pushed together as the Moon contracted.  This causes the near-surface materials to break forming a thrust fault.  The thrust fault carries crustal materials up and sometimes over adjacent crustal materials.  

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    When forces pull a planetary surface apart, the most common landforms created are called a graben. These trough-like features form when a rock is pulled apart, breaks, and drops down between parallel faults. Image courtesy of Arizona State University.

    One of the things that makes Earth unique from the other rocky planets is that its’s “outer shell”—its crust and upper mantle, called the lithosphere—is broken into a mosaic of about 12 plates. They move around and forcibly interact with each other. On Earth, most tectonic landforms are created by the forces that move these tectonic plates.  

    The other rocky planets seem to have unbroken (one-plate) outer shells. What the rocky planets do all have in common is that there are hot interior cores surrounded by hot mantles. As its interior cools, a planet shrinks. On the planets with continuous lithospheres (outer shells) this causes its crust to wrinkle.


    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 - plate surface to contract, creating fault scarps all over the planet. The largest is 620 miles (1,000 kilometers) long, about the size of California’s San Andreas Fault. Image courtesy of NASA.

    Volcanism 

    One way rocky worlds release interior heat is through volcanic activity. This can involve molten rock, or magma, being forced into the crust. The magma may erupt onto the surface to become lava. Scientists have found evidence of volcanic activity on all the rocky planets. 

    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). 

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    These images show an identical area on Venus as imaged by the U.S. NASA Magellan spacecraft in 1991 (left) and the U.S.S.R. Venera 15/16 spacecraft in the early 1980's (right). In the area seen here, approximately 200 small volcanoes, ranging in diameter from 2 to 12 kilometers (1.2 to 7.4 miles) can be identified. Volcanoes form from the buildup of cooled lavas and ash on a planetary surface. Solids, liquids, and gases can erupt from a volcano. The amount and makeup of the erupting material affects the volcano’s size and shape. Venus displays the greatest diversity of volcanic features among the rocky worlds. Image courtesy of NASA.

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    Many of the rocky planets are covered in lavas and other types of volcanic deposits that have erupted from vents and fissures (long linear vents) that may or may not be associated with volcanoes. Many of these eruptions involve a hot mix of ash, gases, and lava, which scientists call pyroclastic. A pyroclastic flow is a mix of hot ash, gases, and lava fragments that moves rapidly downslope. Image courtesy of Wikimedia.

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    Molten rock does not always erupt onto the surface. Sometimes it stalls and cools just beneath the crust, forming what’s called an intrusion. Intrusions often push up and fracture the overlying surface, creating a system of cracks. Erosion sometimes wears away the overlying material, exposing the hardened intrusion. 

    Shiprock in New Mexico is a preserved core of a volcanic vent exposed by erosion. The vent was active 30 million years ago.  Several sheet-like lava intrusions called dikes, in the foreground, radiate from the core on the left. Image courtesy of Wikimedia.

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    Channels and sinuous rilles (winding channels) are common landforms within volcanic terrains. They can form by either the buildup of lava along the side of a lava flow or when a lava flow carves into the surface. Our Moon has many sinuous rilles. Here, a small one lies within an older, larger one on the Aristarchus Plateau. Image courtesy of NASA.

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    Not all volcanic activity involves volcanoes. Many volcanic deposits on the rocky planets were produced by flood lavas, large-volume eruptions that covered vast areas. The lava usually poured out of vents now buried in low-lying areas, such as impact basins and craters.  

    Over 80% of Venus is covered by relatively young volcanic plains, less than 500 million years old. The abundance of these plains suggests that Venus may have experienced catastrophic volcanic events that caused lava to flood large parts of its surface. Image courtesy of NASA.

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    Some lava flows erupt from long cracks, called fissures. This fissure on Mars extends more than 620 miles (1,000 kilometers). Features like this formed from the upward movement of molten rock. Image courtesy of NASA.

    Erosion 

    Gravity, wind, water, ice: all these factors help break down solid rock and scatter the pieces. Wind and water transport dust and sand. Gravity and glacial activity can move large pieces of rock and ice. Erosion wears down land forms, but it also creates new ones. Planets with and without atmospheres experience erosion, but by different geologic processes. On Earth and Mars, water and wind, respectively, are the main agents of erosion. On Mercury, a world without wind or liquid water, impact cratering erodes the landscape. 

    Impacts, crustal movements, volcanic activity, and erosion: all these contribute to the unique surfaces of the rocky planets of our solar system—surfaces that are shaped by the planets geological history.

    Related Topics Science Physical science Solar System
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