Geology of Mars

The geology of Mars is the scientific study of the planet's surface, crust, and interior. This field explores the composition, structure, and history of the Martian world. It also investigates the physical processes that shape the planet over time. This work is very similar to terrestrial geology on Earth. In the broader field of planetary science, geology refers to studying the solid parts of planets and moons. It combines many different disciplines, such as geophysics, geochemistry, mineralogy, geodesy, and cartography.

One of the most striking features of Mars is its hemispheric dichotomy. This refers to the massive difference between the northern and southern hemispheres. The northern hemisphere consists of a vast topographic depression. This lowland area is mostly located in the north and sits 3 to 6 km lower than the southern two-thirds of the planet. This difference in elevation is a first-order relief feature. It is comparable to the elevation difference between Earth's continents and its ocean basins. The southern hemisphere, often called the southern highlands, is very old and rugged. It is heavily covered in impact craters from the period of heavy bombardment.

This dichotomy is also visible in the thickness of the planet's crust. Data from gravity and topography show the southern crust is much thicker. It reaches a maximum thickness of about 70 km. In contrast, the northern crust peaks at only about 32 km. Scientists debate how this happened. Some suggest exogenic theories, where huge impacts early in Martian history caused the split. Others suggest endogenic theories, where internal processes like mantle convection or plate tectonics thinned the northern crust. A new theory involving the Southern Polar Giant Impact suggests that exogenic theories are stronger. This theory suggests Mars never had plate tectonics to modify the dichotomy.

Mars is also home to massive volcanic provinces, most notably the Tharsis region. This area is a huge volcano-tectonic bulge in the western hemisphere. It is thousands of kilometers wide and covers up to 25% of the planet's surface. The Tharsis region averages 7 to 10 km above the Martian datum. It contains the largest known volcanoes in our solar system. Three massive volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons, sit in a line along the crest. These are collectively known as the Tharsis Montes. The famous shield volcano Olympus Mons also sits near the western edge of this province. The massive weight of Tharsis has even caused the crust to fracture. This created immense extensional fractures that radiate halfway around the planet.

Another volcanic area is the Elysium volcanic complex. This region is about 2,000 kilometers in diameter. It includes volcanoes such as Elysium Mons, Hecates Tholus, and Albor Tholus. Scientists believe the development of these volcanoes involved both lavas and pyroclastics.

Large impact basins also define the Martian landscape. The Hellas basin is the largest visible one. It is located in the southern hemisphere and is 1,800 km in diameter. The floor of this basin is very deep. Some areas lie over 8 km below the datum. Other major basins include Argyre and Isidis. The Argyre basin is 800 km wide and sits in the southern highlands. The Isidis basin is roughly 1,000 km wide and sits on the dichotomy boundary. These basins are extremely old and date back to the late heavy bombardment. They are similar in age to the Imbium and Orientale basins found on the Moon.

Near the equator, the Valles Marineris canyon system stretches across the planet. It is a massive network of interconnected troughs and canyons. The system is over 4,000 km long, which is nearly a quarter of the planet's circumference. If it were on Earth, it would span the width of North America. Some parts are 300 km wide and 10 km deep. Unlike the Grand Canyon on Earth, which was made by water, Valles Marineris is of tectonic origin. This means it was formed by faulting in the crust. The canyons likely formed because of the stress caused by the Tharsis bulge.

Finally, Mars features unique chaotic terrain and outflow channels. At the eastern end of Valles Marineris, the land breaks into jumbles of low, rounded hills. This is called chaotic terrain. It marks the start of huge outflow channels that empty into Chryse Planitia. These channels, such as Ares Vallis, are enormous. For example, Ares Vallis is 28 km wide. Scientists estimate the water flow needed to carve it was 14 million cubic meters per second. This is over ten thousand times the average discharge of the Mississippi River. These channels were likely formed by sudden releases of water from underground or melting ice.