The geology of Mars is the scientific study of the planet’s surface, crust, and interior. It emphasizes the composition, structure, history, and physical processes that shape the planet and is analogous to terrestrial geology. In planetary science, the term geology is used in its broadest sense to refer to the study of the solid parts of planets and moons, incorporating aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography. A neologism, areology (from the Greek Arēs, meaning Mars), sometimes appears as a synonym for Mars's geology in popular media and science fiction; the term is also used by the Areological Society.

Mars is a terrestrial planet that has undergone planetary differentiation. The InSight lander, which touched down on 26 November 2018, deployed a sensitive seismometer to enable three-dimensional mapping of the deep interior. Using information from InSight, scientists reported on 25 October 2023 that Mars has a radioactive magma ocean beneath its crust.

Global physiography: Mars exhibits a number of distinct large-scale surface features that indicate the geological processes that have operated on the planet over time. This section introduces several of the larger physiographic regions of Mars. Together, these regions illustrate how geologic processes involving volcanism, tectonism, water, ice, and impacts have shaped the planet on a global scale.

Hemispheric dichotomy

The northern and southern hemispheres of Mars are strikingly different in topography and physiography. This dichotomy is a fundamental global geologic feature of the planet. The northern part is an enormous topographic depression: about one-third of the surface (mostly in the northern hemisphere) lies 3–6 km lower in elevation than the southern two-thirds. This is a first-order relief feature on par with the elevation difference between Earth's continents and ocean basins. The dichotomy is also expressed in two other ways: as a difference in impact crater density and in crustal thickness between the two hemispheres. The hemisphere south of the dichotomy boundary (often called the southern highlands or uplands) is very heavily cratered and ancient, characterized by rugged surfaces that date back to the period of heavy bombardment. In contrast, the lowlands north of the dichotomy boundary have few large craters, are very smooth and flat, and show features indicating extensive resurfacing since the southern highlands formed. Topographic and geophysical gravity data indicate that the crust in the southern highlands has a maximum thickness of about [value], whereas the crust in the northern lowlands peaks at around [value] in thickness. The location of the dichotomy boundary varies in latitude across Mars and depends on which of the three physical expressions of the dichotomy is being considered. The origin and age of the hemispheric dichotomy are still debated. Hypotheses generally fall into two categories: exogenic theories, which propose the dichotomy was produced by a mega-impact event or several large impacts early in the planet's history; and endogenic theories, which propose the dichotomy resulted from crustal thinning in the northern hemisphere due to mantle convection, overturning, or other chemical and thermal processes in the planet's interior. One endogenic model proposes an early episode of plate tectonics producing a thinner crust in the north, similar to spreading plate boundaries on Earth. Whatever its origin, the Martian dichotomy appears to be extremely old. A recent theory based on the Southern Polar Giant Impact and supported by the discovery of twelve hemispherical alignments suggests exogenic mechanisms are more likely than endogenic ones and that Mars never had plate tectonics capable of modifying the dichotomy. Laser altimeters and radar-sounding data from orbiting spacecraft have identified a large number of basin-sized structures previously hidden in visual images. Called quasi-circular depressions (QCDs), these features likely represent derelict impact craters from the period of heavy bombardment that are now covered by a veneer of younger deposits. Crater-counting studies of QCDs suggest that the underlying surface in the northern hemisphere is at least as old as the oldest exposed crust in the southern highlands. The ancient age of the dichotomy places a significant constraint on theories of its origin.

Straddling the dichotomy boundary in Mars's western hemisphere is a massive volcano-tectonic province known as the Tharsis region, or the Tharsis bulge. This immense, elevated structure is thousands of kilometers in diameter and covers up to 25% of the planet's surface. Averaging 7–10 km above datum (Martian "sea" level), Tharsis contains the highest elevations on the planet and the largest known volcanoes in the Solar System. Three enormous volcanoes—Ascraeus Mons, Pavonis Mons, and Arsia Mons (collectively the Tharsis Montes)—sit aligned NE–SW along the crest of the bulge. The vast Alba Mons (formerly Alba Patera) occupies the northern part of the region. The huge shield volcano Olympus Mons lies off the main bulge at the western edge of the province. The extreme massiveness of Tharsis has placed tremendous stress on the planet's lithosphere.