10.4 The Geology of Mars

Mars Exploration and Geology

Mars has been explored by more spacecraft than any other planet besides Earth. Decades of orbiters, landers, and rovers have revealed a world with the solar system's largest volcano, its deepest canyon system, and widespread evidence that liquid water once flowed across its surface. Understanding Mars' geology helps clarify how rocky planets can evolve along very different paths despite similar starting conditions.

Key Mars Exploration Missions

Mariner 9 (1971–1972) was the first spacecraft to successfully orbit another planet. It arrived during a global dust storm, and as the dust cleared, it revealed massive volcanoes, an enormous canyon system, and channels that looked like they were carved by flowing water. This single mission transformed our picture of Mars from a cratered, Moon-like world into something far more dynamic.

Viking 1 and 2 (1976) achieved the first successful landings on Mars. Both landers analyzed soil composition and ran biology experiments designed to detect signs of life. The results were ambiguous and remain debated to this day.

Mars Global Surveyor (1997–2006) mapped the entire Martian surface in high detail. It discovered gullies that suggested geologically recent water activity and detected remnants of an ancient global magnetic field preserved in the oldest crustal rocks.

Spirit and Opportunity Rovers (2004) investigated geology at two different landing sites. Opportunity, in particular, found sedimentary rocks and tiny hematite spherules ("blueberries") that form in the presence of water. Spirit lasted until 2010; Opportunity operated until a dust storm ended its mission in 2018.

Mars Reconnaissance Orbiter (2006–present) carries the most powerful camera ever sent to another planet, returning images that can resolve features smaller than a meter across. It also monitors weather patterns, climate cycles, and seasonal surface changes.

Curiosity Rover (2012–present) landed in Gale Crater to assess whether Mars ever had environments capable of supporting microbial life. At a site called Yellowknife Bay, it found mudstone deposits indicating an ancient lake with neutral pH and the chemical ingredients needed for life.

Insights from Martian Meteorites

We don't need to go to Mars to study its rocks. Large impacts have blasted chunks of Martian crust into space, and some of those rocks eventually fell to Earth as meteorites. Scientists identify them by comparing trapped gas bubbles inside the rocks to the Martian atmosphere measured by Viking.

• SNC meteorites (Shergottites, Nakhlites, Chassignites) are igneous rocks that provide direct information about Martian magma composition, mantle chemistry, and how the planet differentiated into layers.
• ALH84001 is an ancient Martian meteorite (about 4.09 billion years old) found in Antarctica. In 1996, a research team announced it might contain microfossils and organic compounds, but that claim remains highly disputed.
• NWA 7034 ("Black Beauty") is a regolith breccia, meaning it's a jumble of different crustal fragments cemented together. It offers the broadest sampling of Martian crustal materials in a single rock and shows evidence of ancient water-rock interactions.

Diverse Surface Features of Mars

Polar caps are made of layered deposits of water ice and carbon dioxide ice (dry ice). They grow and shrink with the seasons as $CO_2$ ice sublimates in summer and re-deposits in winter. The layered structure records climate variations over time, similar to ice cores on Earth.

Valleys and channels come in two main types:

• Outflow channels (like Kasei Valles) are enormous features carved by catastrophic floods, likely released when subsurface ice or water burst to the surface.
• Valley networks (like Nanedi Valles) branch like river systems on Earth and suggest sustained rainfall or groundwater flow early in Mars' history.

Plains and lowlands dominate the northern hemisphere. The northern lowlands are smooth, sparsely cratered, and sit several kilometers lower than the southern terrain. Some researchers think an ancient ocean once filled this basin. The Hellas Basin, in the south, is the largest confirmed impact basin on Mars at over 2,000 km in diameter.

Highlands and mountains characterize the southern hemisphere. The southern highlands are heavily cratered and geologically much older than the northern lowlands. This stark difference between the two hemispheres is called the Martian crustal dichotomy, and its origin is still debated. The Tharsis region is a massive volcanic plateau that hosts the solar system's largest volcanoes.

Mars vs. Earth: Volcanoes and Canyons

Mars lacks plate tectonics, and that single fact explains why its volcanoes and canyons grew so much larger than Earth's. On Earth, tectonic plates move over volcanic hotspots, so no single volcano sits above the magma source for long. On Mars, the crust stays put, and lava piles up in the same spot for hundreds of millions of years.

Olympus Mons is the largest known volcano in the solar system. It's a shield volcano (broad and gently sloped, like Hawaii's Mauna Loa) but on a completely different scale: about 22 km tall with a base diameter of roughly 624 km. For comparison, Mauna Loa rises about 9 km from the ocean floor.

The Tharsis Montes are three large shield volcanoes (Ascraeus Mons, Pavonis Mons, and Arsia Mons) lined up along the Tharsis ridge. Each one is comparable in shape to terrestrial shield volcanoes but far larger.

Valles Marineris is the solar system's largest canyon system: over 4,000 km long, up to 200 km wide, and as deep as 7 km. For scale, the Grand Canyon is about 450 km long and roughly 1.6 km deep. Valles Marineris formed primarily through tectonic rifting as the Tharsis bulge stretched and cracked the crust, with water erosion widening it further. It was not carved by a river the way the Grand Canyon was.

Current Environmental Conditions on Mars

Atmosphere: Mars' atmosphere is about 95.3% $CO_2$, with small amounts of nitrogen (2.7%) and argon (1.6%), plus traces of oxygen and water vapor. Surface pressure is roughly 1% of Earth's, and the average temperature sits around $-63°C$. That thin atmosphere means very little insulation.

Water: Water ice exists in the polar caps and as subsurface permafrost. Liquid water cannot persist on the surface under current conditions because the atmospheric pressure is so low that water would quickly boil away or freeze. This is a direct consequence of where Mars sits on the phase diagram of water.

Radiation and soil chemistry: Without a global magnetic field and with only a thin atmosphere, Mars' surface is bombarded by cosmic rays and solar radiation. The soil also contains perchlorates, strong oxidizing compounds that would be toxic to most known life forms.

Implications for life: The combination of extreme cold, low pressure, intense radiation, and oxidizing soil makes the current surface hostile to life as we know it. However, subsurface environments (underground aquifers, geothermal zones) could potentially be more hospitable. And the widespread evidence of ancient lakes, rivers, and habitable chemistry tells us that early Mars was a very different place.

Geological Processes Shaping Mars

Five major processes have shaped the Martian surface, though they haven't all been equally active throughout the planet's history:

• Volcanism has been the dominant constructive force, building the Tharsis plateau and its enormous shield volcanoes. Mars may still have residual volcanic activity, though nothing has been directly observed.
• Impact cratering is recorded across the surface, especially in the ancient southern highlands. The heavy cratering there tells us those surfaces are billions of years old.
• Tectonic activity on Mars is limited. There are no moving plates like on Earth, but the crust has fractured and rifted, most dramatically in the formation of Valles Marineris. The absence of plate tectonics also means Mars has no mechanism for recycling its crust, which is why surface features persist for billions of years.
• Aeolian (wind) processes are the most active geological force on Mars today. Wind erosion and sand deposition create dune fields, sculpt rock formations, and drive planet-wide dust storms that can last for months.
• Fluvial (water) processes were important in Mars' early history, carving valley networks, outflow channels, and depositing sediments in ancient lake basins. These features are no longer forming, but they provide the strongest evidence that Mars once had a warmer, wetter climate.