Mars: Crash Course Astronomy #15

Mars has captured human imagination longer and more deeply than almost any other celestial body. Once feared as the God of War, it has evolved in our modern era from a realm of science fiction into a very real world that we have mapped, photographed, and explored with robotic rovers, and which humanity may one day call home.

1. Orbital Characteristics and Physical Dimensions

Mars is the fourth planet from the Sun and the outermost of the terrestrial (rocky) planets.

• Distance from the Sun: Orbits at an average distance of roughly 200 million kilometers.
• Proximity to Earth: Lies completely outside Earth's orbit, approaching no closer than about 60 million kilometers at its closest approach.
• Size: Roughly half the size of Earth in diameter.
• Temperature: Extremely cold, with average surface temperatures hovering around $-60^\circ\text{C}$ (60 degrees Celsius below zero).
• Appearance: When closest to Earth, it shines brightly in the night sky with a distinct reddish hue, resembling an angry eye—which explains why ancient civilizations associated it with war.

2. Why Is Mars Red? (The Rust Factor)

Contrary to popular misconception, the red color of Mars is not caused by blood, but by rust (oxidized iron).

• Composition of Dust: Fine dust particles on the surface, resembling red talcum powder, are rich in oxidized iron.
• Surface Covering: This rusty dust blankets large portions of the planet, tinting its landscape brown-red and blowing into the thin atmosphere.
• Basaltic Rock: Much of the underlying surface consists of dark grey volcanic rocks known as basalt, which blend with iron oxides and other minerals to form the rusty-red Martian landscape.

3. Topography and Dramatic Geological Features

The geography of Mars is characterized by extreme geological contrasts, particularly between its northern and southern hemispheres.

Major Geological Landmarks

• Tharsis Bulge: A massive tectonic plateau that houses the four largest volcanoes on Mars, including Olympus Mons, the largest volcano in the entire solar system.
• Valles Marineris: A colossal canyon system discovered by the Mariner 9 probe in the 1970s. It spans 4,000 kilometers long, 200 kilometers wide, and 7 kilometers deep—making it roughly 10 times longer and wider than Earth's Grand Canyon. Unlike the Grand Canyon, it was not carved by water, but likely tore open as the Tharsis Bulge rose.
• Absence of Active Plate Tectonics: While Mars does not have active tectonic plates today, geological evidence suggests they existed in the past. Tharsis likely formed over a stationary mantle plume, punching through the crust repeatedly.

4. The Martian Polar Ice Caps and Atmosphere

Mars possesses polar ice caps at both poles, composed primarily of water ice several kilometers thick.

• Seasonal Dry Ice (Carbon Dioxide): During the freezing winter months, atmospheric $\text{CO}_2$ freezes directly onto the polar caps, forming a seasonal layer of dry ice 1 to 8 meters thick.
• Summer Sublimation: In summer, sunlight warms the dry ice, turning it directly back into gas and causing it to rush away from the pole, generating violent seasonal winds.
• Thin Atmosphere: The atmospheric pressure on Mars is less than 1% of Earth's, and the air is composed overwhelmingly of carbon dioxide ($\text{CO}_2$). Roughly one-third of the entire Martian atmosphere freezes out into the polar caps every winter.
• Lack of Shielding: Because the atmosphere is so thin, it offers little protection against incoming meteoroids and comets. The surface is heavily cratered, and modern orbiters like the Mars Reconnaissance Orbiter have even documented fresh impact craters forming in real-time.

5. Surface Weather, Winds, and Avalanches

Despite its thin air, the Martian atmosphere is thick enough to drive dynamic weather patterns across the surface.

• Dust Storms and Dunes: Seasonal winds blow dust across craters and sweep eroded basaltic sand into stunning crescent-shaped dunes (barchans) and parallel grooves.
• Dust Devils: Towering, tornado-like dust whirlwinds sweep across the surface, visible from orbit. When they lift red dust over the grey basaltic plains, they carve intricate, beautiful dark patterns.
• Snow and Ice Avalanches: Orbiters have captured dramatic footage of massive avalanches. As buried carbon dioxide ice thaws in the spring along steep canyon walls, it triggers colossal cascades of tumbling rock and dust.

6. The Martian Moons: Phobos and Deimos

Mars is orbited by two tiny, potato-shaped moons that strongly resemble captured asteroids from the nearby asteroid belt.

7. The Hydrological Mystery: Where Did the Water Go?

While surface water is currently absent as liquid, and only locked away as water ice at the poles and subsurface deposits uncovered by impacts, Mars was once a wet world.

• Ancient Mars: Billions of years ago, Mars was warmer, wetter, and possessed a thicker atmosphere.
• Geological Evidence: The surface is scarred by ancient dry riverbeds, dried-up lake beds, sedimentary layering, and mineral formations that only form in the presence of liquid water. Some evidence even points to ancient oceans.
• Planetary Death: So what happened? Billions of years ago, the internal dynamo of Mars shut down, causing its protective global magnetic field to vanish. Without a magnetic field, the solar wind stripped away the Martian atmosphere over millions of years, causing surface water to evaporate and escape into space.

8. The Search for Life and Organic Building Blocks

The question of whether Mars ever hosted life has transformed from science fiction into rigorous astrobiology.

• Early Planetary Cooling: Because Mars is smaller than Earth, it cooled down much faster after formation. Since life on Earth arose rapidly after our surface cooled, life could have theoretically sparked on Mars even earlier than it did on Earth.
• Organic Molecules: NASA's Curiosity rover discovered simple organic molecules inside rock samples. While this does not prove life existed, it confirms that the chemical building blocks for life were and are present.
• Methane Spikes: Curiosity also detected transient spikes in atmospheric methane—a gas that can be produced by biological activity or geological processes. Current evidence remains ambiguous, but the search continues.

9. Human Exploration and Future Outposts

Exploring Mars is extraordinarily difficult—roughly half of all missions sent there have failed. However, robotic success has paved the way for human exploration.

• Robots vs. Humans: Robots are relatively inexpensive and safe, but agonizingly slow. Tasks that take robotic rovers years to accomplish could be completed by human astronauts in a matter of weeks.
• The Challenge of Human Flight: Humans require air, water, food, and protection against cosmic radiation and the vacuum of space.
• Lava Tube Bases: Scientists propose landing humans in lava tubes—natural subterranean caves formed by ancient flowing lava where the roof has caved in (skylights). These volcanic caves would naturally shield human bases from solar radiation, extreme weather, and violent dust storms.

Summary and Key Takeaways

• Physical Profile: Mars is the fourth planet from the Sun, roughly half the size of Earth, and frigidly cold with an average surface temperature of $-60^\circ\text{C}$.
• The Rust Effect: Its characteristic red color is caused by fine, windblown dust rich in oxidized iron (rust) mixed with dark volcanic basalt.
• Extreme Topography: Mars hosts the solar system's largest volcano (Olympus Mons in the Tharsis Bulge) and its largest canyon (Valles Marineris, 10 times wider and longer than Earth's Grand Canyon).
• Thin Atmosphere: The Martian atmosphere is less than 1% as thick as Earth's, composed mostly of $\text{CO}_2$, with polar ice caps that experience seasonal dry-ice freezing and sublimation.
• Active Weather: Despite thin air, Mars features dynamic weather, including seasonal dust storms, towering dust devils, sand dunes, and active rock avalanches.
• Captured Moons: Its two tiny, potato-shaped moons, Phobos and Deimos, resemble captured asteroids, with Phobos spiraling inward toward a catastrophic future impact.
• Ancient Water: Geological features like dry riverbeds, lake beds, and water-formed minerals prove that ancient Mars was once warm, wet, and blanketed by a thicker atmosphere.
• Magnetic Collapse: When the internal planetary dynamo shut down, the global magnetic field vanished, allowing solar winds to strip away the atmosphere and dry up the planet.
• Astrobiology: The search for past or present life is fueled by the discovery of simple organic molecules and methane spikes by rovers like Curiosity.
• Future Habitation: Human colonization is a matter of when, not if, with proposals utilizing subterranean volcanic lava tubes as natural radiation shelters for future Martian bases.