What Is Mars?
Mars is a terrestrial (rocky) planet with a thin atmosphere, polar ice caps, ancient river valleys, and the largest volcano and canyon system known in the Solar System. Its reddish color comes from iron oxide (rust) covering much of the surface.
Although colder and drier than Earth, Mars shows clear evidence that liquid water once flowed across its surface. This history makes it a prime target for understanding whether life ever arose beyond Earth and for future human exploration.
Physical Characteristics
Size & Gravity
Mars has roughly half Earth’s diameter and about 11% of Earth’s mass. Surface gravity is only 0.38 g — a person who weighs 100 kg on Earth would weigh about 38 kg on Mars.
Temperature
Average surface temperature is about –60 °C (–80 °F). Daytime equatorial temperatures can rise above 0 °C, while polar winter nights can drop below –125 °C.
Atmosphere
The atmosphere is thin (about 1% of Earth’s surface pressure) and composed mostly of carbon dioxide (≈95%), with small amounts of nitrogen and argon. It cannot support liquid water for long on the surface today.
Rotation & Seasons
A Martian day (sol) lasts 24 hours 37 minutes. Mars has an axial tilt similar to Earth’s (~25°), so it experiences seasons — though each season lasts nearly twice as long because the year is longer.
Major Surface Features
Olympus Mons
The largest volcano in the Solar System. It rises about 22 km (13.6 miles) high and spans roughly 600 km across — a shield volcano far larger than any on Earth.
Valles Marineris
A vast canyon system stretching nearly 4,000 km long and up to 7 km deep. It dwarfs Earth’s Grand Canyon and is thought to have formed largely through tectonic stretching.
Polar Ice Caps
Both poles have permanent ice caps of water ice, covered seasonally by carbon-dioxide frost. The residual north polar cap is mostly water ice; the south retains a thin CO₂ layer year-round.
Impact Craters & Ancient River Valleys
The southern hemisphere is heavily cratered and older. Networks of dry river valleys and outflow channels record a time when liquid water shaped the landscape.
Moons of Mars
Phobos
The larger and closer moon. It is irregularly shaped, heavily cratered, and orbits so close that it rises in the west and sets in the east, completing an orbit in only 7.7 hours. Phobos is slowly spiraling inward and will eventually break apart or impact Mars.
Deimos
The smaller, more distant moon. Also irregular and cratered, Deimos orbits farther out and appears to move slowly across the Martian sky. Both moons are thought to be captured asteroids or remnants of a larger disrupted body.
Water and the Question of Life
Multiple lines of evidence show that Mars once had significant amounts of liquid water: dried riverbeds, lake deposits, minerals that form in water, and residual water ice at the poles and underground.
Today, liquid water is unstable on the surface because of low pressure and temperature, but subsurface ice is abundant. Whether microbial life ever existed — or still exists in protected niches — remains one of the central questions of Mars exploration.
Rovers and orbiters continue to search for biosignatures and to characterize environments that could once have been habitable.
Exploration of Mars
Orbiters
Spacecraft such as Mars Reconnaissance Orbiter, Mars Express, and MAVEN map the surface in high detail, study the atmosphere, and track climate change over time.
Landers & Rovers
Viking, Pathfinder, Spirit, Opportunity, Curiosity, Perseverance, and others have analyzed rocks, soil, and atmosphere on the ground, discovering evidence of past water and complex chemistry.
Sample Return Plans
Future missions aim to collect and return Martian rock and soil samples to Earth for detailed laboratory study — a major step toward answering whether life ever existed there.
Human Exploration
Multiple space agencies and private organizations are developing technologies for eventual crewed missions. Challenges include radiation, life support, dust, and the long duration of the journey.
Key Takeaways
- 1. Mars is a cold, dry, rocky planet about half the diameter of Earth, with a thin CO₂ atmosphere.
- 2. Its surface preserves evidence of a wetter past, including river valleys, lake beds, and minerals formed in water.
- 3. Olympus Mons and Valles Marineris are among the most extreme geologic features in the Solar System.
- 4. Two small moons, Phobos and Deimos, orbit Mars; both are likely captured asteroids.
- 5. Decades of orbiters, landers, and rovers have transformed Mars from a distant point of light into a well-mapped world.
- 6. The search for past or present life and the planning of human missions make Mars a central focus of planetary science.