Mars, often called the Red Planet, is the fourth planet from the Sun and the second-smallest in the solar system after Mercury. Its distinct reddish hue, visible in the night sky, has captivated human imagination for centuries. Mars is a primary focus in the search for extraterrestrial life and a potential future home. It is defined by its thin atmosphere, colossal geological features, and significant stores of water ice.
Physical Profile and Orbital Characteristics
Mars is a terrestrial planet with a diameter of approximately 6,779 kilometers, which is roughly half that of our home world. Its reddish-orange appearance is the result of iron(III) oxide, commonly known as rust, prevalent in the surface dust and regolith.
The planet’s mass is about 11% of Earth’s, resulting in a surface gravity that is 38% of Earth’s gravity. A Martian day, known as a sol, lasts 24 hours and 37 minutes. The Martian year stretches for 687 Earth days, meaning Mars experiences seasons that are nearly twice as long as those on Earth.
Atmospheric Composition and Climate
The atmosphere of Mars is extremely thin, with an average surface pressure of about 6 to 7 millibars, which is less than one percent of Earth’s sea-level pressure. The composition is dominated by carbon dioxide, which makes up about 95.3% of the atmosphere, with nitrogen and argon making up most of the remaining gas. This provides little insulation, leading to extreme temperature variations.
Temperatures fluctuate widely, ranging from a low of approximately -153°C at the winter pole to a high of about 20°C near the equator during the summer. The Martian climate features dramatic seasonal pressure changes, caused by the cyclical freezing and sublimation of atmospheric carbon dioxide onto the polar ice caps. This is also responsible for global dust storms that can occasionally engulf the entire planet for months.
Defining Surface Geology
The Martian surface is marked by the hemispheric dichotomy: the northern plains are generally smoother and lower in elevation, contrasting sharply with the heavily cratered, higher-elevation southern highlands. Dominating the western hemisphere is the Tharsis Bulge, a massive volcanic plateau roughly 8,000 kilometers across, which rises up to 8 kilometers above the surrounding plains. This immense rise in the crust is thought to have caused the extensive tectonic cracking that created other major features.
The Tharsis region is home to the solar system’s largest known volcano, Olympus Mons, a colossal shield volcano whose summit towers 21.9 kilometers high. This volcano is so massive that its diameter spans over 500 kilometers, dwarfing any comparable feature on Earth. Radiating from the Tharsis Bulge is the Valles Marineris, an extraordinary canyon system that stretches for approximately 4,000 kilometers, extending across nearly one-fifth of the planet’s circumference.
The Valles Marineris plunges to depths of up to 7 kilometers, making it far larger than Earth’s Grand Canyon. This gigantic rift valley is believed to have formed primarily through tectonic processes, with subsequent widening caused by erosion. The geological history is further evidenced by other shield volcanoes in the Tharsis region, such as the Tharsis Montes—Ascraeus Mons, Pavonis Mons, and Arsia Mons—which align southwest to northeast.
Current State of Water and Ice
Liquid water cannot persist on the surface today due to the low atmospheric pressure and cold temperatures, but Mars holds substantial quantities of water in the form of ice. The polar ice caps are composed of layered deposits; the residual caps are primarily water ice, seasonally covered by frozen carbon dioxide (dry ice). Evidence points to extensive subsurface water ice, with more than 5 million cubic kilometers detected near the surface, mostly in the form of permafrost.
Geological features like ancient outflow channels and valleys provide strong evidence that vast amounts of liquid water flowed across the surface billions of years ago. This ancient, warmer period left behind mineralogical signatures. Radar and topographic data recently suggested the possible presence of a stable, highly briny body of liquid water located about 1.5 kilometers beneath the south polar ice cap. The existence of this sub-glacial lake implies that Mars may still possess a greater level of internal geothermal activity than previously assumed.
The Martian Moons
Mars is orbited by two small, irregularly shaped moons named Phobos and Deimos. These two moons are significantly smaller than Earth’s Moon and are named after the Greek mythological personifications of fear and terror, who accompanied the god of war. Phobos is the larger moon, with a diameter of about 22 kilometers, and it orbits incredibly close to the Martian surface.
Phobos completes an orbit in 7 hours and 39 minutes, making it one of the fastest-orbiting moons in the solar system. Deimos is about 12.6 kilometers across and orbits farther away, with an orbital period of just over 30 hours. The prevailing theory suggests that both Phobos and Deimos are captured asteroids, supported by their dark, carbonaceous composition and irregular shapes. Phobos is spiraling inward toward Mars and is predicted to either crash into the planet or break up into a ring in about 50 million years.