Mars has long captivated the human imagination, standing as the most explored and potentially habitable planet beyond Earth. Decades of robotic missions have transformed our understanding, revealing a complex history and surprising physical characteristics. The scale of its geography, the unique passage of time, and the strange nature of its surrounding bodies offer a compelling collection of facts about our neighbor.
The Scale of Martian Geography
The Martian landscape is defined by features that dwarf anything found on Earth, resulting from the planet’s unique geological evolution. Olympus Mons, the largest volcano in the solar system, rises approximately 21.9 kilometers high, nearly three times taller than Mount Everest. Its base stretches about 600 kilometers across, covering an area comparable to Arizona or Poland.
This size is partly due to the absence of plate tectonics, which allowed the volcano to remain stationary over a hot spot for billions of years, building up layers. Mars also hosts Valles Marineris, a canyon system that cuts across the surface for over 4,000 kilometers. This chasm is about 200 kilometers wide and up to 7 kilometers deep, making it roughly ten times the size of Earth’s Grand Canyon.
The planet’s signature reddish color comes from the ubiquitous presence of iron oxide, commonly known as rust, in the soil and dust. This fine, rusty dust is perpetually lofted into the thin atmosphere, coating the surface and giving Mars its famous hue. The massive scale of these stationary features is supported by Mars’s lower gravity, which puts less stress on the crust compared to Earth.
How Time and Sky Differ on Mars
The rotation of Mars dictates a day-night cycle similar to Earth’s. A Martian solar day, called a “sol,” lasts 24 hours, 39 minutes, and 35 seconds, making it only about 40 minutes longer than an Earth day. This minimal difference means a human living on Mars would not face a radical shift in their circadian rhythm.
The Martian year is equivalent to 687 Earth days, meaning seasons are nearly twice as long as they are on our home planet. Mars’s highly elliptical orbit also causes its seasons to vary greatly in length and intensity, unlike Earth’s more consistent seasonal patterns.
The sky presents unique visual phenomena due to the fine dust particles in the thin atmosphere. During the day, the sky appears a reddish-yellow or butterscotch color because of the scattering of rusty dust. At sunset, this color scheme reverses, and the setting Sun is surrounded by a distinct blue glow. This occurs because the fine dust particles scatter blue light forward more efficiently than red light, concentrating the blue wavelengths around the solar disk.
Global dust storms are a regular occurrence, with planet-encircling events happening roughly once every three Earth years. These storms can last for months, engulfing the entire planet and dramatically reducing the sunlight reaching the surface. Although widespread, the thin atmosphere means the wind force is not intense, but the volume of suspended dust is a significant factor in the Martian climate.
Water and the Evidence for Past Habitability
The search for water on Mars has yielded evidence of a much wetter past. Ancient geological features, such as dried-up riverbeds, valley networks, and sedimentary rocks with wave-like ripples, suggest that liquid water once existed on the surface. These findings indicate the Martian climate was once warm and wet enough to support open bodies of water, possibly for extended periods.
Recent discoveries of salt deposits left by evaporating water suggest that liquid water was present on the surface as recently as two billion years ago. This extends the timeline for possible habitability, as scientists previously believed the planet dried out much earlier. The presence of hydrated minerals, such as sulfates, confirms that water interacted with the planet’s crust.
Today, liquid water cannot persist on the surface due to the extremely low atmospheric pressure and cold temperatures, causing it to quickly freeze or boil away. The majority of water now exists as extensive ice deposits beneath the surface, forming a vast permafrost layer. Massive amounts of water ice are also locked away in the polar ice caps.
The Peculiar Nature of Mars’ Moons
Mars is orbited by two small satellites, Phobos and Deimos, which contrast sharply with Earth’s single, large moon. Both moons are irregular, resembling battered potatoes, because their low mass means gravity is too weak to pull them into a round shape. Their dark composition, similar to carbonaceous asteroids, suggests they are captured objects from the asteroid belt.
Phobos, the inner and larger moon, orbits so closely that it completes a revolution in just over seven and a half hours, circling the planet three times daily. This proximity means Phobos is slowly spiraling inward toward Mars. Scientists predict that within about 50 million years, Mars’s tidal forces will either tear Phobos apart to form a temporary ring system or cause it to crash into the planet.
Deimos is smaller and orbits much farther away, taking just over 30 hours to complete one revolution. Unlike Phobos, Deimos is slowly moving away from Mars, echoing the behavior of Earth’s moon. The existence of these two satellites reflects a violent or chaotic past in the early solar system.