Mars

• 1. The Red Wandering Star

  ›

  Mars is the fourth planet from the Sun, a desert-like rocky world that glows orange-red in the night sky. The ancient Sumerians called it Nergal, the god of war and plague. In Mesopotamian texts it was the star of judgement of the fate of the dead. By the time the Romans gave it the name we use today, it had carried the weight of war for thousands of years. The color comes from iron oxide dust, and that dust is everywhere. It coats the surface, hangs in the air, and tints the sky a tawny shade when seen from the ground.

  Mars has just one-tenth the volume of Earth and roughly a third of its gravity. Its atmosphere is a few thousandths the thickness of ours, and no liquid water pools on its surface. Yet humans have been drawn to it for millennia, and at times more than ten spacecraft have operated there at once, more than at any planet beyond Earth. What did people once believe lived there? Why does a world this hostile still anchor dreams of human settlement? And what story do its scars, canyons, and frozen reservoirs tell about a wetter, warmer past?

• 2. A World Of Extremes

  ›

  Olympus Mons rises over 21 kilometers from the foot of its cliffs to its peak. Measured from the plains of Amazonis Planitia more than 1000 kilometers away, the total climb to its summit approaches 26 kilometers, roughly three times the height of Mount Everest. The volcano is over 600 kilometers wide. Its size makes a single clean height figure difficult to pin down. Olympus Mons is either the tallest or second-tallest mountain in the Solar System, possibly rivaled only by the Rheasilvia peak on the asteroid Vesta.

  Valles Marineris stretches 4000 kilometers across the surface, a span equal to the length of Europe. The canyon reaches depths of up to 7 kilometers and crosses one-fifth of the planet's circumference. By comparison, the Grand Canyon on Earth runs only 446 kilometers and is nearly 2 kilometers deep. The Martian dichotomy splits the planet in two. Flat northern plains, smoothed by lava, contrast with cratered southern highlands.

  Hellas is the largest exposed impact crater, 2300 kilometers wide and 7000 meters deep, bright enough to see from Earth. In all, Mars carries 43,000 craters of 5 kilometers or greater. Some scientists believe a far larger wound exists. They propose that four billion years ago, an object struck the Northern Hemisphere and gouged a basin 10600 by 8500 kilometers, an area rivaling Europe, Asia, and Australia combined.

• 3. Reading The Rock

  ›

  Marsquakes tremble beneath the Martian ground, and in 2019 the InSight lander was reported to have detected over 450 of them. The signals revealed a layered world. A metallic core sits beneath a silicate mantle and a crust averaging tens of kilometers thick. That crust thins to just 6 kilometers in Isidis Planitia and thickens to 117 kilometers under the southern Tharsis plateau. The most abundant elements there are silicon, oxygen, iron, magnesium, aluminum, calcium, and potassium.

  The core is iron and nickel, at least partly molten, spanning roughly 1650 to 1675 kilometers, about half the planet's radius. Its temperature is estimated at 2000 to 2400 kelvin, far cooler than Earth's inner core. Whether Mars holds a solid inner core remains unsettled. A 2023 study using InSight data suggested there was none, while a 2025 study using the same data reported a solid inner core 613 kilometers in radius.

  The mantle hides strange debris. Dense fragments up to 4 kilometers across appear scattered deep within, likely driven there by colossal impacts around 4.5 billion years ago. High-frequency waves from eight marsquakes slowed as they crossed these regions. Because Mars lacks plate tectonics and stirs only sluggishly inside, that ancient wreckage was never blended away.

• 4. Echoes Of Water

  ›

  Ma'adim Vallis runs 700 kilometers long, 20 kilometers wide, and 2 kilometers deep in places, carved by flowing water early in the planet's history. Outflow channels like it cut across the surface in about 25 places, thought to record catastrophic floods released from underground aquifers. On the oldest terrain, branching networks of valleys spread like river systems, strongly implying rainfall once fell on early Mars.

  Minerals tell the same story. In 2004, the Opportunity rover detected jarosite, which forms only in acidic water. In December 2011, Opportunity found gypsum, another water-formed mineral. On the 18th of March 2013, NASA reported that the Curiosity rover had found signs of mineral hydration in rocks nicknamed Tintina and Sutton Inlier. Researchers suspect the low northern plains once held an ocean hundreds of meters deep. In March 2015, scientists suggested it might have matched the size of Earth's Arctic Ocean.

  Most Martian water now sits frozen. If the south polar ice cap melted, it could blanket most of the planet to a depth of 11 meters. Near the northern cap, the 81.4-kilometer-wide Korolev Crater holds roughly 2200 cubic kilometers of water ice. In November 2016, NASA reported a vast underground deposit in Utopia Planitia, estimated to rival the water volume of Lake Superior.

• 5. The Thin Veil

  ›

  Mars lost its magnetosphere about four billion years ago, possibly battered away by asteroid strikes. Without that shield, the solar wind strips atoms from the atmosphere's outer edge, and the MAVEN orbiter now studies this slow erosion. The remaining air is about 96 percent carbon dioxide, with small amounts of argon and nitrogen. Surface pressure averages around 600 pascals, only 0.6 percent of Earth's.

  The planet's seasons run uneven. Mars has a pronounced orbital eccentricity, and it reaches perihelion during southern summer. As a result, southern summers can be warmer than their northern counterparts by up to 30 degrees. Surface temperatures swing from lows near -110 degrees Celsius to highs around 35 degrees in equatorial summer. Mars sits 1.52 times as far from the Sun as Earth, receiving just 43 percent of the sunlight.

  Dust dominates the climate. Mars hosts the largest dust storms in the Solar System, with winds topping 100 miles per hour, sometimes swallowing the entire planet. Using the Perseverance rover's recordings, researchers found the speed of sound there is about 240 meters per second below 240 hertz. Auroras occur too, and unlike Earth's they can wrap the whole planet. In September 2017, NASA reported an aurora 25 times brighter than any seen before, triggered by an unexpected solar storm.

• 6. Twins Named For Fear

  ›

  Phobos and Deimos were discovered in 1877 by Asaph Hall, named for the Greek deities of panic and terror who followed their father Ares into battle. Phobos measures about 22 kilometers across and orbits at 9376 kilometers; Deimos is about 12 kilometers across and orbits at 23,460 kilometers. From the surface, Phobos rises in the west and sets in the east, completing its arc in just 11 hours.

  Phobos is doomed. Its orbit sits below synchronous altitude, so Martian tides are slowly dragging it down. In about 50 million years, it could crash into the planet or shatter into a ring. The origins of both moons remain disputed. Their low albedo and carbonaceous chondrite composition resemble asteroids, supporting a capture theory. Yet their circular, near-equatorial orbits are unusual for captured bodies.

  Newer evidence points elsewhere. Phobos appears highly porous and may contain phyllosilicates and minerals known from Mars, suggesting it formed from impact debris that reaccreted in orbit. A 2023 study of Deimos's orbital inclination raised an even stranger idea. Mars may have had a ring system between 3.5 and 4 billion years ago, born from a moon 20 times more massive than Phobos, with Phobos itself a surviving remnant.

• 7. Canals And Imaginations

  ›

  On the 5th of September 1877, a perihelic opposition brought Mars unusually close. The Italian astronomer Giovanni Schiaparelli used a 22-centimeter telescope in Milan to draw the first detailed map, marking features he called canali, meaning channels or grooves. In English the word was mistranslated as canals. Most were later shown to be an optical illusion.

  Percival Lowell took the idea further. The orientalist founded an observatory with 30- and 45-centimeter telescopes and published several books arguing for a dying world crossed by irrigation works built by ancient civilizations. Combined with the seasonal shrinking of polar caps, this fueled what became known as Mars Fever. As telescopes grew larger, the canali faded. During observations in 1909 with an 84-centimeter telescope, Antoniadi saw only irregular patterns.

  The belief in a habitable Mars died slowly. In 1894, W. W. Campbell at Lick Observatory found water vapor and oxygen too scarce to detect. The myth held until W. S. Adams confirmed those findings in 1925. The dream survived in fiction. H. G. Wells imagined Martians invading Earth in The War of the Worlds, Ray Bradbury wrote The Martian Chronicles, and the planetary astronomer Carl Sagan later called Mars a mythic arena onto which we have projected our Earthly hopes and fears.

• 8. The Long Reach Toward Mars

  ›

  In 1963, the Soviet Mars 1 became the first spacecraft sent to the planet, but contact was lost en route. NASA's Mariner 4 launched on the 28th of November 1964 and made its closest approach on the 15th of July 1965, transmitting the first images of another planet from deep space. It also measured a Martian radiation belt about 0.1 percent the strength of Earth's. The Viking probes of the mid-1970s carried experiments to detect microorganisms, and their results sparked a debate that still continues.

  Since 1997, when Mars Pathfinder became the first successful rover beyond the Moon, robots have worked there without interruption. Today Mars hosts ten functioning spacecraft, eight in orbit and two on the ground, the Curiosity and Perseverance rovers. The search for life remains the driving question. In June 2024, NASA designated the Cheyava Falls rock a potential biosignature, and Perseverance core-sampled it for possible return to Earth.

  Human missions remain proposals. The NASA Authorization Act of 2017 directed a feasibility study for a crewed mission in the early 2030s, and the resulting report judged it unfeasible. SpaceX has pushed ahead with its Starship vehicle, and in April 2024 Elon Musk described beginning a Mars colony within the next twenty years. Any crew would have to launch within the optimal window that opens every 26 months, the same orbital rhythm that has governed every spacecraft sent to the fourth planet.