Mars was never “discovered” in the way a telescope reveals a faint, previously unknown world. It is one of the five planets visible to the unaided eye, and every civilization that bothered to look up noticed it. The reddish wanderer has been tracked, charted, and named for at least four thousand years. What makes the history of Mars interesting is not a single discovery moment but a long chain of them, each one peeling back another layer of the planet’s character, from the shape of its orbit to the ice beneath its soil.
Mars in the Ancient Sky
The oldest known records of Mars come from Babylonian astronomers in the second millennium BCE. Cuneiform tablets dating to roughly 1600 BCE contain systematic observations of Mars’s position among the stars, making it one of the earliest celestial objects tracked in writing. The Babylonians called it Nergal, after their god of war and pestilence, a name likely inspired by the planet’s blood-red color. Egyptian records also mention Mars, and by the time of the Greek astronomers, the planet had been watched closely enough that its strange backward loop across the sky, what we call retrograde motion, was a recognized puzzle.
The Greeks named it Ares, again linking it to war. The Roman name Mars stuck, and the association with warfare persisted across cultures: the Chinese called it the “fire star,” and Hindu astronomers named it Mangala, also linked to aggression. None of these civilizations “discovered” Mars in any meaningful sense. They simply recorded what anyone standing outside on a clear night could see. The real discoveries began when people started asking harder questions about what the bright red dot actually was.
From Wandering Star to Planet With an Orbit
For most of recorded history, Mars was understood as a wandering light that moved against the fixed stars. The ancient Greeks debated whether it circled the Earth or the Sun, and Aristarchus of Samos proposed a Sun-centered model around 270 BCE, but the idea did not catch on. For nearly two millennia, the dominant framework placed Earth at the center of everything, with Mars riding on a complicated system of circles-within-circles to account for its retrograde motion.
Nicolaus Copernicus upended this in 1543 when he published his model placing the Sun at the center. Mars’s retrograde motion suddenly had a clean explanation: Earth, on a faster inner orbit, periodically overtakes Mars, making the planet appear to reverse direction against the background stars. But it was Johannes Kepler, working with the exquisitely detailed observations collected by Tycho Brahe, who turned Mars into the key that unlocked planetary mechanics. Kepler spent years trying to fit Mars’s orbit into a perfect circle and kept failing. The data refused to cooperate. His eventual realization, published in 1609, that Mars traces an ellipse with the Sun at one focus became his first law of planetary motion. Mars was, in a real sense, the planet that proved how orbits work.
The Telescope Reveals a World
When Galileo turned his telescope on Mars around 1610, he could see that it showed a disk rather than a point of light, but his instrument was too weak to reveal surface details. The first real telescopic discoveries came a few decades later. In 1659, the Dutch astronomer Christiaan Huygens made a drawing of Mars showing a dark, triangular feature now identified as Syrtis Major, a vast volcanic plain. He also used repeated observations of this feature to estimate Mars’s rotation period at roughly 24 hours, remarkably close to the actual value of 24 hours and 37 minutes.
Huygens also suggested that Mars might have polar caps, but the first clear telescopic observation of them is generally credited to the Italian-French astronomer Gian Domenico Cassini in 1666. Huygens himself did not see the planet’s south polar cap directly until 1672, when Mars made a particularly close approach to Earth.1Research Starter. Mars’s polar caps These bright white patches at the poles were immediately suggestive. They looked like Earth’s ice caps, they appeared to grow and shrink with the Martian seasons, and they launched centuries of speculation about whether Mars might host water or even life.
Canals, Illusions, and the Martian Fever
The most famous episode in the telescopic history of Mars began in 1877, when the Italian astronomer Giovanni Schiaparelli published detailed maps of the Martian surface. He labeled the dark and light areas with names drawn from classical geography, many of which are still in use today. More controversially, he drew a network of fine straight lines crisscrossing the surface and called them “canali,” an Italian word meaning channels or grooves. In English, the word was translated as “canals,” carrying the implication that they were artificially constructed.
The American astronomer Percival Lowell ran with this idea. He built an observatory in Flagstaff, Arizona, largely to study Mars, and from the 1890s onward he championed the theory that the canals were an irrigation system built by an intelligent civilization struggling to survive on a drying planet. Lowell published popular books, gave lectures, and captured the public imagination. The problem was that many other astronomers, using equally good or better telescopes, could not see the canals at all. By the early twentieth century, improved optics and photographic techniques made it clear that the canals were optical illusions, artifacts of the human eye’s tendency to connect faint, disconnected features into lines. Lowell’s Martians did not exist, but the excitement he generated arguably laid the cultural groundwork for the twentieth century’s fascination with Mars exploration.
Finding the Moons
The same opposition of 1877 that inspired Schiaparelli also produced another landmark. Asaph Hall, an astronomer at the U.S. Naval Observatory in Washington, D.C., used the observatory’s 26-inch refractor, one of the largest telescopes in the world at the time, to search systematically for Martian satellites. On August 12, he spotted a faint object near Mars that turned out to be Deimos, the smaller and more distant of Mars’s two moons. Six days later, on August 18, he found Phobos, the larger and closer moon. Both are tiny, irregular bodies, likely captured asteroids, and both were named after the sons of the Greek god Ares: Phobos (fear) and Deimos (dread).
The moons were difficult to find because they are small and orbit very close to Mars, where its glare overwhelms them. Phobos is only about 22 kilometers across at its widest, and Deimos about 13. Their discovery was a triumph of careful, patient observation, and they remained the only known Martian moons. No additional natural satellites have been found since, despite searches with far more powerful equipment.
The Space Age and First Close-Up Views
For all the progress made through telescopes, Mars remained a fuzzy disk with tantalizing but ambiguous features until spacecraft arrived. The first successful flyby was NASA’s Mariner 4 in July 1965. The 22 photographs it returned were a shock. Instead of a world that might support canals or vegetation, Mars looked barren and cratered, more like the Moon than like Earth. The romantic image of a living Mars took a serious hit.
Mariners 6 and 7 followed in 1969 and confirmed the bleak picture, but Mariner 9, which entered orbit around Mars in November 1971, changed everything again. It arrived during a global dust storm and had to wait weeks before the surface cleared, but when it did, the images revealed Olympus Mons, the largest volcano in the solar system, the vast canyon system Valles Marineris (named after the spacecraft), and what appeared to be ancient river channels. Mars was not a dead moonscape after all. It had a dramatic geological history, and those channels hinted strongly at a watery past.
Viking and the Life Question
NASA’s twin Viking landers touched down on Mars in 1976 and carried out the first direct experiments designed to detect life on another planet. Each lander carried three biology experiments that tested samples of Martian soil in different ways. In one, called the labeled release experiment, soil was moistened with a solution containing carbon-based nutrients tagged with a radioactive marker. If microbes were present and metabolizing those nutrients, they would release radioactive gas. Both landers, separated by about 6,400 kilometers, produced positive results: a burst of radioactive gas that diminished when a duplicate soil sample was heat-sterilized first.2PubMed Central. The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment
A second experiment, the gas exchange test, detected a surprising release of oxygen when soil was exposed to water vapor, along with a rise in carbon dioxide. The third, the pyrolytic release experiment, found a small amount of carbon-containing gas converted into organic material, an effect that did not occur in heat-treated samples.3PubMed. The viking biological investigation: preliminary results Taken at face value, some of these results looked biological. But another instrument on the landers, a gas chromatograph–mass spectrometer designed to detect organic molecules directly, found none. Without organic molecules, the biological interpretation was difficult to sustain. The consensus view became that the reactive results were caused by unusual soil chemistry, perhaps highly oxidizing compounds in the Martian surface. The principal investigator of the labeled release experiment, Gilbert Levin, disagreed with this conclusion for the rest of his career, arguing that the experiment had in fact detected life. The debate has never been entirely closed.
Finding Water, Past and Present
After Viking, Mars exploration entered a quieter period, but the late 1990s and 2000s brought a fleet of orbiters and landers that reshaped the picture dramatically. One of the biggest shifts was the mounting evidence that Mars once had abundant liquid water on its surface and still harbors enormous quantities of water ice underground.
NASA’s Mars Odyssey orbiter, which arrived in 2001, carried instruments that could detect hydrogen just below the surface, a proxy for water ice. Observations of how the surface temperature changed with the seasons revealed significant variability in subsurface ice deposits, detectable at scales smaller than a kilometer, and in some areas the ice distributions matched predictions from atmospheric models of how water vapor would migrate into the soil.4PubMed. High-resolution subsurface water-ice distributions on Mars Later, the Phoenix lander in 2008 dug into the Martian arctic and directly confirmed the presence of water ice centimeters below the surface, watching it sublimate after exposure.
From orbit, the Mars Reconnaissance Orbiter’s spectrometer identified a wide variety of water-bearing minerals scattered across ancient Martian terrain. These included clays formed in the presence of liquid water, with smectite-family minerals like nontronite and saponite being the most widespread, alongside chlorite, kaolinite, and even hydrated silica.5Nature. Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument The variety of these minerals points to diverse watery environments during Mars’s earliest geological period, some acidic, some alkaline, some warm, some cold. That diversity matters because it implies the planet once offered multiple distinct habitats, not just a single wet phase.
Rovers and the Search for Habitability
The question driving modern Mars exploration is no longer whether water existed but what kinds of environments the water created and whether any of them could have supported life. NASA’s Curiosity rover, which landed in Gale crater in 2012, found compelling geological evidence that the crater once held a lake. Instruments aboard the rover detected organic molecules in sedimentary mudstone, along with nitrate compounds in the sediments, raising the possibility that a metabolism based on nitrate-dependent iron oxidation could have been viable for hypothetical Martian microbes.6PubMed. Quantifying the Potential for Nitrate-Dependent Iron Oxidation on Early Mars: Implications for the Interpretation of Gale Crater Organics
The Perseverance rover, which landed in Jezero crater in 2021, has pushed the investigation further. Jezero was chosen because orbital data showed it once contained a lake fed by a river delta, an environment with strong potential for both habitability and the preservation of biosignatures.7PubMed Central. In Situ Identification of Paleoarchean Biosignatures Using Colocated Perseverance Rover Analyses: Perspectives for In Situ Mars Science and Sample Return Among the rover’s findings are iron-phosphate minerals formed under watery conditions, representing the most definitive identification yet of phosphate minerals on the Martian surface.8PubMed Central. Fe-phosphates in Jezero Crater as evidence for an ancient habitable environment on Mars Phosphorus is a building block of DNA and cell membranes, so confirming its availability in an ancient aqueous environment is a meaningful piece of the habitability puzzle.
Perseverance is also collecting and caching rock samples intended for eventual return to Earth by a future mission. If those samples make it back, they will be the first Martian rocks analyzed in terrestrial laboratories, where instruments orders of magnitude more sensitive than anything a rover can carry could search for chemical signatures of past life.
The Methane Mystery
One of the more puzzling chapters in Mars science opened in 2004, when the European Space Agency’s Mars Express orbiter reported detecting methane in the Martian atmosphere at a global average of about 10 parts per billion by volume, with concentrations varying from near zero to roughly 30 parts per billion across different regions.9PubMed. Detection of methane in the atmosphere of Mars On Earth, methane is overwhelmingly produced by living organisms and by geological processes like hydrothermal reactions. On Mars, it should be destroyed by solar ultraviolet radiation within a few hundred years, so any methane detected now implies an active source replenishing it.
Curiosity’s instruments later detected sporadic spikes of methane at ground level inside Gale crater, adding to the mystery. The pattern of variable, intermittent detections from both surface and orbit is consistent with what planetary scientists describe as active gas seepage from the subsurface, a process well studied on Earth but never confirmed on Mars.10PubMed Central. Methane Seepage on Mars: Where to Look and Why The source could be biological, geological, or both. Adding to the confusion, the ESA’s Trace Gas Orbiter, specifically designed to map trace gases with high sensitivity, initially struggled to detect methane from orbit at the levels Curiosity was measuring on the ground. The discrepancy remains unresolved and is one of the more active debates in Mars science.
What “Discovery” Means for a Planet You Can See With Your Eyes
The story of Mars does not fit the tidy narrative of discovery that works for, say, Neptune, where a mathematician predicted it and an astronomer pointed a telescope at the right spot in 1846. Mars has been known to every human culture that looked at the night sky. Its “discovery” is really a series of conceptual breakthroughs spread across millennia: recognizing it as a planet rather than a star, understanding its elliptical orbit, seeing surface features through a telescope, debunking the canals, finding its moons, landing on it, detecting its chemistry, and slowly assembling the evidence for a wet, potentially habitable past.
Each of these moments redefined what Mars was understood to be. The Babylonians knew it as an omen. Kepler knew it as proof that orbits are ellipses. Schiaparelli and Lowell knew it, mistakenly, as a world of engineered waterways. Viking scientists knew it as a chemically perplexing surface. Curiosity and Perseverance know it as a place that once had lakes, phosphorus, and organic molecules. The discovery of Mars is not a date on a calendar. It is a process still underway, and the next major chapter may arrive when sealed sample tubes from Jezero crater finally reach a lab on Earth.
Mars in Comparative Planetary Science
Mars occupies a unique position among the planets explored so far. It is the only world beyond Earth where scientists have conducted biological experiments on the surface, and the only one where the question of past habitability is investigated with ground-truth data from multiple rovers and landers. Venus, despite being closer to Earth in size, has a surface too hostile for any lander to survive more than a couple of hours. The outer solar system moons with subsurface oceans, like Europa and Enceladus, remain tantalizingly out of reach for surface missions. Mars, by contrast, is accessible enough that we have driven robotic vehicles across its surface for more than two decades.
The geological record on Mars is also unusually well preserved. Earth’s tectonic plates constantly recycle the crust, erasing most rocks older than a few billion years. Mars has no plate tectonics, so ancient terrain dating back more than three and a half billion years is still exposed on the surface. The Martian crust is often characterized through its dominant water-altered minerals, with ancient clay-rich terrains from the earliest era and sulfate-bearing deposits from a later, more acidic period.11Icarus. Presence of clay minerals can obscure spectral evidence of Mg sulfates: implications for orbital observations of Mars Reading that mineral record is like reading a diary of how Mars’s climate changed, from wetter and warmer conditions early on to the cold, dry world we see today. Understanding that transition matters well beyond Mars itself: it speaks to how rocky planets in general lose their water and atmospheres, a question with obvious relevance to Earth’s own long-term future.