4.5 Billion Years of History
Mars has hung in the human sky since the dawn of our species, a pinprick of restless, reddish light against the steadfast stars. To our ancestors, it was a wandering god of war, its color a reminder of fire and blood. To early astronomers, its looping path across the heavens was a cosmic puzzle that helped unravel the very laws of planetary motion. To a generation captivated by the power of new telescopes, it was a dying world, crisscrossed by the grand engineering works of a fading civilization. And to the first robotic explorers, it was a significant disappointment – a cratered, Moon-like wasteland.
Today, we know Mars as none of these things, and yet, as all of them. It is a world of history, a planet that has undergone a transformation more dramatic than any other in our solar system. Its story is a 4.5-billion-year epic of planetary evolution, written in stone and ice. But there is a second history of Mars: the story of our own evolving understanding. This is a journey of human curiosity that has taken us from myth-making in ancient Mesopotamia to the deployment of robotic geologists on the rust-colored plains of Jezero Crater. These two histories are now inextricably linked. The story of Mars is the story of a world that was once warm, wet, and perhaps habitable, but lost it all. The story of our pursuit of Mars is the quest to understand why.
This article traces these parallel histories. It begins with the planet itself, from its violent birth and watery youth to its volcanic, transitional adolescence and its long, frozen adulthood. It then turns to the human gaze, following our perception of Mars from the naked-eye observations of antiquity to the grand illusions of the canal era. From there, it chronicles the modern age of robotic exploration, a multi-decade campaign of orbiters, landers, and rovers that has peeled back the planet’s secrets, layer by layer. Finally, it explores Mars’s enduring grip on our imagination and looks ahead to the next chapter in our shared story – a future that may see pieces of the Red Planet brought to Earth, and eventually, human footprints left in its crimson dust.
The Making of a World: Mars Through Geologic Time
The story of Mars is written in its rocks, its canyons, and its craters. Planetary scientists have learned to read this story by dividing it into three great epochs, named for regions on the planet that typify them: the Noachian, the Hesperian, and the Amazonian. These divisions are defined by a simple but powerful principle: crater counting. An older surface has been exposed to impacts from asteroids and comets for longer, and so it bears more scars. The heavily cratered southern highlands are ancient, while the smooth northern plains are relatively young. By mapping the density of these craters, we can piece together a global timeline of the planet’s dramatic past.
A Planet is Born
Like its terrestrial siblings, Mars coalesced from the swirling disk of gas and dust surrounding the young Sun approximately 4.6 billion years ago. Within its first few tens of millions of years, it underwent differentiation, with heavy elements like iron sinking to form a core, surrounded by a rocky mantle and a primitive crust. This primordial era, known as the Pre-Noachian, was a time of unimaginable violence. The young solar system was a chaotic shooting gallery, and Mars was subjected to a period of intense bombardment by asteroids and comets.
This early chaos shaped the planet’s most fundamental feature: its global dichotomy. Mars is a world of two faces. The southern hemisphere is a rugged, ancient highland, standing several kilometers above the mean planetary elevation. The northern hemisphere, in contrast, is a vast, smooth lowland. The prevailing theory for this stark division is a single, cataclysmic event known as the giant-impact hypothesis. Early in its history, Mars was likely struck a glancing blow by a protoplanet the size of Pluto. The impact blasted away the crust of the northern hemisphere, creating the enormous Borealis basin, which covers 40% of the planet’s surface. This colossal scar would define the planet’s geography and destiny, creating a natural basin where water would later pool. The end of this violent era and the beginning of the next is often marked by another giant impact – the one that formed the Hellas basin, a 2,300-kilometer-wide crater in the southern hemisphere, sometime between 4.1 and 3.8 billion years ago.
The Noachian Epoch: A World of Water
Named for Noachis Terra, or the “Land of Noah,” the Noachian Epoch lasted from roughly 4.1 to 3.7 billion years ago. Terrains from this period are the most ancient on Mars, characterized by a very high density of impact craters. They cover about 45% of the planet, composing the bulk of the southern highlands. The Noachian is the period that holds the most tantalizing evidence of a Mars that was startlingly Earth-like – a world with a thicker atmosphere, warmer temperatures, and abundant liquid water.
The evidence for this ancient hydrosphere is carved into the very landscape. The most compelling features are the valley networks, dense, branching systems of channels that dissect the Noachian highlands. Their intricate, tree-like patterns strongly resemble terrestrial river basins formed by persistent rainfall and surface runoff. These were not scars from one-off floods but integrated drainage systems that collected and channeled water over long periods.
This water did not just disappear. It pooled in impact craters and regional depressions, forming large, long-lived lakes. Scientists have identified over 200 such paleolakes in the southern highlands, some rivaling the size of Earth’s Caspian Sea. Where the ancient rivers flowed into these standing bodies of water, they slowed down and deposited their sediment load, building up classic fan-shaped deltas. The beautifully preserved deltas in craters like Jezero – the landing site of the Perseverance rover – and Eberswalde are smoking guns for a past aqueous cycle.
Further evidence comes from orbit. Spectrometers on spacecraft have detected widespread deposits of phyllosilicates, or clay minerals, across Noachian terrains. Clays form through the chemical alteration of volcanic rock by prolonged contact with water. Their presence indicates that water was not just a transient feature but a persistent agent of geological change.
The collective evidence for rivers, lakes, and water-weathered minerals leads to a grand hypothesis: the existence of a primordial Martian ocean. The northern lowlands, created by the ancient giant impact, form a natural basin. It is here, in this vast depression, that the water draining from the southern highlands would have collected, forming an ocean dubbed Oceanus Borealis. This body of water could have covered nearly a third of the planet’s surface, containing more water than Earth’s Arctic Ocean. Evidence supporting this idea includes not only the location of deltas at a consistent elevation that could mark a former coastline, but also atmospheric measurements. The ratio of deuterium (a heavy isotope of hydrogen) to normal hydrogen in the Martian atmosphere today is much higher than on Earth. This indicates that a vast amount of the lighter hydrogen has been lost to space over eons – hydrogen that was once locked in water molecules. More recently, radar data from China’s Zhurong rover, exploring the southern part of the Utopia Planitia basin, has revealed subsurface layers with structures consistent with coastal sediments, providing some of the strongest support yet for an ancient northern ocean.
This picture of a wet, early Mars presents a significant climate puzzle known as the “faint young Sun paradox.” During the Noachian, the Sun was only about 75% as luminous as it is today. Under such faint sunlight, Mars should have been a frozen ice ball, not a world of flowing rivers. For liquid water to have been stable, Mars must have had a much thicker atmosphere capable of trapping heat through a powerful greenhouse effect. This ancient atmosphere was likely rich in carbon dioxide, supplied by intense and sustained volcanic activity, particularly in the growing Tharsis region.
Even with a thick atmosphere, the exact nature of the Noachian climate is debated. Was Mars a continuously warm and wet planet, with global rainfall and a stable ocean? Or was it a largely cold and icy world, punctuated by brief, violent episodes of melting and water flow triggered by the heat of large meteorite impacts or massive volcanic eruptions? The geological record suggests that much of the most intense fluvial activity occurred in a relatively short interval toward the end of the Noachian, perhaps hinting at a climate that was episodic rather than sustained. Whatever the precise mechanism, the evidence is clear: for a time, billions of years ago, Mars was a water world.
The Hesperian Transition: An Age of Fire and Flood
The Hesperian Epoch, which spanned from approximately 3.7 to 3.0 billion years ago, was the great transition in Martian history. It was during this period that Mars transformed from the wetter, warmer world of the Noachian into the cold, dry, and dusty planet we see today. The name comes from Hesperia Planum, a broad lava plain that exemplifies the dominant geological force of this era: volcanism.
As the relentless bombardment of the early solar system subsided, the planet’s internal heat took over as the primary engine of change. The Hesperian was an age of fire. Volcanism on an epic scale reshaped the planet, erupting vast plains of flood basalts known as “ridged plains.” These low-viscosity lavas poured from massive fissures in the crust, burying older Noachian terrain and resurfacing at least a third of the planet. At the same time, the colossal Tharsis Bulge continued to grow, and the great shield volcanoes that litter its expanse, including Alba Mons and the giants that would become Olympus Mons and the Tharsis Montes, began to form in earnest.
This planetary-scale volcanic activity had a significant and complex effect on the climate, acting as a final, violent bridge between the wet past and the frozen future. The erupting volcanoes released not only carbon dioxide but also immense quantities of sulfur gases, such as sulfur dioxide and hydrogen sulfide. These sulfur compounds created a potent, albeit temporary, greenhouse effect. This volcanic warming may have been responsible for a final flourish of water activity near the Noachian-Hesperian boundary, raising global temperatures just enough to melt remaining snow and ice deposits, carving the last of the valley networks and even causing basal melting beneath the south polar ice sheet.
However, this volcanic reprieve came at a cost. The same sulfur gases that warmed the atmosphere also reacted with water to form sulfuric acid. The surface water on Mars became acidic, fundamentally changing the planet’s chemistry. This is etched into the geological record: the era of clay formation gave way to an era of sulfate deposition. Widespread sulfate minerals, detected by orbiters in regions like Valles Marineris and Meridiani Planum, are the chemical signature of the Hesperian’s acidic waters.
While volcanism was reshaping the surface from above, water was reshaping it from below. The Hesperian is also defined by a different kind of water activity: catastrophic outflow floods. As the planet cooled, a global subsurface layer of ice, the cryosphere, began to form. Tectonic stresses from the growing Tharsis bulge or heat from rising magma would occasionally fracture this cryosphere, releasing enormous quantities of pressurized groundwater in violent, short-lived floods. These were not meandering rivers but deluges on an unimaginable scale, with discharges thousands of times greater than the Mississippi River. They scoured the landscape, carving the immense outflow channels that are a hallmark of the Hesperian, and poured into the northern lowlands, perhaps forming large, transient, ice-covered seas.
By the end of the Hesperian, the transformation was nearly complete. The planet’s internal heat was waning, volcanic activity was declining, and the atmosphere had thinned to near its present density. The last of the surface water either evaporated, was lost to space, or became permanently locked away as ice in the polar caps and as a thick layer of permafrost beneath the surface. The wet world of Noah was gone, replaced by a volcanic, acidic, and rapidly freezing planet.
The Amazonian Epoch: The Long, Cold Quiet
The Amazonian Epoch began around 3.0 billion years ago and continues to the present day. It is the longest of Mars’s geological eras, but also the quietest. Surfaces from the Amazonian are characterized by a sparse number of impact craters, testifying to their relative youth and the dramatic slowdown in the planet’s geological activity.
During the Amazonian, the pace of change dropped precipitously. Volcanism, while not entirely extinct, became a much rarer and more localized phenomenon, largely confined to the great volcanic provinces of Tharsis and Elysium. The main era of catastrophic flooding was over. Erosion and weathering rates, once high enough to wear down crater rims in the Noachian, fell to almost zero. For billions of years, Mars has been a planet largely in stasis, a geological museum preserving the evidence of its more dynamic past.
The defining characteristic of the Amazonian is the overwhelming influence of ice. With surface water long gone, ice became the primary agent of geological change. The modern polar caps, brilliant white swirls of water and carbon dioxide ice, were built up during this time. These caps are not static; they are composed of thousands of distinct layers of ice and dust, known as the polar layered deposits. These layers record a history of cyclical climate change, driven by long-term variations in the planet’s tilt and orbit that alter the pattern of solar heating. By studying these layers, scientists can read the climate history of Mars over millions of years, much like reading tree rings or ice cores on Earth.
Beyond the poles, ice has sculpted the landscape in more subtle ways. Glacial deposits are found on the flanks of the high volcanoes. A thin, ice-rich veneer of material covers the ground at high latitudes. In the mid-latitudes, between 30 and 55 degrees, a variety of strange landforms – lobate debris aprons, lineated valley fill, concentric crater fill – are all interpreted as evidence for the slow creep and flow of buried glaciers and ice sheets. Radar data has confirmed that vast quantities of water ice remain hidden just beneath the surface in these regions. Even the small, fresh-looking gullies carved into steep crater walls and slopes, which hint at recent water flow, are thought to have formed late in the Amazonian, likely from the melting of seasonal snowpacks during periods when Mars’s axial tilt was higher than it is today. The Amazonian is the story of a world in a deep freeze, where the slow, patient work of ice has replaced the fire and flood of its youth.
Architects of a Landscape: The Great Features of Mars
The surface of Mars is dominated by three features of such colossal scale that they have no true parallel on Earth: the Tharsis Bulge, Olympus Mons, and Valles Marineris. These are not separate, unrelated wonders. They are deeply interconnected, a family of geological titans born from the same underlying planetary condition: Mars’s lack of plate tectonics. On Earth, the crust is broken into plates that drift over the hot mantle. When a plume of hot magma, a “hotspot,” rises from the mantle, the crustal plate moves over it, creating a chain of volcanoes like the Hawaiian Islands. On Mars, the crust is a single, static shell. This simple difference allowed for the creation of features on a scale that dwarfs anything on our own world.
The Tharsis Bulge is a vast volcanic plateau in the planet’s western hemisphere, covering a quarter of its surface. It is an immense dome of rock and lava, thousands of kilometers across and rising an average of 7 to 10 kilometers above the surrounding plains. Its formation began in the Noachian and continued through the Hesperian, a slow-motion construction project that lasted over a billion years. It was built up in stages, first through a combination of volcanic eruptions and isostatic uplift, and then by such a continuous and massive outpouring of lava that the crust beneath it began to sag under the sheer weight.
Perched on the northwestern flank of this bulge is Olympus Mons, the largest volcano in the solar system. It is a shield volcano, like those in Hawaii, built up layer by layer from countless eruptions of highly fluid basaltic lava. Because Mars’s crust did not move, the volcano remained parked over the same mantle hotspot for hundreds of millions of years, allowing it to grow to its current, staggering proportions. It stands nearly 22 kilometers (13.6 miles) high, almost three times the height of Mount Everest, and its base would cover the entire state of Arizona. Its summit is capped by a complex caldera 80 kilometers across. Olympus Mons is the ultimate expression of stationary hotspot volcanism.
The formation of Valles Marineris, the grandest canyon in the solar system, is a direct consequence of the formation of Tharsis. As the Tharsis Bulge swelled with magma and piled on trillions of tons of lava, it placed unimaginable stress on the surrounding crust. The crust stretched, strained, and ultimately failed. It cracked open, forming a series of enormous rift valleys. Valles Marineris is not a canyon carved by a river, like Earth’s Grand Canyon; it is a tectonic “crack” in the planet’s crust, born from the immense weight of its volcanic neighbor. This initial rift system, which stretches for 4,000 kilometers – nearly a quarter of the planet’s circumference – was then widened and deepened over eons by other processes. Massive landslides, some of the largest in the solar system, collapsed its walls. Erosion, perhaps aided by the catastrophic outflow floods of the Hesperian, further sculpted its form. But its origin lies in the tectonic stresses induced by Tharsis. Together, these three features tell a single, coherent story of a planet whose geology is defined by its stillness.
The Great Escape: The Disappearance of an Atmosphere and Ocean
The central mystery of Mars’s history is its dramatic climate change. How did a planet with rivers, lakes, and perhaps an ocean become a frozen desert? The answer lies in the loss of its atmosphere, a process that began early in the planet’s history and was sealed by the death of its magnetic field.
Like Earth, early Mars had a global magnetic field generated by a dynamo of molten iron convecting in its core. This magnetic field acted as a protective shield, deflecting the solar wind – a constant stream of charged particles flowing from the Sun. But Mars is only about half the size of Earth, and smaller bodies cool faster. Sometime around 4.1 billion years ago, as the planet’s interior cooled and solidified, the core dynamo shut down. The global magnetic field vanished.
Without this magnetic shield, the atmosphere was left vulnerable. The high-energy particles of the solar wind began to slam directly into the upper atmosphere, knocking atmospheric gas molecules away one by one and stripping them into space. This process, known as sputtering, was relentless. It was also far more intense in the distant past, when the young Sun was more active and produced a much stronger solar wind and higher levels of extreme ultraviolet radiation.
This long-held theory has been confirmed and quantified by NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) mission, which has been orbiting Mars since 2014. MAVEN has measured the current rate of atmospheric loss, finding that Mars loses about 2 to 3 kilograms of gas per second, a rate that spikes dramatically during solar storms. By measuring the ratio of two isotopes of the noble gas argon – a heavy gas that can only be lost through sputtering – MAVEN’s science team was able to calculate how much argon has been lost over the planet’s history. The result: at least 65% of all the argon Mars ever possessed has been stripped away into space. Since carbon dioxide, the main component of the atmosphere, is lighter than argon, it would have been stripped away even more easily. This confirms that atmospheric sputtering was the dominant process responsible for transforming Mars’s climate.
The story has an ironic twist. While the loss of the global magnetic field was the trigger, the “ghost” of that field, preserved in patches of magnetized rock in the ancient crust, may have actually accelerated the process. Instead of providing a weak shield, these localized magnetic fields can be bent and funneled by the solar wind, creating pathways that actively channel atmospheric ions out into space, potentially at a higher rate than from a completely unmagnetized planet.
As the atmosphere thinned, the pressure at the surface dropped. Liquid water became unstable; it either boiled away in the low pressure or froze as the greenhouse effect weakened and temperatures plummeted. The water vapor that entered the atmosphere was broken down by solar ultraviolet radiation into hydrogen and oxygen. The light hydrogen atoms easily escaped into space, forever lost to the planet. The oxygen either escaped as well or reacted with surface rocks, contributing to Mars’s characteristic red color. The great Noachian ocean and the rivers that fed it were gone, their water either lost to space or locked away in the planet’s vast reservoirs of subsurface ice and the polar caps, where they remain to this day.
An Eye on the Red Star: Mars in Human History
Long before Mars was known as a planet, it was a presence in the human sky. Its distinct reddish hue and its peculiar, looping motion against the backdrop of fixed stars set it apart. For millennia, humanity watched this “wandering star,” first with mythic reverence and later with scientific curiosity. This second history of Mars – the story of our observation of it – is a journey that reflects our own technological and intellectual evolution, from naked-eye sky-watchers to builders of interplanetary probes.
The Wandering Star of Antiquity
Mars has been a part of human culture since before the beginning of recorded history. Ancient Egyptian astronomers, more than 4,000 years ago, charted its movements, noting its apparent retrograde motion – the illusion that it temporarily reverses its course in the sky as Earth overtakes it in its orbit. They called it “Her Desher,” the red one. In ancient India, it was seen as a fire in the sky.
The astronomers of the Neo-Babylonian Empire made systematic, quantitative observations. They calculated that Mars completes 37 synodic periods (returns to the same position relative to the Sun and Earth) every 79 years, a remarkably accurate measurement. They developed arithmetic methods to predict its position, associating the planet with Nergal, their god of war and pestilence. This association of the red planet with conflict was a common thread across cultures. The Greeks, inheriting knowledge from Mesopotamia, linked it to their god of war, Ares. When the Romans adopted and adapted the Greek pantheon, they named the planet for their own war god, Mars. The name has stuck, and in Roman mythology, Mars was also the divine father of the city’s legendary founders, Romulus and Remus.
Elsewhere, other civilizations were conducting their own sophisticated observations. Chinese records of Mars date back to the Zhou dynasty, before 1045 BCE. In the Americas, the Maya civilization meticulously tracked the planet’s cycles, as recorded in the Dresden Codex. They understood not only its 780-day synodic period but also its more complex sidereal motion against the stars, using this knowledge to align their intricate calendars with the cosmos. For these ancient cultures, Mars was not just a point of light; it was a celestial actor, a timekeeper, and a deity whose movements held meaning.
A World Revealed: The First Telescopic Views
The invention of the telescope in the early 17th century transformed Mars from a mythological symbol into a physical place. In 1609, Galileo Galilei turned his new instrument toward the heavens and was the first human to see Mars as a small, ruddy disk. It was no longer just a wandering star; it was a world.
Over the next two centuries, as telescopes improved, the features of this new world slowly came into focus. In 1659, the Dutch astronomer Christiaan Huygens sketched the first recognizable surface feature, a large, dark, triangular patch now known as Syrtis Major. By tracking its movement across the disk from night to night, he made a remarkable discovery: Mars rotates on its axis, and its day is approximately 24 hours long, almost the same as Earth’s. This discovery powerfully reinforced the idea of Mars as a sister planet.
In 1666, Giovanni Domenico Cassini observed bright white patches at the planet’s poles. More than a century later, in the 1780s, William Herschel confirmed that these were polar caps. He watched them grow and shrink with the Martian seasons and correctly deduced that Mars must have an axial tilt similar to Earth’s, which gave it a similar seasonal cycle, albeit one where each season lasted twice as long. The analogy to Earth seemed almost perfect. By the 19th century, astronomers were creating the first full maps of the planet. They labeled the dark areas maria (seas) and the bright, reddish areas continua (continents), assuming they were seeing oceans and landmasses, just like on Earth.
This growing body of evidence – a day of similar length, seasons, polar ice caps, and what appeared to be oceans and continents – all pointed to one conclusion: Mars was an Earth-like world. And if it was like Earth, perhaps it also harbored life. This idea, which had been a subject of philosophical speculation for centuries, now seemed to have a firm observational basis.
The Grand Illusion: Schiaparelli, Lowell, and the Martian Canals
The perception of Mars as a potential abode of life reached its zenith – and its most spectacular error – in the late 19th century. The story begins in 1877, when Mars made a particularly close approach to Earth. An Italian astronomer named Giovanni Schiaparelli, using a powerful new telescope in Milan, meticulously observed the planet and produced a detailed map. On it, he depicted a network of fine, linear features crisscrossing the bright “continents.” He called these features canali.
In Italian, canali means “channels,” a term that can describe natural features like the English Channel. However, in the English-speaking world, the word was widely and fatefully mistranslated as “canals,” a term that implies artificial construction. This simple linguistic slip ignited a global sensation. The idea that the lines on Mars were vast engineering works built by an intelligent civilization seized the public imagination.
The most fervent and influential champion of this idea was Percival Lowell, a wealthy Bostonian who became obsessed with the Martian canals. In 1894, he established a major observatory in Flagstaff, Arizona, for the express purpose of studying Mars. For the next two decades, Lowell dedicated his life to observing and mapping the planet. He produced intricate globes showing hundreds of canals, which he argued were part of a planet-wide irrigation system. In his view, Mars was a dying, desert world, and a wise, ancient civilization had built this colossal network to carry water from the melting polar caps to their thirsty cities in the equatorial regions.
Lowell was a brilliant popularizer, and he promoted his theories in a series of widely read books, including Mars (1895) and Mars as the Abode of Life (1908). His vision of a heroic, dying Martian race battling for survival captured the spirit of the age, an era of great engineering feats like the Suez and Panama Canals. To many, Lowell’s theory seemed a logical extension of terrestrial ambition and ingenuity.
However, from the beginning, the canals were deeply controversial within the scientific community. Many of the most experienced astronomers, using the world’s most powerful telescopes, simply could not see them. They saw a mottled, patchy surface of irregular dark and light areas, but no fine, straight lines. The great observer Eugène Antoniadi, using the large refractor at the Meudon Observatory in Paris during the 1909 opposition, definitively stated that the canals did not exist. His detailed drawings showed a complex, natural-looking surface that bore no resemblance to Lowell’s geometric webs.
Other lines of scientific evidence also began to mount against Lowell’s vision. The naturalist Alfred Russel Wallace, co-discoverer of the theory of evolution, published a book in 1907 titled Is Mars Habitable? in which he used the principles of physics to argue that Mars was far too cold and its atmosphere far too thin to support liquid water or any form of complex life. Early spectroscopic studies of the Martian atmosphere failed to detect any sign of water vapor, a critical blow to the irrigation theory.
The canals of Mars were, in the end, a grand illusion. Later experiments demonstrated that the human brain has a natural tendency to perceive lines and patterns when viewing a complex, low-contrast image at the very limit of visibility. Observers were subconsciously connecting unrelated dots and smudges on the Martian surface into straight lines. Lowell, driven by his passionate belief in Martian life, was predisposed to see what he wanted to see. The controversy simmered for decades, but the scientific case against the canals was overwhelming. The final, definitive refutation would have to wait for the first spacecraft to visit the planet and show us what was really there. The story of the canals is a powerful cautionary tale in the history of science, a demonstration of how observation can be shaped by expectation, and how a compelling narrative can, for a time, overshadow a lack of concrete evidence. It also reveals how our perception of other worlds is often a reflection of our own. The Martians of the canal era, with their grand engineering and their struggle against a changing climate, looked remarkably like the humans of the industrial age who invented them.
Touching the Red Soil: The Robotic Exploration of Mars
The second half of the 20th century opened a new chapter in the history of Mars. The human eye, limited by Earth’s turbulent atmosphere and the vast distances of space, gave way to the robotic eye of the space probe. In a few short decades, our understanding of Mars would be transformed, not by slow, incremental gains, but by a series of revolutionary leaps. This new era has been a dramatic story of discovery, disappointment, and rediscovery, as each new mission has painted a more detailed and complex portrait of the Red Planet.
First Contact: The Mariner Missions and a New Mars
The age of robotic exploration began with NASA’s Mariner program. On July 15, 1965, the Mariner 4 spacecraft flew past Mars, capturing the first-ever close-up images of another planet. The 22 grainy, black-and-white photos it transmitted back to an astonished world were a significant shock. There were no canals, no cities, no signs of a dying civilization. Instead, the images revealed a barren, desolate surface, pockmarked with impact craters. Mars, it seemed, was not like Earth at all; it looked like the Moon. Data from the flyby also confirmed that the atmosphere was incredibly thin – less than one percent of Earth’s – and that the planet lacked a global magnetic field to shield it from cosmic radiation. In a single afternoon, the romantic Mars of Percival Lowell was obliterated, replaced by the image of a cold, dead world.
Flybys by Mariners 6 and 7 in 1969 reinforced this bleak picture, though they provided hints of a more complex geology, including a strange, jumbled region dubbed “chaotic terrain.” But the true turning point came with Mariner 9. Launched in 1971, it was not a flyby mission but an orbiter, designed to map the planet over many months. When it arrived at Mars, a new surprise awaited: the entire planet was engulfed in a massive, global dust storm. Mission controllers at the Jet Propulsion Laboratory could see nothing but a featureless, hazy globe with four mysterious dark spots poking through the dust near the equator.
This could have been a disaster, but the mission’s orbital nature allowed them to simply wait. As the dust began to settle over the next few months, a “New Mars” was unveiled. The four dark spots resolved into the summits of colossal shield volcanoes, dwarfing any on Earth. The largest, which coincided with a bright spot Schiaparelli had called Nix Olympica (“the Snows of Olympus”), was named Olympus Mons. As the air cleared further, an even more spectacular feature emerged: a gigantic canyon system stretching for thousands of kilometers, which was named Valles Marineris in honor of the mission.
Most importantly, Mariner 9’s cameras revealed unmistakable evidence of what Lowell had sought in vain: channels. Not the straight, artificial canals of his imagination, but sinuous, branching valleys that were clearly the dried-up beds of ancient rivers. For the first time, there was undeniable proof that liquid water had once flowed across the surface of Mars. The mission mapped 85% of the planet, photographed its two tiny moons, Phobos and Deimos, up close, and completely overturned the disappointing image from Mariner 4. Mars was not a dead, Moon-like world after all. It was a complex planet with a dynamic and mysterious past.
Life on Mars? The Viking Landers
The discoveries of Mariner 9 set the stage for the most ambitious Mars mission yet undertaken: the Viking program. In 1975, NASA launched two identical spacecraft, Viking 1 and Viking 2, each consisting of an orbiter and a lander. On July 20, 1976 – seven years to the day after humans first walked on the Moon – the Viking 1 lander touched down safely on the plains of Chryse Planitia. Its sister craft, Viking 2, landed weeks later in Utopia Planitia. Humanity had arrived on the surface of Mars.
The first images sent back were breathtaking. They showed a desolate, rock-strewn desert of red soil under a pale pink sky. The landers were sophisticated scientific stations, equipped with cameras, a weather station, a seismometer, and a robotic arm to scoop up soil samples. For more than six years, they sent back a continuous stream of data, providing our first long-term look at conditions on the Martian surface.
But the primary goal of the Viking mission was to answer the ultimate question: is there life on Mars? To this end, each lander carried a miniature, automated biological laboratory. A robotic arm would collect a soil sample and deliver it to three separate experiments designed to look for signs of metabolism. The results were both thrilling and confounding. One experiment, called the Labeled Release, came back positive. When a nutrient broth was added to the soil, a radioactive gas was released, just as would be expected if microorganisms were consuming the nutrients.
The excitement was short-lived. A different instrument, a gas chromatograph-mass spectrometer designed to identify organic molecules – the chemical building blocks of life as we know it – found nothing. The soil appeared to be completely devoid of organics. This contradiction was difficult to resolve. How could there be metabolism without any organic molecules? The scientific consensus gradually settled on a non-biological explanation. The positive result, most scientists concluded, was not caused by Martian microbes, but by the unusual, highly reactive chemistry of the Martian soil itself. Later missions, such as the Phoenix lander in 2008, discovered perchlorate salts in the soil, which are strong oxidants that could have produced the same chemical reaction seen by Viking. The question of life on Mars remains unanswered, but the Viking experiments provided a stark lesson: the search would be far more difficult than anyone had imagined.
While the landers conducted their search, the Viking orbiters were busy mapping the planet in unprecedented detail, imaging 97% of the surface. Their high-resolution photos expanded on the discoveries of Mariner 9, revealing vast outflow channels carved by catastrophic floods, intricate networks of valleys suggestive of ancient rainfall, and volcanoes and canyons in stunning clarity. Together, the Viking missions painted a comprehensive portrait of Mars: a planet that is cold, dry, and likely lifeless today, but which bears the unmistakable scars of a warmer, wetter, and more dynamic past.
A Fleet of Observers: The Modern Orbital Campaign
After the Viking missions, there was a long hiatus in Mars exploration. But beginning in the late 1990s, a new and sustained campaign began, with a fleet of sophisticated orbiters from multiple space agencies transforming our understanding of the planet from above.
The renaissance began with NASA’s Mars Global Surveyor (MGS), which arrived in 1997. Over its nine-year mission, MGS used a laser altimeter to create the first high-resolution 3D map of the entire planet. Its magnetometer discovered that while Mars lacks a global magnetic field, its ancient crust retains patches of strong, localized magnetism – a fossil record of its long-dead dynamo. Its camera, the Mars Orbiter Camera, imaged the surface with stunning clarity, discovering thousands of fresh-looking gullies on crater walls that suggested the possibility of recent liquid water seeps, and revealing a surprisingly dynamic planet with shifting sand dunes and seasonal changes in the polar caps.
In 2001, NASA’s Mars Odyssey arrived, and it is still operating today. Its most significant discovery came from its gamma-ray spectrometer, which mapped the distribution of hydrogen in the top meter of the Martian soil. The map revealed vast reservoirs of water ice buried just beneath the surface at high latitudes. The amount of ice detected was enormous, equivalent to the volume of Lake Superior. Odyssey’s thermal emission imaging system (THEMIS) has also created a global mineral map of the planet and continues to serve as the primary communications relay for rovers on the surface.
The European Space Agency’s Mars Express joined the fleet in 2003. It quickly confirmed that the south polar cap is a mixture of water ice and frozen carbon dioxide. Its instruments made the first tentative detections of methane in the Martian atmosphere – a tantalizing discovery, as methane can be produced by both geological activity and biological processes. Its subsurface sounding radar (MARSIS) has probed beneath the surface, finding evidence of what may be large bodies of liquid, briny water trapped beneath the thick ice of the south pole.
NASA’s Mars Reconnaissance Orbiter (MRO), which entered orbit in 2006, carries the most powerful telescopic camera ever sent to another planet. The High Resolution Imaging Science Experiment (HiRISE) can resolve features on the surface as small as a coffee table. MRO has provided a new, dynamic view of Mars as a world that is still active today. It has captured images of massive avalanches collapsing down polar cliffs, documented the formation of new impact craters, and tracked seasonal changes in detail. It has closely monitored dark, narrow streaks called Recurring Slope Lineae (RSL) that appear on warm slopes in the summer. Initially thought to be flows of briny water, they are now believed to be flows of dry sand and dust. MRO’s spectrometer has mapped the distribution of a wide variety of minerals formed in water, revealing that ancient Mars hosted diverse aqueous environments – salty seas, neutral-pH lakes, and hydrothermal systems – some of which would have been far more friendly to life than others.
The most recent addition to the orbital fleet is MAVEN, which arrived in 2014. Its mission is to understand how Mars lost its atmosphere. By studying the interaction between the solar wind and the planet’s upper atmosphere, MAVEN has confirmed that solar wind sputtering is the primary mechanism of atmospheric loss and has measured how this process is accelerated by solar storms, providing the final pieces to the puzzle of Mars’s great climate transition.
The Geologists on Wheels: A Generation of Rovers
While orbiters provide the global context, the detailed, ground-truth story of Mars has been uncovered by a series of robotic rovers that have acted as our remote geologists on the surface.
The era of rovers began with a small, microwave-sized vehicle named Sojourner, which landed with the Mars Pathfinder mission in 1997. Primarily a technology demonstration, Pathfinder proved the viability of a novel airbag landing system, and Sojourner became the first wheeled vehicle to explore another planet. For 83 days, it roamed the area around the lander in Ares Vallis, analyzing the chemistry of rocks and soil with its Alpha Proton X-ray Spectrometer. Its findings confirmed that the rocks were volcanic in nature and that the landing site was, as suspected, the floodplain of a massive, ancient flood.
The true geological exploration began with the arrival of the twin Mars Exploration Rovers, Spirit and Opportunity, in January 2004. These golf-cart-sized rovers were sent with a clear directive: “follow the water.” Their mission was to find definitive, on-the-ground evidence of past aqueous environments.
Opportunity landed in Meridiani Planum, a region chosen because orbiters had detected the mineral hematite, which often forms in water. The rover scored a scientific “hole-in-one.” Within the small crater where it landed, it found layered sedimentary rocks and an abundance of small, hematite-rich spherules that the science team nicknamed “blueberries.” Analysis proved that these rocks had formed in a standing body of salty, acidic water. Over a mission that lasted an astonishing 14 years, Opportunity explored crater after crater, building an overwhelming case for a watery past. Near the end of its journey, it discovered clay minerals that had formed in neutral-pH water, an environment far more conducive to life.
Spirit landed on the other side of the planet in Gusev Crater, which appeared from orbit to be a former lakebed. To the science team’s initial disappointment, the crater floor was covered by a thick layer of volcanic rock, obscuring any ancient lake deposits. Undeterred, Spirit undertook a long trek to a nearby range of hills, the Columbia Hills. There, it found a treasure trove of geological diversity. It discovered rocks that had been extensively altered by water, and in a patch of soil churned up by a malfunctioning wheel, it made its most important discovery: a deposit of nearly pure silica. On Earth, such deposits form in hot springs or steam vents – environments known to be hotspots for microbial life. Spirit had found evidence of a past habitable environment.
Building on the success of Spirit and Opportunity, NASA sent a far more capable rover in 2012: the car-sized Mars Science Laboratory, Curiosity. Its mission was not just to find evidence of water, but to determine if Mars ever possessed environments that were truly habitable – that is, containing all the necessary ingredients for life. Landing in Gale Crater, Curiosity soon drilled into a fine-grained mudstone at a location named Yellowknife Bay. The analysis of this rock powder in its onboard chemistry lab was a landmark moment. It revealed the presence of sulfur, nitrogen, hydrogen, oxygen, phosphorus, and carbon – the key chemical building blocks for life. It also showed that the rock had formed at the bottom of an ancient, freshwater lake with a neutral pH, a place where microbes could have thrived. Curiosity had achieved its primary goal: it had found definitive proof of a past habitable environment on Mars. It has since been slowly ascending the central peak of Gale Crater, Mount Sharp, reading the layers of rock like the chapters of a book to understand how this life-friendly environment eventually dried out and transformed into the Mars of today.
The most recent rover, Perseverance, landed in Jezero Crater in 2021. Its mission represents the next logical step in the scientific progression. With habitability established, Perseverance’s primary goal is astrobiology: to actively search for signs of past microbial life, or biosignatures. Jezero Crater was chosen because it contains a beautifully preserved river delta, a place where sediments – and any potential evidence of life – would have been deposited in an ancient lake. Perseverance is equipped with a suite of advanced instruments designed to detect organic molecules and analyze mineral textures at a microscopic scale. But its most important job is to be the first leg of a historic relay race. It is drilling and collecting the most scientifically compelling rock and soil samples, sealing them in pristine tubes, and caching them on the surface for a future mission to retrieve and return to Earth. Perseverance has already discovered a variety of organic molecules and minerals altered by water in the crater floor, setting the stage for the ultimate analysis of Martian materials in laboratories back on Earth.
The history of robotic exploration has been a systematic, multi-decade investigation. It has progressed from initial reconnaissance to asking the big question about life, then to a more fundamental search for water, to a detailed assessment of habitability, and now, finally, to a focused search for the signs of life itself. Each mission has built upon the discoveries of its predecessors, transforming Mars in our understanding from a static, Moon-like object into a complex, dynamic world with a history that may, one day, intersect with the history of life.
Mars in the Human Imagination
As our scientific understanding of Mars has evolved, so too has its place in our culture. The Red Planet has served as a powerful and versatile muse, a blank canvas onto which we have projected our hopes, our fears, and our dreams. The history of Mars in fiction and film is a mirror, reflecting our changing knowledge of the planet and the anxieties of our own world.
From Wells to Weir: Mars in Science Fiction
The first great wave of Martian fiction was a direct product of the canal controversy. H.G. Wells’s seminal 1898 novel, The War of the Worlds, brought Percival Lowell’s theories to their terrifying logical conclusion. Wells imagined the Martians as a technologically advanced but desperate race, their own world dying of thirst, who looked upon a lush, water-rich Earth with envious eyes. His depiction of a ruthless invasion, with humanity cast as the helpless natives facing a superior colonial power, was a brilliant inversion of the British imperial mindset of the time and established the “alien invasion” trope that has endured for over a century.
In a very different vein, Edgar Rice Burroughs saw Lowell’s dying Mars not as a source of horror, but as a backdrop for high adventure. His A Princess of Mars (1912) and its sequels transported the hero John Carter to “Barsoom,” a fantasy version of Mars populated by multi-limbed green warriors, beautiful red-skinned princesses, and bizarre alien beasts. Burroughs’s Mars was a world of planetary romance, a pulp fiction stage for sword fights and heroic deeds that influenced generations of science fiction and fantasy writers.
By the mid-20th century, as the canal theory faded but before the first spacecraft had returned data, Mars became a destination for stories about human exploration and colonization. The most iconic work of this era is Ray Bradbury’s The Martian Chronicles (1950). Less a novel than a collection of poetic, interconnected stories, Bradbury’s book uses the settlement of Mars as a mirror to reflect on quintessentially human themes: loneliness, nostalgia, racism, environmental destruction, and the fear of nuclear annihilation. His Mars is a dreamscape, a place of telepathic, ghost-like Martians and human settlers who try to recreate their small Ohio towns on an alien world, only to find themselves changed by it. Other authors of the “Golden Age” of science fiction, like Arthur C. Clarke (The Sands of Mars, 1951) and Robert Heinlein (Red Planet, 1949), took a more grounded approach, beginning to explore the practical challenges of establishing a human presence on the Red Planet.
The stark, cratered images returned by Mariner 4 in 1965 had a “chilling effect” on Martian fiction. The revelation of a cold, airless, and seemingly lifeless world made stories of ancient civilizations and breathable atmospheres obsolete. The focus of Martian literature shifted from encounters with aliens to the grim struggle for human survival against a hostile environment.
This trend toward realism has culminated in a modern renaissance of Martian fiction, fueled by the wealth of new data from the modern fleet of orbiters and rovers. The undisputed masterpiece of this new era is Kim Stanley Robinson’s epic Mars Trilogy (Red Mars, Green Mars, Blue Mars), published in the 1990s. This series is a monumental, scientifically rigorous saga that chronicles the colonization and terraforming of Mars over two centuries, exploring in painstaking detail the science, politics, sociology, and philosophy of creating a new human society on another world. More recently, Andy Weir’s The Martian (2011) took this realism to its extreme, presenting a gripping survival story grounded in meticulously researched, real-world science and engineering. The protagonist, stranded astronaut Mark Watney, survives not by fighting monsters, but by solving a series of complex technical problems using the knowledge and technology we have today. The evolution of Mars in literature is a clear barometer of our scientific progress; as our image of the planet has come into sharper focus, so too have the stories we tell about it.
Mars on Screen: From Monsters to Missions
The portrayal of Mars in film and television has followed a similar trajectory, evolving from fanciful B-movie fodder to scientifically grounded drama. Early films, like the Soviet silent classic Aelita: Queen of Mars (1924), were products of their time, blending science fiction with political allegory. The 1950s brought a wave of Cold War-era creature features, with films like The Angry Red Planet (1959) and Invaders from Mars (1953) depicting the planet as a source of monstrous threats.
As the space age dawned, depictions began to incorporate more realistic elements. Robinson Crusoe on Mars(1964) was a classic survival story, updating the familiar tale for a new frontier. But it was the post-Viking era that cemented Mars’s cinematic identity as a perilous, but achievable, destination for human exploration. Films of the late 20th and early 21st centuries, such as Total Recall (1990), Mission to Mars (2000), and Red Planet(2000), focused on the dangers of colonization, from corporate malfeasance to technological failure and the unforgiving nature of the Martian environment itself.
In recent years, mirroring the trend in literature, there has been a strong push toward greater scientific authenticity. The 2015 film adaptation of The Martian, directed by Ridley Scott, was widely praised for its commitment to scientific accuracy, working closely with NASA to realistically portray the challenges of a human mission to Mars. Similarly, the National Geographic docudrama series MARS (2016) blended a fictional narrative of the first human colony with documentary-style interviews with real-world scientists and space exploration advocates like Elon Musk. These modern productions treat Mars not as a fantasy backdrop, but as a real place, a destination that is within our technological reach. They seek to inspire and educate as much as to entertain, seducing us with a vision of a future on Mars that is no longer just science fiction.
The Next Chapter: The Future of Mars Exploration
After more than six decades of robotic exploration, our journey to understand Mars is approaching a pivotal moment. We have mapped its surface, plumbed its interior, analyzed its atmosphere, and confirmed its past habitability. The next logical step, and the highest priority for the international planetary science community, is to bring pieces of Mars back to Earth.
Bringing Mars to Earth: The Sample Return Campaign
The Mars Sample Return (MSR) campaign is an ambitious, multi-mission endeavor planned jointly by NASA and the European Space Agency (ESA). Its goal is to retrieve the rock and soil samples currently being collected by the Perseverance rover and return them to Earth for intensive study.
The rationale for sample return is compelling. While rovers like Curiosity and Perseverance are marvels of engineering, the scientific instruments they can carry are limited by mass, power, and complexity. The most powerful and sophisticated analytical tools exist only in laboratories on Earth. Bringing samples back would allow scientists from around the world to scrutinize them using techniques far beyond the capabilities of any rover, from precise radiometric dating to determine the absolute age of the rocks, to searching for complex organic molecules and microscopic fossils that could be definitive proof of past life. Furthermore, the samples could be archived and studied by future generations of scientists using technologies that have not yet been invented, as has been done with the Apollo Moon rocks for over 50 years. Sample return represents a fundamental shift in planetary science, from remote observation to direct, hands-on laboratory investigation.
The proposed architecture for MSR is a complex, multi-stage relay race. The first leg is already underway, with the Perseverance rover drilling rock cores and sealing them in titanium tubes in Jezero Crater. The next stage would involve a Sample Retrieval Lander touching down nearby. This lander would deploy either a small “fetch rover” or a pair of helicopters to retrieve the sample tubes. A robotic arm would then transfer the tubes into a basketball-sized container aboard a small rocket called the Mars Ascent Vehicle (MAV). The MAV would launch from the lander, becoming the first rocket ever to lift off from another planet, and place the sample container into orbit around Mars. The final leg of the journey would be performed by an Earth Return Orbiter, built by ESA, which would rendezvous with and capture the sample container in Mars orbit, then fire its engines for the long flight back to Earth.
As of the mid-2020s, the Mars Sample Return campaign faces significant challenges. Independent reviews have highlighted soaring cost estimates and an extended timeline, prompting NASA to re-evaluate the original plan and solicit new, more innovative and affordable concepts from industry and its own research centers. While the specific architecture may change, the scientific goal remains paramount. The commitment to bringing the first pristine samples of Mars to Earth is a cornerstone of the future of solar system exploration.
New Explorers and the Human Horizon
While sample return is the flagship effort, other robotic missions are also on the horizon. ESA’s ExoMars rover, named Rosalind Franklin, is slated for launch in 2028. A key European-led mission with significant NASA contributions, its primary objective is to search for signs of life. Its most important instrument is a drill capable of extracting samples from up to two meters beneath the surface. This is a critical capability, as the harsh radiation environment on the Martian surface would have destroyed any complex organic molecules over time. By drilling into the subsurface, the rover will access material that has been shielded for billions of years, offering a much better chance of finding preserved biosignatures.
Other missions are also in development. NASA’s twin ESCAPADE orbiters, set to launch in 2024, will work in tandem to create a 3D map of Mars’s magnetosphere and study atmospheric loss in greater detail. China is also planning its own ambitious sample-return mission, Tianwen-3, for the end of the decade.
All of these robotic missions are essential precursors for the long-term goal of sending human explorers to Mars. They are our scouts, mapping the terrain, identifying resources like water ice, assessing potential hazards such as radiation and toxic dust, and helping to select safe and scientifically interesting landing sites. The data they collect on weather, atmospheric density, and soil properties are all critical for designing the systems that will one day land astronauts on the Red Planet and sustain them there. The journey to Mars will be the most challenging undertaking in the history of exploration, but it is a journey that has already begun, carried forward by our tireless robotic emissaries.
Summary
The history of Mars is a tale of two worlds. The first is the planet itself, born in fire and chaos 4.5 billion years ago. It was a world that, for a time, was much like our own – with a thick atmosphere, a protective magnetic field, and a surface awash in liquid water, hosting rivers, lakes, and perhaps even a vast northern ocean. It was a world that possessed all the necessary ingredients for life. But this vibrant, habitable Mars was not destined to last. As its small core cooled and its magnetic field died, the solar wind relentlessly stripped away its atmosphere, and its water was lost to space or frozen into the crust. The planet underwent a significant transformation, from a warm, wet world to the cold, arid, and seemingly sterile desert we see today.
The second world is the Mars of the human imagination, a history of our own evolving perception. It began as a god of war, a baleful red light in the night sky. With the advent of the telescope, it became a sister planet, an object of scientific inquiry that helped us discover the laws of the cosmos. It then became a canvas for our fantasies, a world of heroic engineers and dying civilizations. The dawn of the space age shattered these illusions, revealing a cratered, Moon-like wasteland, only for that image to be replaced, in turn, by the complex and dynamic planet we know today – a world with a rich, watery past and a compelling story to tell.
Our journey to understand Mars has been a reflection of the scientific process itself: a cycle of theory, observation, surprise, and revision. We have moved from speculation to certainty, from flybys to orbiters, from landers to sophisticated roving laboratories. And yet, for all we have learned, the most significant questions remain. We know that Mars was once habitable, but was it ever inhabited? Did life arise on this second genesis in our solar system, only to be extinguished by a changing climate? Or does it, perhaps, persist in some hidden niche beneath the frozen surface?
The answers may be waiting for us, locked inside the rocks of Jezero Crater. The history of Mars is not yet fully written. We are now a part of its story, and the next chapter – one that may involve bringing pieces of that world to our own, and eventually, sending pieces of our world to it – is about to begin.