Mars: A Portrait of the Red Planet

An Introduction to the Red Planet

Mars hangs in the night sky as a distinct, reddish point of light, a feature that has captured human imagination for millennia. Ancient civilizations, noting its blood-like hue, named it for their gods of war; the Egyptians called it “Her Desher,” the red one, while the Romans gave it the name that endures today. This striking color comes from the vast plains of iron-rich dust that cover its surface, a world of rock and sand that has slowly oxidized, or rusted, over eons.

As the fourth planet from the Sun, Mars is a small, rocky world, about half the size of Earth. It is a planet of profound contrasts. From a distance, it can appear as a cold, static desert. Early robotic flybys in the 1960s reinforced this view, sending back images of a bleak, cratered surface that seemed disappointingly lifeless. Yet decades of continued exploration have peeled back this simple facade to reveal a world of surprising complexity and dynamism. Mars has seasons, shifting polar ice caps, an atmosphere with clouds and powerful winds, and a landscape scarred by features of an almost unimaginable scale, including the largest volcanoes and deepest canyons in the solar system.

The story of Mars is inseparable from the story of our efforts to understand it. Our perception of the planet has shifted dramatically, from a fantasized world of canals and vegetation to a cratered wasteland, and now to what it is today: a primary target in the search for life beyond Earth. While its surface is now cold and its air too thin to breathe, evidence gathered by a fleet of robotic explorers points to a distant past when Mars was a much warmer, wetter place, with a thick atmosphere and conditions that may have been suitable for life to emerge. It is this tantalizing possibility of a lost, habitable world that drives our enduring fascination and relentless exploration of our planetary neighbor.

A World in Motion: Orbit, Time, and Seasons

The fundamental rhythms of Mars are both strangely familiar and distinctly alien. A Martian day, called a “sol,” lasts 24.6 hours, just slightly longer than a day on Earth, giving the two planets a comparable daily cycle. A year on Mars, however, is a much longer affair. Because it is farther from the Sun, Mars takes 687 Earth days to complete one full orbit, making its year nearly twice as long as our own.

Like Earth, Mars has seasons. This is because its axis of rotation is tilted at about 25 degrees relative to its orbital plane, very close to Earth’s 23.4-degree tilt. This axial tilt means that for parts of the year, one hemisphere receives more direct sunlight while the other receives less, creating summer and winter, with spring and autumn as transitional periods.

This simple similarity is complicated by another key feature of Mars’s journey around the Sun: its orbit is significantly more elliptical, or oval-shaped, than Earth’s. Throughout its long year, Mars’s distance from the Sun varies dramatically, from a closest point (perihelion) of about 206 million kilometers to a farthest point (aphelion) of 249 million kilometers. This varying distance has a profound effect on its climate.

The combination of axial tilt and an elliptical orbit creates seasons of unequal length and intensity. When Mars is closer to the Sun, it travels faster in its orbit; when it is farther away, it moves more slowly. For the southern hemisphere, summer occurs when Mars is near its closest approach to the Sun. This makes southern summers intensely hot but relatively short. Conversely, southern winter happens when Mars is at its farthest point, resulting in a season that is exceptionally long and cold. This asymmetry is quite pronounced: in the northern hemisphere, for example, spring is the longest season at 194 sols, while autumn is the shortest, lasting only 142 sols.

This orbital arrangement is more than a celestial curiosity; it is the primary engine of Mars’s modern climate system. The intense solar heating the southern hemisphere receives during its short, hot summer is the main trigger for the planet’s most extreme weather events. The largest and most violent dust storms, which can grow to envelop the entire planet, almost always begin in the southern hemisphere during this period of maximum heat. The planet’s path through space directly dictates the timing and severity of the tempests that scour its surface.

The Martian Landscape: A Tour of Extremes

The surface of Mars is a gallery of geological superlatives. Its features are sculpted on a scale that dwarfs their terrestrial counterparts, a testament to a planetary history governed by different rules. The low gravity and a crust that has remained largely unbroken for billions of years have allowed monumental structures to grow, creating a landscape of breathtaking extremes.

Giants of Tharsis

Near the Martian equator lies the Tharsis Montes region, a vast volcanic plateau crowned by some of the most colossal mountains in the solar system. Towering above them all is Olympus Mons, a shield volcano of truly epic proportions. It stands approximately 25 kilometers (16 miles) high, nearly three times the height of Earth’s Mount Everest, with a base so wide it would cover an area roughly the size of the state of Arizona. If you were to stand at its edge, its slopes would be so gentle that the summit would lie beyond the horizon, hidden by the curvature of the planet itself.

Olympus Mons is a shield volcano, meaning it was not formed by violent, explosive eruptions but was built up over immense spans of time by countless flows of slow-moving lava that spread out to form its broad, shield-like shape. At its peak is a complex of collapsed craters, known as a caldera, that stretches 85 kilometers across—a chasm wide enough to swallow a large city. While the volcano has been dormant for a very long time, some of its lava flows appear to be only a few million years old, a geologically recent period. This suggests that Olympus Mons may not be entirely extinct, but could simply be a sleeping giant, biding its time until a future eruption.

The sheer size of Olympus Mons and its neighbors in the Tharsis region is a direct result of Mars’s unique geological conditions. First, the planet’s surface gravity is only about 38% of Earth’s, which allows mountains to grow much taller before they collapse under their own weight. Second, and more importantly, Mars lacks plate tectonics. On Earth, the crust is broken into moving plates. As a plate drifts over a stationary plume of magma, or “hotspot,” a chain of volcanoes is formed, like the Hawaiian Islands. No single volcano gets to grow for too long before the plate moves it away from the magma source. On Mars, the crust is a single, solid shell. A hotspot in the mantle below could thus remain fixed under the same spot for billions of years, erupting again and again, piling layer upon layer of lava to build a single, monstrous volcanic edifice. These giant mountains are not just large; they are geological fossils preserving a record of a planet that evolved without the constant churning and recycling of its crust that defines our own world.

The Great Scar of Valles Marineris

Stretching east of the Tharsis region is another feature of planetary scale: Valles Marineris, the grandest canyon system in the solar system. This is not a single canyon but a vast network of interconnected chasms that runs for over 4,000 kilometers (2,500 miles) along the Martian equator—a distance equivalent to the width of the United States. In places, the canyon is up to 600 kilometers wide and plunges to depths of 8 kilometers (4 miles). By comparison, Earth’s Grand Canyon is about 800 kilometers long and just 1.8 kilometers deep. Valles Marineris is so immense that it spans nearly a fifth of the entire circumference of Mars.

Unlike the Grand Canyon, Valles Marineris was not carved primarily by a river. Its origin is thought to be tectonic, a direct consequence of the same forces that built the Tharsis bulge next door. The leading hypothesis holds that billions of years ago, as the colossal mass of the Tharsis volcanoes welled up, it placed unimaginable stress on the surrounding crust. The crust buckled and tore apart, forming a gigantic rift valley, or crack, in the planet’s surface. Over subsequent eons, this initial fracture was widened and modified by erosion, landslides, and possibly some outflow of water from subsurface channels. But its fundamental nature is that of a planetary-scale scar. Valles Marineris and the Tharsis volcanoes are not separate wonders; they are two sides of the same coin, a testament to immense geological forces that shaped ancient Mars and have long since fallen silent.

The Frozen Poles

At its north and south poles, Mars possesses ice caps that, like Earth’s, grow and shrink with the seasons. These polar caps are complex, multi-layered structures that play an active role in the planet’s modern climate. Each cap consists of two main parts. The topmost layer is a seasonal cap, which is made of carbon dioxide from the atmosphere that freezes directly onto the surface during the frigid polar winter. This is essentially a planet-sized layer of dry ice. As spring arrives and temperatures rise, this CO2 ice does not melt; it sublimates, turning directly from a solid into a gas and returning to the atmosphere. This process is so vast that it causes about a quarter of the entire Martian atmosphere to cycle between a gaseous state and a solid state at the poles each year, driving huge seasonal swings in global atmospheric pressure.

Beneath this fleeting seasonal cap lies a permanent, or residual, cap that persists through the summer. This permanent cap is composed primarily of water ice. The south polar ice cap, in particular, is immense. Radar data from orbiting spacecraft has shown that its layered deposits of ice and dust are up to 3.7 kilometers (2.3 miles) thick. The amount of water locked away in these southern deposits is staggering—enough to cover the entire surface of Mars in a liquid layer 11 meters (36 feet) deep if it were all to melt.

These deep layers of ice and dust, known as the polar layered deposits, are one of the most compelling features on Mars. They are built up over hundreds of thousands of years, with each layer representing a different climatic period, much like the rings of a tree or ice cores drilled in Antarctica and Greenland. By studying the thickness and composition of these layers, scientists hope to read the history of Mars’s climate, uncovering a detailed record of past ice ages and warming periods driven by long-term changes in the planet’s orbit and tilt. The polar caps are not just frozen reservoirs; they are active engines of the current climate and the keepers of a detailed historical archive, holding the secrets to Mars’s deep past.

A Thin Veil: Atmosphere and Weather

The atmosphere of Mars is a mere whisper of what Earth possesses. It is incredibly thin—more than 100 times less dense than our own—with a surface pressure less than 1% of what we experience at sea level. A human standing on the surface would not be able to breathe the air, which is composed almost entirely of carbon dioxide (about 95%), with minor amounts of nitrogen and argon. Instead of the familiar blue sky of Earth, the Martian sky is a hazy, butterscotch-colored expanse, its tint derived from the fine, rusty dust particles that are perpetually suspended in the air.

This tenuous atmosphere is unable to retain much heat. As a result, Mars is a world of extreme temperatures. The planet-wide average is a frigid -60°C (-80°F). However, temperatures can fluctuate wildly depending on location and time of day. Near the equator at noon in the summer, the surface can warm to a comfortable 20°C (70°F), while during the polar winter, temperatures can plummet to a stunning -125°C (-195°F).

Despite its thinness, the Martian atmosphere is far from static. It supports an active weather system, complete with winds, clouds of both water ice and carbon dioxide ice, and spiraling columns of dust known as dust devils, which can tower kilometers high. Most dramatically, Mars is home to the most ferocious dust storms in the solar system. While some are continent-sized regional events, every few years a local storm can explode in scale, growing into a planet-encircling event that shrouds the entire globe in a thick haze of dust. These global dust storms can rage for weeks or even months at a time.

The winds in these storms can reach speeds of around 100 kilometers per hour (60 mph), but because the air is so thin, they lack the raw force of hurricane-force winds on Earth. The primary danger they pose to robotic explorers is not from the wind itself, but from the dust. The fine, electrostatic particles get into mechanisms and cling to every surface, most critically to solar panels. By blocking out the sun, a global dust storm can starve a solar-powered rover of energy, as was the fate of the Opportunity rover in 2018.

These colossal storms are powered by an elegant and powerful feedback loop. The process starts when sunlight warms the Martian ground, which in turn heats the layer of air just above it. This warm air rises, carrying fine dust particles with it. This is where the feedback kicks in: the suspended dust particles are dark and absorb sunlight very efficiently, heating the atmosphere around them even more. This creates stronger updrafts and more powerful winds, which lift even more dust from the surface. This cycle can escalate with astonishing speed, allowing a storm to grow exponentially. The storm’s eventual demise is, paradoxically, a product of its own success. Once the dust shroud becomes so thick that it blankets the planet, it blocks sunlight from reaching the ground. This cools the surface, cutting off the heat source that powers the storm. The winds gradually die down, and over a period of months, the fine dust slowly settles out of the sky, returning Mars to its usual state of cold, clear tranquility.

The Story of Water on Mars

The narrative of water on Mars is a tale of two planets: a world that was once warm, wet, and potentially habitable, and the cold, dry desert we see today. The evidence for this dramatic transformation is etched across the Martian landscape and locked within its rocks, while the fate of its ancient water is a mystery whose solution is only now beginning to emerge from deep beneath the surface.

Echoes of an Ancient, Wetter World

For decades, scientists have been assembling a mountain of evidence that points to a bygone era, billions of years ago, when liquid water was abundant on the surface of Mars. Orbiting spacecraft have mapped vast, branching networks of valleys that look for all the world like ancient riverbeds, as well as enormous outflow channels that suggest catastrophic floods once scoured the landscape. Some of the most compelling images show fan-shaped deltas where rivers appear to have emptied into large, standing bodies of water, and even features that have been interpreted as the shorelines of ancient oceans.

While orbital images were suggestive, the “ground truth” delivered by robotic rovers has been decisive. The twin rovers Spirit and Opportunity, which landed in 2004, were sent as robotic geologists with the primary mission to “follow the water.” They succeeded beyond all expectations. Opportunity landed in a crater and almost immediately discovered small, iron-rich spheres nicknamed “blueberries.” On Earth, these mineral concretions, made of hematite, almost always form in the presence of water. The rover also found layered sedimentary rocks that appeared to have been laid down at the bottom of a shallow, salty sea. On the other side of the planet, the Spirit rover found evidence of an even more dynamic environment: deposits of pure silica and minerals that on Earth are characteristic of hot springs and volcanic steam vents—environments that are known to be teeming with microbial life.

The Curiosity rover, which arrived in 2012, took this investigation to the next level. Its mission was not just to find evidence of water, but to assess whether that water created a truly habitable environment. In Gale Crater, Curiosity found definitive proof that the crater once hosted a large, long-lived lake. By drilling into the ancient lakebed, it analyzed the rock chemistry and found not only the essential building blocks of life—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—but also determined that the water was fresh, not harshly acidic or salty. It was an environment that a microbe from Earth could have comfortably survived in. This progression of discovery, from seeing water-carved shapes from orbit to analyzing the chemistry of a habitable lakebed on the ground, has fundamentally reshaped our understanding of Mars. It has transformed the planet from a simple curiosity into a prime candidate in the search for a second genesis of life.

Water’s Hiding Places Today

The Mars of today is a stark contrast to its ancient, watery past. The planet’s thin atmosphere and frigid temperatures mean that any liquid water on the surface would almost instantly freeze or boil away into space. The vast majority of Mars’s known water is now locked away in solid form, either as water ice in the massive polar caps or as permafrost mixed into the soil just beneath the surface. There are some tantalizing hints of modern liquid water in the form of dark streaks that appear seasonally on some steep, warm slopes. These are thought to be flows of extremely salty water, or brines, whose high salt content allows them to remain liquid at temperatures below the normal freezing point of water.

For years, a major puzzle for planetary scientists was the “missing water” problem. Calculations based on the planet’s geology suggested that ancient Mars had far more water than could be accounted for in the polar caps and atmosphere today. The prevailing theory was that most of it had been lost to space over billions of years as the planet’s atmosphere thinned. However, recent discoveries have offered a dramatic new possibility: the water may not have all escaped, but instead, a large portion of it may have gone down.

Data from seismic instruments, like those on the InSight lander, have provided evidence for the existence of vast reservoirs of liquid water deep within the Martian crust. Analysis of seismic waves traveling through the planet suggests the presence of a thick layer of porous rock, saturated with liquid water like a sponge, located somewhere between 10 and 20 kilometers (about 6 to 12 miles) below the surface. The potential volume of this hidden aquifer is enormous, possibly containing enough water to flood the entire planet in an ocean hundreds of meters deep. This discovery could solve the mystery of the missing water, suggesting it was sequestered deep underground, protected from the harsh conditions that rendered the surface uninhabitable. This prospect is revolutionary for the search for life. While the surface of Mars is hostile, a deep, stable, liquid water environment, shielded from deadly radiation, could be the last and best refuge for any life that may have survived from the planet’s warmer past, or that might still exist today.

The Two Companions: Phobos and Deimos

Mars is not alone on its journey through the solar system. It is accompanied by two small, misshapen moons, Phobos and Deimos. Named for the mythological attendants of the Greek god of war, Ares, their names translate to “Fear” and “Dread,” respectively. These moons are nothing like Earth’s large, spherical companion. They are tiny, potato-shaped bodies, among the smallest moons in the solar system, and are thought to be captured asteroids that strayed too close to Mars and were trapped by its gravity. They are composed of dark, carbon-rich rock mixed with ice and are covered in craters, bearing the scars of billions of years of impacts.

Phobos, the larger of the two, orbits Mars in a breathtakingly close and rapid path. It skims just 6,000 kilometers (3,700 miles) above the Martian surface—no other moon in the solar system orbits so close to its planet—and completes a full circuit in less than eight hours, whipping around Mars three times every Martian day. Deimos, by contrast, is about half the size of Phobos and orbits much farther out, taking a more leisurely 30 hours to circle the planet.

The fate of Phobos is sealed. Its close orbit places it deep within the grip of Mars’s gravity, and tidal forces are causing it to slowly spiral inward. At a rate of about 1.8 meters (6 feet) per century, it is drawing ever closer to the Red Planet. In about 50 million years, this gravitational dance will reach its dramatic conclusion. Phobos will either crash into the Martian surface in a spectacular impact or, more likely, it will be torn apart by Mars’s tidal forces once it crosses a critical threshold known as the Roche limit. The resulting debris will spread out to form a narrow, dense ring around Mars, giving the planet a feature similar to, though much smaller than, the rings of Saturn. The existence of these two small moons provides a dynamic, real-time example of orbital mechanics in action, and the inevitable demise of Phobos serves as a reminder that even on a planetary scale, nothing is permanent.

The Robotic Vanguard: A History of Exploration

Our knowledge of Mars has been painstakingly assembled over more than half a century by an international fleet of robotic emissaries. Each mission has built upon the successes and failures of its predecessors, engaging in a systematic, multi-generational conversation with the Red Planet. This history reveals a clear, logical progression of scientific inquiry, evolving from simple reconnaissance to a sophisticated search for the signs of life.

The story of direct exploration began in the 1960s with a series of flyby missions, but the modern era was inaugurated in 1976 with the arrival of the twin Viking 1 and 2 landers. They were the first spacecraft to successfully touch down and operate on the Martian surface, and they returned the first stunning, panoramic images of its rock-strewn, reddish plains. The Vikings were equipped with a suite of instruments to analyze the thin atmosphere and volcanic soil, but their most ambitious objective was to conduct the first-ever experiments to search for life. The results were both tantalizing and deeply puzzling. One experiment, designed to detect microbial metabolism, returned a positive signal. Yet another, a gas chromatograph-mass spectrometer, found no trace of organic molecules—the carbon-based building blocks of life. This contradiction, the “Viking paradox,” led most scientists at the time to conclude that the positive result was due to some exotic, non-biological chemistry in the soil. Decades later, the discovery of compounds called perchlorates in the Martian soil offered a potential explanation: they could have destroyed any organic molecules during the heating process of the Viking experiment, suggesting the original result might have been a false negative. The Viking results remain a subject of debate, a scientific cliffhanger that highlighted the need for a more foundational understanding of Mars before tackling the ultimate question of life.

After a long hiatus, the next major step in surface exploration came in 1997 with the Mars Pathfinder mission and its small, microwave-sized rover, Sojourner. While its scientific instruments were limited, Sojourner’s successful demonstration of mobile exploration technology was a watershed moment, proving that rovers could effectively navigate and study the Martian terrain.

This paved the way for the much more ambitious Mars Exploration Rovers, Spirit and Opportunity, which landed in 2004. These twin, golf-cart-sized geologists were designed for a 90-day mission to “follow the water”. They exceeded this goal spectacularly. Spirit operated for over six years, and Opportunity for nearly fifteen, traveling dozens of kilometers across the Martian surface. They found a wealth of definitive evidence for a past wet environment, from the hematite “blueberries” and sulfate salts discovered by Opportunity to Spirit’s discovery of minerals formed in ancient hot springs. They answered the foundational question left hanging after Viking: Mars was, without a doubt, once a watery world.

With the presence of water confirmed, the next logical question was whether that water created a truly habitable environment. This was the task of the Curiosity rover, a car-sized, nuclear-powered mobile science laboratory that landed in Gale Crater in 2012. Curiosity’s mission was to assess past habitability. It succeeded brilliantly, discovering that Gale Crater once contained a long-lived freshwater lake with a neutral pH and all the key chemical elements necessary for life. It also made the first definitive detection of complex organic molecules preserved in ancient Martian mudstone, confirming that the building blocks of life were present on the planet.

This logical progression has culminated in the current Perseverance rover, which landed in 2020. Building on all the discoveries that came before, its primary mission is to take the final step: to actively search for direct signs of past microbial life, or biosignatures. It is exploring an ancient river delta in Jezero Crater, a location considered ideal for preserving such evidence. Critically, Perseverance is also the first stage of a Mars Sample Return campaign; it is drilling and caching rock cores that a future mission will collect and bring back to Earth for analysis in the most advanced laboratories our planet has to offer. Accompanying Perseverance was the Ingenuity helicopter, a small drone that proved powered, controlled flight was possible in Mars’s thin atmosphere, opening up a new dimension for future exploration. This half-century of exploration reveals a patient, deliberate scientific strategy, moving step-by-step from reconnaissance to habitability assessment and, finally, to the direct search for life and the return of physical evidence.

The Enduring Question of Life

The central question driving our exploration of Mars is whether life ever arose on its surface. The modern search for an answer is far more nuanced than simply looking for “Martians.” It is a forensic investigation into the planet’s past, guided by the concepts of habitability and biosignatures. Scientists are not focused on finding life thriving on the hostile surface today; the intense solar radiation, extreme cold, and thin, dry atmosphere make that highly unlikely. Instead, the search is concentrated on finding evidence of microbial life that may have existed billions of years ago, when Mars was a warmer and wetter world.

The search follows a clear logic based on life as we know it, which requires three key ingredients: liquid water, a source of energy, and organic compounds based on the element carbon. The work of rovers like Spirit, Opportunity, and Curiosity has largely confirmed that these conditions were met on ancient Mars. They found evidence for long-standing bodies of liquid water, from lakes to hot springs, and confirmed the presence of both energy sources and organic compounds.

With habitability established, the goal now is to find “biosignatures.” A biosignature is a feature, substance, or pattern whose origin specifically requires a biological explanation. This is a challenging task, as non-biological geological processes can sometimes mimic the signs of life. Potential biosignatures could include microscopic fossils, specific types of complex organic molecules that are unlikely to form without biology, or distinct patterns in the composition of rock layers that were altered by microbial activity.

The discovery by the Curiosity rover that Mars once had an active carbon cycle is a significant piece of this puzzle. On Earth, the carbon cycle is fundamental to regulating the climate and providing the raw material for life. Finding evidence of a similar, albeit different, cycle on Mars strengthens the case that the planet once operated in a way that could have supported a biosphere. This methodical approach—first establishing that the conditions for life were present, and now searching for the specific chemical fingerprints that life may have left behind in the rock record—is at the heart of the Perseverance rover’s mission. The ultimate goal of returning samples to Earth is a recognition that the final, definitive answer to this profound question may require the full analytical power of our terrestrial laboratories, capable of detecting the subtlest signs of a life that may have vanished billions of years ago.

Earth’s Neighbor: A Comparison

While Mars shares many characteristics with Earth, such as polar ice caps and seasons, it is a distinctly different world in terms of its physical properties. The following table provides a direct comparison of key metrics between the two planets, highlighting the scale of these differences.

This comparison underscores the fundamental realities of Mars. It is a smaller, lighter planet with significantly weaker gravity. While its day is remarkably similar to ours, its year is nearly twice as long. The most dramatic differences lie in its climate and atmosphere. Mars is substantially colder, and its air is a thin, unbreathable mix of carbon dioxide, in stark contrast to Earth’s thick, nitrogen-oxygen atmosphere that supports a temperate climate and abundant life.

The Human Horizon: Challenges of a Manned Mission

Sending humans to Mars represents one of the greatest technological and physiological undertakings in the history of exploration. The challenges extend far beyond simply building a powerful rocket. They encompass the perilous final moments of landing on the planet and the profound difficulties of keeping a crew safe and healthy during a mission that could last up to three years.

The “Six Minutes of Terror”: Entry, Descent, and Landing (EDL)

The process of getting from the top of the Martian atmosphere to a soft touchdown on the surface, known as Entry, Descent, and Landing (EDL), is arguably the single most difficult phase of any Mars mission. For robotic missions, this phase is often called the “six minutes of terror,” and the challenge escalates exponentially when considering the scale required for human exploration.

The core of the problem lies in the nature of Mars’s atmosphere. It is just thick enough to generate immense frictional heat as a spacecraft plummets through it at hypersonic speeds, necessitating a robust heat shield to prevent the vehicle from burning up. Yet, it is far too thin to slow a heavy spacecraft down sufficiently using parachutes alone. Current robotic landers like Curiosity and Perseverance weigh about one metric ton. A human mission would need to land payloads of 20 to 80 metric tons, including habitats, life support systems, surface vehicles, and the ascent rocket for the return journey to Earth. This leap in mass is well beyond the capabilities of any system ever flown.

Scaling up current technologies will not work. A human-scale lander will require a revolutionary approach, likely a complex, multi-stage sequence. This could involve an initial deceleration using a large aeroshell, followed by the deployment of massive supersonic parachutes or innovative inflatable aerodynamic decelerators to slow the craft further. The final approach and soft touchdown would have to be accomplished using a powerful set of retrorockets in a propulsive descent. Adding to the complexity, human missions will require pinpoint landing accuracy, needing to touch down within tens of meters of pre-positioned supplies or habitats, a feat made difficult by unpredictable winds and a variable atmosphere. Furthermore, the powerful rocket plumes of a large lander will interact with the surface, kicking up a cloud of dust and rocks that could damage the vehicle or obscure the landing sensors. Compounding all these issues is the near impossibility of conducting a full-scale test on Earth, as no terrestrial environment can simultaneously replicate Mars’s unique combination of atmospheric pressure, composition, and gravity. Solving the EDL problem is the critical gateway to placing humans on Mars.

The Five Hazards of Human Spaceflight

Once on the ground, and during the long journey there and back, astronauts would face a relentless assault from the space environment. NASA has identified five primary, interconnected hazards of human spaceflight that are particularly acute for a Mars mission.

1. Space Radiation: Away from the protection of Earth’s magnetic field, the crew will be constantly exposed to a bath of high-energy galactic cosmic rays and unpredictable bursts of solar radiation from the Sun. This radiation can damage DNA, increasing the lifetime risk of cancer, and can also affect the central nervous system and other organs.
2. Isolation and Confinement: A small crew of astronauts will be confined together in a space no larger than a small house for up to three years. The psychological strain of this prolonged isolation can lead to depression, interpersonal conflict, and sleep disorders, all of which can severely impact crew cohesion and performance.
3. Distance from Earth: Mars is, on average, 225 million kilometers away. This vast distance imposes a communication delay of up to 20 minutes each way. There can be no real-time conversation with Mission Control, no immediate expert guidance in an emergency. The crew must be exceptionally autonomous, capable of solving complex problems and handling medical emergencies on their own.
4. Altered Gravity: The mission involves three distinct gravitational regimes: the weightlessness of deep space during the months-long transit, the 38% gravity of Mars for the surface stay, and the return to Earth’s full gravity. These transitions wreak havoc on the human body, causing bone density loss, muscle atrophy, shifts in cardiovascular function, and balance disorders.
5. Hostile/Closed Environments: The spacecraft and surface habitat are closed-loop systems—machines that must perfectly maintain a breathable atmosphere, comfortable temperature, and safe levels of pressure and noise. The risk of equipment failure, fire, or contamination from microbes is constant. The Martian surface itself is hostile, with fine, abrasive dust and toxic perchlorates in the soil that pose a health risk to astronauts.

These five hazards do not exist in a vacuum; they feed off one another. The psychological stress of isolation can weaken the immune system, making an astronaut more susceptible to the effects of radiation. A medical emergency caused by altered gravity becomes far more dangerous when expert help is 40 minutes away. A human mission to Mars is therefore not just an engineering problem, but a profound human challenge, testing the limits of our technology and the resilience of the human body and mind.

Summary

Mars is a world of captivating duality. It is the “Red Planet,” a cold, windswept desert with a tenuous atmosphere and a surface hostile to life as we know it. Yet, it is also a planet that bears the unmistakable scars of a warmer, wetter past, a time when liquid water carved valleys and filled crater lakes, creating environments that could have supported the emergence of life. Our understanding of this complex history is a testament to a half-century of relentless robotic exploration, a journey that has transformed Mars in our minds from a simple point of light into a world of immense geological and scientific importance.

Each successive mission has built upon the knowledge of the last, peeling back layers of the Martian mystery in a patient, logical sequence. We have progressed from asking if there was ever water, to confirming that there was, to determining that the watery environments were habitable, and now to actively searching for the fossilized or chemical remains of any life that may have taken hold. The planet’s giant volcanoes and canyons speak to a unique geological evolution, while its dynamic polar caps and globe-spanning dust storms reveal a climate system that is both alien and elegantly complex. The recent discovery of vast potential reservoirs of liquid water deep underground has opened an entirely new frontier, suggesting that the story of water on Mars may be one of sequestration, not just loss.

As we look to the future, Mars represents the next great horizon for human exploration. The challenges are monumental, from the “six minutes of terror” involved in landing a heavy spacecraft to the profound physiological and psychological hazards of a multi-year journey so far from home. Overcoming these obstacles will require unprecedented technological innovation and a deep understanding of how to sustain human life in the most unforgiving of environments. The quest to reach Mars is driven by the same spirit of inquiry that has animated our exploration thus far—the desire to understand our place in the cosmos and to answer one of the most fundamental questions we can ask: are we alone?