Billions of years ago, rivers flowed across Mars. Today, the Red Planet is a frozen desert with an atmosphere so thin that liquid water can barely survive on its surface. What happened to the air that once helped make those rivers possible?
For decades, scientists have been searching for an answer.
Was the ancient Martian atmosphere gradually absorbed into the planet's rocks? Did it disappear into space? Or did something far more powerful slowly strip it away?
NASA sent a spacecraft called MAVEN to investigate this mystery. What scientists discovered revealed an extraordinary chapter in the history of Mars.
The Red Planet did not simply lose its atmosphere in one catastrophic event. Instead, much of its atmospheric gas escaped into space over billions of years.
And one of the main forces responsible was the same star that once helped warm its surface: the Sun.
Mars Was Once a Very Different Planet
Imagine standing on Mars billions of years ago.
Instead of the dry, dusty landscape photographed by modern rovers, you might have seen flowing rivers, ancient lakes, and water collecting inside enormous impact craters.
This is not simply science fiction.
NASA's missions have uncovered extensive geological evidence that liquid water once existed on the Martian surface.
Ancient river valleys stretch across the landscape. Sedimentary rocks preserve traces of former lakes. Certain minerals found by spacecraft and rovers could only have formed through interactions with water.
Together, these discoveries reveal that early Mars experienced environmental conditions dramatically different from those we see today.
But there is a problem.
Liquid water requires suitable temperatures and pressure to remain stable.
Modern Mars has neither.
Its average surface temperature is approximately −60°C (−80°F), although temperatures vary considerably by location and season.
Its atmospheric pressure is typically less than one percent of Earth's sea-level pressure.
To understand just how thin that atmosphere is, consider Mount Everest.
At the summit of Everest, atmospheric pressure is roughly one-third of what we experience at sea level. Climbers often need supplemental oxygen because the air is so thin.
Now imagine an atmosphere far thinner than even the air at Everest's summit.
That is the environment on Mars.
Under these conditions, exposed liquid water is generally unstable. Depending on temperature and pressure, it can freeze, evaporate, or boil.
Yet the ancient Martian landscape tells us that water once flowed across the planet.
Something enormous must have changed.
And the atmosphere became one of the most important clues.
The Mystery of Mars' Missing Atmosphere
Scientists have identified several ways a planet can lose atmospheric gases.
Some gases can become trapped in minerals through chemical reactions. Carbon dioxide, for example, can react with rocks and contribute to the formation of carbonate minerals.
Other gases can escape directly into space.
These processes leave very different clues.
If much of Mars' ancient carbon dioxide became locked inside rocks, scientists might find evidence of that missing atmospheric reservoir in the planet's crust.
But if atmospheric gases escaped into space, the evidence would be more difficult to recover.
After all, how do you investigate something that disappeared billions of years ago?
Scientists needed a way to study the processes removing atmospheric particles today and then reconstruct what might have happened in the distant past.
That challenge led to one of NASA's most important missions to Mars.
Meet MAVEN: The Spacecraft Sent to Investigate Mars' Missing Air
On September 21, 2014, NASA's Mars Atmosphere and Volatile Evolution spacecraft, better known as MAVEN, successfully entered orbit around Mars.
Unlike rovers such as Curiosity and Perseverance, MAVEN was not designed to explore the Martian surface.
Its mission was to investigate something almost invisible: the planet's upper atmosphere.
Led by principal investigator Bruce Jakosky of the University of Colorado Boulder, the mission studied how atmospheric gases interact with solar radiation and particles arriving from the Sun.
MAVEN carried instruments capable of measuring charged particles, atmospheric composition, ultraviolet emissions, and other properties of the Martian environment.
By studying these interactions, researchers hoped to answer a fundamental question:
Could the Sun have gradually stripped away enough atmospheric gas to transform the climate of an entire planet?
The answer required understanding an invisible stream of particles moving through our solar system.
The Solar Wind: An Invisible Force That Never Stops
The Sun does much more than provide light and heat.
It continuously releases a stream of electrically charged particles, primarily electrons and protons, into space.
Scientists call this stream the solar wind.
Near Earth's orbit, its speed commonly reaches hundreds of kilometers per second. A typical value of about 400 kilometers per second is equivalent to approximately 1.44 million kilometers per hour.
At such speeds, particles can travel enormous distances through the solar system.
Earth is also exposed to the solar wind.
Fortunately, our planet possesses a global magnetic field that helps redirect much of this incoming charged-particle flow.
Mars is different.
Although its crust contains regions of ancient magnetism, the planet no longer has an Earth-like global magnetic field.
This leaves its upper atmosphere more directly exposed to interactions with the solar wind.
And those interactions can gradually remove atmospheric particles.
But how?
How the Sun Strips Away the Martian Atmosphere
Imagine throwing a fast-moving ball into a collection of smaller objects.
The ball strikes one object, transferring energy. That object then collides with others, sending some of them flying.
A somewhat similar process occurs in the upper atmosphere of Mars.
Solar ultraviolet radiation and interactions with the solar wind can create electrically charged atmospheric particles called ions.
Some of these ions are accelerated by electric and magnetic fields associated with the solar wind.
When energetic ions travel back toward the atmosphere, they can collide with neutral atmospheric particles.
These collisions transfer energy.
Occasionally, an atmospheric atom receives enough energy to escape Mars' gravitational pull and travel into space.
Scientists call this process atmospheric sputtering.
It is one of several mechanisms responsible for atmospheric escape.
Other important processes include the escape of ions carried away by electromagnetic forces and the loss of particles through photochemical reactions.
Together, these mechanisms have helped reshape Mars over billions of years.
And MAVEN was designed to investigate them.
NASA Measured Mars Losing Atmospheric Gas
In November 2015, NASA announced important results from MAVEN.
Measurements indicated that the solar wind was removing atmospheric gas from Mars at a rate of approximately 100 grams per second under the conditions studied.
That may not sound impressive.
One hundred grams is roughly the mass of a small apple.
But consider what happens when that loss continues.
At a constant rate of 100 grams per second, Mars would lose approximately:
6 kilograms every minute.
360 kilograms every hour.
8,640 kilograms every day.
More than 3,100 metric tons every year.
And these figures describe only a representative measured rate, not a constant value throughout Martian history.
The actual escape rate varies with solar activity and other atmospheric conditions.
MAVEN also observed that atmospheric loss increased during solar storms in March 2015.
This was an important discovery.
It showed that the Sun was not merely warming the Martian atmosphere.
Solar activity could also help remove its particles.
But the discovery raised another question.
If Mars is losing atmospheric gas today, how much did it lose billions of years ago?
Today's measurements alone could not provide the complete answer.
Scientists needed evidence that preserved information about the planet's ancient atmosphere.
Surprisingly, that evidence came from a gas that rarely reacts with anything.
Argon: The Gas That Helped Reveal Mars' Past
One of the most fascinating parts of the MAVEN investigation involves a chemical element called argon.
Argon is a noble gas.
Unlike carbon dioxide, which can participate in chemical reactions and become incorporated into minerals, argon is chemically inert under ordinary Martian conditions.
This makes it particularly useful for studying atmospheric loss.
If atmospheric argon becomes depleted, chemical reactions with surface rocks cannot easily explain its disappearance.
Scientists can therefore use argon to investigate how atmospheric gases escaped into space.
But argon has another remarkable property.
It exists in different forms called isotopes.
Two important isotopes are argon-36 and argon-38.
Both are argon atoms, but their nuclei contain different numbers of neutrons.
Argon-38 is slightly heavier than argon-36.
And that tiny difference in mass helps scientists investigate events that occurred billions of years ago.
How Two Types of Argon Became Clues to a Planetary Mystery
Imagine a mixture of lightweight and heavyweight particles being exposed to a process that removes the lighter particles more efficiently.
Over time, the material left behind becomes relatively richer in heavier particles.
Something similar can happen in a planetary atmosphere.
During certain atmospheric escape processes, lighter isotopes are removed more efficiently than heavier isotopes.
As atmospheric gas escapes, the ratio between the remaining isotopes gradually changes.
Scientists call this effect isotopic fractionation.
The ratio of argon-36 to argon-38 can therefore preserve information about atmospheric loss.
MAVEN measured argon isotopes in the upper Martian atmosphere using its Neutral Gas and Ion Mass Spectrometer, or NGIMS.
Researchers combined those measurements with observations made near the surface by the Sample Analysis at Mars instrument, known as SAM, aboard NASA's Curiosity rover.
By studying how the isotope ratio changed with altitude and modeling the escape process, scientists could estimate how much argon had been removed over time.
The result was remarkable.
NASA Found That Mars Lost About 65% of Its Atmospheric Argon
In March 2017, researchers published a major study in the journal Science.
Their analysis indicated that approximately 65–66% of the argon that had been present in the Martian atmosphere was lost to space.
This was an important clue about the planet's atmospheric history.
Because argon is chemically inert, its depletion provides strong evidence that atmospheric escape played a major role in Mars' evolution.
The researchers also concluded that substantial quantities of other gases, including carbon dioxide, had escaped into space.
However, an important distinction is necessary.
The 65–66% figure refers specifically to atmospheric argon. It does not mean scientists directly measured the same percentage of loss for every atmospheric gas.
Carbon dioxide, for example, can also be removed from the atmosphere through chemical interactions with surface materials.
Its history is therefore more complicated.
Nevertheless, the argon measurements provided compelling evidence that escape into space was a major contributor to the disappearance of Mars' once-thicker atmosphere.
The Sun had helped change the environment of an entire planet.
And the story was not finished.
A 2025 Discovery Provided Direct Evidence of Atmospheric Sputtering
For years, scientists had strong theoretical and indirect observational evidence that sputtering contributed to atmospheric loss on Mars.
But directly observing this process was extremely challenging.
That changed in 2025.
On May 29, NASA announced that MAVEN researchers had reported the first direct observation of atmospheric sputtering at Mars.
The discovery represented an important advance in understanding how energetic particles can eject atoms from the Martian atmosphere.
It strengthened the connection between the physical processes observed today and models used to reconstruct the planet's ancient climate.
The significance extends beyond Mars.
Atmospheric escape is a fundamental process in planetary science.
By understanding how it works on Mars, scientists can improve their understanding of atmospheric evolution on other planets, including worlds orbiting distant stars.
A planet's ability to retain an atmosphere can influence its climate, surface conditions, and potential habitability.
Mars provides an extraordinary natural laboratory for investigating these questions.
What Happened to Mars' Magnetic Field?
The disappearance of Mars' ancient global magnetic field is another important part of the story.
Scientists believe that early Mars once possessed a magnetic dynamo generated by motion within its interior.
Evidence of ancient magnetic fields remains preserved in parts of the Martian crust.
However, the global dynamo eventually ceased operating.
Without a strong global magnetic field like Earth's, the Martian atmosphere became more exposed to interactions with the solar wind.
This likely influenced atmospheric escape over geological timescales.
But the relationship is not as simple as saying that a magnetic field guarantees atmospheric survival.
Venus, for example, lacks an Earth-like global magnetic field yet retains an extremely dense atmosphere.
Planetary gravity, atmospheric composition, solar radiation, geological activity, and other factors all influence how atmospheres evolve.
Mars' smaller size and lower gravity also matter because atmospheric particles can escape its gravitational pull more easily than they can from a more massive planet.
Its atmospheric history resulted from several interacting processes rather than one isolated event.
Did Mars Lose All Its Water When Its Atmosphere Disappeared?
Not exactly.
The disappearance of Mars' thicker atmosphere helped make stable surface water increasingly difficult to maintain.
But scientists have found evidence that some Martian water remains today.
Large quantities of water ice exist in the polar regions and beneath parts of the surface.
Water is also present in hydrated minerals.
Meanwhile, some water molecules were broken apart by sunlight, allowing hydrogen and other atmospheric constituents to escape into space through different processes.
Researchers continue investigating how much ancient Martian water escaped, how much became trapped in minerals, and how much remains underground.
These questions are closely connected to the search for ancient Martian life.
If Mars once supported rivers and lakes, could microorganisms have existed in those environments?
Scientists have not confirmed that life ever existed on Mars.
But understanding the planet's changing atmosphere helps researchers determine when and where conditions might once have been suitable for life.
What Happened to NASA's MAVEN Mission?
MAVEN entered Mars orbit in September 2014 and operated for more than 11 years, far exceeding its original planned science mission.
During that time, it transformed our understanding of the Martian upper atmosphere and its interactions with the Sun.
The spacecraft also supported communications with missions operating on the surface.
However, NASA lost contact with MAVEN in December 2025.
Following recovery efforts and a technical review, NASA declared the mission over.
Although the spacecraft is no longer operating, the scientific data collected during its years around Mars remains valuable.
Researchers can continue analyzing those observations to investigate atmospheric escape, solar activity, and the evolution of the Red Planet.
MAVEN's legacy is therefore much larger than the lifetime of the spacecraft itself.
Could Mars Ever Become Habitable Again?
The discovery that Mars once had a thicker atmosphere naturally raises another question.
Could humans restore it?
The idea of transforming Mars into a warmer, more Earth-like environment is known as terraforming.
In theory, a thicker atmosphere could help increase surface pressure and alter the planet's climate.
But creating such an environment would be extraordinarily difficult.
Mars has limited readily accessible atmospheric resources, and its low gravity and exposure to space-weather processes present long-term challenges.
Even if humans could release significant amounts of trapped carbon dioxide, current scientific understanding suggests that available resources would not be sufficient to produce an Earth-like atmosphere using foreseeable technology.
Future human explorers would therefore need protected habitats, reliable life-support systems, and carefully controlled environments.
The ancient Martian climate cannot simply be switched back on.
The Bigger Lesson: A Planet's Climate Can Change Forever
Perhaps the most remarkable lesson from MAVEN is that planetary environments are not permanent.
A world can begin with conditions that allow liquid water to flow across its surface and gradually evolve into a frozen desert.
These changes can unfold over billions of years.
To human observers, the process may appear almost motionless.
Yet atmospheric particles can continue escaping into space, one by one.
The ancient rivers of Mars are long gone.
Their valleys remain etched into the surface, preserving evidence of a time when the planet was very different.
Above those valleys lies a thin atmosphere that continues to interact with sunlight and the solar wind.
And thanks to MAVEN, scientists now understand far more about how Mars reached its present condition.
The Red Planet did not simply become dry because its rivers disappeared.
Its atmosphere, climate, geology, and water were connected through a long history of planetary change.
Mars reminds us that a planet does not need to explode, collide with another world, or experience a single catastrophic event to become unrecognizable.
Sometimes, the transformation happens slowly.
Atom by atom.
Over billions of years.
And that leaves us with one fascinating question:
If an entire planet can change so dramatically, what might the atmospheres of other worlds reveal about their past—and their future?
Frequently Asked Questions
1. Why did Mars lose its atmosphere?
Mars lost much of its atmosphere through several processes, including interactions with solar wind, ultraviolet radiation, and atmospheric escape. Its relatively weak gravity and lack of an Earth-like global magnetic field influenced how these processes affected the planet.
2. Does Mars still have an atmosphere?
Yes. Mars still has an atmosphere, composed mainly of carbon dioxide, with smaller amounts of nitrogen, argon, and other gases. However, its surface pressure is typically less than one percent of Earth's sea-level pressure.
3. Is Mars still losing atmospheric gas?
Yes. Atmospheric escape remains an ongoing natural process. The rate varies depending on solar activity, atmospheric conditions, and the escape mechanism involved.
4. What did NASA's MAVEN mission discover?
MAVEN measured atmospheric escape, investigated interactions between the solar wind and Mars, and helped establish that a substantial portion of the planet's ancient atmospheric gas escaped into space. Its findings included isotope-based estimates of argon loss and direct observations of atmospheric sputtering.
5. Did Mars once have oceans?
Evidence strongly supports the existence of ancient rivers and lakes on Mars. Some researchers also propose that a large ocean once occupied parts of its northern hemisphere, although the extent and duration of any ancient Martian ocean remain subjects of scientific investigation.
6. Could Mars support life in the past?
Ancient Mars may have possessed environments suitable for microbial life, including liquid water and potentially useful chemical energy sources. However, scientists have not yet confirmed evidence of past or present Martian life.
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