Quick Answer
Mars lost its atmosphere because it lost its global magnetic field. When Mars’ core cooled and its protective magnetic shield switched off around four billion years ago, the solar wind — a stream of charged particles from the Sun — slowly stripped away the planet’s once-thick air, molecule by molecule, over billions of years. What was once a warm, wet world became the cold, dry desert we see today, with an atmosphere less than 1% as dense as Earth’s.
Mars is a planetary cautionary tale: proof that a world much like the early Earth can have its air torn away and its oceans freeze. Understanding how Mars died tells us a great deal about what keeps our own planet alive. This guide explains what happened to the Martian atmosphere, the role of its vanished magnetic field, and what the MAVEN mission discovered about the slow suffocation of the Red Planet.
What Mars’ Atmosphere Is Like Today
The modern Martian atmosphere is thin, cold, and unbreathable. It is composed of about 95% carbon dioxide, with small amounts of nitrogen and argon, and the surface pressure averages only around 0.6% of Earth’s at sea level. That is so thin that liquid water cannot remain stable on the surface, and a human without a pressure suit would not survive — body fluids would begin to vaporise.
Average temperatures hover near −60°C, swinging wildly between day and night because the wispy air holds almost no heat. Despite its thinness, this atmosphere still produces clouds, dust storms that can engulf the entire planet, and seasonal frost. But it is a ghost of what Mars once had.
The Magnetic Field Mars Lost
Early Mars almost certainly had a global magnetic field, generated deep inside the planet much as Earth’s is today. We know this because spacecraft have detected strong “fossil” magnetism locked into the oldest regions of the Martian crust — a permanent record of a field that has since vanished. That field was Mars’ invisible shield, deflecting the charged particles of the solar wind before they could reach the atmosphere.
How a dying core killed the magnetosphere
A planet generates a global magnetic field through a dynamo: the churning of molten, electrically conductive metal in its core. Mars is only about half Earth’s diameter, so it cooled far faster. As its core lost heat, the convection that powered the dynamo weakened and then stopped, roughly four billion years ago. With the dynamo dead, the global magnetic field collapsed — and the atmosphere was left exposed. The same machinery still runs inside Earth, as explained in our article on how Earth’s magnetic field works.
How the Solar Wind Stripped the Air Away
The Sun constantly blows out a stream of charged particles called the solar wind. On Earth, our magnetic field channels this wind around the planet. On Mars, with no global field to stop it, the solar wind slammed directly into the upper atmosphere. There, it energised gas molecules and carried them off into space through several processes: charged particles were “picked up” and dragged away, and impacts knocked neutral atoms out in a process called sputtering.
This stripping was not a single dramatic event but a slow bleed over billions of years. Crucially, the young Sun was far more active than it is today, with a stronger solar wind and more intense radiation, so the early loss rate was much higher. Bit by bit, Mars exhaled its atmosphere into the void, and as the air thinned, the greenhouse warming it provided faded — letting the planet freeze.
The MAVEN Mission’s Findings
To measure this process directly, NASA sent the MAVEN spacecraft (Mars Atmosphere and Volatile Evolution) into orbit in 2014. MAVEN was designed specifically to study how Mars loses gas to space, and its findings confirmed the central story: the solar wind is indeed the main culprit.
- Active stripping: Mars is still losing gas to space today, at a rate of roughly a few hundred grams per second.
- Storm surges: during solar storms, the escape rate jumps dramatically.
- A faster past: when the Sun was young and violent, Mars lost its air far more quickly.
- Enough lost: the cumulative loss is sufficient to explain the transition from a thick early atmosphere to today’s thin one.
In other words, MAVEN caught the crime in progress. The atmosphere Mars has today is the small remainder left after eons of erosion by the Sun.
Could We Give Mars an Atmosphere Back?
If the Sun stripped Mars’ air away, could humans put it back? This is the heart of terraforming, and it is the scenario we explore in what if a space mirror melted Mars’ ice caps. The idea would be to release Mars’ frozen carbon dioxide and water to thicken the atmosphere and warm the planet through a greenhouse effect.
The obstacles are immense. First, studies suggest Mars may simply not have enough accessible CO2 left to build a thick, warm atmosphere on its own. Second — and more fundamentally — even if we rebuilt the atmosphere, Mars still has no global magnetic field, so the solar wind would slowly strip it away again. Any new atmosphere would need either constant replenishment or some form of artificial protection, such as a magnetic shield placed between Mars and the Sun. The very thing that killed Mars would keep trying to kill it again. Where that water and ice sit today is covered in our companion piece on liquid water on Mars.
What Mars Teaches Us About Earth
Mars is the clearest demonstration of why Earth’s magnetic field matters. Both planets likely started with atmospheres and water. The crucial difference is that Earth is larger, so its core has stayed hot and molten, keeping the protective magnetic dynamo running for billions of years. That shield, combined with Earth’s stronger gravity, has let our planet hold onto its air and oceans while Mars lost everything.
It is a humbling comparison: the boundary between a living world and a dead one can come down to a planet’s size and the heat in its core. Mars shows what Earth might have become — and reminds us how much we owe to the churning iron beneath our feet.
Q&A
Not any time soon. Earth’s stronger gravity and its active magnetic field protect our atmosphere from the solar wind. Earth does lose a tiny amount of gas to space, but the rate is negligible, and our atmosphere is safe for billions of years — until the aging Sun eventually changes the picture.
The surface pressure on Mars averages about 0.6% of Earth’s sea-level pressure. That is comparable to the near-vacuum found roughly 35 kilometres above Earth’s surface — far too thin to breathe or to keep liquid water stable.
Not naturally, and not easily. Even ambitious terraforming concepts would take centuries to millennia and face the problem that Mars has little accessible carbon dioxide and no magnetic shield. For the foreseeable future, humans on Mars will need pressurised habitats and oxygen made on site.
Estimates range from centuries to many thousands of years, and all are highly speculative. Warming the planet and thickening the air enough for liquid water might be conceivable over long timescales, but creating breathable, Earth-like air is far beyond any technology we currently possess.
The Bigger Question
Mars lost its atmosphere because it lost its magnetic shield and its internal heat — a slow death by solar wind. The dream of reversing that, of warming the planet and melting its ice to bring back rivers and air, runs straight into the same physics that killed it. That is the tension at the centre of what if a space mirror melted Mars’ ice caps, where we put the terraforming fantasy to a scientific test.
For more on planetary habitability and humanity’s future in space, explore our Space & Cosmos hub.
Watch the terraforming scenario and see whether we could ever give the Red Planet a second chance.