Mars is a desert today — thin-aired, frozen, bone-dry. Yet its surface is carved with dry riverbeds, ancient lakebeds, and minerals that only form in water. That contradiction points to one of the great questions in planetary science: if Mars was once warm and wet, where did its air go? For more than a decade, a NASA spacecraft called MAVEN circled the planet to find out — and the answer it sent home is a cautionary tale about what keeps a world alive.

What MAVEN was for

MAVEN’s job was to answer one of the great questions about Mars: where did the air go? Today the Martian atmosphere is thin, cold and almost useless for liquid water. But the evidence carved into the planet’s surface — dry riverbeds, ancient lakebeds, minerals that only form in water — tells us that Mars was once warmer and wetter, with a much thicker atmosphere.

Something stripped that atmosphere away. MAVEN, whose name stands for Mars Atmosphere and Volatile Evolution, was built to study exactly how. Orbiting through the upper layers of the Martian atmosphere, it measured how the solar wind — the constant stream of charged particles from the Sun — peels gas away from a planet that, unlike Earth, lost its protective global magnetic field billions of years ago.

What it found

The mission’s central insight is both simple and profound: Mars lost its atmosphere largely because it had no magnetic shield. Earth’s magnetic field deflects the solar wind, protecting the air we breathe. Mars, with its field long gone, was left exposed. Over eons, the solar wind carried the Martian atmosphere into space, molecule by molecule.

MAVEN watched this happening in real time, measuring how the rate of atmospheric loss spikes during solar storms. It turned a story about the ancient past into a process scientists could actually observe — a planet slowly being undressed by its own star.

Why it matters for Earth

It is tempting to file all of this under “interesting things about Mars.” But MAVEN’s findings are really about what makes any world habitable — and that includes ours. Earth holds onto its atmosphere for the same reason Mars could not: our planet still has a churning molten core that generates a global magnetic field. That invisible shield deflects the solar wind, keeping our air, and the liquid water it allows, in place.

Mars shows what happens when that protection fails. The two planets likely started out far more similar than they look now; the difference in their fates comes down largely to a magnetic field one of them kept and the other lost. Studying a dead Martian atmosphere is, in effect, studying the fragile conditions we take for granted every time we breathe.

The bigger picture

MAVEN turned a story about the ancient past into a process scientists could actually watch — a planet slowly undressed by its own star. Its long record of measurements will be mined by researchers for years to come, and it laid the groundwork for every future mission trying to understand how rocky planets gain and lose their air. The lesson it leaves behind is a humbling one: an atmosphere is not a permanent feature of a world. It is something a planet has to be able to hold.