Astronomy
Why Mars Lost Its Magnetic Field and Atmosphere
Quick fact
Mars lost most of its atmosphere over billions of years, and a key culprit is that its global magnetic field shut down about 4 billion years ago.
Why this is interesting
Mars was once a warm, wet world with rivers and lakes. So what turned it into the freezing, dusty desert we see today?
Read the full explanation
Understanding Why Mars Lost Its Magnetic Field and Atmosphere
Think of Mars's magnetic field as an invisible umbrella that deflects the Sun's constant outflow of charged particles (the solar wind). When Mars was young, its molten iron core churned and generated a global magnetic field, much like Earth does today. That field shielded the atmosphere from being directly eroded. However, as Mars's interior cooled, the core solidified or stopped convecting, and the dynamo that powered the magnetic field shut off. Once the umbrella disappeared, the solar wind—a stream of high-speed particles—began striking the upper atmosphere directly. Over time, these collisions knocked atoms and molecules into space, gradually thinning the atmosphere. With a thinner atmosphere, surface pressure dropped, liquid water could not remain stable, and the planet became the cold, arid world we know.
A deeper explanation
The magnetic field of a planet is generated by a dynamo process: convection of electrically conducting fluid (molten iron) in the core, combined with the planet's rotation, creates electric currents and a magnetic field. On Mars, the core is smaller and probably solid or partially liquid, but convection ceased because the planet lost heat quickly due to its smaller size (compared to Earth). The loss of the magnetic field meant the magnetosphere—the protective bubble—disappeared. Without it, the solar wind, traveling at hundreds of kilometers per second, could interact directly with the upper atmosphere. This interaction drives several escape mechanisms, such as ion escape (charged particles being swept away by electric fields of the solar wind) and sputtering (high-energy neutral particles knocking atmospheric atoms into space). Measurements by NASA's MAVEN mission show that solar wind erosion continues today, stripping about 100 grams of atmosphere per second. This is why the atmospheric pressure on Mars is less than 1% of Earth's, which is insufficient for liquid water to persist on the surface. Understanding this process is crucial for assessing a planet's habitability and for planning missions to Mars, including how to protect future human settlers from radiation and how to create a breathable atmosphere.