Mars doesn’t have a global magnetic field anymore. Lost it about 4 billion years ago, which—if you’re keeping track—is roughly when Earth’s oceans were just getting started and our planet was still being pummeled by asteroids the size of small countries.
But here’s the thing: Mars isn’t entirely defenseless.
When Dead Planets Keep Tiny Umbrellas Over Their Scars
Scattered across the Martian surface are these localized magnetic anomalies called shield fields, and they’re basically the planetary equivalent of wearing mismatched socks. NASA’s Mars Global Surveyor mapped them starting in 1997, finding the strongest concentrations in the southern highlands—ancient terrain that remembers when Mars actually had its act together magnetically speaking. These patches of magnetized crust generate their own mini-magnetospheres, some extending up to 1,200 kilometers into space, which is frankly impressive for fossilized magnetism.
The leading theory? When Mars still had a churning liquid core generating a proper magnetic field, iron-rich minerals in the crust got magnetized and locked in place as they cooled. Then the core solidified, the global field collapsed, but these crustal remnants stayed magnetized like geological time capsules. Terra Cimmeria and Terra Sirenum host some of the most powerful shield fields, with magnetic strengths reaching 1,500 nanoteslas—about 30 times stronger than Earth’s surface field in some spots.
Wait—maybe that’s not the whole story.
Some researchers, including a team led by Jafar Arkani-Hamed in 2004, proposed these shield fields might trace back to ancient asteroid impacts that remagnetized the crust through sheer thermal violence. The Europa Lander mission concept even considered using similar shield field mapping techniques, though that’s a different frozen world entirely and I’m getting distracted.
The Solar Wind Problem That Makes Everything Complicated
Without a global magnetic umbrella, Mars gets absolutely hammered by solar wind—charged particles streaming from the Sun at speeds around 400 kilometers per second. Earth’s magnetosphere deflects this onslaught, but Mars? The solar wind strips away about 100 grams of atmosphere every single second, according to NASA’s MAVEN spacecraft data from 2015. That’s how you go from potentially habitable to freeze-dried desert planet.
Turns out these shield fields offer microscopic protection. They create small-scale magnetic bubbles that deflect some solar wind particles, though calling it “protection” is like using a cocktail umbrella in a hurricane. Still, researchers from the University of Colorado Boulder found in 2017 that regions beneath strong shield fields experience measurably less atmospheric stripping than unprotected areas.
The Reull Vallis region shows this effect clearly—a canyon system where shield fields correlate with slightly higher atmospheric retention. It’s not enough to save the planet, but it’s geological defiance nonetheless.
China’s Tianwen-1 orbiter, which arrived in 2021, carries magnetometers specifically designed to study these crustal magnetic signatures at unprecedented resolution. The data suggests some shield fields are more complex than simple frozen magnetism—possibly involving multiple layers of magnetized material from different epochs, like a geological lasagna of ancient magnetic history.
Nobody’s suggesting terraforming Mars by amplifying these natural shield fields, though the idea surfaces occasionally in speculative engineering papers. You’d need to artificially generate a magnetic field strong enough to encompass the entire planet, which would require energy outputs comparable to Earth’s entire electrical grid running continuously for centuries. Slightly impractical.
But understanding shield fields matters for future Mars settlements. Knowing where these magnetic anomalies provide even marginal radiation shielding could influence hab placement decisions. The European Space Agency’s ExoMars Trace Gas Orbiter has been mapping correlations between shield fields and surface radiation levels since 2016, building a database that might actully prove useful when humans finally show up with their inflatable habitats and hydroponic gardens.
Mars kept pieces of its magnetic past embedded in its skin. Not enough to matter on planetary scales, but enough to remind us that this rusty world was once something more dynamic, more alive in the geological sense, before its core went cold and the solar wind started its long, patient work of erasure.
