CAN you flip a planet’s magnetic field on and off like a light switch? An asteroid could have done just that to Mars 4 billion years ago.

Mars once had a magnetic field, which may have been driven by a dynamo formed from the convection of material in the core, much like the Earth’s is today. Yet crater records suggest the Martian dynamo died quickly, over a few tens of thousands of years, something researchers struggle to explain.

Now Jafar Arkani-Hamed of the University of Toronto, Canada, and colleagues say the gravitational tug of an orbiting asteroid may have powered …