A pebble from a distant asteroid can remember the magnetic field that acted on it billions of years ago. Such pebbles, particles from asteroid Ryugu, were brought to Earth by Japan's Hayabusa2 probe. Inside them, like the needle of an ancient compass, the direction of the magnetic field from the solar system's youth is frozen.

Scientists took 28 particles and gradually "erased" their magnetization with an alternating magnetic field, recording how it changed. In 23 particles, they found one or two stable magnetic traces, like two layers of writing in an old diary. Five particles showed no such traces at all. The strength of the ancient field, calculated from these traces, varied by more than a factor of ten.

This scatter is not a measurement error but a reflection of real history. The magnetic traces formed at different times and in different places on Ryugu's parent body. Most likely, the record was created when magnetite, a mineral that forms in the presence of water, grew in the rock's pores. Water altered the rock, and at that moment it "remembered" the surrounding magnetic field.

Now these pebbles are like found pages of a chronicle. They tell us what the magnetic environment was like in the first tens of millions of years after the Sun's birth. And although the pages are incomplete and partially erased, they are enough to understand: the magnetic history of the solar system began long before the planets formed.