Hayabusa2 brought home pieces of asteroid Ryugu — the most pristine extraterrestrial material ever studied. No atmospheric entry, no terrestrial contamination, no exposure to Earth's magnetic field during collection. Twenty-eight fine-grained particles, measured under controlled demagnetization.
Twenty-three retained stable magnetization. But the directions disagree.
Not just between particles — within individual particles, the magnetization directions are inconsistent. Different grains in the same fragment point different ways. Whatever magnetic field imprinted these records was spatially heterogeneous at the microscale. The particles didn't magnetize in a uniform ambient field and then get reshuffled. They magnetized in place, surrounded by a field that varied over distances smaller than the particles themselves.
The explanation: chemical remanent magnetization during aqueous alteration. Ryugu's parent body — a larger asteroid that later broke apart — had liquid water flowing through its interior. As the water reacted with rock, it produced framboidal magnetite: tiny iron oxide clusters that grew in the presence of a magnetic field and locked that field's direction into their crystal structure. The field they recorded was the parent body's own — generated by its internal dynamics, varying on the scale of individual hydrothermal channels.
This isn't a compass reading of the solar system's ambient field. It's a microscopic portrait of a dead asteroid's interior, taken during the brief window when water flowed and minerals grew. The magnetic heterogeneity tells you the parent body's field was local — driven by convection or chemical gradients in the water, not by a coherent dynamo like Earth's.
The paleointensity varies by more than an order of magnitude across samples. Some grains recorded a strong field; others, almost nothing. The spatial structure of the magnetic environment was as complex as a geological landscape — ridges and valleys of field strength, frozen into mineral grains that then traveled for billions of years inside a rubble pile orbiting the Sun.
What Hayabusa2 brought back isn't just rock. It's a photograph of something that happened once, in one place, four and a half billion years ago — the inside of a wet asteroid, taken by the atoms themselves.