The second most massive asteroid in the Solar System, Vesta, may once have had a rotating liquid metallic core that generated a magnetic field at the asteroid's surface.
Space scientists had previously established that the 525-kilometre-wide rock had differentiated — forming a dense core and a lighter mantle and crust. Roger Fu at the Massachusetts Institute of Technology in Cambridge and his colleagues studied the meteorite Allan Hills A81001, which probably originated on Vesta, and detected signatures of a magnetic field that formed 3.69 billion years ago. The team surmises that Vesta's rotating core produced a dynamo powerful enough to generate a magnetic field with a strength of 10 to 100 microteslas.
The discovery suggests that dynamos might have formed in other small, differentiated bodies in the early Solar System.
Science 338, 238–241 (2012) http://dx.doi.org/10.1126/science.1225648