Final answer:
The discovery of a substantial magnetic field around Mercury was surprising due to its small size and slow rotation. The magnetic field was detected by the MESSENGER spacecraft.
Step-by-step explanation:
The discovery of a substantial magnetic field around Mercury was a surprise because Mercury is a small planet with a slow rotation, and it was believed that small, slowly rotating objects would not have a significant magnetic field.
Additionally, Mercury is closer to the Sun and experiences intense solar wind, which was expected to strip away any magnetic field. The magnetic field around Mercury was detected by the MESSENGER spacecraft, which observed changes in the planet's magnetic field as it flew by Mercury.