Final answer:
We should be surprised by Mercury's magnetic field because it was believed that Mercury's core had solidified, making it incapable of producing one. The field implies a partially liquid iron-nickel core, challenging our understanding of Mercury's thermal and geological history, and suggests a dynamo is at work despite Mercury's slow rotation and lack of a significant atmosphere.
Step-by-step explanation:
We should be surprised to find that Mercury has a magnetic field because it challenges previous assumptions about the planet's interior. According to the dynamo theory, a magnetic field is generated by the movement of a conductive fluid, usually molten metal, within a body's core. For Earth, this is created by its liquid metallic core. Given Mercury's small size and proximity to the Sun, it was thought that its interior had solidified and would not support the liquid core necessary for generating a magnetic field.
The presence of Mercury's magnetic field suggests that its core is partially liquid, despite expectations. The magnetic field is likely due to circulating electric currents within Mercury's large iron-nickel core, which constitutes about 60% of its mass. This is particularly intriguing as Mercury does not have the same conditions that typically support a dynamo action, such as rapid rotation or a significant atmosphere.
Some theories suggest that a significant collision event in Mercury's past might have stripped away much of its silicate mantle, creating a body dominated by its iron core, which contributes to the persistence of its magnetic field. This explains Mercury's unique composition among the terrestrial planets.