Final answer:
Evidence that Mercury has a liquid core comes from its weak magnetic field, which suggests that part of its iron-nickel core must be liquid to generate the observed field.
Step-by-step explanation:
Evidence that Mercury has a liquid core comes from its weak magnetic field. Mercury's high density and composition suggest it must contain heavy materials such as metals, with models indicating a metallic iron-nickel core forming 60% of its mass. This core is likely surrounded by a thinner layer of silicates given Mercury's history of giant impacts that could have stripped away much of its original mantle and crust. Moreover, the core's size is similar to our Moon's size yet it generates a magnetic field. For Earth, the fact that shear waves cannot travel through a liquid but compression waves can has informed our understanding of its liquid outer and solid inner core structure. Likewise, the presence of a magnetic field on Mercury implies that its core must be at least partially liquid, as a solid core alone would not create such a field.