What discovery by Henri Becquerel inspired Marie Curie’s research?

Marie Curie’s research was inspired by Henri Becquerel’s 1896 discovery of a new phenomenon, which she later named radioactivity.12

Becquerel discovered that uranium salts emitted penetrating, invisible radiation—a property he initially interpreted as a form of "invisible phosphorescence" after observing that the salts could affect photographic plates even without stimulation by sunlight.3

Intrigued by this discovery, Marie Curie decided to investigate whether this property of emitting radiation was unique to uranium or if it could be found in other matter. This inquiry led her to test various substances, eventually discovering that thorium also exhibited radioactivity and that the mineral pitchblende possessed an activity level far superior to that of pure uranium. These findings prompted her and her husband, Pierre Curie, to search for the unknown, highly active substances within the ore, ultimately resulting in the discovery of the elements polonium and radium.12