Europa's surface color suggests an ocean rich with sodium chloride

The composition of Europa's surface may be representative of the subsurface ocean; however, considerable debate persists regarding the endogenous or exogenous nature of a hydrated sulfate feature on Europa. Direct evidence of oceanic salts on Europa's surface has been largely inconclusive. We show that the observed color within geologically young features on Europa's surface can be explained by sodium chloride delivered from the ocean below. We find that sodium chloride, when exposed to Europa surface conditions, accumulates electrons in F and M centers, yielding a yellow-brown discoloration comparable to Europa's surface. Irradiation of sodium chloride from Europa's ocean thus provides a simple and elegant solution to the color of the non-ice material observed on Europa. This evidence for endogenous salts suggests that Europa's ocean is interacting with a silicate seafloor, a critical consideration for assessing habitability.