Question

### Europa's Subsurface Ocean

Two scientists discuss the thermal mechanisms that maintain a liquid water ocean beneath the icy crust of Jupiter's moon, Europa.

Scientist 1
Europa's subsurface ocean is kept liquid primarily by tidal heating resulting from its eccentric orbit around Jupiter, which is maintained by orbital resonances with Io and Ganymede. This gravitational flexing generates friction within Europa's metallic core and silicate mantle, but most significantly within its ductile lower ice shell. This tidal dissipation produces a heat flux of approximately $50\text{ mW/m}^2$, which is sufficient to maintain a liquid ocean beneath a $15\text{--}25\text{ km}$ thick ice shell. Seafloor hydrothermal venting is minor and does not contribute significantly to the ocean's thermal budget. Radioactive decay within Europa's rocky mantle provides less than $5\text{ mW/m}^2$ of heat flux, which is negligible.

Scientist 2
Tidal dissipation within Europa's ice shell is inefficient and cannot exceed $15\text{ mW/m}^2$ of heat flux, which would cause the ocean to freeze completely. Instead, the primary source of Europa's thermal energy is hydrothermal activity at the seafloor. This is driven by tidal dissipation occurring exclusively within the rocky mantle and core, combined with radiogenic decay. This localized heating at the ocean floor drives vigorous hydrothermal circulation, transporting hot fluids into the ocean. This seafloor hydrothermal heat flux exceeds $80\text{ mW/m}^2$, sustaining the ocean and leading to a thin ice shell of only $3\text{--}5\text{ km}$.

Scientist 1 and Scientist 2 differ in their views regarding which of the following?