Geophysical measurements can reveal the structures and thermal states of icy ocean worlds. The interior density, temperature, sound speed, and electrical conductivity thus characterize their habitability. We explore the variability and correlation of these parameters using 1‐D internal structure models. We invoke thermodynamic consistency using available thermodynamics of aqueous MgSO4, NaCl (as seawater), and NH3; pure water ice phases I, II, III, V, and VI; silicates; and any metallic core that may be present. Model results suggest, for Europa, that combinations of geophysical parameters might be used to distinguish an oxidized ocean dominated by MgSO4 from a more reduced ocean dominated by NaCl. In contrast with Jupiter's icy ocean moons, Titan and Enceladus have low‐density rocky interiors, with minimal or no metallic core. The low‐density rocky core of Enceladus may comprise hydrated minerals or anhydrous minerals with high porosity. Cassini gravity data for Titan indicate a high tidal potential Love number (k2>0.6), which requires a dense internal ocean (ρocean>1,200 kg m−3) and icy lithosphere thinner than 100 km. In that case, Titan may have little or no high‐pressure ice, or a surprisingly deep water‐rock interface more than 500 km below the surface, covered only by ice VI. Ganymede's water‐rock interface is the deepest among known ocean worlds, at around 800 km. Its ocean may contain multiple phases of high‐pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's interior structure may be intermediate to those of Titan and Europa, with a water‐rock interface 250 km below the surface covered by ice V but not ice VI.

Yellowstone's missing magmatic link Yellowstone is an extensively studied “supervolcano” that has a large supply of heat coming from a pool of magma near the surface and the mantle below. A link between these two features has long been suspected. Huang et al. imaged the lower crust using seismic tomography (see the Perspective by Shapiro and Koulakov). Their findings provide an estimate of the total amount of molten rock beneath Yellowstone and help to explain the large amount of volcanic gases escaping from the region. Science , this issue p. 773 ; see also p. 758

No abstract