DIX Planetary Science Seminar

Saturn and Jupiter are nearly the same size and composition, but their magnetic fields behave very differently. Jupiter exhibits a tilted, highly complex, multipolar field; meanwhile Saturn's field is perfectly aligned with its spin axis, and completely axisymmetric. In this work we rationalize these differences by considering the thermodynamics of the helium rain layers of both planets. Using computational characterization of the hydrogen-helium phase diagram and equation of state, we find that moist convection and diffusive convection are inhibited, implying a stable helium rain layer in both Jupiter and Saturn. We estimate the helium rain latent heat flux on both planets, finding that while on Saturn the latent heat flux is sufficient to accommodate nearly its entire heat flow, on Jupiter it is not. Therefore, we predict a thin (~tens of km) stable layer on Jupiter, while Saturn's helium rain layer can be thick (a large fraction of its total radius). Finally, we use Juno spacecraft data to present evidence of a systematic attenuation of the non-azimuthally symmetric components of Jupiter's magnetic field relative to both the Earth and a theoretical fully stochastic model. Such behavior is consistent with a thin (~70km) stable layer in Jupiter's deep interior. By contrast, Saturn's magnetic field is completely axisymmetric, consistent with a thick stable layer. We therefore suggest the differences between the two planets' magnetic fields may be attributed to the way their different masses and luminosities interact with the thermodynamics of the hydrogen-helium system.