The sources of sodium escaping from Io revealed by spectral high definition imaging

On Jupiter's moon Io, volcanic plumes and evaporating lava flows provide hot gases to form an atmosphere that is subsequently ionized. Some of Io's plasma is captured by the planet's strong magnetic field to form a co-rotating torus at Io's distance; the remaining ions and electrons form Io's ionosphere. The torus and ionosphere are also depleted by three time-variable processes that produce a banana-shaped cloud orbiting with Io, a giant nebula extending out to about 500 Jupiter radii, and a jet close to Io. No spatial constraints exist for the sources of the first two; they have been inferred only from modelling the patterns seen in the trace gas sodium observed far from Io. Here we report observations that reveal a spatially confined stream that ejects sodium only from the wake of the Io-torus interaction, together with a visually distinct, spherically symmetrical outflow region arising from atmospheric sputtering. The spatial extent of the ionospheric wake that feeds the stream is more than twice that observed by the Galileo spacecraft and modelled successfully. This implies considerable variability, and therefore the need for additional modelling of volcanically-driven, episodic states of the great jovian nebula.     

An early lunar core dynamo driven by thermochemical mantle convection

Although the Moon currently has no internally generated magnetic field, palaeomagnetic data, combined with radiometric ages of Apollo samples, provide evidence for such a magnetic field from approximately 3.9 to 3.6 billion years (Gyr) ago, possibly owing to an ancient lunar dynamo. But the presence of a lunar dynamo during this time period is difficult to explain, because thermal evolution models for the Moon yield insufficient core heat flux to power a dynamo after approximately 4.2 Gyr ago. Here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo. Using a three-dimensional spherical convection model, we show that a dense layer, enriched in radioactive elements (a 'thermal blanket'), at the base of the lunar mantle can initially prevent core cooling, thereby inhibiting core convection and magnetic field generation. Subsequent radioactive heating progressively increases the buoyancy of the thermal blanket, ultimately causing it to rise back into the mantle. The removal of the thermal blanket, proposed to explain the eruption of thorium- and titanium-rich lunar mare basalts, plausibly results in a core heat flux sufficient to power a short-lived lunar dynamo.