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Modelling Galilean moon atmospheres finds evidence that Galileo detected a water plume on Europa

By modelling interactions between Jupiter’s plasma flow and the atmosphere of its moons – Io and Europa – AuroraMHD advanced interpretation of previous data. This included hints that Galileo had registered a Europa water plume.

Space

The two Galilean moons – Io and Europa – are both embedded in Jupiter’s vast magnetosphere. This is a swirling magnetised plasma of electrically conductive ionised gas. The gas interacts with the moons’ atmospheres generating aurorae, as observed by the Hubble Space Telescope. Changes to the brightness and morphology of these aurorae correlate with the moons’ changing plasma environment. For example, bright spots near Io’s equator oscillate in relation to Jupiter’s changing magnetospheric field. Europa’s aurora is faint in the equatorial region, with bright patches in the northern and southern polar regions. Io’s plasma interaction electromagnetically couples Io and Jupiter through an invisible electric current of charged particles leaving auroral ‘footprints’ in Jupiter’s southern and northern hemispheres. The brightness of these vary according to Io’s local interaction with the magnetospheric plasma. The AuroraMHD project, supported by the Marie Skłodowska-Curie Actions, developed a computer model to study the interaction between the moons and the moving magnetised plasma. “Comparing our results to actual plasma measurements increased our knowledge about the interiors and atmospheres of these moons. We also possibly identified a Europa water plume detected by Galileo,” says research fellow Aljona Blöcker from the KTH Royal Institute of Technology in Stockholm.

Modelling plasma interactions

AuroraMHD modelled different physical phenomena from moon and plasma interactions, such as collisions between particles in the moons’ atmospheres and plasma particles from Jupiter’s magnetosphere. As these phenomena fluctuate over time, the simulations were generated from different configurations. The results were compared with measurements from NASA’s Galileo spacecraft, through a collaboration with ESTEC-ESA. Data about the electromagnetic fields in Europa’s plasma environment, from each simulation, were input to further simulations of the flux of energetic protons. Water molecules can modify the electromagnetic environment of the atmosphere and so affect the trajectories of energetic particles. As Europa is hypothesised to have a water ocean under its crust, proton movements were simulated within different configurations of Europa’s atmosphere, with a water vapour plume present. The results were then compared to energetic particle measurements made by NASA’s Galileo spacecraft. “We were excited to see that including a plume in our simulation leads to a similar proton depletion as measured during one fly-by of Galileo,” adds Blöcker. “With no direct observations of water plumes, hints like this are extremely important. Energetic proton depletions had not been analysed in relation to different atmospheric features of Europa before.” The team also modelled plasma interaction with Io to investigate how variations in Io’s plasma environment and atmosphere could affect the energy flux radiating away from Io, which influences its aurora footprint. “We related changes in atmospheric and plasma densities, alongside magnetic field alterations, to the variations in the brightness of Io’s footprint observed by NASA’s Juno spacecraft,” explains Blöcker.

Helping future missions

Io is the most volcanically active body in our solar system with lava flows and eruptions present all over its surface. The moon has a thin atmosphere of sulfur dioxide and might harbour a magma ocean. Europa has a weak oxygen atmosphere and, covered in ice, has the smoothest surface in our solar system. As it harbours a salty water ocean under its icy crust, it is believed to have the key ingredients to sustain life. AuroraMHD’s results help the planning of future spacecraft observations, such as ESA’s JUpiter ICy moons Explorer (JUICE) and NASA’s Europa Clipper mission.

Keywords

AuroraMHD, plasma, Europa, Jupiter, Io, plume, water, Galileo, aurora, atmosphere, moon, proton

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