A new international study has found that Enceladus, one of Saturn's smallest moons, leaves behind a vast trail of electromagnetic ripples stretching more than 500,000 kilometres into space. Using 13 years of data from the NASA/ESA/ASI Cassini spacecraft, researchers discovered a crisscross, lattice-like pattern of reflected waves flowing behind the icy moon. The findings show how powerful plumes of water vapour and dust erupting from Enceladus' southern surface become electrically charged in space, interacting with Saturn's magnetic field and playing a key role in shaping the planet's space environment.
"Enceladus, Saturn's small icy moon, is famous for its water geysers, but its actual impact and interaction with the giant planet has remained partly unknown. This result from Cassini transforms our vision of the moon's role in the Saturnian system," said Lina Hadid of the Laboratoire de Physique de Plasmas (LPP) in France, who led the study.
The study, published in the Journal of Geophysical Research: Space Physics, shows how wave structures, known as ‘Alfvén wings', travel like vibrations on a string along magnetic field lines connecting Enceladus to Saturn's pole. The initial ‘main' Alfvén wing is reflected back-and-forth both by Saturn's ionosphere and the plasma torus that encircles Enceladus's orbit, resulting complex and structured system. By using a multi-instrumental approach, researchers were able to show that the influence of Enceladus extends over a record distance of over 504,000 km – more than 2,000 times the moon's radius.
"This is the first time such an extensive electromagnetic reach by Enceladus has been observed, proving that this small moon acts as a giant planetary-scale Alfvén wave generator," said Thomas Chust of LPP, co-author of the study. "This work sets the stage for future studies of other systems, such as the icy moons of Jupiter or exoplanets, by showing that a small moon with an electrically-conducting atmosphere can influence its host over vast distances on the scale of the giant planet itself."
The researchers examined archive data from the suite of instruments carried by Cassini to study electromagnetic wave and particle interactions, looking for flyby and non-flyby paths near Enceladus that showed evidence of magnetic connections between the moon and Saturn. On 36 occasions, they found signatures related to Alfvén waves, including at much further distances than they originally anticipated.
As well as the large-scale structures, the team found evidence that turbulence teases out the waves into filaments within the main Alfvén wing. This fine-scale structure helps the waves bounce off Enceladus's plasma torus and reach the high-latitudes in Saturn's ionosphere where auroral features associated with the moon form.
"These results highlight the importance for future missions to Enceladus, such as the planned ESA orbiter and lander in the 2040s, to carry instrumentation that can study these electromagnetic interactions in even more detail," said Hadid.
