Charged nanograins in the Enceladus plume



Enceladus, which orbits Saturn in the planet's E ring, is one of the few geologically active moons in the outer solar system. It emits a large plume that contains water-ice dust grains. Hill et al. used instruments on the Cassini spacecraft to observe charged nanometer sized grains of water ice emanating from Enceladus. The authors measured the charge-to-mass ratio of each grain and found that the most likely charge per grain is 1 electron charge. In addition, negatively charged grains outnumber positively charged ones by a factor of more than 2000. On the basis of their analysis, the researchers argue that the nanograins are not charged when they leave Enceladus but become charged as they encounter electrons in the plasma around Enceladus. (Journal of Geophysical Research-Space Physics, doi:10.1029/2011JA017218, 2012)