UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS

During the Cassini spacecraft’s first encounter with Saturn’s 500km-wide moon Enceladus, clear indications were detected by the spacecraft’s magnetometer that the way that this body was interacting with Saturn’s magnetosphere was highly unusual. Further observations in 2005 showed that the moon was expelling gas and dust from its south polar region.

Observations from the Cassini Plasma Spectrometer (CAPS) made during the Cassini fly-by of Enceladus on 12 March 2008, superimposed on Cassini’s path. As the spacecraft passed the moon, CAPS detected streams of charged particles in individual jets within the plume; negative particles are shown in this view. Each ribbon in the image gives an indication of the measured particle energy per charge: high energy particle fluxes are shown nearest Enceladus, and lower energy particles are farthest. The red points marked on Enceladus show the locations of known jet sources found by other Cassini instruments.

Credit: MSSL/UCL

Cassini has encountered Enceladus several times since then. During some of the encounters, the spacecraft has passed directly through the plume of gas and dust, and the MSSL-led portion of the Cassini Plasma Spectrometer – the Electron Spectrometer, ELS, has directly sampled this material by being pointed in the direction of travel while traversing the plume.

Rather than only detecting electrons as it was designed to do, CAPS-ELS detected negatively charged, nanometer-sized grains when crossing the plume. These particles were structured into individual jets, and another part of CAPS, the ion mass spectrometer, IMS, detected positively-charged grains, which were structured differently to their negatively-charged counterparts.

These surprising observations reveal the low end of the range of masses present in Enceladus’s plume, complementing observations made by the spacecraft’s dust instrument, CDA. The differences between the positively- and negatively-charged grains suggest that the trajectories of these charged grains are strongly influenced by forces associated with the flow of plasma past Enceladus.

For more information, see:

G. H. Jones, C. S. Arridge , A. J. Coates , G. R. Lewis , S. Kanani , A. Wellbrock , D. Young , F. Crary , R. L. Tokar , R. Wilson , T. Hill , R. Johnson , D. Mitchell , J. Schmidt , S. Kempf , U. Beckmann , C. T. Russell , Y. Jia , M. K. Dougherty , J. H. Waite , B. Magee, Fine jet structure of electrically charged grains in Enceladus's plume Geophys. Res. Lett. 36, L16204, doi:10.1029/2009GL038284, 2009.