Saturn moon spouts plasma unlike any seen before

A NEW form of matter surrounds Saturn - a plasma put there by Enceladus, the planet's tiny moon.

"It's a type of charged particle that has never been observed before," says Tom Hill of Rice University in Houston, Texas.

Shortly after it arrived at the Saturn system in 2004, the Cassini spacecraft discovered that the small icy moon Enceladus was spouting a watery geyser. The plume contained water vapour, as well as micrometre-sized dust grains.

Yet in 2009, Cassini saw something else in the plume: nanometre-sized grains that each carried an electric charge. That meant the plume was a powerful source of plasma, a form of matter in which positively and negatively charged particles move around separately. It seems that Enceladus provides most of the plasma in the magnetic bubble, or magnetosphere, surrounding Saturn.

But it was unclear how the particles got their charges. Now, after three fly-bys during which Cassini's plasma detectors could investigate the nanograins, Hill and colleagues think they have an answer.

The sun's ultraviolet light strips electrons from the gas and other material in the plume, creating a cloud of free electrons. As the uncharged nanograins leave Enceladus and move through this charged cloud, they pick up about one electron each to create a plasma.

But that means that the structure of the plasma is backwards, says Hill. Most plasmas contain positive ions, which carry mass, and negative free electrons, which carry almost no mass. Here, most of the mass is in the form of negatively charged grains (Journal of Geophysical Research, DOI: 10.1029/2011JA017218).

"That changes the basic behaviour of the plasma," says Hill - although we will have to wait until Cassini next flies by the plume in more than a year to see in what way.

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