ESA Cassini-Huygens: Cassini samples the icy spray of Enceladus' water plumes

The NASA/ESA/ASI Cassini-Huygens mission has directly sampled the water plumes jetting into space from Saturn’s moon Enceladus.

The findings from these fly-throughs are the strongest evidence yet for the existence of large-scale saltwater reservoirs beneath the moon’s icy crust.

Enceladus’ water plumes shoot water vapour and tiny grains of ice into space.

They originate from the ‘tiger stripe’ surface fractures at the moon’s south pole, and create the faint E-ring, which traces the orbit of Enceladus around Saturn.

The Cassini spacecraft discovered the plumes in 2005 and more recently has been able to fly directly through them.

During three of Cassini’s passes in 2008 and 2009, the Cosmic Dust Analyser measured the composition of freshly ejected plume grains.

The icy particles hit the detector target at speeds of 6.5–17.5 km/s, and vaporised instantly. Electrical fields inside the instrument then separated the various constituents of the resulting impact cloud for analysis.

Far away from Enceladus, the data show that the ejected grains are relatively small and mostly salt-poor, closely matching the composition of the E-ring. Closer to the moon, however, Cassini has found that relatively large, salt-rich ice grains dominate.

It appears as though more than 99% of the total mass of ejected solids is in salt-rich grains, but most of these are heavy and fall back to the moon, so never make it into the E-ring.

The salt-rich particles have an ‘ocean-like’ composition which indicates that most, if not all, of the expelled ice comes from liquid saltwater, rather than from the icy surface of the moon.

When salty water freezes slowly, the salt is squeezed out, leaving pure water ice behind. So, if the plumes were coming from the surface ice, there should be very little salt in them.

“There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across the tiger stripes other than from saltwater under Enceladus’ icy surface,” says Frank Postberg, Universität Heidelberg, Germany, who is the lead author on the paper announcing these results.

ESA - Cassini-Huygens - Cassini samples the icy spray of Enceladus' water plumes