Negative Ions a Positive Sign for Liquid Water in Enceladus

In recent dives through the water ice plume of Enceladus, the Cassini plasma spectrometer has found unexpected populations of charged molecules and dust that strengthen arguments for the presence of liquid water and the ingredients for life inside the icy moon.

The Cassini plasma spectrometer, originally designed to take data in Saturn’s magnetic environment, measures the density, flow velocity and temperature of ions and electrons that enter the instrument. But since the discovery of Enceladus’ water ice plume, the instrument has also successfully captured and analyzed samples of material in the jets.

Early in its mission, Cassini discovered the plume that fountains water vapor and ice particles above Enceladus. Since then, scientists have found that these water products dominate Saturn’s magnetic environment and create Saturn’s huge E ring.

Now, Cassini scientists report they have found negatively charged ions in the plume, many of which are water, but also including other hydrocarbon species. Their findings, based on analysis from data taken in plume fly-throughs in 2008, are reported in the journal Icarus.

Enceladus thus joins Earth, Titan and comets where negatively charged ions are known to exist in the solar system. Negative oxygen ions were discovered in Earth’s ionosphere at the dawn of the space age. At Earth’s surface, negative water ions are present where liquid water is in motion, such as waterfalls or crashing ocean waves.

“While it’s no surprise that there is water there, these short-lived ions are extra evidence for sub-surface water and where there’s water, carbon and energy, some of the major ingredients for life are present,” said lead author Andrew Coates from University College London’s Mullard Space Science Laboratory.

“The surprise for us was to look at the mass of these ions. There were several peaks in the spectrum, and when we analyzed them we saw the effect of water molecules clustering together one after the other.” The measurements were made as Cassini plunged through Enceladus’ plume on March 12, 2008.
Looking toward high northern latitudes on Titan, the Cassini spacecraft spies a banded pattern encircling the pole.

At Titan, the same instrument detected extremely large negative hydrocarbon ions with masses up to 13,800 times that of hydrogen. A paper in Planetary and Space Science by Coates and colleagues in December 2009. They found showed that, at Titan, the largest hydrocarbon or nitrile ions are seen at the lowest altitudes of the atmosphere that Cassini flew (950 kilometers, or 590 miles). They suggest these large ions are the source of the smog-like haze that blocks most of Titan’s surface from view. They may be representative of the organic mix called “tholins” by Carl Sagan when he produced the reddish brew of prebiotic chemicals in the lab from gases that were known to be present in Titan’s atmosphere. Tholins that may be produced in Titan’s atmosphere could fall to the moon’s surface and may even make up the sand grains of the dunes that dominate part of Titan’s equatorial region.

The findings add to our growing knowledge about the detailed chemistry of Enceladus’ plume and Titan’s atmosphere, giving new understanding of environments beyond Earth where pre-biotic or life-sustaining environments might exist.

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information above was derived from the following publications:

1) “Negative ions in the Enceladus plume,” A.J.Coates, (Mullard Space Science Lab. University College of London); G.H. Jones, G.R. Lewis, A. Wellbrock (Mullard Space Science Lab and Centre for Planetary Sciences at University College of London); D.T. Young, F.J. Crary (Southwest Research Institute, San Antonio Texas); R.E. Johnson, T.A. Cassidy (University of Virginia, Charlottesville); T.W. Hill (Rice University, Houston, Texas), Icarus, in press, online July 17, 2009

2) “Heavy negative ions in Titan’s ionosphere: Altitude and latitude dependence,” A.J.Coates, A.Wellbrock, G.R. Lewis, G.H. Jones (Mullard Space Science Lab and Centre for Planetary Sciences at University College of London), D.T. Young, F.J. Crary, J.H. Waite Jr. (Southwest Research Institute, San Antonio, Texas), Planetary and Space Science, Volume 57, Issues 14-15, December 2009, Pages 1866-1871

3) “Heavy ion formation in Titan’s ionosphere: Magnetospheric introduction of free oxygen and a source of Titan’s aerosols?” E.C. Sittler Jr., (NASA Goddard Space Flight Center (GSFC), Maryland); A. Ali (NASA GSFC and University of Maryland, College Park, Maryland); J.F. Cooper, R.E. Hartle (NASA GSFC); R.E. Johnson (University of Virginia, Charlottesville), A.J. Coates (Mullard Space Science Lab. University College of London) and D.T. Young (Southwest Research Institute, San Antonio, Texas), Planetary and Space Science, Volume. 57, Issue 13, November 2009, Pages 1547-1557.

4) “On the amount of heavy molecular ions in Titan’s ionosphere,” J.-E. Wahlund (Swedish Institute of Space Physics, Uppsala); M. Galand, I. Mueller-Wodarg, J. Cui (Imperial College of London); R.V. Yelle (University of Arizona); F.J. Crary, K. Mandt, B. Magee, J.H. Waite Jr., D.T. Young (Southwest Research Institute, San Antonio, Texas); A.J. Coates (Mullard Space Science Laboratory, University College of London); P. Garnier (Swedish Institute of Space Physics, Uppsala, University of Toulouse, France, and CNRS, Toulouse); K. Agren, M. Andre, A.I. Eriksson (Swedish Institute of Space Physics, Uppsala); T.E. Cravens (University of Kansas); V. Vuitton (Laboratory of Planetology of Grenoble, France); D.A. Gurnett and W.S. Kurth (University of Iowa), Planetary and Space Science, Volume 57, Issues 14-15, December 2009, Pages 1857-1865

5) “Negative ion chemistry in Titan’s upper atmosphere,” V. Vuitton (Laboratory of Planetology of Grenoble, France and University of Arizona, Tuscon); P. Lavvas, R.V. Yelle (University of Arizona, Tuscon); M. Galand (Imperial College, London), A. Wellbrock, G.R. Lewis, A.J. Coates (Mullard Space Science Laboratory and Centre for Planetary Sciences, University College of London) and J.-E. Wahlund (Swedish Institute of Space Physics, Uppsala, Sweden) Laboratory of Planetology of Grenoble, France) aboratoire de Planetologie de Grenoble, CNRS, Grenoble, France, Planetary and Space Science, Volume 57, Issues 14-15, December 2009, Pages 1857-1865