Kicking Up Dust – Saturn and the Solar Wind Send Tiny Particles Flying

The story begins in 1992 when the dust detector aboard the Ulysses spacecraft detected dust streams coming from Jupiter during its gravity assist flyby of the planet. The Galileo spacecraft, which orbited Jupiter, and Cassini, which used a gravity assist from Jupiter, also detected dust streams there. Cassini went on to find fast-moving dust streams when it reached Saturn.

Such observations are counter-intuitive because one would expect dust particles to be drawn toward the planets, not sent flying away at high speeds. The dust particles in the streams ejected by Jupiter and Saturn are tiny and swift. About one-hundred-thousandth the diameter of a human hair, they are accelerated by the solar wind to speeds more than 100 times faster than a rifle bullet

H.-W. (Sean) Hsu, of the University of Colorado, and his colleagues used data collected by Cassini’s cosmic dust analyzer during Cassini’s initial three orbits of Saturn with supporting data from the spacecraft’s magnetometer to develop a model and explanation of the process that creates the dust streams coming from both giant planets.

They found that several steps are involved. The tiny dust particles must acquire an electric charge, and then the magnetosphere of the planet and the interplanetary magnetic field carried by the solar wind become players. The electromagnetic forces greatly exceed the force of gravity and govern the behavior of the dust particles. The accompanying animation illustrates how the components of the phenomenon work together.

At Jupiter, the source of the dust particles is volcanism on the satellite Io. At Saturn, the building blocks of these nanoparticles are metal-poor silicates. These may be generated by meteoroid collisions with Saturn’s moons and rings. The abundant water ice on Saturn’s moons and rings also experiences plasma sputtering erosion that may free silicate particles of their icy shells. Solar ultraviolet radiation, via the photoelectric effect, and charge transfer by solar wind ions could leave the dust particles with a residual electric charge.

The planetary magnetosphere comes into play with the charged dust particles. The rotation of Saturn’s magnetic field generates an electric field directed radially outward from the planet. This field will eject small, electrically charged particles, into interplanetary space. Once that happens, the interplanetary magnetic field “frozen” into the solar wind can accelerate the dust particles to high speeds, creating the bursts of particles recorded by cosmic dust detectors. Evidence for this interpretation came from the changes in direction of the stream particles when the interplanetary magnetic field changed direction, in close timing with the Sun’s rotation.

Additional findings from Cassini have added more to the story. Besides detecting high-velocity dust particles accelerated by the solar wind, Cassini’s cosmic dust analyzer also found “faint” impacts coming from the line of sight to Saturn. These impacts varied with Cassini’s proximity to Saturn. Researchers think they may be the result of “freshly” ejected dust particles that had not yet been fully accelerated by the solar wind field away from their original trajectories.

Simulations suggest that sputtered E ring particles are the source of the silicate nanoparticles detected by the cosmic dust analyzer. But that’s not all. The products of the sputtering, among them neutral and charged atoms and molecules like oxygen, hydroxyl (OH) and water, have been observed by Cassini’s other instruments, including the ultraviolet imaging spectrograph. It seems that E ring grains, whose origins are the jets of Enceladus, serve as a source of both neutral and electrically charged particles in Saturn’s mid- to outer magnetosphere.

Cassini’s study of the puzzling dust stream phenomenon has helped illuminate a new mechanism for accelerating dust particles in dusty plasmas, an area of science currently receiving particular attention. Given that dust and dusty plasmas are found in regions where new stars form, Hsu et al.’s results from Saturn may provide new insight into the process of star and planet formation in the Milky Way galaxy and elsewhere.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory in Pasadena manages the mission for the agency's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology, Pasadena.

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 or informed by the following publications:

“Cassini Dust Stream Particle Measurements during the First Three Orbits at Saturn,”H.-W. Hsu,(MPI für Kernphysik, Heidelberg, Germany; Institut für Geowissenschaften, Ruprecht-Karls Universität, Heidelberg, Germany; Laboratory for Atmospheric and SpacePhysics, University of Colorado at Boulder, Boulder, Colorado, USA), S. Kempf, (MPI für Kernphysik, Heidelberg, Germany, Institut für Geophysik und extraterrestrische Physik, Universität Braunschweig, Braunschweig, Germany), F. Postberg (Institut für Geowissenschaften, Ruprecht-Karls Universität, Heidelberg, Germany; MPI für Kernphysik, Heidelberg, Germany), M. Trieloff, Institut für Geowissenschaften, Ruprecht-Karls Universität, Heidelberg, Germany), M. Burton, (Jet Propulsion Laboratory, Pasadena, California, USA), M. Roy (Jet Propulsion Laboratory, Pasadena, California, USA), G. Moragas-Klostermeyer (MPI für Kernphysik, Heidelberg, Germany; Laboratory for Atmospheric and SpacePhysics, University of Colorado at Boulder, Boulder, Colorado, USA), R. Srama (MPI für Kernphysik, Heidelberg, German; Institut für Raumfahrtsysteme ,Universität Stuttgart, Stuttgart, Germany), J. Geophys. Res., Vol. 116, No. A8, A08213, http://dx.doi.org/10.1029/2010JA015959.

“Stream Particles as the Probe of the Dust-Plasma-Magnetosphere Interaction at Saturn,”H.-W. Hsu,(MPI für Kernphysik, Heidelberg, Germany; Institut für Geowissenschaften, Ruprecht-Karls Universität, Heidelberg, Germany; Laboratory for Atmospheric and SpacePhysics, University of Colorado at Boulder, Boulder, Colorado, USA), F. Postberg (Institut für Geowissenschaften, Ruprecht-Karls Universität, Heidelberg, Germany; MPI für Kernphysik, Heidelberg, Germany), S. Kempf, (MPI für Kernphysik, Heidelberg, Germany, Institut für Geophysik und extraterrestrische Physik, Universität Braunschweig, Braunschweig, Germany, Laboratory for Atmospheric and SpacePhysics, University of Colorado at Boulder, Boulder, Colorado, USA), M. Trieloff, Institut für Geowissenschaften, Ruprecht-Karls Universität, Heidelberg, Germany), M. Burton, (Jet Propulsion Laboratory, Pasadena, California, USA), M. Roy (Jet Propulsion Laboratory, Pasadena, California, USA), G. Moragas-Klostermeyer (MPI für Kernphysik, Heidelberg, Germany; Laboratory for Atmospheric and SpacePhysics, University of Colorado at Boulder, Boulder, Colorado, USA), R. Srama (MPI für Kernphysik, Heidelberg, German; Institut für Raumfahrtsysteme ,Universität Stuttgart, Stuttgart, Germany), J. Geophys. Res., Vol. 116, No. A9, A09215, http://dx.doi.org/10.1029/2011JA016488.

The research group website can be found at http://www.mpi-hd.mpg.de/dustgroup/

-- Stephen J. Edberg, Cassini science communication coordinator