Jets of fine, icy particles streaming from Saturn's moon
Enceladus were captured in recent images from NASA's Cassini
spacecraft. The images provide unambiguous visual evidence that the
moon is geologically active.
"For planetary explorers like us, there is little that can
compare to the sighting of activity on another solar system body,"
said Dr. Carolyn Porco, Cassini imaging team leader at the Space
Science Institute in Boulder, Colo. "This has been a heart-stopper,
and surely one of our most thrilling results."
The Cassini images clearly show multiple jets emanating from the
moon's south polar region. Based on earlier data, scientists
strongly suspected these jets arise from warm fractures in the
region. The fractures, informally dubbed "tiger stripes," are
viewed essentially broadside in the new images.
The fainter, extended plume stretches at least 186 kilometers
(300 miles) above the surface of Enceladus, which is only 186
kilometers wide. Cassini flew through the plume in July, when it
passed a few hundred kilometers above the moon. During that flyby,
Cassini's instruments measured the plume's constituent water vapor
and icy particles.
Imaging team members analyzed images of Enceladus taken earlier
this year at similar viewing angles. It was a rigorous effort to
demonstrate that earlier apparitions of the plumes, seen as far
back as January, were in fact real and not due to imperfections in
The recent images were part of a sequence planned to confirm the
presence of the plumes and examine them in finer detail. Imaging
team member Dr. Andrew Ingersoll from the California Institute of
Technology in Pasadena, said, "I think what we're seeing are ice
particles in jets of water vapor that emanate from pressurized
vents. To form the particles and carry them aloft, the vapor must
have a certain density, and that implies surprisingly warm
temperatures for a cold body like Enceladus."
Imaging scientists are comparing the new views to earlier
Cassini data in hopes of arriving at a more detailed,
three-dimensional picture of the plumes and understanding how
activity has come about on such a small moon. They are not sure
about the precise cause of the moon's unexpected geologic
"In some ways, Enceladus resembles a huge comet," said Dr.
Torrence Johnson, imaging team member from NASA's Jet Propulsion
Laboratory in Pasadena. "Only, in the case of Enceladus, the energy
source for the geyser-like activity is believed to be due to
internal heating by perhaps radioactivity and tides rather than the
sunlight which causes cometary jets." The new data also give yet
another indication of how Enceladus keeps supplying material to
Saturn's gossamer E ring.
The Cassini-Huygens mission is a cooperative project of NASA,
the European and Italian Space Agencies. JPL, a division of the
Caltech, manages the mission for NASA's Science Mission
Directorate. The Cassini orbiter and its two onboard cameras were
designed, developed and assembled at JPL. The imaging team is based
at the Space Science Institute.