CASSINI FINDS RECENT, UNUSUAL GEOLOGY ON ENCELADUS
New detailed images taken by NASA's Cassini spacecraft of the south polar region of Saturn's moon Enceladus reveal distinctive geological features, and the most youthful terrains of any seen on Enceladus. These findings point to a very complex evolutionary history for Saturn's brightest, whitest world.
Cassini's flyby on July 14 brought it within 175 kilometers (109 miles) of the surface of the icy moon. The close encounter revealed a landscape near the south pole littered with house-sized ice boulders, carved by tectonic patterns unique to Enceladus, and almost entirely free of impact craters. These features set the southerly region apart from the rest of the moon.
As white as fresh snow, Enceladus (505 kilometers, 314 miles across) has the most reflective surface in the solar system. Previous Cassini flybys have revealed that Enceladus, in contrast to the other icy moons of Saturn, possesses lightly cratered regions, fractured plains and wrinkled terrain.
The new findings add to the story of a body that has undergone multiple episodes of geologic activity spanning a considerable fraction of its lifetime, and whose southern-most latitudes have likely seen the most recent activity of all. These same latitudes may also bear the scars of a shift in the moon's rate of spin. This speculation which, if true, may help scientists understand why Enceladus has a tortured looking surface, with pervasive crisscrossing faults, folds and ridges.
The most remarkable images show ice blocks, about 10 to 100 meters (33 to 328 feet) across, lying in a region that is unusual in its lack of the very fine-grained frost that seems to cover the rest of Enceladus.
"A landscape littered with building-sized blocks was not expected," said Dr. Peter Thomas, an imaging team member from Cornell University, Ithaca, N.Y. "The minimal cover of finer material and the preservation of small, crossing fracture patterns in the surrounding areas indicate that this region is young compared to the rest of Enceladus."
False color composites of this region, created from the most recent images, show the largest exposures of coarse-grained ice fractures seen anywhere on the satellite. It also supports the notion of a young surface at southern latitudes.
"These southern exposures of coarse-grained ice are aligned with tectonic "stripes" and not covered by the fine-grained materials that one would expect from various space weathering processes," said Dr. Alfred McEwen, an Imaging Team member at the University of Arizona, Tucson.
The apparent absence of sizable impact craters also suggests that the south pole is younger than other terrains on Enceladus. These indications of youth are of great interest to scientists who have long suspected Enceladus as one possible source of material for Saturn's extensive and diffuse E ring, which coincides with the icy moon's orbit. Young surfaces represent a challenge as it has generally been believed that Enceladus is too small and too cold to generate the heat required to modify its surface.
Some of the latest images, which have revealed additional examples of a distinctive "Y-shaped" tectonic feature on Enceladus in which parallel ridges and valleys appear to systematically fold and deform around the south polar terrains, may hint at the answer.
"These tectonic features define a boundary that isolates the young, south polar terrains from older terrains on Enceladus," noted Dr. Paul Helfenstein, an associate of the Imaging Team also at Cornell University. "Their placement and orientation may tell us a very interesting story about the way the rotation of Enceladus has evolved over time and what might have provided the energy to power the geologic activity that has wracked this moon."
Cassini will explore Enceladus further in a future close flyby planned for March 2008.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team leader (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.