This false-color mosaic combines Imaging Science Subsystem (ISS) narrow-angle camera images obtained through ultraviolet, green, and near-infrared camera filters. Areas that are greenish in appearance are believed to represent deposits of coarser grained ice and solid boulders that are too small to be seen at this scale, but which are visible in the higher resolution views, while whitish deposits represent finer grained ice. The mosaic shows that coarse-grained and solid ice are concentrated along valley floors and walls, as well as along the upraised flanks of the “tiger stripe” fractures, which may be covered with plume fallout that landed not far from the sources. Elsewhere on Enceladus, this coarse water ice is concentrated within outcrops along cliff faces and at the top of ridges. The sinuous boundary of scarps and ridges that encircles the south polar terrain at about 55 degrees south latitude is conspicuous. Much of the coarse-grained or solid ice along this boundary may be blocky rubble that has crumbled off of cliff faces as a result of ongoing seismic activity. This mosaic complements the imaging coverage acquired during Cassini’s July 2005 flyby of Enceladus by showing portions of the moon’s south polar region and tiger stripes, or sulci, that were in darkness during that flyby (PIA06247).
The reversed lighting conditions over the polar region (compared to the July 2005 images) highlight features, such as fractures and ridges, that are barely visible in the July 2005 views, and vice versa. The four most prominent sulci (from top to bottom: Damascus, Baghdad, Alexandria and Cairo) appear as generally horizontal fractures near lower right, and they extend into the moon’s night side. The mosaic is an orthographic projection centered at 63.0 degrees south latitude, 281.3 degrees west longitude, and has an image scale of 60 meters (196 feet) per pixel. The original images ranged in resolution from 28 to 154 meters (92 to 505 feet) per pixel and were taken at distances ranging from 5,064 to 25,949 kilometers (3,140 to 15,468 miles) from Enceladus.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
PASADENA, Calif. — In a feat of interplanetary sharpshooting, NASA’s Cassini spacecraft has pinpointed precisely where the icy jets erupt from the surface of Saturn’s geologically active moon Enceladus.
New carefully targeted pictures reveal exquisite details in the prominent south polar “tiger stripe” fractures from which the jets emanate. The images show the fractures are about 300 meters (980 feet) deep, with V-shaped inner walls. The outer flanks of some of the fractures show extensive deposits of fine material. Finely fractured terrain littered with blocks of ice tens of meters in size and larger (the size of small houses) surround the fractures.
“This is the mother lode for us,” said Carolyn Porco, Cassini imaging team leader at the Space Science Institute, Boulder, Colo. “A place that may ultimately reveal just exactly what kind of environment — habitable or not — we have within this tortured little moon.”
One highly anticipated result of this flyby was finding the location within the fractures from which the jets blast icy particles, water vapor and trace organics into space. Scientists are now studying the nature and intensity of this process on Enceladus, and its effects on surrounding terrain. This information, coupled with observations by Cassini’s other instruments, may answer the question of whether reservoirs of liquid water exist beneath the surface.
The high-resolution images were acquired during an Aug. 11, 2008, flyby of Enceladus, as Cassini sped past the icy moon at 64,000 kilometers per hour (40,000 miles per hour). A special technique, dubbed “skeet shooting” by the imaging team, was developed to cancel out the high speed of the moon relative to Cassini and obtain the ultra-sharp views.
“Knowing exactly where to point, at just the right time, was critical to this event,” said Paul Helfenstein, Cassini imaging team associate at Cornell University, Ithaca, NY., who developed and used the skeet-shoot technique to design the image sequence. “The challenge is equivalent to trying to capture a sharp, unsmeared picture of a distant roadside billboard with a telephoto lens out the window of a speeding car.”
Helfenstein said that from Cassini’s point of view, “Enceladus was streaking across the sky so quickly that the spacecraft had no hope of tracking any feature on its surface. Our best option was to point the spacecraft far ahead of Enceladus, spin the spacecraft and camera as fast as possible in the direction of Enceladus’ predicted path, and let Enceladus overtake us at a time when we could match its motion across the sky, snapping images along the way.”
For scientists, having the combination of high-resolution snapshots and broader images showing the whole region is critical for understanding what may be powering the activity on Enceladus.
“There appears to have been extensive fallout of icy particles to the ground, along some of the fractures, even in areas that lie between two jet source locations, though any immediate effects of presently active jets are subtle,” said Porco.
Imaging scientists suggest that once warm vapor rises from underground to the cold surface through narrow channels, the icy particles may condense and seal off an active vent. New jets may then appear elsewhere along the same fracture.
“For the first time, we are beginning to understand how freshly erupted surface deposits differ from older deposits,” said Helfenstein, an icy moons expert. “Over geologic time, the eruptions have clearly moved up and down the lengths of the tiger stripes.”
The new images, with jet source locations labeled, are available at: http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov and http://ciclops.org .