March 16, 2005
Atmosphere Found on Saturn’s Moon Enceladus
JPL -- The Cassini spacecraft's two close flybys of Saturn's icy moon Enceladus have revealed that the moon has a significant atmosphere. Scientists, using Cassini's magnetometer instrument for their studies, say the source may be volcanism, geysers, or gases escaping from the surface or the interior.
When Cassini had its first encounter with Enceladus on Feb. 17 at an altitude of 1,167 kilometers (725 miles), the magnetometer instrument saw a striking signature in the magnetic field. On March 9, Cassini approached to within 500 kilometers (310 miles) of Enceladus' surface and obtained additional evidence.
The observations showed a bending of the magnetic field, with the magnetospheric plasma being slowed and deflected by the moon. In addition, magnetic field oscillations were observed. These are caused when electrically charged (or ionized) molecules interact with the magnetic field by spiraling around the field line.
This interaction creates characteristic oscillations in the magnetic field at frequencies that can be used to identify the molecule. The observations from the Enceladus flybys are believed to be due to ionized water vapor.
"These new results from Cassini may be the first evidence of gases originating either from the surface or possibly from the interior of Enceladus," said Dr. Michele Dougherty, principal investigator for the Cassini magnetometer and professor at Imperial College in London.
This is the first time since Cassini arrived in orbit around Saturn last summer that an atmosphere has been detected around a moon of Saturn, other than its largest moon, Titan. Enceladus is a relatively small moon. The amount of gravity it exerts is not enough to hold an atmosphere very long. Therefore, at Enceladus, a strong continuous source is required to maintain the atmosphere.
The need for such a strong source leads scientists to consider eruptions, such as volcanoes and geysers. If such eruptions are present, Enceladus would join two other such active moons, Io at Jupiter and Triton at Neptune.
"Enceladus could be Saturn's more benign counterpart to Jupiter's dramatic Io," said Dr. Fritz Neubauer, co-investigator for the Cassini magnetometer, and a professor at the University of Cologne in Germany.
Since the Voyager flyby, scientists have suspected that this moon is geologically active and is the source of Saturn's icy E ring.
Enceladus is the most reflective object in the solar system, reflecting about 90 percent of the sunlight that hits it. If Enceladus does have ice volcanoes, the high reflectivity of the moon's surface might result from continuous deposition of icy particles originating from the volcanoes.
Enceladus' diameter is about 500 kilometers (310 miles), which would fit in the state of Arizona. Yet despite its small size, Enceladus exhibits one of the most interesting surfaces of all the icy satellites.
Images and Captions
Image 1: Atmosphere on Enceladus -- This artist concept shows the detection of an atmosphere on Saturn's icy moon Enceladus. The Cassini magnetometer instrument is designed to measure the magnitude and direction of the magnetic fields of Saturn and its moons. During Cassini's two close flybys of Enceladus -- Feb. 17 and March 9 -- the instrument detected a bending of the magnetic field around Enceladus.
The graphic shows the magnetic field observed by Cassini along its trajectory plotted in a vector form. Even though the spacecraft altitude was almost 500 kilometers (310 miles) at closest approach and the flyby was upstream of the moon (where the interaction is expected to be weaker) Cassini's magnetometer observed bending of the magnetic field consistent with its draping around a conducting object, which indicates that the Saturnian plasma is being diverted away from an extended atmosphere.
Image 2: Icy Enceladus -- This infrared color image of Enceladus was obtained by the Cassini visual infrared mapping spectrometer on March 9, 2005, when the Cassini spacecraft was 9,145 kilometers (5,716 miles) away from Enceladus.
Enceladus shows substantial differences in composition or, more likely, particle size on its surface. Redder areas correspond to larger grain sizes, and appear to be correlated with craters and ridged regions. The surface of Enceladus is nearly pure water ice; no other components have been identified yet.
The middle of the image is located at the equator near a longitude of 210 degrees. The image is about 100 kilometers (63 miles) square. The image shows the ratio of reflected light at 1.34 and 1.52 microns, wavelengths that are not visible to the human eye.
Image 3: Cracked Face of Enceladus -- The finest details on the surface of Saturn's moon Enceladus are revealed in this 30-meter (100-foot) per-pixel, enhanced-color image taken during Cassini's closest-ever encounter with Enceladus on March 9, 2005.
The surface of Enceladus is almost uniformly white and even though the natural color of this scene has been exaggerated in intensity, no obvious departure from the uniform hue is apparent. The image was also processed to enhance contrast while avoid saturation of the brightest parts of the scene. Hence, the surface does not have the brightness of fresh snow, as it would appear to the human eye.
The Sun is illuminating the surface from the left of the image and at a low enough angle that the rugged ridge crests near upper left (which range in height from 50 to 100 meters or 164 to 328 feet) cast dramatic shadows, as at the top center of the image. The origin of the very small dark spots in the ridged terrain is uncertain. They could be shadows cast by small, building-sized outcrops (approximately 60-meter or 200-feett high) just at the limits of resolution.
Intriguingly, the craters in this scene are quite subdued, indicating that they have been degraded by some process. The craters clearly predate most of the fractures.
Additionally, multiple sets of fractures running in different directions can be seen. One set above the lower right has a gentle appearance similar to that of the craters. In contrast, the fractures running along the left are fresher. By studying differences in the morphology and patterns of the fractures, scientists will be able to learn about Enceladus' crust and how it, and geologic processes acting within it, have changed over time.
Images obtained using red, green and blue spectral filters were combined to create this view. The image was taken with the Cassini spacecraft narrow-angle camera at a distance of approximately 5,200 kilometers (3,200 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 39 degrees. The scene is centered on a region at -3 degrees latitude and 218 degrees longitude.
Image 4: Painting on the Walls -- During its closest flyby of Saturn's wrinkled, icy moon Enceladus, Cassini obtained multi-spectral images of its cratered terrain that have been put together to create this false-color view.
To human eyes, Enceladus appears almost completely white, but false color reveals intriguing details. This view is a composite of images taken using filters sensitive to ultraviolet (centered at 338 nanometers), green (centered at 568 nanometers), and near-infrared (centered at 930 nanometers) light, and has been processed to accentuate subtle color differences. The uppermost surface of these terrains appears uniformly grey in this picture, suggesting that they are covered with materials of homogeneous composition and grain size. However, the walls of many of the fractures appear to be somewhat bluer than typical surface materials. It is possible that the difference in color identifies outcrops of solid ice on the walls of fractures, or ice with different grain-sizes, compared to powdery surface materials. It is also possible that the color identifies some compositional difference between buried ice and ice at the surface.
The surface is peppered with craters of all sizes, from the 21-kilometer (13-mile) diameter crater at the top of the image, down to tiny craters near the limit of resolution. The prominent crater at the top contains a central, domelike structure more than 11 kilometers (7 miles) in diameter. The dome, the crater -- and indeed the entire scene -- is sliced by a complex network of fractures ranging in width from hundreds of meters in some places, to over three kilometers (2 miles) in others.
The prominent, complex fracture in the bottom of the frame extends over 85 kilometers (53 miles) in length across the field of view. From Cassini's oblique vantage point, the walls of the large fracture are clearly visible. A pervasive network of narrow, parallel grooves can be seen in many places in the image, and they appear to slice the surface into parallel slabs of ice approximately 500 meters (1,600 feet) in thickness.
The image has been rotated so that north is at the top of the scene. The terrain in this scene is located on the side of Enceladus that faces away from Saturn, centered on latitude 28.7 north, longitude 192.5 west.
The image was taken during Cassini's closest-ever approach to Enceladus on March 9, 2005. It was taken with the Cassini spacecraft narrow-angle camera at a distance of approximately 21,300 kilometers (13,200 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Resolution in the image is about 130 meters (430 feet) per pixel.
Image 5: Deep Color -- This false-color, close-up look at Saturn's moon Enceladus yields new insight into the different processes that have shaped the moon's icy surface.
Extending through the center of this image is a system of rifts 3 kilometers wide (2 mile) and lanes of grooved terrain 20 kilometers wide (12 mile), which separate two distinct geological provinces. To the right of the boundary is older, cratered terrain -- a region peppered with craters ranging from 10 kilometers (6 miles) in diameter, down to craters near the limit of resolution. The region is believed to be old because it has accumulated a relatively high density of impact craters over time and the topography is soft and muted, suggesting that it is covered by a layer of particulate materials. The cratered terrain is cut crosswise by numerous faults and fractures ranging in width from hundreds of meters to a few kilometers.
On the left side of the scene are grooved, icy plains. This broad, relatively flat region is scored by an extensive band of parallel grooves that appear to subdivide the surface into narrow lanes approximately 1 kilometer or half a mile wide. The low abundance of impact craters and crisp relief on topographic features here imply that this region is geologically much younger than the cratered terrain at the right.
This view is a composite of images taken using filters sensitive to ultraviolet (centered at 338 nanometers), green (centered at 568 nanometers), and near-infrared (centered at 930 nanometers) light, and has been processed to accentuate subtle color differences.
The uppermost surface of these terrains has a relatively uniform pinkish cast in this picture, suggesting that it is covered with materials of homogeneous composition and grain size. However, many of the fractures reveal a distinctly different color (represented by greenish tones in this false-color image) than the typical surface materials in this region. The fractures seem to penetrate down to a material that is texturally or compositionally different than most surface materials. One possibility is that the walls of the fractures expose outcrops of solid ice or ice with different grain-sizes compared to powdery surface materials that mantle flat-lying surfaces. It is also possible that the color identifies some compositional difference between buried ice and ice at the surface.
The scene is located on the side of Enceladus that faces away from Saturn. The images were obtained with the Cassini spacecraft narrow-angle camera when the spacecraft was at a distance of approximately 25,700 kilometers (15,969 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 46 degrees. Resolution in the image is about 150 meters (490 feet) per pixel.
Image 6: Blue Clues -- During its very close flyby on March 9, 2005, the Cassini spacecraft captured this false-color view of Saturn's moon Enceladus, which shows the wide variety of this icy moon's geology. Some geological regions on Enceladus are old and retain large numbers of impact craters; younger areas exhibit many generations of tectonic troughs and ridges. Subtle differences in color may indicate different ice properties, such as grain sizes, that will help unravel the sequence of geologic events leading to the current strange landscape.
This false-color view is a composite of individual frames obtained using filters sensitive to green (centered at 568 nanometers) and infrared light (two infrared filters, centered at 752 and 930 nanometers respectively). The view has been processed to accentuate subtle color differences. The atmosphere of Saturn forms the background of this scene (its color has been rendered grey to allow the moon to stand out).
The Sun illuminates Enceladus from the left, leaving part of it in shadow and blocking out part of the view of Saturn. This view shows the anti-Saturn hemisphere, centered nearly on the equator.
The images comprising this view were taken with the Cassini spacecraft narrow-angle camera at a distance of approximately 94,000 kilometers (58,000 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 48 degrees. Resolution in the image is about 560 meters (1,800 feet) per pixel.
Image 7: Old and New Again -- This false-color Cassini mosaic of Saturn's moon Enceladus captures in a single view, much of the frigid moon's diverse geology.
Cratered terrain dominates most of the scene. The relatively dense accumulation of impact craters implies that this terrain is among the oldest on the moon's surface. Near the bottom of the picture is a crater 20 kilometers wide (12-mile) with a prominent dome-shaped structure in its center. The entire area is transected by a complex web of fractures and faults; some are as narrow as a few hundred meters, others as wide as 5 kilometers (3 miles).
The rims and interiors of many craters seem to be sliced by a pervasive system of narrow, parallel grooves into slabs or lanes that typically are a kilometer (about a half-mile) in width. The widely varied appearances of fractures in this region attest to the fact that the surface of Enceladus has been shaped by a long history of intense tectonic activity. The oldest fractures are characterized by a soft, muted appearance and are overprinted by numerous, superimposed impact craters. More recent fractures exhibit topographic relief that is relatively "crisp" in appearance, and they appear to slice through pre-existing impact craters and older fractures.
On the right side of the image is a conspicuous and twisted network of ridges and troughs forming a distinct tectonic region on Enceladus. The paucity of craters and the sharp appearance of the topography in this area indicate that this is a relatively young terrain on Enceladus.
This view is a composite of images taken using filters sensitive to ultraviolet (centered at 338 nanometers), green (centered at 568 nanometers), and near-infrared (centered at 930 nanometers) light, and has been processed to accentuate subtle color differences. The uppermost surface of these terrains has a relatively uniform grayish color in this picture, suggesting that it is covered with materials of homogeneous composition and grain size. However, many of the fractures reveal a distinctly different color (represented by pale-bluish tones in this false-color image) than the typical surface materials. These "colored" fractures seem to penetrate down to a material that is texturally or compositionally different than most of the material at the surface.
One possibility is that the walls of the fractures expose outcrops of solid ice, or ice with different grain-sizes compared to powdery surface materials that blanket flat-lying surfaces. It is also possible that the color identifies some compositional difference between buried ice and ice at the surface. The distinct coloration of "youthful" fracture walls are nearly absent in the oldest fractures. This is consistent with the possibility that the older fractures are covered with a drape of particulate material which mantles nearly all the oldest features on the satellite.
In the early 1980's, NASA's Voyager mission to the outer planets revealed a strikingly similar arrangement of terrains on Miranda, an icy moon of Uranus (see PIA 00141). Miranda is 470-kilometers-wide (290 miles), nearly as large as Enceladus (504 kilometers, or 313 miles wide). The similarities in size and tectonic history on these objects may suggest that remarkably similar physical processes have controlled the separate geological evolutions of these bodies.
The images that comprise this mosaic were obtained during Cassini's closest approach to Enceladus on March 9, 2005. The images was taken in visible green light with the Cassini spacecraft narrow-angle camera at a distance of approximately 29,000 kilometers (18,000 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 46 degrees. Resolution in the original images is about 170 meters (560 feet) per pixel.
Image 8: Moon With a Past -- The complex history of Enceladus' surface is revealed in great detail in this mosaic of images taken during Cassini's closest encounter with this intriguing icy moon.
Fractures are nearly ubiquitous in this terrain, cutting across each other and across impact craters. Scientists can use the relationships between different features to determine the order in which they formed, thereby unraveling the moon's past. For example, almost all the craters in this mosaic have fractures running through their rims and floors, indicating that the craters formed first. This means that Enceladus has been geologically active relatively recently, especially compared to some of its neighbors in the Saturn system.
There is an impressive variety of fractures visible here -- from the wide east-west rifts near the upper left of the mosaic to the very fine north-south fractures in the center (which are approximately 100 to 400 meters, or 330 to 1,300 feet, across). Due to the complexity of this terrain, the task of unraveling Enceladus' history promises to be a worthy challenge for planetary scientists.
The images in this mosaic were taken on March 9, 2005, in visible green light with the Cassini spacecraft narrow-angle camera at distances ranging from approximately 13,000 to 5,200 kilometers (8,000 to 3,200 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle ranging from 44 to 38 degrees. Resolution in the original images ranges from about 80 to 30 meters (260 to 100 feet) per pixel.
For images and information on the Cassini mission visit http://saturn.jpl.nasa.gov/ and http://www.nasa.gov/cassini.
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 Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.