Iapetus: Saturn's Walnut-shaped Moon

[Waspie_Dwarf]

Approaching Iapetus

October 8, 2007

The slim crescent of Iapetus looms before the Cassini spacecraft as it approaches the mysterious moon.

Iapetus, 1,468 kilometers (912 miles) across, seen here in false color, is unique in its dramatic variation in brightness between the northern polar region and the middle and low latitudes. Equally prominent is the moon's equatorial ridge of towering mountains. The profile of the ridge against the darkness of space reveals that it is topped by a cratered plateau approximately 15 kilometers (9 miles) wide. Further west, the profile of the ridge changes from a long plateau to discrete peaks.

The mosaic consists of four image footprints across the surface of Iapetus and has a resolution of 489 meters (0.3 miles) per pixel.

A full-resolution clear filter image was combined with half-resolution images taken with infrared, green and ultraviolet spectral filters (centered at 752, 568 and 338 nanometers, respectively) to create this full-resolution false color mosaic.

The color seen in this view represents an expansion of the wavelength region of the electromagnetic spectrum visible to human eyes. The intense reddish-brown hue of the dark material is far less pronounced in true color images. The use of enhanced color makes the reddish character of the dark material more visible than it would be to the naked eye. In addition, the scene has been brightened to improve the visibility of surface features.

This view was acquired with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at a distance of about 83,000 kilometers (51,600 miles) from Iapetus.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

A Scene of Craters

October 8, 2007

This high-resolution view shows a vast range of crater sizes in the dark terrain of the leading hemisphere of Saturn's moon Iapetus.

Across the scene, a few small bright spots indicate fresh, rayed craters where impactors have punched through the thin blanket of dark material to the cleaner ice beneath.

The slight elevation on the bottom half of the image is part of the giant equatorial ridge that spans a wide fraction of Iapetus' circumference. The numerous craters on top of the ridge indicate that it is an old surface feature.

The mosaic consists of three image footprints across the surface of Iapetus. The view is centered on terrain near 0.5 degrees north latitude, 141.6 degrees west longitude. Image scale is approximately 22 meters (72 feet) per pixel. Illumination is from the left.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at a distance of approximately 63,000 kilometers (39,000 miles) from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of about 125 degrees.

Iapetus is 1,468 kilometers (912 miles) across.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Closest View of Iapetus

October 8, 2007

This mosaic of Cassini images shows the smallest details ever observed on Saturn's moon Iapetus.

Visible here are small craters as well as the base of a large mountain ridge located just south of the mosaic. At several places, bright spots about 20 to 50 meters (66 to 164 feet) across are visible. At these locations, more recent impactors have punched through the overlying blanket of dark material to reveal brighter, cleaner ice beneath.

Since the bright craters are relatively small and very shallow, it is likely that the dark blanket is rather thin in this area; it is assumed that its actual average thickness might be on the order of a foot.

The small crater at the upper left edge of the mosaic has a diameter of about 50 meters (164 feet) and shows a distinct ray pattern from excavated ice. This feature is so bright in comparison to the dark surrounding terrain that it had to be darkened manually so as not to look overexposed in this mosaic.

The mosaic consists of eight image footprints across the surface of Iapetus, presented here in simple cylindrical projection. The view is centered on terrain near 0 degrees north latitude, 164.9 degrees west longitude, within the dark leading hemisphere of Iapetus. Image scale is approximately 10 meters (33 feet) per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at distances ranging from 1,627 to 2,040 kilometers (1,011 to 1,268 miles) from Iapetus. Iapetus is 1,468 kilometers (912 miles) across.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Towering Peaks of Iapetus

October 8, 2007

This stereo image, or anaglyph, shows huge mountains on Saturn's moon Iapetus, imaged by NASA's Cassini spacecraft during its very close flyby in Sept. 2007. These mountains are located at the moon's equator in the westward-most part of the dark terrain.

Here, the brightness pattern on the surface is very complex. The mountain in the center of this view is part of the range informally named "the Voyager mountains" that were first detected on the limb of the moon in NASA Voyager spacecraft images. Interestingly, its eastern (right) flank is dark, while the other flanks are bright. This suggests that external material arriving on Iapetus from its orbital motion might play a role in the darkening process. One plausible source, the outer moons of Saturn, might provide a very thin but steady stream of very dark particles from the eastern direction as seen from this mountain.

The mosaic consists of six image footprints across the surface of Iapetus. The view is centered on terrain near 0.1 degree north latitude, 199 degrees west longitude. Image scale is approximately 46 meters (151 feet) per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow- angle camera on Sept. 10, 2007. Distances for the blue portion of the image range from 7,744 to 9,135 kilometers (4,812 to 5,676 miles) from Iapetus; distances for the red portion of the image range from 20,267 to 21,595 kilometers (12,593 to 13,418 miles) from the moon.

A separate, non-stereo version of the scene is included for comparison.

Iapetus is 1,468 kilometers (912 miles) across.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

The Transition Zone

October 8, 2007

Soaring above the alien, icy wastelands of Saturn's moon Iapetus, NASA's Cassini spacecraft captured a series of high-resolution images of the transition region from dark to bright terrain at southern middle latitudes that have been mosaicked together in this view.

An important characteristic of the terrain in the boundary region is that the isolated bright patches are mainly found on slopes facing toward the bright trailing hemisphere or toward the south pole. The same polarity is found within the bright terrain, where the dark material can be seen at the bottom of craters and on equator-facing slopes. These indicate that thermal effects are at play in painting the surface of Iapetus, 1,468 kilometers (912 miles) across. The mosaic consists of eight image footprints across the surface of Iapetus. The view is centered on terrain near 38.6 degrees south latitude, 171.3 degrees west longitude. Image scale is approximately 52 meters (171 feet) per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow- angle camera on Sept. 10, 2007, at a distance of approximately 5,000 kilometers (3,100 miles) from Iapetus.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

A Complex Transition

October 8, 2007

The transition region from Saturn's moon Iapetus' dark leading hemisphere to its bright trailing hemisphere is a complicated patchwork of craters and highlands, with low elevations filled in by dark material.

An explanation of the pattern visible here might be key to a full understanding of the bright/dark dichotomy on Iapetus, 1,468 kilometers (912 miles) across.

The view is centered on the equator and covers an area 711 kilometers wide by 417 kilometers tall (442 by 259 miles).

The giant equatorial ridge visible on the dark leading hemisphere is not present anymore in this region. Instead, large, isolated mountains more than 10 kilometers (6 miles) tall are spread along the equator. These mountains show bright western flanks, while the surrounding lowlands are generally dark.

The bright mountains at center right, surrounded by dark terrain, are also visible in the stereo view Towering Peaks of Iapetus. The region of Iapetus seen in this mosaic is also visible in the color full-disk mosaic The Other Side of Iapetus.

The mosaic is an orthographic projection consisting of 21 image footprints across the surface of Iapetus. The view is centered on terrain near 0.1 degree north latitude, 199 degrees west longitude, in the quadrant of Iapetus that faces away from Saturn. Image scale is approximately 83 meters (272 feet) per pixel. An orthographic view is most like the view seen by a distant observer looking through a telescope.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow- angle camera on Sept. 10, 2007, at distances ranging from 13,857 to 21,846 kilometers (8,610 to 13,574 miles) from Iapetus.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Spotty Iapetus

October 8, 2007

At high resolution, terrain in the transition region between bright and dark hemispheres on Saturn's moon Iapetus reveals a spotty appearance reminiscent of a Dalmatian. The bright material on the frozen surface of Iapetus, 1,468 kilometers (912 miles) across, is water ice, and the dark material is likely carbonaceous in composition.

The dark material is preferentially found at the bottoms of craters. Bright water ice forms the "bed rock" on Iapetus, while the dark, presumably loose material apparently lies on top of the ice.

The terrain seen here is also visible in Speckled Surface, but it is viewed here at higher resolution.

The mosaic consists of two image footprints across the surface of Iapetus. The view is centered on terrain near 42 degrees south latitude and 209.3 degrees west longitude, on the anti-Saturn facing hemisphere. Image scale is approximately 32 meters (105 feet) per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow- angle camera on Sept. 10, 2007, at distances ranging from 5,363 to 5,884 kilometers (3,332 to 3,656 miles) from Iapetus.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Speckled Surface

October 8, 2007

Dark material has coated the low-elevation terrain and the interiors of craters in the southern portions of the quadrant on Iapetus that faces away from Saturn. This is part of the boundary region separating the dark leading and bright trailing hemispheres. The dark coating is thought to be no more than a few tens of centimeters thick (10 centimeters equals 4 inches) and, as seen here, predominately appears on the northern-facing walls of craters in the south.

Farther south, the dark splotches are less numerous and appear almost absent at the highest latitudes (near the bottom of the frame). This is a strong indicator that thermal effects play a role in the darkening process of parts of Iapetus' surface: the colder the surface, the less common is the dark terrain. As on Earth, the higher latitudes on Iapetus receive less heating by sunlight.

At left, below center, the eastern rim of a great and ancient impact basin can be seen. With a diameter of almost 500 kilometers (310 miles), it is one of the largest impact structures on Iapetus, 1,468 kilometers (912 miles) across, and in the entire Saturn system.

This monochrome view shows terrain also seen in The Other Side of Iapetus but at higher resolution.

The mosaic consists of three narrow-angle camera footprints across the surface of Iapetus. This view is centered on terrain near 35.1 degrees south latitude, 218.5 degrees west longitude. Image scale is approximately 231 meters (758 feet) per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft on Sept. 10, 2007, at a distance of approximately 40,000 kilometers (25,000 miles) from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of 31 degrees.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

The Other Side of Iapetus

October 8, 2007

Cassini captures the first high-resolution glimpse of the bright trailing hemisphere of Saturn's moon Iapetus.

This false-color mosaic shows the entire hemisphere of Iapetus (1,468 kilometers, or 912 miles across) visible from Cassini on the outbound leg of its encounter with the two-toned moon in Sept. 2007. The central longitude of the trailing hemisphere is 24 degrees to the left of the mosaic's center.

Also shown here is the complicated transition region between the dark leading and bright trailing hemispheres. This region, visible along the right side of the image, was observed in many of the images acquired by Cassini near closest approach during the encounter.

Revealed here for the first time in detail are the geologic structures that mark the trailing hemisphere. The region appears heavily cratered, particularly in the north and south polar regions. Near the top of the mosaic, numerous impact features visible in NASA Voyager 2 spacecraft images (acquired in 1981) are visible, including the craters Ogier and Charlemagne.

The most prominent topographic feature in this view, in the bottom half of the mosaic, is a 450-kilometer (280-mile) wide impact basin, one of at least nine such large basins on Iapetus. In fact, the basin overlaps an older, similar-sized impact basin to its southeast.

In many places, the dark material -- thought to be composed of nitrogen-bearing organic compounds called cyanides, hydrated minerals and other carbonaceous minerals -- appears to coat equator-facing slopes and crater floors. The distribution of this material and variations in the color of the bright material across the trailing hemisphere will be crucial clues to understanding the origin of Iapetus' peculiar bright-dark dual personality.

The view was acquired with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at a distance of about 73,000 kilometers (45,000 miles) from Iapetus.

The color seen in this view represents an expansion of the wavelengths of the electromagnetic spectrum visible to human eyes. The intense reddish-brown hue of the dark material is far less pronounced in true color images. The use of enhanced color makes the reddish character of the dark material more visible than it would be to the naked eye.

This mosaic consists of 60 images covering 15 footprints across the surface of Iapetus. The view is an orthographic projection centered on 10.8 degrees south latitude, 246.5 degrees west longitude and has a resolution of 426 meters (0.26 miles) per pixel. An orthographic view is most like the view seen by a distant observer looking through a telescope.

At each footprint, a full resolution clear filter image was combined with half-resolution images taken with infrared, green and ultraviolet spectral filters (centered at 752, 568 and 338 nanometers, respectively) to create this full-resolution false color mosaic.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Iapetus' Equatorial Region

October 8, 2007

Cassini made a close flyby of Saturn's moon Iapetus on Sept. 10, 2007, and the visual and infrared mapping spectrometer obtained these images during that event.

These two images show a higher resolution version of the equatorial region shown in Tiny Grains on Iapetus. The equatorial region includes the equatorial bulge which shows no differences in these compositions compared to surrounding regions.

The color image on the right shows the results of mapping for three components of Iapetus' surface: carbon dioxide that is trapped or adsorbed in the surface (red), water in the form of ice (green), and a newly-discovered effect due to trace amount of dark particles in the ice creating what scientists call Rayleigh scattering (blue). The Rayleigh scattering effect is the main reason why the Earth's sky appears blue.

There is a complex transition zone from the dark region, on the right, which is high in carbon dioxide, to the more ice-rich region on the left. Some crater floors are filled with carbon dioxide-rich dark material. As the ice becomes cleaner to the left, the small dark particles become more scattered and increase the Rayleigh scattering effect, again indicative of less than 2 percent dark sub-0.5-micron particles.

The visual and infrared mapping spectrometer is like a digital camera, but instead of using three colors, it makes images in 352 colors, or wavelengths, from the ultraviolet to the near-infrared. The many wavelengths produce a continuous spectrum in each pixel, and these spectra measure how light is absorbed by different materials. By analyzing the absorptions expressed in each pixel, a map of the composition at each location on the moon can be constructed.

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 was designed, developed and assembled at JPL. The Visual and Infrared Mapping Spectrometer team is based at the University of Arizona.

For more information about the Cassini-Huygens mission, visit: http://saturn.jpl.nasa.gov/home/index.cfm. The visual and infrared mapping spectrometer team home page is at: http://wwwvims.lpl.arizona.edu.

Credit: NASA/JPL/University of Arizona /USGS

Source: NASA/JPL - Cassini

Edited October 8, 2007 by Waspie_Dwarf

[Waspie_Dwarf]

Two Ultraviolet Views of Iapetus

October 8, 2007

The far left image shows the bright-dark boundary region on Saturn's moon Iapetus at far-ultraviolet wavelengths, viewed by Cassini's ultraviolet imaging spectrograph on Sept. 10, 2007.

These wavelengths represent reflected solar light and indicate where the surface is brightest and highest in water ice abundance. (Red indicates the brightest regions, purple the darkest.) The bright "Voyager Mountains", part of the equatorial ridge, are seen as bright spots against a dark background. The dark material that covers one hemisphere of Iapetus is indicated in purple and is seen on the right side of this image.

The middle image is a color composite: blue-green (longer ultraviolet wavelengths) indicates where the surface is bright and probably richest in water ice. Red (short ultraviolet wavelengths) indicates where the surface is low in water ice and relatively high in dirty material. The sky background is also bright at these wavelengths, making the limb, or edge, of Iapetus where the surface is dark indistinguishable from the sky background.

The image on the right, taken by the imaging science subsystem, is for reference, with the regions observed by Cassini¿s ultraviolet imaging spectrometer outlined in red.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's 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 was designed, developed and assembled at JPL. The ultraviolet imaging spectrograph was designed and built at, and the team is based at the University of Colorado, Boulder. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov/home/index.cfm. The ultraviolet imaging spectrograph team home page is at http://lasp.colorado.edu/cassini. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/University of Colorado/Space Science Institute

Source: NASA/JPL - Cassini

Edited October 8, 2007 by Waspie_Dwarf

[Waspie_Dwarf]

Tiny Grains on Iapetus

October 8, 2007

Cassini made a close flyby of Saturn's moon Iapetus on Sept. 10, 2007, and the visual and infrared mapping spectrometer obtained these images showing surface composition and particle size.

The visual and infrared mapping spectrometer is like a digital camera, but instead of using three colors, it makes images in 352 colors, or wavelengths, from the ultraviolet to the near-infrared. The many wavelengths produce a continuous spectrum in each pixel, and these spectra measure how light is absorbed by different materials. By analyzing the absorptions expressed in each pixel, a map of the composition at each location on the moon can be constructed.

The left image in the figure shows the amount of reflected light at a wavelength of 1.75 microns in the infrared (green light seen by our eyes is 0.53 microns). The color image on the right shows the results of mapping for three components of Iapetus' surface: carbon dioxide that is trapped or adsorbed in the surface (red), water in the form of ice (green), and a newly-discovered effect due to trace amount of dark particles in the ice creating what scientists call Rayleigh scattering (blue). The Rayleigh scattering effect is the main reason why the Earth's sky appears blue.

The Rayleigh scattering effect on Iapetus provides evidence that tiny grains, less than the wavelength of visible light (less than 0.5 microns) have been embedded in the surface of Iapetus. The tiny grains must be well-separated for the Rayleigh effect to become prominent, so the abundance of particles must be less than about 2 percent. The Rayleigh scattering effect shows in all areas, although weakly in dark regions (the red carbon dioxide dominates the color image), and it appears stronger away from the equator. Investigating the trend from dark to bright areas, the Rayleigh effect changes with the amount of dark material in the ice, and becomes weaker as more dark material is added. This points to cleaner ice as one moves north or south from the equator and away from the dark leading side of the moon (toward the right in the image).

This provides additional evidence for an external source for the dark material coating Iapetus, and for ice transport away from the warm dark regions and equator to the cooler poles. The ice transport away from the equator increases the concentration of dark material there and reduces the Rayleigh effect. With the volatile transport from the dark warm regions, the strong carbon dioxide signature is a surprise because frozen carbon dioxide is more volatile than water ice. Therefore, the carbon dioxide must be trapped, making its presence stable in the warm equatorial region. The trapping mechanism is currently under study.

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 was designed, developed and assembled at JPL. The Visual and Infrared Mapping Spectrometer team is based at the University of Arizona.

For more information about the Cassini-Huygens mission, visit: http://saturn.jpl.nasa.gov/home/index.cfm. The visual and infrared mapping spectrometer team home page is at: http://wwwvims.lpl.arizona.edu.

Credit: NASA/JPL/University of Arizona /USGS

Source: NASA/JPL - Cassini

Edited October 8, 2007 by Waspie_Dwarf

[Waspie_Dwarf]

Warm and Dry on Iapetus

October 8, 2007

This image compares midday temperatures on Saturn's moon Iapetus, recorded by the composite infrared spectrometer instrument during Cassini's close Sept. 10, 2007 flyby, with images of the same region recorded during the same flyby by the Cassini imaging science subsystem, shown on the right. See The Other Side of Iapetus for full imaging mosaic.

Smallest features visible in the composite infrared spectrometer image (on the left) are about 8 kilometers (5 miles) across. The red rectangle on the visible light (right) image shows the region covered by infrared spectrometer, which extends a distance of 385 kilometers (240 miles) from 36 north, 212 west to 22 south, 220 west. The composite infrared spectrometer determined surface temperatures by measuring the spectrum of infrared radiation emitted by Iapetus in the 9 to 16 micron wavelength range. The dark regions are warmer because they absorb more of the sunlight shining on Iapetus, so dark spots in the visible (right) image show up as warm spots in the infrared image on the left. Temperatures near the equator vary between about 128 Kelvin (minus 229 degrees Fahrenheit) in the darkest regions and about 113 Kelvin (minus 256 degrees Fahrenheit) in the brightest regions.

This relatively small temperature difference has a large effect on Iapetus, because at the temperature of the dark regions, a large amount of water ice, which is abundant on most moon surfaces in the Saturn system, can be lost by evaporation over the several-billion year age of Iapetus' surface. Composite infrared spectrometer scientists calculate that when daytime temperatures reach 128 Kelvin (minus 229 degrees Fahrenheit), about 20 meters (65 feet) of ice can be lost per billion years. In the bright regions, with peak temperatures of 113 Kelvin (minus 256 degrees Fahrenheit), only about 10 centimeters, or 2.5 inches, of ice is lost in the same period. It is thus likely that the ice has evaporated completely from the surface of the dark regions of Iapetus, darkening them further, and has collected in the neighboring bright regions, making them brighter, thereby exaggerating initially modest brightness variations. This process is known as thermal segregation.

Models by the composite infrared spectrometer team also show that ice evaporated from the warm dark terrain at low latitudes can collect at higher latitudes, and can thus explain the bright polar caps on the dark leading side of Iapetus as well as the relatively dark equatorial regions on the bright trailing side.

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 was designed, developed and assembled at JPL. The composite infrared spectrometer team is based at NASA's Goddard Space Flight Center, Greenbelt, Md. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The composite infrared spectrometer team homepage is http://cirs.gsfc.nasa.gov/. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/GSFC/SwRI/SSI

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Iapetan Geography

October 15, 2007

Cassini soars above the many pits and basins in the rolling landscape of Saturn's moon Iapetus. This mosaic view looks out onto an area close to the northern bright/dark boundary, but still within the dark region, Cassini Regio.

Near upper left is a large crater with terraced walls, a mostly flat floor and a prominent group of peaks in its center. The sharp features make this likely one of the youngest craters in this area of Iapetus. Cassini imaged another similarly flat-floored and relatively fresh crater during its Dec. 2004 Iapetus flyby (see Giant Landslide on Iapetus).

The mosaic consists of three image footprints across the surface of Iapetus (1,468 kilometers, or 912 miles across). The view is centered on terrain near 43.3 degrees north latitude, 138 degrees west longitude. Image scale is approximately 75 meters (246 feet) per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at a distance of approximately 13,500 kilometers (8,400 miles) from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of 139 degrees.

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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini

[Waspie_Dwarf]

Flyby Follow-up

October 24, 2007

Following Cassini's highly successful flyby of Iapetus in September 2007, the spacecraft repeatedly glanced back at the two-toned moon for some time. As Cassini receded from Iapetus, more and more of the bright trailing hemisphere rotated into view.

This image shows terrain farther west of that visible in The Other Side of Iapetus. Most notably in this view, it can be seen that the dark equatorial terrain reaches onto the moon's trailing side by the same amount on the western and eastern sides.

This view looks toward Iapetus (1,468 kilometers, or 912 miles, across) from about 10 degrees south of the moon's equator and is centered on 284 degrees west longitude. North is up and rotated 16 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 19, 2007. The view was acquired at a distance of approximately 1.7 million kilometers (1 million miles) from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of 32 degrees. Image scale is 10 kilometers (6 miles) per pixel.

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.

For more information about the Cassini-Huygens mission visit _http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at _http://ciclops.org.

Credit: NASA/JPL/Space Science Institute

Source: NASA/JPL - Cassini