 | | Two-Faced Moon - This false color picture of Iapetus from December 27, 2004, is composed of two infrared images (928 nanometers, displayed as red, and 750 nanometers, displayed as green) and a green image, displayed as blue. The colors were oversaturated some for increased visibility. Iapetus is a two-colored moon. One side is light grey, the other dark brown. The dark area is called Cassini Regio, for the French astronomer Jean (Giovanni) Cassini (as is our spacecraft), who saw that the moon completely disappeared when this side faced earth, since it is so dark. The bright side is a very light grey, perhaps as reflective as many flat white paints, or dirty snow. The dark side varies from as dark as new asphalt to charcoal. The reason for the two faces of Iapetus is still a mystery as of December 2004, but might not be for long. |
 | | Straight Lines in Nature? - This is the same image above, contrast enhanced to show detail in the dark area. Continuing a tradition established by Mimas, Iapetus has a large crater reminiscent of the Death Star's weapon dish, only on this moon, the equatorial trench (actually a ridge) appears also. This is really strange, since long straight lines are rare in natural formations. The ridge/trench on Iapetus is part of a great circle, which appears straight when viewed from above as in this image. This arc crosses over half of the circumference of the moon. It also nearly evenly splits the dark area. Are the two related? This remains to be seen, though a correlation of two never seen elsewhere types of features begs for more than chance. (Some of the blockier splotches in the dark region are JPEG artifacts that have been enhanced, unfortunately, along with the real features.) |
 | | Cassini Regio and the Great Wall - This mosaic is composed of images taken on December 31, 2004, near Cassini's closest appreach to Iapetus on that date. The resolution is about one kilometer per pixel. It shows Cassini Regio and some of the light bordering areas. It can be seen in the bordering areas that dark material appears to streak away from craters in generally one direction. This kind of pattern is seen in places where wind blows, such as Mars, but Iapetus does not nor is likely to have ever had an atmosphere. The most obvious feature in Cassini Regio is the "straight line", a mountain range that follows a great circle and looks like the dorsal ridge on an animal. The mountains are nearly ten kilometers tall at the horizon, taller than any mountain on Earth. The overall appearance of the moon suggests a hypothesis that Iapetus cracked open and volatile material erupted out and covered the region around the crack, perhaps suddenly, perhaps over a very long period. In large craters further from the ridge, landslides of white can be seen, suggesting that cliffsides broke loose, exposing white material underneath. The confirmation of any hypothesis will likely require high resolution images of crater rims, the central ridge, and the regions that look "windblown". |
 | | Cassini Regio in Color - This image from December 31, 2004 shows part of the region in the image above in exaggerated color, to show that Cassini Regio is brownish. To sight, the color would barely be noticeable. Many asphalts have this color. Brown is actually our word for very dark orange or yellow, and in this case results from there being slightly more red and green light than blue reflecting from the dark area. This trend of Cassini Regio reflecting more long-wavelength light continues across the entire range of the Cassini spacecraft's vision. This color is seen on many outer Solar System objects, including some of the moons of Uranus, and possibly comets. It is generally hypothesized to be some volatile small organic molecule like Methane which is altered by ultraviolet light to become dark and non-volatile. (The greenish tinge on the right hand side of the image is an artifact of the mosaicing process that I have been unable to remove.) |
 | | Passing Iapetus - This image from December 31, 2004 is made from the highest resolution data from Iapetus on this passage, and has a resolution of about 730 meters/pixel. It shows a region that is essentially the upper left half of the image "Cassini Regio and the Great Wall" above. Note that the spacecraft has rotated some, so the terminator (the boundary of day and night) appears to have rotated. The area visible is nearly all of dark material, dominated by a large crater at center and half of an even larger one at upper right. The central crater has landslides visible on its rim. |
 | | Receding Crescent - This mosaic from Jan 1, 2005 shows the crescent Iapetus after Cassini passed it the day before. resolution is about 1 kilometer/pixel. The region shown is the upper left of the previous image, plus some over the horizon to the left. The spacecraft has again rotated, so the terminator again appears rotated. The large crater in the middle of the previous image is now on the limb at lower left. The higher rim of this crater is about five kilometers (three miles) tall. |
 | | Mimas and Herschel Crater - This approximately true color image from Jan 16, 2005 shows the Mimas near Cassini's closest approach of the moon on this orbit. It shows the large crater Herschel, which makes the moon look like the Death Star. The color is at best approximate, since the brightness and contrast in the available blue image is so much different than the red and green, which makes a simple color balancing impossible. The glow on the dark side is reflected light from Saturn. When we see the glow on the dark side of our own moon in crescent phase we call it "Earthshine", hence "Saturnshine" here. Were we to stand on this side of Mimas, Saturn would loom brilliantly in the sky, illuminating the ground in yellow. |
 | | Enceladus in False Color - This image from Jan 16, 2005 shows the moon Enceladus in false color. The image from Cassini's 928nm IR filter was used for red, true green for green, and 343 nm UV for blue, so this composite shows a much wider range of colors that we can see with the unaided eye. In true color, Enceladus is all a little bit orangeward of frosty white, but these extra colors show some contrasts of blueish and reddish. Colors were oversaturated to bring out more distinctions. Note that there are very few craters. The surface of Enceladus has suffered some reprocessing which has obliterated older, larger craters. |
 | | Crescent Enceladus in False Color - This image, from minutes after the previous one, was made in a similar way. The middle color in this case, represented by green, is from the red filter image, but the effect is similar to using green. |
 | | Rhea - This mosaic of four images shows the moon Rhea near closest approach on January 16, 2005. Image scale is about 930 meters per pixel. Rhea is heavily cratered, and also has large whitish fissures or grabens that can be seen near the limb at lower left. |
 | | Rings and Mimas in Saturnshine - This approximately true color composite from Jan 17, 2005 shows the moon Mimas and the A Ring. The yellow glow of reflected light from Saturn, or "Saturnshine", can be seen on the left side of the night side of Mimas. The F Ring is the outermost ("lowest") of the rings in the image. A rainbow like feature is visible in it, but this is an accident of photography. A bright region in this ring moved significantly between the red, green, and blue "exposures" that make up the image, so a spread of color appears that is not really there. If you magnify this image and look at the outer edge of the Encke Division (the large dark gap in the rings) near the top of the picture, you can see another rainbow pattern, due to density variations moving along the outer edge of the division producing false color in the same way as in the F Ring. |
 | | An Interloper Crashes the Party - This images comes from one orbit of Mimas later. The spacecraft is further from Saturn, and another moon - probably Pandora - has entered the field. Pandora is one of the shepherd moons whose gravity and motion constrain the F Ring. The outer edge of the Encke Division shows a density variation producing false color, as in the previous image. |
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