Cassini shows off new global maps of six moons of Saturn
Sen—NASA's Cassini spacecraft is the mission that keeps on giving. The latest gift from its scientists is new global maps of Saturn's six major icy moons, Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus.
Aside from a gap in the north polar region of Enceladus (to be filled in next year), and some areas of Iapetus, this objective is now more or less complete.
Compare the image above from Cassini's first flyby of Enceladus in 2004 with the new map below. Image credit: NASA/JPL/Space Science Institute
The new maps are the best global colour maps of the major icy moons to date, and the first to show natural brightness variations and high-resolution colour together. Colours in the maps represent a broader range than human vision, extending slightly into infrared and ultraviolet wavelengths.
Colour map of Enceladus, 2014. Image credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
Enceladus displays at least five different types of terrain. Parts of the moon show craters no larger than 35 kilometres (about 22 miles) in diameter. Other areas show regions with no craters indicating major resurfacing events in the geologically recent past. The yellowish and magenta tones in the map are believed to be due to differences in the thickness of surface deposits there.
In addition, many of the most recently formed fractures on Enceladus have a stronger ultraviolet signature (meaning they are brighter), and they appear bluish in these maps. The famous "tiger stripe" fractures, which are the sources of the plumes venting gas and dust into space have a similar colour.
Researchers think this colour could be due to large-grained ice exposed on the surface. Some of the gas and dust being vented returns to the surface and paints Enceladus with a fresh coating.
Colour map of Mimas, 2014. Image credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
The most striking feature on Mimas is the huge 140-kilometer (88-mile) wide Herschel Crater, which is one-third the diameter of the moon itself. The walls of the crater are approximately 5 kilometres (3 miles) high and the central peak towers are almost 6 kilometers (4 miles) above the floor of the crater.
Colour map of Tethys, 2014. Image credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
Another dramatic feature is the equatorial band on the moon's leading hemisphere. Cassini revealed a similar feature on Tethys. Cassini found these areas on both Mimas and Tethys to be significantly brighter in the ultraviolet than surrounding terrains, and it appears somewhat bluish here. This pattern on the leading hemispheres correlates with the predicted pattern of bombardment of high-energy electrons trapped in Saturn's magnetic field.
Colour map of Rhea, 2014. Image credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
These enhanced colour views have yielded several important discoveries about the icy moons. The most obvious are differences in colour and brightness between the two hemispheres of Tethys, Dione and Rhea.
The dark reddish colours on the moons' trailing hemispheres are due to alteration by charged particles and radiation in Saturn's magnetosphere. The blander leading hemispheres, the sides that always face forward as the moons orbit Saturn, are all coated with icy dust from Saturn's E-ring, formed from tiny particles erupting from the south pole of Enceladus.
Colour map of Dione, 2014. Image credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
Cassini arrived at Saturn on 30 June, 2004, for a four-year primary mission. NASA has since granted the mission three extensions, allowing scientists an unprecedented opportunity to observe the planet and its 53 officially named moons while they completed one-third of their almost 30-year-long orbit around the Sun.