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making measurements and transmitting data to Cassini for 72 minutes after landing until power limitations and deterioration of the spacecraft due to extreme surface conditions on Titan resulted in loss of signal.


Exploring the Ringed Planet Aside from Titan, more moons of greater variety orbit Saturn than any other planet. So far, observations from Earth and by spacecraft have found Saturnian satellites ranging from small asteroid-size bodies to those as large as Titan. Saturn is the second-largest planet in the


solar system. Like the other gaseous outer planets— Jupiter, Uranus and Neptune— it has an atmosphere made up mostly of hydrogen and helium, and like them, it is ringed. Saturn’s distinctive bright rings are made up of ice and rock particles ranging in size from grains of sand to small houses.


Although the face of Saturn appears calm, the planet has a windswept atmosphere where an equatorial jet stream blows at 1,800 km/h


[ 1,118 mi/h] , and swirling storms churn beneath the cloud tops. Early explorations by NASA’s Pioneer 11 spacecraft in 1979, and the Voyager 1 and 2 spacecraft in 1980 and 1981, found Saturn to have a huge and complex magnetic environ- ment where trapped protons and electrons interact with each other, the planet, the rings and the surfaces of many of Saturn’s moons. From Earth, Saturn’s rings appear as only a few monolithic bands, while in reality, they consist of thousands of rings and ringlets, with particles sometimes arranged in complicated orbits by the gravitational interaction of small moons previously unseen from Earth (right). Scientists are using data from the UVIS in detailed computer models to simulate the complex motion of these rings. Second in size only to Jupiter, Saturn has more than 75 0 times the volume of Earth. Combined with the planet’s low density, less than half that of water, its fast rotation promotes a bulge of material near the equator. Saturn is shaped like a flattened ball; its pole-to- pole diameter is only 108,728 km [ 67,5 60 miles] , compared to about 120,5 36 km [ about 74,898 miles] for the equatorial diameter.


4 1. European Space Agency – Cassini-Huy gens: http: / / huy gens. esa. int/ science-e/ w w w / ob j ect/ index . cfm ? fob j ectid= 3 63 9 6 ( accessed April 13 , 2006) .


> Under Titan’ s atm osphere. The perspective view of the surface of Titan near the Huy gens prob e landing site ( top) is color-coded, w ith b lue the low est altitude and red the highest. The total area covered b y the im age is ab out 1 b y 3 k m [ 0. 6 b y 2 m iles] . A pair of im ages ( inset) w as acq uired from the Huy gens descent im ager/ spectral radiom eter. The left im age w as acq uired from 14 . 8 k m [ 9 m iles] ab ove the surface w ith the high-resolution im ager and the right from 6. 7 k m [ 4 m iles] altitude w ith the m edium -resolution im ager. ( Im ages courtesy of ESA/ NASA/ J PL/ University of Arizona/ USGS. )


> Ex ploring Saturn’ s rings. Im ages tak en during the Cassini spacecraft’ s orb it around Saturn show com positional variation in Saturn’ s rings ( top) . The red in the im age indicates sparser ringlets that prob ab ly com prise “ dirty , ” and possib ly sm aller particles than those in the icier turq uoise ringlets. The red b and roughly three-fourths of the w ay outw ard is k now n as the Enck e Gap. This im age w as tak en w ith the Ultraviolet Im aging Spectrograph ( UVIS) instrum ent, w hich is capab le of resolving the rings to show features up to 9 7 k m [ 60 m i] across, roughly 100 tim es the resolution of ultraviolet data ob tained b y the Voy ager 2 spacecraft. The false-color view of Saturn’ s A ring ( b ottom left) w as also tak en b y the UVIS. The ring is the b luest in the center, w here the gravitational clum ps are the largest. The thick est b lack b and in the ring is the Enck e Gap, and the thin b lack b and farther to the right is the


K eeler Gap. The insert ( b ottom right) is a com puter sim ulation ab out 15 0 m [ 4 9 0 ft] across, illustrating a clum py region of icy particles in the A ring. ( Im ages courtesy of NASA/ J PL/ University of Colorado. )


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