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Unlike rocky inner planets such as Earth, Saturn has no surface on which to land. A spacecraft descending into its atmosphere would simply find the surrounding gases becoming denser, and the temperature progressively hotter; eventually the craft would be crushed and melted. Detailed analysis of Saturn’s gravita- tional field leads astronomers to believe that the deepest interior of Saturn must consist of a molten rock core about the same size as the planet Earth, but much denser. Spectroscopic studies by the Voyager spacecraft found Saturn to be made up of about 94% hydrogen and 6% helium. Hydrogen and helium are the primary constituents of all the giant gas planets, the Sun and the stars. Gravity at the top of Saturn’s clouds is similar to that near the surface of Earth. The temperature near the cloud tops is about -139° C [ -218° F] , increasing toward the planet’s core due to increased atmospheric pressure. At the core, Saturn’s temperature is predicted to be about 10,000° C [ 18,000° F] . On June 21, 2005 , the UVIS detected auroral


emissions from both Saturn’s northern and southern poles (above right).4 2


These emissions


are believed to be similar to Earth’s Northern Lights yet are invisible to the naked eye. Ultraviolet images captured the entire oval of the auroral emissions from hydrogen gas excited by electron


bombardment. Time-lapse images


indicate that aurora lights are dynamic, responding rapidly to changes in the solar wind.


New Moons


There were only 18 known moons orbiting Saturn when the Cassini spacecraft began its mission to Saturn in 1997. During Cassini’s seven-year journey, Earth-based telescopes uncovered 13 more moons. Soon after the spacecraft reached Saturn, the Cassini team discovered two more tiny moons, Methone and Pallene. The two new moons are approximately 3 km [ 1.8 miles] and 4 km [ 2.5 miles] across.


Scientists suspected that more tiny Saturnian moons might be found within the gaps in Saturn’s rings. On May 1, 2005 , using a sequence of time- lapse images from Cassini’s cameras, astrono- mers confirmed the presence of a tiny moon hidden in a gap in Saturn’s A ring.4 3


The images


4 2. Lab oratory for Atm ospheric and Space Phy sics– Cassini-UVIS Mission to Saturn and Titan: http: / / lasp. colorado. edu/ cassini/ w hats_ new / ( accessed April 13 , 2006) .


4 3 . NASA/ J et Propulsion Lab oratory – Cassini Finds an Active, Watery World at Saturn’ s Enceladus: http: / / w w w . nasa. gov/ m ission_ pages/ cassini/ m edia/ cassini-07 29 05 . htm l ( accessed April 13 , 2006) .


4 4 . NASA/ J et Propulsion Lab oratory , reference 4 3 .


> Perturb ations caused b y a tiny m oon. This im age con rm ed earlier suspicions that a sm all m oon w as orb iting w ithin the narrow K eeler Gap in Saturn’ s A ring. The K eeler Gap is located ab out 25 0 k m


[ 15 5 m iles] inside the outer edge of Saturn’ s A ring, w hich is also the outer edge of the b right m ain rings. The new m oon, Daphnis, is ab out 7 k m across and re ects ab out 5 0% of incident sunlight. Scientists predicted the m oon’ s presence and its orb ital distance from Saturn after J uly 2004 , w hen they saw perturb ations in the ring structure of the K eeler Gap’ s outer edge. These im ages w ere ob tained w ith the Cassini spacecraft narrow -angle cam era on May 1, 2005 , at a distance of approx im ately 1. 1 m illion k m [ 680, 000 m iles] . ( Im age courtesy of NASA/ J PL/ Space Science Institute. )


M oon


P erturbations caused by moon


> The southern lights of Saturn. Im ages of Saturn ob tained b y Cassini’s UVIS show auroral em issions at its poles sim ilar to Earth’ s Northern Lights. The tw o UV im ages are the  rst from the Cassini-Huy gens m ission to capture the entire


“ oval” of the auroral em issions at Saturn’ s southern pole. They also show sim ilar em issions at Saturn’ s north pole. In these false-color im ages, b lue represents aurora em issions from hy drogen gas ex cited b y electron b om b ardm ent, w hile red-orange represents re ected sunlight. These im ages


w ere tak en 1 hour apart; during this tim e the b rightest spot in the left aurora im age fades and a b right spot appears in the m iddle of the aurora in the right im age. ( Im ages courtesy of NASA/ J PL/ University of Colorado. )


show the tiny object in the center of the Keeler Gap and the wavy patterns in the gap edges that are generated by the moon’s gravitational influence (above).


The new object, Daphnis, is about 7 km


[ 4 miles] across and reflects about half the light falling on it— a brightness that is typical of the particles in the nearby rings. As Cassini continues to explore Saturn and its moons, scientists expect to uncover more of the secrets of this vast planetary system.


Signs of an Atmosphere


Although the moon Enceladus is covered with ice composed of water, like Saturn’s other moons, it displays an abnormally smooth surface with very few impact craters. With a diameter of only 5 00 km [ 310 mi] , Enceladus would fit


into


the state of Arizona. Yet despite its small size, Enceladus exhibits one of the most interesting surfaces of all the icy satellites. Enceladus reflects about 90% of the incident sunlight as if


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Oilfield Review


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