Les lunes de Jupiter
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Though the details of that transport are not clear yet, mounting evidence suggests that some mechanism has spread the mysterious dark material found on several of the moons from one to another; the material may even have a common cometary origin. Along those lines, several of the new papers focus on similarities between the dark material found on different moons on Hyperion and Iapetus, for example, or between Phoebe and Iapetus. Roger Clark of the United States Geological Survey in Denver, USA goes further, saying, We see the same spectral signature on all the moons that have coatings of dark material. Clark is lead author of one of the new papers, which focuses on Saturns moon Dione. His team found the dark material there to be extremely fine-grained, making up only a very thin layer on the moons trailing side. Its distribution and composition, as measured by the Cassini visual and infrared mapping spectrometer, indicate that the dark material is not native to Dione. And scientists see many of the same signatures there that appear on the moons Phoebe, Iapetus, Hyperion and Epimetheus, and also in Saturn's F-ring. As for where this material comes from and what the dark material is, Clark said, Its a mystery, which makes it intriguing. Were still trying to find the exact match. The visual and infrared spectrometer detected unique absorption bands in the dark material within the Saturn system, which scientists have not seen anywhere else in the Solar System. The data keeps getting better and better, he said. Were ruling things out and figuring out pieces. So far, the team has identified bound water and, possibly, ammonia in the dark material. Ongoing geological activity is another component of Saturns ecology as some of the moons continue to feed the planets rings, which in turn affect many of the moons. Clarks team reports tentative evidence to support the hypothesis presented earlier this year that Dione is still geologically active. In one series of observations, the infrared spectrometer detected a cloud of methane and water ice encircling Dione in its orbit within the outer portions of Saturn's E-ring. Of course the big story is the icy plumes spewing from the warm, south polar region of Enceladus. These plumes are believed to be feeding the E-ring. A paper led by Frank Postberg of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, says there are traces of organic compounds or silicate materials within the water ice-dominated E-ring, close to Enceladus. This implies that the moons rocky core and liquid water are dynamically interacting. The finding could bolster a theory that Dennis Matson and Julie Castillo of JPL put forth this year, which said that a warm, organic brew might lie just below Enceladus surface. Cassinis next close study of an icy moon is the highly-anticipated flyby of Enceladus scheduled for 12 March. During that flyby, Cassini will pass by the active moon at a distance of only 50 km at its point of closest approach, and at a distance of around 200 km when it passes through the plumes.
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En savoir plus
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"Nous commençons à
percer les mystères de ces différentes et étranges
lunes", a déclaré Rosaly Lopes, Cassini scientifique
de la NASA's Jet Propulsion Laboratory (JPL), USA. Elle a coordonné
une section spéciale de 14 articles sur les lunes glacées
de Saturne qui apparaît dans le numéro de février de
la revue Icarus. |
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Retour au sommaire astro
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