PASADENA, California - Grazie alle ultime osservazioni della sonda Cassini, gli scienziati hanno determinato la stratificazione geologica interna di Titano. I suoi sensibili rilevatori gravitazionali che hanno misurato la luna, suggeriscono l'idea che gli strati interni sono stati troppo freddi e privi di incisività per dividersi completamente tra ghiaccio e roccia. La scoperta, che sarà pubblicata sul numero del 12 marzo della rivista Science, dimostra che Titano si è evoluto in maniera diversa da pianeti interni, come la Terra, o come la lune ghiacciate di Giove tipo Ganimede, i cui interni sono divisi in livelli distintivi.
Questi risultati sono fondamentali per comprendere la storia delle lune del Sistema Solare esterno, ha detto Bob Pappalardo del Project Cassini e del JPL, commentando la ricerca dei suoi colleghi. "Ora possiamo meglio collocare Titano tra i satelliti ghiacciati nel nostro Sistema Solare.
"Gli scienziati sanno che il Titano, la più grande luna di Saturno, è composto a metà di ghiaccio e roccia , ma c'era bisogno dei dati gravitazionali per capire come i materiali fossero stati distribuiti. Si scopre dunque che l'interno di Titano è un sorbetto di ghiaccio costellato di rocce che probabilmente non si sono mai riscaldate oltre una temperatura relativamente tiepida. Solo nelle regioni ultraperiferiche a 500 chilometri (300 miglia) vi è di ghiaccio privo di roccia, mentre ghiaccio e roccia sono mescolati in varia misura a una profondità maggiore. Affinchè il ghiaccio e roccia restino separati, è necessario evitare un riscaldamento troppo elevato, ha detto David J. Stevenson, uno dei co-autori del rapporto e professore di scienza planetaria presso il California Institute of Technology di Pasadena.
Questo significa che Titano si è formato abbastanza lentamente in circa un milione di anni, subito dopo la formazione del Sistema Solare. Questa separazione incompleta di ghiaccio e roccia rende Titano differente dalla luna di Giove, Ganimede, dove ghiaccio e roccia sono invece completamente separati, ed è forse più simile all'altra sua luna, Callisto, che si suppone abbia un misto di ghiaccio e roccia al suo interno. Anche se le lune sono tutte all'incirca delle stesse dimensioni, hanno avuto chiaramente delle storie diverse.
Le misurazioni di Cassini hanno contribuito a costruire una mappa gravitazionale che può aiutare a spiegare perché Titano ha una ridotta topografia, infatti all'interno il ghiaccio deve essere abbastanza caldo per scorrere lentamente in risposta al peso delle pesanti strutture geologiche, come le montagne.
Per creare la mappa di gravità sono state richieste delle modifiche dell'ultimo minuto in termini di velocità per Cassini. La sonda ha sorvolato Titato tra i 1.300 e i 1.900 km (da 800 a 1.200 miglia). Le onde di gravità di Titano sono stare registrate da Cassini e inviate a terra al Deep Space Network entro 5 millesimi di millimetro al secondo [0,2 millesimi di pollice al secondo], mentre la sonda era ad oltre un miliardo di chilometri [oltre 600 milioni di miglia] di distanza, ha detto Luciano IESS, un membro del team di Radio Sapienza dell'Università di Roma in Italia e autore principale delll'articolo. E' stato un esperimento difficile. I risultati non ci dicono se Titano ha un oceano sotto la superficie, ma gli scienziati dicono questa ipotesi è molto plausibile e che intendono indagare.
La rilevazione delle maree indotte da Saturno, un obiettivo del team scientifico radio, fornirebbe la prova più chiara per la presenza di acqua sotto la sua superfice. Un investigatore interdisciplinare di Cassini, Jonathan Lunine, ha detto parlando dei risultati dei suoi colleghi, che degli incontri aggiuntivi potranno dirci se la crosta è spessa o sottile. Lunine è con l'Università di Roma, Tor Vergata, in Italia, e con la University of Arizona, Tucson. Con queste informazioni potremmo avere una migliore comprensione di come il metano, influisca nella formazione di fiumi, laghi e nuvole, nel corso del suo tempo geologico. Come lo è stata l'acqua sulla Terra, il metano è importante per una visione profonda della natura di Titano nel tempo.
Traduzione a cura di Arthur McPaul
English
PASADENA, Calif. -- By precisely tracking NASA's Cassini spacecraft on its low swoops over Saturn's moon Titan, scientists have determined the distribution of materials in the moon's interior. The subtle gravitational tugs they measured suggest the interior has been too cold and sluggish to split completely into separate layers of ice and rock.
The finding, to be published in the March 12 issue of the journal Science, shows how Titan evolved in a different fashion from inner planets such as Earth, or icy moons such as Jupiter's Ganymede, whose interiors have split into distinctive layers.
"These results are fundamental to understanding the history of moons of the outer solar system," said Cassini Project Scientist Bob Pappalardo, commenting on his colleagues' research. Pappalardo is with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We can now better understand Titan's place among the range of icy satellites in our solar system."
Scientists have known that Titan, Saturn's largest moon, is about half ice and half rock, but they needed the gravity data to figure out how the materials were distributed. It turns out Titan's interior is a sorbet of ice studded with rocks that probably never heated up beyond a relatively lukewarm temperature. Only in the outermost 500 kilometers (300 miles) is Titan's ice devoid of any rock, while ice and rock are mixed to various extents at greater depth.
"To avoid separating the ice and the rock, you must avoid heating the ice too much," said David J. Stevenson, one of the paper's co-authors and a professor of planetary science at the California Institute of Technology in Pasadena. "This means that Titan was built rather slowly for a moon, in perhaps around a million years or so, back soon after the formation of the solar system."
This incomplete separation of ice and rock makes Titan less like Jupiter's moon Ganymede, where ice and rock have fully separated, and perhaps more like another Jovian moon, Callisto, which is believed to have a mixed ice and rock interior. Though the moons are all about the same size, they clearly have diverse histories.
The Cassini measurements help construct a gravity map, which may help explain why Titan has a stunted topography, since interior ice must be warm enough to flow slowly in response to the weight of heavy geologic structures, such as mountains.
Creating the gravity map required tracking minute changes in Cassini's speed along a line of sight from Earth to the spacecraft as it flew four close flybys of Titan between February 2006 and July 2008. The spacecraft took paths between about 1,300 to 1,900 kilometers (800 to 1,200 miles) above Titan.
"The ripples of Titan's gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the ground antennas of the Deep Space Network within 5 thousandths of a millimeter per second [0.2 thousandths of an inch per second] even as the spacecraft was over a billion kilometers [more than 600 million miles] away," said Luciano Iess, a Cassini radio science team member at Sapienza University of Rome in Italy, and the paper's lead author. "It was a tricky experiment."
The results don't speak to whether Titan has an ocean beneath the surface, but scientists say this hypothesis is very plausible and they intend to keep investigating. Detecting tides induced by Saturn, a goal of the radio science team, would provide the clearest evidence for such a hidden water layer.
A Cassini interdisciplinary investigator, Jonathan Lunine, said of his colleagues' findings, "Additional flybys may tell us whether the crust is thick or thin today." Lunine is with the University of Rome, Tor Vergata, Italy, and the University of Arizona, Tucson. "With that information we may have a better understanding of how methane, the ephemeral working fluid of Titan's rivers, lakes and clouds, has been resupplied over geologic time. Like the history of water on Earth, this is fundamental to a deep picture of the nature of Titan through time."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of Caltech, manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL. Cassini's radio science subsystem has been jointly developed by NASA and the Italian Space Agency (ASI).
The finding, to be published in the March 12 issue of the journal Science, shows how Titan evolved in a different fashion from inner planets such as Earth, or icy moons such as Jupiter's Ganymede, whose interiors have split into distinctive layers.
"These results are fundamental to understanding the history of moons of the outer solar system," said Cassini Project Scientist Bob Pappalardo, commenting on his colleagues' research. Pappalardo is with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We can now better understand Titan's place among the range of icy satellites in our solar system."
Scientists have known that Titan, Saturn's largest moon, is about half ice and half rock, but they needed the gravity data to figure out how the materials were distributed. It turns out Titan's interior is a sorbet of ice studded with rocks that probably never heated up beyond a relatively lukewarm temperature. Only in the outermost 500 kilometers (300 miles) is Titan's ice devoid of any rock, while ice and rock are mixed to various extents at greater depth.
"To avoid separating the ice and the rock, you must avoid heating the ice too much," said David J. Stevenson, one of the paper's co-authors and a professor of planetary science at the California Institute of Technology in Pasadena. "This means that Titan was built rather slowly for a moon, in perhaps around a million years or so, back soon after the formation of the solar system."
This incomplete separation of ice and rock makes Titan less like Jupiter's moon Ganymede, where ice and rock have fully separated, and perhaps more like another Jovian moon, Callisto, which is believed to have a mixed ice and rock interior. Though the moons are all about the same size, they clearly have diverse histories.
The Cassini measurements help construct a gravity map, which may help explain why Titan has a stunted topography, since interior ice must be warm enough to flow slowly in response to the weight of heavy geologic structures, such as mountains.
Creating the gravity map required tracking minute changes in Cassini's speed along a line of sight from Earth to the spacecraft as it flew four close flybys of Titan between February 2006 and July 2008. The spacecraft took paths between about 1,300 to 1,900 kilometers (800 to 1,200 miles) above Titan.
"The ripples of Titan's gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the ground antennas of the Deep Space Network within 5 thousandths of a millimeter per second [0.2 thousandths of an inch per second] even as the spacecraft was over a billion kilometers [more than 600 million miles] away," said Luciano Iess, a Cassini radio science team member at Sapienza University of Rome in Italy, and the paper's lead author. "It was a tricky experiment."
The results don't speak to whether Titan has an ocean beneath the surface, but scientists say this hypothesis is very plausible and they intend to keep investigating. Detecting tides induced by Saturn, a goal of the radio science team, would provide the clearest evidence for such a hidden water layer.
A Cassini interdisciplinary investigator, Jonathan Lunine, said of his colleagues' findings, "Additional flybys may tell us whether the crust is thick or thin today." Lunine is with the University of Rome, Tor Vergata, Italy, and the University of Arizona, Tucson. "With that information we may have a better understanding of how methane, the ephemeral working fluid of Titan's rivers, lakes and clouds, has been resupplied over geologic time. Like the history of water on Earth, this is fundamental to a deep picture of the nature of Titan through time."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of Caltech, manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL. Cassini's radio science subsystem has been jointly developed by NASA and the Italian Space Agency (ASI).
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