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| Piú viste - Uranus and His Moons |
AB-Ariel-V2-PIA00041_modest.jpgAriel (real colors)124 visiteUrano ha (circa) 20 Lune naturali, ma le sue Lune Maggiori sono soltanto 5: Ariel, Umbriel, Titania, Oberon e Miranda.
Si tratta, in tutti i casi, di corpi rocciosi, privi di atmosfera e sottoposti da ere ad un costante bombardamento meteorico. Certo, le caratteristiche superficiali di alcune di queste lune ci incuriosiscono, ma presumiamo che si tratti, in ogni caso, di mondi totalmente inospitali i quali, assai difficilmente, saranno teatro di future missioni e - magari - tentativi di atterraggio.
Anche se non si può mai dire...
In questa prima immagine, vediamo la Luna più conosciuta di Urano: Ariel. Si tratta di un vero e proprio "pianetino" il quale presenta delle caratteristiche superficiali che lo rendono, in parte, simile alla nostra Luna. Si possono vedere dei crateri da impatto (quasi tutti molto antichi), delle vaste distese prive di rilievi ("mari") e poi dei lunghi e profondi crepacci (in questo frame guardate in basso verso Dx) i quali si sono formati, probabilmente, durante il periodo in cui Ariel ha iniziato a raffreddarsi e, come si pensa, detto raffreddamento dovrebbe essere avvenuto in maniera decisamente rapida.
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Titania-PIA01979-PCF-LXTT-IPF.jpgTitania (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)124 visiteCaption NASA:"Voyager 2 obtained this full-disk view of Uranus' moon Titania in the early morning hours of Jan. 24, 1986, from a distance of about 500.000 Km (approx. 300.000 miles). Many circular depressions -- probably impact craters -- are visible in this clear-filter image returned by the Voyager narrow-angle camera.
Other bright spots are distinguished by radiating rays and are probably halo craters that mark relatively more recent impacts. Even more interesting are Linear Troughs (right) that are probably Fault Canyons (Tectonic Fractures). The Troughs break the crust in two directions, an indication of some tectonic extension of Titania's crust. These features indicate that this icy satellite has a dynamic, active interior. Titania is about 1600 Km (approx. 1000 mi) in diameter; the resolution of this image is about 9 Km (approx. 6 mi) per pixel". MareKromium
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Miranda-PIA00044_modest.jpgMiranda (super close-up of "Verona Rupes") - HR123 visiteCaption NASA originale:"This HR image of Miranda was acquired by Voyager 2 on Jan. 24, 1986, when the spacecraft was 36.250 Km (22.500 miles) from the Uranian moon. In this clear-filter, narrow-angle image, Miranda displays a dramatically varied surface. Well shown at this resolution of 660 meters (2.160 feet) are numerous ridges and valleys - a topography that was probably produced by compressional tectonics. Cutting across the ridges and valleys are many faults. The largest fault scarp, or cliff, is seen below and right of center; it shows grooves probably made by the contact of the fault blocks as they rubbed against each other (leaving what are known as slickensides). Movement of the down-dropped block is shown by the offset of the ridges. The fault may be 5 Km (3 mi) high, or higher than the walls of the Grand Canyon on Earth. NASA's Voyager 2 probe encountered the planet Uranus and its collection of moons in 1986. The small moon Miranda, with a diameter of 472 km, has a surface made up of a jumble of bizarre geological features. One of the most prominent of these features is an enormous cliff with a vertical relief of about 20 km (12 miles). This cliff, named Verona Rupes, is 10 times higher than the walls of the Grand Canyon on Earth. That this towering cliff is so huge is made even more remarkable by the small size of Miranda itself. The white material exposed in the cliff face is probably mostly water ice".
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ZA-Uranus-LP-PIA00143.jpgFarewell Uranus... (2)122 visiteCaption NASA originale:"This view of Uranus was recorded by Voyager 2 on Jan 25, 1986, as the spacecraft left the planet behind and set forth on the cruise to Neptune. Voyager was 1 MKM (about 600.000 miles) from Uranus when it acquired this wide-angle view. The picture - a color composite of blue, green and orange frames - has a resolution of 140 Km (90 mi) per pixel".
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VA-UranusRings-3.jpgUranus from HST (Clouds, Rings & Moons - False Colors; credits: NASA)120 visiteMa laddove fotografato, per esempio, in luce infrarossa, Urano rivela alcuni dei suoi segreti: nuvole, anelli e tante piccole lune.
Anche i venti che soffiano su Urano sono decisamente violenti: attraverso analisi compiute di recente e fondate sull'osservazione continua dello spostamento di cospicui ammassi nuvolosi sulla superficie del Pianeta, è stato possibile stimare la velocità di questi "brezze" (!) intorno ai 500 Km orari.
Non si tratta dei venti più violenti del Sistema Solare, però...
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URANUS.jpgUranus from European Southern Obs.119 visiteCome avete potuto vedere Voi stessi nel frame precedente, Urano assomiglia tantissimo ad una biglia di alluminio: un disco completamente liscio e privo di qualsiasi punto di riferimento.
In realtà le foto più spettacolari del Pianeta sono state scattate dall'HST e dal Southern European Observatory, ma la scoperta del suo sistema di anelli avvenne nel 1977, senza l'impiego di sonde o di telescopi orbitali o di telescopi terrestri raffinatissimi ed ultramoderni.
Per quanto attiene la composizione di questi anelli, possiamo solo avanzare delle ipotesi.
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VA-UranusRings-2.jpgUranus, from HST (Clouds, Rings & Moons - False Colors; credits: NASA)117 visiteLa causa di un simile evento rimane tuttora sconosciuta anche se molti Studiosi pensano che questa abnorme inclinazione dell'asse di rotazione del Pianeta (guardate questa immagine per renderVi conto di che cosa intendiamo) sia anch'essa - al pari del sistema di anelli - il "ricordo" di un impatto cosmico con un altro (grande!) corpo celeste, avvenuto qualche centinaio di milioni di anni fa.
Se sottoposto ad un'osservazione visuale diretta Urano, come già accennavamo in precedenza, si mostra come un disco grigiastro, assolutamente privo di caratteristiche superficiali.
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Miranda-vg2_2684617-B.jpgThe beautiful "Verona Rupes" from a different angle (detail mgnf)116 visiteUn detail mgnf dedicato ad un frame raro e stupendo che ci rammenta (ancora una volta) una delle Meraviglie del Sistema Solare: le Verona Rupes.
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UMBRIEL.jpgUmbriel from Voyager 2115 visiteLa quinta Luna Maggiore di Urano: Umbriel.
La caratteristica superficiale che rende unico questo Satellite è data dalla curiosa "aureola bianca" che sembra essere "poggiata" sul suo Polo Nord. In un certo senso un'aureola di questo tipo è visibile anche al Polo Nord di Titania, ma la nitidezza e la definizione possedute da quella di Umbriel sono davvero uniche.
Si tratta di ghiacciai antichissimi?
O magari di una formazione montuosa "circolare" e leggermente infossata?
Non lo sa nessuno: ogni ipotesi è valida e quindi - ovviamente - nessuna, in fondo, lo è...
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Miranda-PIA00038_modest.jpg"Chevron grooves" on Miranda - HR115 visiteCaption NASA originale:"This image of Miranda, obtained by Voyager 2 on approach, shows an unusual 'chevron' figure and regions of distinctly differing terrain on the Uranian moon. Voyager was 42.000 Km (26.000 miles) away when its narrow-angle camera acquired this clear-filter view. Grooved areas baring light and dark bands, distinct from other areas of mottled terrain, are visible at this resolution of about 600 meters (2.000 feet). The bright V-shaped feature in the grooved areas is the 'chevron' observed in earlier, lower-resolution images. Cutting across the bands are sinuous scarps, probably faults. Superimposed on both types of terrain are many bowl-shaped impact craters less than 5 Km (3 mi) wide. The entire picture spans an area about 220 Km (140 mi) across".
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Miranda-V2-PIA00141_modest.jpgMiranda's limb from Voyager 2 - January 24, 1986114 visiteEd ecco ora Miranda, la Luna più interna di Urano. Essa ha una forma decisamente irregolare ed evidenzia molto chiaramente - come in questo frame - delle profonde "cicatrici" superficiali le quali rivelano, come dicono alla NASA, una "complessa storia geologica". EccoVi parte della caption originale NASA per questa foto:"...At least 3 terrain types of different age and geologic style are evident at this resolution of about 700 (2.300 feet) meters. Visible in this clear-filter, narrow-angle image are, from left: (1) an apparently ancient, cratered terrain consisting of rolling, subdued hills and degraded medium-sized craters (2) a grooved terrain with linear valleys and ridges developed at the expense of, or replacing, the first terrain type and (3) a complex terrain seen along the terminator, in which intersecting curvilinear ridges and troughs are abruptly truncated by the linear, grooved terrain. Voyager scientists believe this third terrain type is intermediate in age between the first two".
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Miranda-PIA00042-PCF-LXTT-IPF.jpgMiranda's Southern Hemisphere (Possible Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)113 visiteThis image-mosaic of the Uranian moon Miranda was taken by the NASA - Voyager 2 Spacecraft on January 24, 1986, from a distance of approx. 147.000 Km (such as about 91.287 miles). This image-mosaic was constructed from images taken through the Spacecraft's Narrow-Angle Camera's Green, Violet and UltraViolet Filters.
Miranda, just about 480 Km (such as approx. 299 miles) across, is the smallest of Uranus' five Major natural satellites. Miranda's Regional Geologic Provinces are shown very well in this view of its Southern Hemisphere, imaged at a resolution of roughly 2,7 Km (such as 1,67 miles) per pixel. The dark- and bright-Banded Region with its curvilinear traces, covers about half of the frame. Higher-resolution pictures taken some time later showed many Fault Valleys and Ridges, all parallel to these "Bands". Near the Terminator Line (on the right - Dx), another system of Ridges and Valleys abuts the Banded Terrain; furthermore, many Impact Craters pockmark the Surface of Miranda all over this Region. The largest of these Craters are about 30 Km (approx. 18,63 miles) in diameter and many more of them lie in the range of 5 to 10 Km (such as from 3,1 to 6,2 miles) in diameter.
This frame (which is an Original NASA - Voyager 2 Spacecraft Natural Color image-mosaic published on the NASA - Planetary Photojournal with the ID n. PIA 00042) has been additionally processed, contrast enhanced, magnified to aid visibility of the Surface details and then re-colorized, according to an educated guess carried out by Dr Paolo C. Fienga/LXTT/IPF, in what they should be its Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Voyager 2 Spacecraft and then looked outside, towards the Uranian moon Miranda), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among others, the existence of different Elements present on the Surface of Miranda, each having a different Albedo (---> Reflectivity) and Chemical Composition.
Note: it is possible (but we, as IPF, have no way to be one-hundred-percent sure of such a circumstance), that the actual luminosity of Miranda - as it is in this frame - would appear, to an average human eye, a little bit lower than it has been shown (or, better yet: interpreted) here.MareKromium
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