| Piú viste - Uranus and His Moons |
VA-Uranus_ Rings-V2-PIA01977_modest.jpgUranus' Rings (1)81 visiteEcco gli Anelli di Urano, dei quali si è sovente parlato, ma senza mai approfondire l'argomento in maniera particolare.
Noi Vi offriamo 3 istantanee provenienti dalla Sonda Voyager 2 e relative a questa ennesima Meraviglia del Cosmo. Non ci sono segni evidenti di anomalie e le immagini, occorre dirlo, non sono di grandissima qualità. Tuttavia, in attesa di nuovi passaggi ravvicinati accanto a questo strano Gigante Gassoso (passaggi, purtroppo, non previsti a breve...), noi guardiamo ancora al passato ed a ciò che abbiamo raccolto negli anni trascorsi.
Il valore storico di queste fotografie è indiscutibile; quello scientifico può essere meglio compreso confrontandole con le immagini ad Alta Definizione degli Anelli di Saturno che ci arrivano - possiamo dire "quotidianamente" - dalla Sonda NASA-ESA Cassini-Huygens.
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VO-Uranus-PIA00032.jpgUranus in natural and false colors81 visiteCaption NASA originale:"These two pictures of Uranus - one in true color (left) and the other in false color - were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. The spacecraft was 9,1 MKM (5,7 MMs) from the planet, several days from closest approach. The picture at left has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. The picture is a composite of images taken through blue, green and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. Beyond this boundary lies the hidden northern hemisphere of Uranus, which currently remains in total darkness as the planet rotates.
The blue-green color results from the absorption of red light by methane gas in Uranus' deep, cold and remarkably clear atmosphere. The picture at right uses false color and extreme contrast enhancement to bring out subtle details in the polar region of Uranus. Images obtained through ultraviolet, violet and orange filters were respectively converted to the same blue, green and red colors used to produce the picture at left. The very slight contrasts visible in true color are greatly exaggerated here. In this false-color picture, Uranus reveals a dark polar hood surrounded by a series of progressively lighter concentric bands. One possible explanation is that a brownish haze or smog, concentrated over the pole, is arranged into bands by zonal motions of the upper atmosphere. The bright orange and yellow strip at the lower edge of the planet's limb is an artifact of the image enhancement. In fact, the limb is dark and uniform in color around the planet".
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AB-AB-vg2_2686312.gifThe limb of Uranus81 visiteOriginal caption:"Voyager 2 image of the limb of Uranus from about 694.000 Km. This narrow angle view is about 5.100 Km across and was taken 12 hours after closest approach. The image is centered at -10, 212 and North is up. (Voyager 2, FDS 26863.12)".
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VAA-Uranus Rings-1.jpgUranus' Rings (3)81 visiteCiò che intendiamo dire (l'ipotesi di lavoro che Vi proponiamo) è questo: forse gli Anelli di Giove, Urano e di Nettuno hanno un'origine diversa (almeno in parte) rispetto a quelli di Saturno. Gli Scienziati che vanno per la maggiore, infatti, pensano che la differenza nella "tessitura" degli Anelli tipici dei Giganti Gassosi dipenda solo dalle dimensioni originali del corpo celeste (o dei corpi celesti) dal cui disintegrarsi essi vennero generati: più grande era questo/i corpo/i, maggiore era il quantitativo di materiale che si distribuiva lungo l'orbita, più grande e maestosa (scenicamente) risultava la trama finale degli Anelli. Noi opiniamo che, forse, questa costruzione spiega gli Anelli di Giove, Urano e Nettuno ma non è sufficiente - da sola! - per spiegare l'origine degli Anelli di Saturno i quali ci appaiono troppo grandi e troppo complessi per essere derivati soltanto da un evento come quello dianzi ipotizzato.
Ne riparleremo ancora.
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Titania-vg2_p29509.jpgTitania in natural colors (HR)81 visitenessun commento
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Titania-PIA01979.jpgTitania (possible Natural Colors; credits: Lunar Explorer Italia)81 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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Uranus-HST-PIA01282.jpgUranus from HST80 visiteThis NASA HST image of Uranus reveals the Planet's Rings and bright clouds and a high altitude haze above the Planet's South Pole.
Hubble's view was obtained on August 14, 1994, when Uranus was 1,7 BM (about 2,8 BKM) from Earth. These details, as imaged by the Wide Field Planetary Camera 2, were only previously seen by the Voyager 2 spacecraft, which flew by Uranus in 1986. Since then, none of these inner satellites has been further observed, and detailed observations of the Rings have not been possible.
Though Uranus' Rings were discovered indirectly in 1977 (through stellar occultation observations), they have never before been seen in visible light through a ground-based telescope.
Hubble resolves several of Uranus' Rings, including the outermost Epsilon-Ring. The Planet has a total of 11 concentric Rings of dark dust. Uranus is tipped such that its rotation axis lies in the plane of its orbit, so the Rings appear nearly face-on.
Three of Uranus' inner moons each appear as a string of three dots at the bottom of the picture. This is because the picture is a composite of three images, taken about six minutes apart, and then combined to show the moons' orbital motions. The satellites are, from left to right, Cressida, Juliet, and Portia. The moons move much more rapidly than our own Moon does as it moves around the Earth, so they noticeably change position over only a few minutes.
One of the four gas giant planets of our solar system, Uranus is largely featureless. HST does resolve a high altitude haze which appears as a bright "cap" above the planet's south pole, along with clouds at southern latitudes (similar structures were observed by Voyager). Unlike Earth, Uranus' south pole points toward the Sun during part of the planet's 84-year orbit. Thanks to its high resolution and ability to make observations over many years, Hubble can follow seasonal changes in Uranus's atmosphere, which should be unusual given the planet's large tilt.
The Wide Field/Planetary Camera 2 was developed by the Jet Propulsion Laboratory and managed by the Goddard Spaced Flight Center for NASA's Office of Space Science.
MareKromium
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UMBRIEL-vg2_p29502-Wunda.jpgUmbriel's extremely unusual surface feature: Wunda Crater (Natural Colors; credits: NASA)80 visiteHere is a closeup systhesis from all the close-ups of Wunda Crater.
It is hard to tell if the craters dug up the bright material, or if they had something to do with the impacts. Maybe it was derived from eruptions or melting caused by impacts.
Or maybe they are some sort of frost left over from impacts.
This issue may not be resolved until another spacecraft observes Umbriel. Here is the best view of Wunda that can be generated with the available data.MareKromium
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Miranda-vg2_2684626.jpgThe "limb" of Miranda79 visitenessun commento
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VP-URANUS-PIA01360.jpgUranus in natural colors78 visiteCaption NASA originale"These 2 pictures of Uranus were compiled from images recorded by Voyager 2 on Jan. 1O, 1986, when the NASA spacecraft was 18 MKM (11 MMs) from the Planet. The images were obtained by Voyager 2 narrow-angle camera; the view is toward the planet's pole of rotation, which lies just left of center. The picture on the left has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. The second picture is an exaggerated false-color view that reveals details not visible in the true-color view - including indications of what could be a polar haze of smog-like particles. The true-color picture was made by combining pictures taken through blue, green and orange filters. The dark shading of the upper right edge of the disk is the terminator, or day-night boundary. The blue-green appearance of Uranus results from methane in the atmosphere; this gas absorbs red wavelengths from the incoming sunlight, leaving the predominant bluish color seen here".
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Miranda.jpgExtremely anomalous surface features on Miranda (HR)78 visitenessun commento
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UranusandAriel-Eclypse.jpgUranus and Ariel; Earth and Moon78 visiteCaption NASA originale"This illustrates the relative sizes of Uranus, Earth and Earth's Moon. The images are shown at the proper relative size, but not the correct relative distance from each other. Uranus is approx. 31.000 miles (about 50.000 Km) in diameter, or about 4 times the size of Earth. The Earth is approx. 7.900 miles (about 12.800 Km) in diameter, or about 4 times the diameter of the Moon (2.100 miles, such as about 3.500 Km)".
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