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| Ultimi arrivi - Neptune and His Moons |
Triton-PIA00059_modest.jpgThe South Polar Region of Triton: "dark plumes" and "seasonal winds"61 visiteCaption NASA originale:"This image of the South Polar terrain of Triton, taken on Aug. 25, 1989 reveals about 50 dark plumes or 'wind streaks' on the icy surface. The plumes originate at very dark spots generally a few miles in diameter and some are more than 100 miles long. The spots which clearly mark the source of the dark material may be vents where gas has erupted from beneath the surface and carried dark particles into Triton's nitrogen atmosphere. Southwesterly winds then transported the erupted particles, which formed gradually thinning deposits to the northeast of most vents. It is possible that the eruptions have been driven by seasonal heating of very shallow subsurface deposits of volatiles and the winds transporting particles similarly may be seasonal winds. The polar terrain, upon which the dark streaks have been deposited, is a region of bright materials mottled with irregular, somewhat dark patches. The pattern of irregular patches suggests that they may correspond to lag deposits of moderately dark material that cap the bright ice over the polar terrain".Gen 13, 2005
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Triton-PIA00061_modest.jpgTriton's Northern Hemisphere in HR62 visiteCaption NASA originale:"This is one of the most detailed views of the surface of Triton taken by Voyager 2 on its flyby of the large satellite of Neptune early in the morning of Aug. 25, 1989. The picture was stored on the tape recorder and relayed to Earth later. Taken from a distance of only 40.000 Km (25.000 mi), the frame is about 220 Km (140 miles) across and shows details as small as 750 meters (0.5 miles). Most of the area is covered by a peculiar landscape of roughly circular depressions separated by rugged ridges. This type of terrain, which covers large tracts of Triton's Northern Hemisphere, is unlike anything seen elsewhere in the Solar System. The depressions are probably not impact craters: they are too similar in size and too regularly spaced. Their origin is still unknown, but may involve local melting and collapse of the icy surface. A conspicuous set of grooves and ridges cuts across the landscape, indicating fracturing and deformation of Triton's surface. The rarity of impact craters suggests a young surface by solar system standards, probably less than a few billion years old".Gen 13, 2005
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Triton-PIA00317_modest.jpgTriton (False Colors; credits: NASA/JPL/USGS)64 visiteCaption NASA originale:"The color was synthesized by combining HR images taken through orange, violet and ultraviolet filters; these images were displayed as red, green and blue images and combined to create this color version. With a radius of 1.350 Km, about 22% smaller than Earth's Moon, Triton is by far the largest satellite of Neptune. It is 1 of only 3 objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan). Triton has the coldest surface known anywhere in the Solar System (38 K, about -391 degrees F); it is so cold that most of Triton's nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cantaloupe terrain, whose origin is unknown, and a set of "cryovolcanic" landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior".Gen 13, 2005
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Triton-PIA01537_modest.jpgGeologic processes on Triton (1)63 visiteCaption NASA originale:"This image of Triton was taken from a distance of about 130.000 Km (80.000 miles) at 12:20 a.m. PDT Aug. 25 1989. The image was received at JPL four hours later at about 4:20 a.m. The smallest detail that can be seen is about 2,5 Km (or 1,5 miles) across. The long linear feature extending vertically across the image is probably a graben (a narrow down dropped fault block) about 35 Km (20 miles) across. The ridge in the center of the graben probably is ice that has welled up by plastic flow in the floor of the graben. The surrounding terrain is a relatively young icy surface with few impact craters".Gen 13, 2005
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Triton-PIA01538_modest.jpgGeologic processes on Triton (2)62 visiteCaption NASA originale:"Part of the complex geologic history of icy Triton, Neptune's largest satellite, is shown in this Voyager 2 photo, which has a resolution of 900 meters (2.700 feet) per picture element. The photo was received as part of a Triton-mapping sequence between 3:30 and 5:30 a.m. (PDT). This view is about 500 Km (300 miles) across.
It encompasses 2 depressions, possibly old impact basins, that have been extensively modified by flooding, melting, faulting and collapse. Several episodes of filling and partial removal of material appear to have occurred. The rough area in the middle of the bottom depression probably marks the most recent eruption of material. Only a few impact craters dot the area, which shows the dominance of internally driven geologic processes on Triton".Gen 13, 2005
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Triton-PIA01536_modest.jpgTriton (through green, violet and ultraviolet filters)64 visiteCaption NASA originale:"This color photo of Neptune's large satellite Triton was obtained on Aug. 24 1989 at a range of 530.000 Km (or 330.000 miles). The resolution is about 10 Km (6,2 miles), sufficient to begin to show topographic detail. The image was made from pictures taken through the green, violet and ultraviolet filters. In this technique, regions that are highly reflective in the ultraviolet appear blue in color. In reality, there is no part of Triton that would appear blue to the eye. The bright southern hemisphere of Triton, which fills most of this frame, is generally pink in tone as is the even brighter equatorial band. The darker regions north of the equator also tend to be pink or reddish in color".Gen 13, 2005
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s Rings-PIA02207_modest.jpgNeptune's Rings (and a crescent Neptune) from 1,1 MKM65 visiteCaption NASA originale:"This wide-angle Voyager 2 image, taken through the camera's clear filter, is the first to show Neptune's rings in detail. The 2 main rings, about 53.000 Km and 63.000 Km from Neptune, are 5 to 10 times brighter than in earlier images and the difference is due to lighting and viewing geometry. In approach images, the rings were seen in light scattered backward toward the spacecraft at a 15° phase angle. However, this image was taken at a 135° phase angle as Voyager 2 left the planet. That geometry is ideal for detecting microscopic particles that forward-scatter light preferentially. The fact that Neptune's Rings are so much brighter at that angle means the particle-size distribution is quite different from most of Uranus' and Saturn's rings, which contain fewer dust-size grains. However, a few components of the Saturnian and Uranian ring systems exhibit forward-scattering behavior: the F-Ring and the Encke Gap ringlet at Saturn, and 1986U1R at Uranus".Gen 13, 2005
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Triton-PIA02246_modest.jpgTriton from Voyager 2 (real colors)63 visiteCaption NASA originale:"Features as small as 100 Km across can be seen in this color image of Neptune's satellite Triton, photographed by Voyager 2 on Aug. 20, 1989, while it was still 5.4 MKM (3.3 million miles) from Neptune. Triton's overall pinkish color may be due to reddish materials produced by irradiation of methane gas and ice on the satellite. The dark areas near the top of the image seem to be part of a belt of dark markings observed near Triton's equator at different longitudes. Generally, darker areas on Triton appear to be somewhat redder in color than brighter areas. The central longitude in the image is 123°. Here the south pole is at about 6 o'clock, approximately one sixth of the way up from the bottom. The color image was made from three black and white frames, taken through clear, violet and green filters".Gen 13, 2005
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Triton-PIA02212_modest.jpgTriton from Voyager 2 (false colors?)60 visiteCaption NASA originale:"The smallest features that can be seen in this false color image of Neptune's largest satellite, Triton, are about 47 Km across. The image, taken by Voyager 2 early in the morning of Aug. 23, 1989, is a composite of three images taken through ultraviolet, green, and violet filters. The image offers an example of the kinds of puzzles scientists face on the eve of an encounter: mottling in the bright southern hemisphere may be the result of topography, if Triton's crust is predominantly water ice, which is rigid at Triton's surface temperature. Alternatively, the mottling could be due to markings on a smooth surface, if the crust is composed of nitrogen, carbon monoxide, or methane ice, since they are soft at the same temperature". Nota: in questo frame la NASA parla chiaramente di "false colors" mentre nel successivo dice "natural colors". Ora a noi i "colors" di Tritone, nei due frames, sembrano identici.
Sono dunque entrambi "natural" o "false"? Dov'è la verità?Gen 13, 2005
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AA-Neptune-HST-PIA01287_modest.jpgNeptune from HST88 visiteLa seconda serie di immagini del Pianeta Nettuno, ripreso dal Telescopio Spaziale Hubble. Poco da aggiungere rispetto a quanto già detto in precedenza se non che la (quasi totale) impossibilità di fotografare amatorialmente Nettuno dalle nostre latitudini dipende da un continuo e costante degrado dell'atmosfera che ci circonda: inquinamento gassoso ed inquinamento luminoso fanno a gara per vedere quale dei due sia il peggiore.
Purtroppo, sebbene per motivi ed in ambiti diversi, sono entrambi "letali"!Ott 13, 2004
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Nereid-V2-PIA00054.jpgNereid64 visiteUn'immagine certo non magnifica, ma di grande valore e molto rara: si tratta di Nereide, la seconda delle due Lune Maggiori di Nettuno. I dettagli che possono percepirsi da questa fotografia ottenuta dal Voyager 2 sono troppo indefiniti per poter azzardare delle valutazioni, ma l'ipotesi più probabile relativamente alle fattezze di Nereide, è che si tratti di un piccolo mondo fatto di roccia e ghiaccio, dalla forma irregolare, ricoperto di crateri da impatto e costellato da crepacci che ne rendono la superficie completamente inospitale.
Forse, come sostengono alcuni Scienziati, si potrebbe trattare di un KBO catturato da Nettuno in tempi assai remoti.Ott 13, 2004
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AA-Neptune-HST-PIA01285_modest.jpgNeptune from HST76 visitePer incominciare il nostro breve viaggio verso l'ultimo dei Giganti Gassosi, ci sembra giusto mostrarVi due serie di immagini del "Re dei Mari" che sono state ottenute dall'Hubble Space Telescope. Nettuno ci appare come un disco dal colorito azzurro vivo, sul quale spiccano - ora bianche, ora con colori e sfumature diverse - le grandi formazioni nuvolose che attraversano rapidamente gli strati alti dell'atmosfera del Pianeta.
Nettuno, per gli amanti della fotografia spaziale, è un obbiettivo difficile ed al confine estremo di visibilità dalla Terra, allorchè si faccia uso di strumenti amatoriali. A "fuoco diretto", usando un buon telescopio (tipo C-8), esso appare leggermente più definito di una stella, la forma sferica è appena percepibile ed il colorito, purtroppo, grigio-azzurrato, molto lontano - purtroppo - dal suo colore naturale, come vedremo sia nelle foto HST, sia nelle foto Voyager.Ott 13, 2004
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