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Saturn is composed mostly of hydrogen. Its atmosphere has a brownish hue, and its markings are muted by high-altitude haze. Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.
Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed.
Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.
Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christian Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus and shortly thereafter around Jupiter and Neptune).
Physical characteristics
Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water.
Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K at the core, and it radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.
Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and they're also much wider near the equator. Saturn's cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made. Saturn exhibits long-lived ovals and other features common on Jupiter; in 1990 the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters and in 1994 another, smaller storm was observed. Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only planet in the solar system known to do so.
Rotational behaviour
Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been assigned to it, like in Jupiter's case: System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close in value to System II, it has largely superseded it.
While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s). [2] The cause of the change is unknown.
Saturn's rings
Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system. See rings of Saturn for a list of the planet's rings.
History
The rings were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of them. He wrote to the Grand Duke of Tuscany that "Saturn is not alone but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one [Saturn itself] is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo.
The riddle of the rings was not solved until 1655 by Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time.
In 1675 Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.
Physical characteristics of the rings
The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material that Saturn formed out of. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin.
While the largest gaps in the rings, such as the Cassini division and Encke division, could be seen from Earth, the Voyagers discovered the rings to have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise from the gravitational pull of Saturn's many moons in several different ways. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be undiscovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings actually consists of spiral waves raised by the moons' periodic gravitational perturbations.
Data from the Cassini space probe indicates that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O2) and is thought to be a product of the disintegration of water ice from the rings into its components, oxygen and hydrogen.
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Sta je Cassini naso na Enceladus-u
Saturn's tiny icy moon Enceladus, which ought to be cold and dead, instead displays evidence for active ice volcanism.
NASA's Cassini spacecraft has found a huge cloud of water vapor over the moon's south pole, and warm fractures where evaporating ice probably supplies the vapor cloud. Cassini has also confirmed Enceladus is the major source of Saturn's largest ring, the E-ring.
"Enceladus is the smallest body so far found that seems to have active volcanism," said Dr. Torrence Johnson, Cassini imaging-team member at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Enceladus' localized water vapor atmosphere is reminiscent of comets. 'Warm spots' in its icy and razbijaced surface are probably the result of heat from tidal energy like the volcanoes on Jupiter's moon Io. And its geologically young surface of water ice, softened by heat from below, resembles areas on Jupiter's moons, Europa and Ganymede."
Cassini flew within 175 kilometers (109 miles) of Enceladus on July 14. Data collected during that flyby confirm an extended and dynamic atmosphere. This atmosphere was first detected by the magnetometer during a distant flyby earlier this year.
The ion and neutral mass spectrometer and the ultraviolet imaging spectrograph found the atmosphere contains water vapor. The mass spectrometer found the water vapor comprises about 65 percent of the atmosphere, with molecular hydrogen at about 20 percent. The rest is mostly carbon dioxide and some combination of molecular nitrogen and carbon monoxide. The variation of water vapor density with altitude suggests the water vapor may come from a localized source comparable to a geothermal hot spot. The ultraviolet results strongly suggest a local vapor cloud.
The fact that the atmosphere persists on this low-gravity world, instead of instantly escaping into space, suggests the moon is geologically active enough to replenish the water vapor at a slow, continuous rate.
"For the first time we have a major clue not only to the role of water at the icy moons themselves, but also to its role in the evolution and dynamics of the Saturn system as a whole," said Dr. Ralph L. McNutt, ion and neutral mass spectrometer-team member, Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
Images show the south pole has an even younger and more fractured appearance than the rest of Enceladus, complete with icy boulders the size of large houses and long, bluish razbijacs or faults dubbed "tiger stripes."
Another Cassini instrument, the composite infrared spectrometer, shows the south pole is warmer than anticipated. Temperatures near the equator were found to reach a frigid 80 degrees Kelvin (minus 316 Fahrenheit), as expected. The poles should be even colder because the Sun shines so obliquely there. However, south polar average temperatures reached 85 Kelvin (minus 307 Fahrenheit), much warmer than expected. Small areas of the pole, concentrated near the "tiger stripe" fractures, are even warmer: well over 110 Kelvin (minus 261 Fahrenheit) in some places.
"This is as astonishing as if we'd flown past Earth and found that Antarctica was warmer than the Sahara," said Dr. John Spencer, team member of the composite infrared spectrometer, Southwest Research Institute, Boulder, Colo.
Scientists find the temperatures difficult to explain if sunlight is the only heat source. More likely, a portion of the polar region, including the "tiger stripe" fractures, is warmed by heat escaping from the interior. Evaporation of this warm ice at several locations within the region could explain the density of the water vapor cloud detected by other instruments. How a 500-kilometer (310-mile) diameter moon can generate this much internal heat and why it is concentrated at the south pole is still a mystery.
Cassini's cosmic dust analyzer detected a large increase in the number of particles near Enceladus. This observation confirms Enceladus is a source of Saturn's E-ring. Scientists think micrometeoroids blast the particles off, forming a steady, icy, dust cloud around Enceladus. Other particles escape, forming the bulk of the E ring.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.
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Titan je suv
LOS ANGELES - Scientists peering through a ground-based telescope say the surface of Saturn's planet-sized moon Titan appears dry and not awash in oceans of liquid hydrocarbons as is commonly believed.
itan — one of two moons in the solar system known to have a significant atmosphere — has long baffled scientists because it's surrounded by a thick blanket of nitrogen and methane. Scientists have speculated that the atmospheric methane probably came from seas of liquid methane and ethane.
But telescopes and orbiting spacecraft have yet to turn up evidence of a global ocean of methane on Titan.
In the latest study, scientists using the Keck II telescope in Hawaii failed to see any reflections of sunlight that would indicate a body of liquid on the frozen moon during several viewings in 2003 and 2004, said lead researcher Robert West of the Jet Propulsion Laboratory in Pasadena.
Results appear in Thursday's issue of the journal Nature.
The latest Earth-based observations were focused on Titan's southern hemisphere. It's possible the northern region may still contain pools of liquid organic material, West said.
"I would not say that the surface is devoid of liquid methane," he said.
Scientists believe Titan's smoggy atmosphere may be similar to that of the primordial Earth and studying it could provide clues to how life began.
Early radar studies showed that Titan was covered with pools of methane — a flammable gas on Earth but liquid on Titan because of the intense atmospheric pressure and cold.
The continuing search for oceans of liquid hydrocarbons has also proved vexing for the orbiting international Cassini spacecraft, which arrived at Saturn last year on a mission to study the ringed planets and its many moons. Cassini has yet to detect abundant reflections that would indicate areas of liquid on Titan.
But last month, Cassini photographed what appeared to be the best evidence yet of a liquid hydrocarbon lake. The spacecraft noticed a dark spot on Titan's south pole about the size of Lake Ontario that could be the site of a past or present lake, scientists had said. More flybys are needed to confirm the finding.
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Cassini prolete pored Mimas-a
On its recent close flyby of Mimas (MY-muss), the Cassini spacecraft found the Saturnian moon looking battered and bruised, with a surface that may be the most heavily cratered in the Saturn system.
The Aug. 2 flyby of Saturn's 'Death Star' moon returned eye-catching images of its most distinctive feature, the spectacular 140-kilometer diameter (87-mile) landslide-filled Hershel crater. Numerous rounded and worn-out craters, craters within other craters and long grooves reminiscent of those seen on asteroids are also seen in the new images.
The closest images show Mimas, measuring 397 kilometers (247 miles) across, in the finest detail yet seen. One dramatic view acquired near Cassini's closest approach shows the moon against the backdrop of Saturn's rings. A false color composite image reveals a region in blue and red of presumably different composition or texture just west of, and perhaps related to, the Hershel crater.
Scientists hope that analysis of the images will tell them how many crater-causing impactors have coursed through the Saturn system, and where those objects might have come from. There is also the suspicion, yet to be investigated, that the grooves, first discovered by NASA's Voyager spacecraft but now seen up close, are related to the giant impact that caused the biggest crater of all, Herschel, on the opposite side of the moon.
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Cassini snimio Saturnovu auroru
The Cassini spacecraft has taken its best images yet of Saturn's auroras. Like their counterparts on Earth, the polar lights on the Ringed Planet change rapidly in response to activity on the Sun.
Saturn's shifting auroras have been imaged before by both the Hubble Space Telescope and the Cassini spacecraft currently in orbit around the planet. But Cassini previously had only a peripheral view of the phenomenon, as it was focused on taking images of Saturn's equatorial region.
Now, scientists using the spacecraft's Ultraviolet Imaging Spectrograph (UVIS) have taken their best images yet of the shimmering lights. In two images snapped an hour apart, the lights form an oval around the planet's south pole, mimicking a pattern often seen in auroras on Earth. A similar glow can be glimpsed around the north pole in the images.
The auroras "change their shape a little over just one hour," says UVIS principal investigator Larry Esposito, a planetary scientist at the University of Colorado in Boulder, US.
Shock wave
The variability can be traced to charged particles blown off the Sun in the solar wind. "When the solar wind is stronger, or there's a shock wave from the Sun, the auroras are also stronger," says Esposito.
The solar wind compresses the magnetic field around Saturn, dislodging charged particles trapped in radiation belts around the planet. These particles then stream along magnetic field lines down into the top of the atmosphere around the poles. There, they collide with atoms and molecules of hydrogen, making them glow at ultraviolet wavelengths.
"This is a way to remotely see the upper atmosphere of Saturn," Esposito told New Scientist. "We see the interaction between the thin, tenuous upper atmosphere and the lower parts of the radiation belts."
He hopes these and future observations will reveal the composition, density, and any temperature variations in this region, which lies about 1000 kilometres above what is often considered the "top" of the gas giant's main atmosphere. "We hope to learn more about the auroral variations, and make comparisons with the auroras on Earth," says Esposito.
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Saturn
Saturn, šestu po redu planetu od Sunca, okružuje sistem prstenova koji ga čine najljepšom planetom Sunčevog sistema. Vjekovima je ova planeta označavala granicu Sunčevog sistema i mada su je ljudi od davnina gledali, u svoj svojoj ljepoti se predstavila tek kada se u XVII vijeku čovjekovo oko naoružalo teleskopom. Sistem prstenova oko planete koji se tada ukazao podsticao je maštu mnogih sanjara. Napredak optike povećavao je naša znanja o ovoj planeti. Prije početka kosmičkog istraživanja ove planete, bilo je poznato čak deset njegovih satelita. Sa razvojem radio- astronomije otkrivena je veoma aktivna magnetosfera. Saturn je privlačio pažnju mnogih naučnika i sa teorijske tačke gledišta.. Ovakvi razlozi naveli su naučnu javnost da predloži programe kosmičkih istraživanja koje je NASA prihvatila uprkos njihovoj izuzetno velikoj cijeni.
Posle prolaska “Pionira 11”, septembra 1979. i “Vojadžera 1” novembra 1980, pored Saturna je 26. avgusta 1981 prošao i “Vojadžer 2”. Ova tri susreta donijeli su obilje izvanrednih snimaka i podataka o ovoj planeti.
Kao i Jupiter, ova planeta je gotovo potpuno tečno tijelo. U centru planete nalazi se malo kameno jezgro prečnika 20000 km, okruženo slojem leda debljine 5000 km. Sve ovo nalazi se u okeanu vodonika sa nešto helijuma, čija dubina ide i do 35 000 km na ekvatoru. Srednja gustina planete je 704 kg/m3, što je znatno lakše od vode. Zbog toga, iako je Saturn 815 puta veći od Zemlje, teži je samo 95 puta.
Planeta je udaljena od Sunca 1,4 milijarde kilometara, odnosno 9,5 puta više nego Zemlja i prima samo 1% Sunčeve energije koju prima naša Planeta. Ipak, Saturn kao i Jupiter zrači energiju i to 2,8 puta više nego što primi od Sunca. Ova planeta je znatno spljoštena. Prečnik na polovima iznosi 108 000 km a na ekvatoru 119 300 km, što se objašnjava djelovanjem snažne centrifugalne sile, pošto se Saturn okrene oko svoje ose za 10 časova i 14 minuta. Usljed ovako brze rotacije, u ekvatorijalnim predelima Saturna duvaju stalni vjetrovi u pravcu obrtanja planete, koji dostižu brzine od 1800 km/h. To opet uzrokuje veoma malu temperaturnu razliku. Prosječna temperatura u Saturnovoj atmosferi iznosi –180°C i razlika izmedju dnevne i noćne strane je svega nekoliko stepeni. Atmosfera Saturna je mnogo složenija od Jupiterove, to se može vidjeti po tome što ima pet do šest puta više “pruga” sa slojevima promjenljive visine. Vidljivi sloj oblaka Saturna predstavljaju oblaci kristalića amonijaka pomješanih sa kapljicama metana, kojima živopisne boje daju ugljovodonici, sumpor i fosfor. Ovi oblaci plivaju nad slojem aerosoli debljine 80 km. Duboko u unutrašnjosti planete, nalazi se sloj metalnog vodonika, doveden u ovakvo stanje ogromnim pritiscima. Ovaj sloj stvara magnetno polje planete, koje je hiljadu puta jače od Zemljinog, a 20 puta jače od Jupiterovog. Saturnovo magnetno polje je jedinstveno u Sunčevom sistemu. Ose magnetnih polja ostalih planeta su nagnute u odnosu na osu rotacije planete, dok se kod Saturna ove dvije ose poklapaju.
Sistem Saturnovih prstenova predstavlja jedno od čuda vidljivog univerzuma koje se u povoljnim prilikama može vidjeti jačim dvogledom. Prema klasičnoj podjeli, sistem Saturnovih prstenova se sastoji od tri glavna podprstena koji se oznacavaju slovima A, V i S. Oni leže u ekvatorijalnoj ravni u sloju koji se prostire od 2600 do 77 400 km iznad vrhova oblaka. Četvrti prsten koji je dobio oznaku D otkrili su francuski astronomi 1979. godine. Još 1966. godine se predpostavljalo da postoji još jedan prsten, ali je postojanje drugih prstenova potvrdio tek prolet “Pionira 11” pored Saturna. Ova letilica je otkrila i peti F prsten, koji se nalazi 2000 km dalje od A prstena, a širok je 500 km. Osim toga podaci sa “Pionira 11” ukazivali su na postojanje jos dva prstena, G i E, što su u potpunosti potvrdili tek rezultati misije “Vojadžera 1”. Istraživanja su pokazala da se prstenovi najvjerovatnije sastoje od silikatnih čestica oko kojih se akumulirala voda. Na osnovu analize prostiranja radio-talasa ustanovljeno je da veličina čestica varira od 1 cm do 5 m i one odbijaju svjetlost u svim pravcima. S obzirom da je osa Saturna nagnuta pod uglom od 26°45', izgled Saturnovih prstenova sa Zemlje se mijenja. Najbolje se vide kada je jedan od polova najviše okrenut Zemlji što se dešava dva puta za vrijeme njegovog obilaska oko Sunca koji traje 29 i po godina. Kada se Zemlja nalazi u ravni prstenova, nevidljivi su i za velike opservatorijske instrumente.
Danas, znamo da najmanje 30 svjetova kruži oko Saturna. Najveći od njih je Titan, koga je Hajgens otkrio još 1665. godine. On s pravom nosi ovo ime koje mu je dao ser Džon Hersel dva vijeka kasnije. Veći je od Merkura i Plutona i dugo se vjerovalo da je najveći satelit u Suncčevom sistemu. “Vojadžer 1” je pokazao da to nije bilo tačno. Titan ima gustu atmosferu koja skriva njegovu pravu veličinu. Poluprečnik čvrstog tijela je 2575 km dok Jupiterov satelit Ganimed ima poluprecnik 2640 km. Još početkom vijeka javila se pretpostavka da Titan ima atmosferu, pošto je utvrdjeno da je disk Titana tamniji prema periferiji nego u centru. Sunčeva svjetlost koju Titan reflektuje prema Zemlji, prolazi duži put kroz atmosferu na periferiji nego u centru. Zato je slabljenje svjetlosti na periferiji usljed apsorpcije veće pa je rub diska tamniji.
U atmosferi ovog satelita Kujper je 1944. otkrio metan, a pretpostavljalo se da je crvenkasta boja Titana indirektna posljedica prisustva azota. Prve slike koje su stigle sa “Vojadžera” bile su prilično razočaravajuće. Titan je izgledao kao teniska lopta i kroz njegovu gustu atmosferu ništa se nije moglo vidjeti. Jedino se moglo primjetiti da je južna hemisfera znatno svjetlija od sjeverne. Rezultati dobijeni pomoću Vojadžera pokazali su da je atmosfera Titana gušća od Zemljine, i kao i Zemljina bogata je azotom, a pritisak je 1500 milibara. Najvjerovatnije je da u atmosferi postoje oblaci metana i da povremeno padaju metanske kiše. Sama atmosfera se sastoji od azota, argona, metana i vodonika kao i čitavog niza organskih jedinjenja. To je ustanovljeno analizom zračenja koje dolazi iz atmosfere ovog satelita. Naime svaki atom ili molekul zrači ili apsorbuje samo na nekim, odredjenim, talasnim dužinama koje su karakteristične za njega i na osnovu kojih se može prepoznati.
Mjerenja “Vojadžera 1” su pokazala da je temperatura na površini -179°S i da varijacije između ekvatora i polova nisu veće od 3°. Uzrok tome je gusta atmosfera koja apsorbuje svjetlost i ima toplotnu inerciju sto jako smanjuje promjene temperature. Na ovakvim temperaturama metan može da bude tečan i moguće je da je cijeli Titan pokriven okeanom od metana. On bi na Titanu mogao da ima ulogu koju na Zemlji ima voda. Na površini on je tečan, a u atmosferi gasovit i vjerovatno može da stvara oblake i kišu. Izgleda da smo na kraju našli jedini svijet osim Zemlje sa okeanima, oblacima i kišom.
Prisustvo azota i metana na ovom satelitu omogučuje nastanak cijanovodonične kiseline i cijanoacetilena sto omogučuje pojavu amino kiselina, te pruža pogodne uslove za razvoj primitivnog života.
Jos mnogo ostaje da se otkrije o ovom prstenastom svetu i njegovim satelitima. Prvi sledeci korak na tom putu naciniti ce kosmicki brod “Kasini” koji vec hita ka Saturnu i stici ce u njegovu blizinu 2004. godine. Ova misija, u ciji sastav ulazi i sonda “Hajgens” koja ce biti spustena u Titanovu atmosferu, donece odgovore na mnoga pitanja o ovom fantasticnom svetu i njegovim pratiocima.
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Dopuna: 15 Avg 2005 21:48
Zagonetna mrlja na Enceladus-u
Dead moon now appears to be geologically active.In July, NASA's Cassini spacecraft made its latest flyby of Saturn's moon Enceladus, revealing an unexpected hot spot on the moon's south pole.
The finding flipped everything scientists knew about Enceladus on its head, because what should have been a dead moon appeared to be geologically active and what was supposed to be the moon's coldest region turned out to be its warmest.
"This is as astonishing as if we'd flown past Earth and found that Antarctica was warmer than the Sahara," said John Spencer, an astronomer from the Southwest Research Institute in Colorado and a co-investigator of the Cassini mission.
The finding could explain an old mystery concerning Enceladus, but it also presents a new puzzle of its own.
Discovered in 1789 by a British astronomer, Enceladus is named after a mythological Greek giant. Despite its namesake, it is a tiny moon, only about 300 miles in diameter, and small enough to fit snugly inside the state of Arizona.
The surface of Enceladus is coated in a thin layer of ice that reflects back nearly all of the sunlight striking it, making it the brightest object in the solar system apart from the Sun.
Cassini's July flyby of Enceladus had it dipping within 109 miles of the moon's icy surface, its closest approach yet. In addition to the south pole hot spot, Cassini also revealed that the "icy veins" were actually a series of fractures on the moon's surface. Even more surprising, the fractures appeared to be active, violently spewing a slushy jet of warm water and ice into space.
Together, the venting fractures and the hot spot provide strong evidence for geologic activity on Enceladus. If true, the findings could explain the moon's connection with one of Saturn's rings, a relationship that has puzzled scientists for years.
Ovde
Dopuna: 16 Avg 2005 22:17
Mimas
False color images of Saturn's moon, Mimas, reveal variation in either the composition or texture across its surface.
During its approach to Mimas on Aug. 2, 2005, the Cassini spacecraft narrow-angle camera obtained multi-spectral views of the moon from a range of 228,000 kilometers (142,500 miles).
The image at the left is a narrow angle clear-filter image, which was separately processed to enhance the contrast in brightness and sharpness of visible features. The image at the right is a color composite of narrow-angle ultraviolet, green, infrared and clear filter images, which have been specially processed to accentuate subtle changes in the spectral properties of Mimas' surface materials. To create this view, three color images (ultraviolet, green and infrared) were combined into a single black and white picture that isolates and maps regional color differences. This "color map" was then superimposed over the clear-filter image at the left.
The combination of color map and brightness image shows how the color differences across the Mimas surface materials are tied to geological features. Shades of blue and violet in the image at the right are used to identify surface materials that are bluer in color and have a weaker infrared brightness than average Mimas materials, which are represented by green.
Herschel crater, a 140-kilometer-wide (88-mile) impact feature with a prominent central peak, is visible in the upper right of each image. The unusual bluer materials are seen to broadly surround Herschel crater. However, the bluer material is not uniformly distributed in and around the crater. Instead, it appears to be concentrated on the outside of the crater and more to the west than to the north or south. The origin of the color differences is not yet understood. It may represent ejecta material that was excavated from inside Mimas when the Herschel impact occurred. The bluer color of these materials may be caused by subtle differences in the surface composition or the sizes of grains making up the icy soil.
The images were obtained when the Cassini spacecraft was above 25 degrees south, 134 degrees west latitude and longitude. The Sun-Mimas-spacecraft angle was 45 degrees and north is at the top.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
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Dopuna: 17 Avg 2005 22:35
Saturnovi prsteni imaju atmosferu
Data from the NASA/ESA/ASI Cassini spacecraft indicate that Saturn's majestic ring system has its own atmosphere - separate from that of the planet itself.
During its close fly-bys of the ring system, instruments on Cassini have been able to determine that the environment around the rings is like an atmosphere, composed principally of molecular oxygen.
This atmosphere is very similar to that of Jupiter's moons Europa and Ganymede.
The finding was made by two instruments on Cassini, both of which have European involvement: the Ion and Neutral Mass Spectrometer (INMS) has co-investigators from USA and Germany, and the Cassini Plasma Spectrometer (CAPS) instrument has co-investigators from US, Finland, Hungary, France, Norway and UK.
Saturn's rings consist largely of water ice mixed with smaller amounts of dust and rocky matter. They are extraordinarily thin: though they are 250 000 kilometres or more in diameter they are no more than 1.5 kilometres thick.
Despite their impressive appearance, there is very little material in the rings - if the rings were compressed into a single body it would be no more than 100 kilometres across.
The origin of the rings is unknown. Scientists once thought that the rings were formed at the same time as the planets, coalescing out of swirling clouds of interstellar gas 4000 million years ago. However, the rings now appear to be young, perhaps only hundreds of millions of years old.
Another theory suggests that a comet flew too close to Saturn and was broken up by tidal forces. Possibly one of Saturn's moons was struck by an asteroid smashing it to pieces that now form the rings.
Though Saturn may have had rings since it formed, the ring system is not stable and must be regenerated by ongoing processes, probably the break-up of larger satellites.
Water molecules are first driven off the ring particles by solar ultraviolet light. They are then split into hydrogen, and molecular and atomic oxygen, by photodissocation. The hydrogen gas is lost to space, the atomic oxygen and any remaining water are frozen back into the ring material due to the low temperatures, and this leaves behind a concentration of oxygen molecules.
Dr Andrew Coates, co-investigator for CAPS, from the Mullard Space Science Laboratory (MSSL) at University College London, said: "As water comes off the rings, it is split by sunlight; the resulting hydrogen and atomic oxygen are then lost, leaving molecular oxygen.
"The INMS sees the neutral oxygen gas, CAPS sees molecular oxygen ions and an ‘electron view’ of the rings. These represent the ionised products of that oxygen and some additional electrons driven off the rings by sunlight."
Dr Coates said the ring atmosphere was probably kept in check by gravitational forces and a balance between loss of material from the ring system and a re-supply of material from the ring particles.
Last month, Cassini-Huygens mission scientists celebrated the spacecraft's first year in orbit around Saturn. Cassini performed its Saturn Orbit Insertion (SOI) on 1 July 2004 after its six-year journey to the ringed planet, travelling over three thousand million kilometres.
Ovde
Atlas small satellite of the planet Saturn. Atlas is the second known moon from the planet. It orbits Saturn at a distance of about 133,000 km (about 82,600 mi) and completes one orbit every 14 hours. Atlas’s orbit is circular and parallels Saturn’s equator.
Atlas is egg-shaped, with a long diameter of about 40 km (about 25 mi) and a short diameter of about 20 km (about 12 mi). It could easily fit into a medium-sized crater on Earth’s moon. Nothing is known of Atlas’s interior structure, composition, or density. Atlas orbits inside Saturn’s ring system, which is made up of hundreds of thin ringlets of ice and dust separated by thin gaps. There are several major gaps in the ring system called divisions, which are almost entirely devoid of ring particles. Atlas orbits on the inside edge of the Encke Division, one of the most prominent gaps. Scientists believe that Atlas’s gravity “shepherds” the ring particles by attracting them and keeping them out of the Encke Division. For this reason, Atlas is called a shepherd moon.
Atlas was discovered in 1980 by American astronomer Richard Terrile using pictures taken of the rings by the United States Voyager probes. The moon is named for a character in Greek mythology who was condemned to support the weight of the heavens on his shoulders.
Calypso small satellite of the planet Saturn. Calypso is the same distance from Saturn as the moons Tethys and Telesto, but by convention it is considered the 11th moon from the planet. Calypso orbits Saturn at an average distance of about 295,000 km (about 183,000 mi), completing one orbit in about two Earth days. The moon’s orbit is circular and parallels Saturn’s equator.
Calypso is irregular in shape, measuring about 34 km (about 20 mi) at its widest point and about 22 km (about 14 mi) at its narrowest point. The moon could easily fit inside a
medium-sized crater on Earth’s moon. Calypso’s mass, density, interior structure, and composition are all unknown.
Calypso’s most distinctive feature is its orbital relationship with the large moon Tethys. Calypso orbits near Tethys’s trailing Lagrangian point, which is a spot one-sixth of an orbit (60°) behind the larger moon. At this distance, gravitational forces balance well enough to allow Calypso to use the same orbit without crashing into Tethys. Some astronomers theorize that Calypso and Telesto, the other moon that shares Tethys’s orbit, might have been one moon long ago. However, this theory does not explain how the two moons became separated in Tethys’s two Lagrangian points.
A team of American astronomers, including William Baum, Douglas Currie, Dan Pascu, P. Kenneth Seidelman, and Brad Smith, discovered Calypso in 1981 using pictures taken by a prototype of the Faint Object Camera now aboard the Hubble Space Telescope. Calypso is named after a sea nymph in Homer’s epic poem the Odyssey.
Dione large moon of the planet Saturn. Dione shares its orbit with the moon Helene, so both are the same distance from Saturn, but by convention Dione is considered the 12th known satellite from the planet. Dione orbits Saturn at a distance of about 377,000 km (about 234,000 mi). The moon completes one orbit about every three Earth days. Its orbit parallels Saturn’s equator and is only slightly elliptical.
Dione is spherical, measuring about 1120 km (about 672 mi) across, about one-third the diameter of Earth’s moon. After Titan, Dione is Saturn’s densest moon. Planetary scientists theorize that Dione has a rocky core making up about one-third of its diameter, with the outer layers made up of dirty water ice.
Dione’s trailing hemisphere—the side of the moon that faces rearward as the moon orbits Saturn—is heavily cratered and covered by white, wispy features. The leading hemisphere is generally lighter and more lightly cratered. Most moons’ leading hemispheres are more heavily cratered than their trailing hemispheres, so astronomers theorize that Dione was spun around by a colliding asteroid. The wispy features were probably formed when water vapor jetted through razbijacs in the crust and froze on the surface. The center of the side of the moon that faces Saturn is a relatively smooth plain. The two largest craters are Dido, 125 km (78 mi) across, and Aeneas, 150 km (93 mi) across. The tiny moon Helene shares Dione’s orbit around Saturn.
Dione was discovered in 1684 by Italian-born French astronomer Giovanni Cassini, who named his find for the mother of Aphrodite in Greek mythology. Individual features on Dione are named for characters and places from the Italian poet Virgil’s epic poem the Aeneid.
Enceladus large moon of the planet Saturn. Enceladus is the eighth known moon from the planet’s cloud tops. The moon orbits Saturn at a distance of about 238,000 km (about 143,000 mi), completing an orbit about once every 1.3 Earth days. Enceladus’s orbit parallels Saturn’s equator and is only slightly elliptical.
Enceladus is spherical, measuring about 500 km (about 300 mi) in diameter, or about one-sixth the diameter of Earth’s moon. Enceladus’s internal structure and composition are unknown, but measurements of its density and its bright surface cause planetary scientists to theorize that it is made almost entirely of water ice, with a small rocky core. Enceladus is too small to hold onto an atmosphere.
Enceladus’s surface is extremely smooth, with few craters. This means that Enceladus’s surface is young compared to the surfaces of most of the other moons of Saturn—in fact, planetary scientists estimate that Enceladus’s surface is younger than 100 million years. The surface is very reflective, like newly fallen snow. Planetary scientists theorize that Enceladus is continually resurfaced by water bursting through its ice crust. The gravitational forces created by the nearby moons Tethys and Dione, combined with Saturn’s gravitational pull, may squeeze and stretch Enceladus enough to keep it warm inside. Rocks inside Enceladus rub together, producing heat, which melts ice to produce dramatic geologic features on the moon. Portions of the crust are covered by alternating ridges and valleys similar to those observed on Jupiter’s moon, Ganymede. Planetary scientists believe these ridges and valleys formed when sections of the crust broke into plates of sheet ice and bumped into each other. Some of Enceladus’s few craters are flooded by water that washed in from outside the crater and froze. Geysers on Enceladus may contribute material to Saturn’s rings.
Enceladus was discovered in 1789 by German-born English astronomer Sir William Herschel, who named the moon for a Titan defeated in battle by the goddess Athena. Individual features on Enceladus are named for characters and places from English writer Sir Richard Burton’s translation of the collection of stories Arabian Nights.
Epimetheus small satellite of the planet Saturn. Epimetheus is the fifth known moon from the planet. It orbits Saturn at a distance of about 151,000 km (about 94,000 mi) about once every 17 hours. Epimetheus’s orbit is circular and parallels Saturn’s equator.
Epimetheus is irregular in shape, measuring about 144 km (about 89 mi) at its widest point and about 98 km (about 61 mi) at its narrowest point. It could fit inside of a large crater on Earth’s moon. Epimetheus’s density is uncertain, but appears to be very low. The moon might be made of fluffy, porous water ice, or it may contain many cavities. Epimetheus is much too small to maintain an atmosphere.
Epimetheus has an ancient surface, with several asteroid impact craters wider than 30 km (19 mi) across. Other features include grooves, valleys, and ridges. Epimetheus orbits about 10,000 km (about 6000 mi) from the outer edge of Saturn’s ring system. Epimetheus and the neighboring moon Janus probably originated from a single moon split apart by an asteroid collision long ago. The two little moons have orbits only 50 km (30 mi) apart, which causes them to trade places every four years. The outer moon slows down, the inner moon speeds up, and the moons swing past each other, with the moon closer to Saturn moving into the slightly more distant orbit and the moon farther from Saturn moving into the slightly closer orbit. Because of this behavior, Epimetheus and Janus are called co-orbital moons.
Epimetheus was first spotted in 1966 by American astronomer Richard Walker and French astronomer Audouin Dollfus, but they confused it with the moon Janus. In 1978 American astronomers Stephen Larson and John Fountain discovered that there were really two moons in nearly the same orbit. This was confirmed in 1980 using photographs taken by the United States Voyager probes and from earth-based telescopes. Epimetheus is named after a character in Greek mythology who was the son of Iapetus and the brother of Prometheus and Atlas. Individual features on Epimetheus are named for characters from the Greek myth about Castor and Pollux.
Helene small satellite of the planet Saturn. Helene shares its orbit with the larger moon Dione, so both moons are the same distance from Saturn, but by convention Helene is called the 13th moon from the planet. Helene orbits Saturn at an average distance of about 377,000 km (about 226,000 mi), completing one orbit in a little less than three Earth days. Helene’s orbit is circular and parallels Saturn’s equator.
Helene is irregular in shape, measuring about 36 km (about 22 mi) across its widest diameter and about 30 km (about 19 mi) through its narrowest diameter. The moon could easily fit inside a medium-sized crater on Earth’s moon. Helene’s mass, density, interior structure, and composition are all unknown. Helene’s most distinctive feature is its orbital relationship with Dione. Helene occupies one of Dione’s Lagrangian points. A Lagrangian point is a spot one-sixth of an orbit (60°) ahead of or behind an orbiting body. At a Lagrangian point, gravitational forces balance well enough to allow another orbiting object, if the object in the Lagrangian point is considerably smaller than the host body. Since Dione is about 33 times larger than Helene, the moons stay in the same orbit without crashing into each other. Astronomers do not know how the two moons came to occupy the same orbit, but some scientists believe both moons may have been part of a larger body that broke apart.
French astronomers Pierre Laques and Jean Lecacheux discovered Helene in 1980, when Saturn’s rings were tipped edge-on as viewed from Earth and the little moon became visible against the ring edge. The moon is named for Helen of Troy, a character in Greek mythology who was the daughter of Leda and Zeus.
Mimas large moon of the planet Saturn. Mimas is the seventh known moon from the planet. Mimas orbits Saturn at a distance of about 186,000 km (about 116,000 mi) about once every 23 hours. Mimas’s orbit is circular and tilted only 1.5° relative to Saturn’s equator.
Mimas is spherical and measures about 392 km (about 235 mi) across, or about one-ninth the diameter of Earth’s moon. Mimas’s internal structure and composition are unknown, but measurements of its density cause planetary scientists to theorize that it is made almost entirely of water ice, with very little rock.
Mimas’s most distinctive feature is the asteroid impact crater, Herschel. The 130-km (78-mi) crater is one-third as wide as Mimas itself. Herschel was first seen in pictures taken by the United States Voyager probes in 1980. Herschel has walls 5 km (3 mi) high, a floor 10 km (6 mi) deep, and a central peak 6 km (3.6 mi) high. Mimas also has many razbijacs, called chasma. Most occur on the side of Mimas opposite Herschel crater. These were probably generated by stresses caused by the asteroid impact. The largest chasma are called Ossa, Pelion, Qeta, and Pangea. The rest of Mimas’s surface is covered with smaller craters. The largest of these, Arthur crater, is less than half as big as Herschel. There are few signs of geological activity, and Mimas is too small to hold onto an atmosphere.
Mimas was discovered in 1789 by German-born British astronomer William Herschel, who named his find for a Titan slain by the Greek hero Hercules. Except for Herschel crater, individual features on Mimas are named for people and places from English writer Sir Thomas Malory’s prose epic Le morte d’Arthur.
Phoebe moon of the planet Saturn. Phoebe is the 18th known satellite from the planet. Phoebe orbits Saturn as a distance of about 13 million km (about 8 million mi). The moon completes an orbit about once every 550 Earth days. Phoebe is four times more distant from Saturn than the next moon, Iapetus. Phoebe’s orbit is tilted about 5° to Saturn’s equator and is elliptical, or oval-shaped. The moon has a retrograde orbit, which means that it moves clockwise as seen from Saturn’s north pole. Most moons and planets in the solar system, including all of Saturn’s other moons, travel counterclockwise as seen from the north pole of the body they orbit.
Phoebe is roughly spherical, measuring about 220 km (about 132 mi) across. Its diameter is about one-fifteenth the diameter of Earth’s moon. Phoebe’s internal structure and composition are unknown, but its density is extremely low, so it may be made of fluffy water ice or have many cavities.
Phoebe’s surface is extremely dark. The little moon reflects only about 2 percent of the light that falls on it. Phoebe’s surface is covered by dark reddish material similar but not identical to material on Saturn’s moons Iapetus and Hyperion. The United States Voyager probes never passed close enough to Phoebe to take detailed photographs, but some astronomers theorize that the moon’s surface resembles that of Halley’s Comet, a dark, icy body that originated in the cold outer part of the solar system.
Some planetary scientists believe that Phoebe was captured by Saturn’s gravity from a population of odd objects called Centaurs, which orbit the sun near Saturn. The best known of these objects is Chiron, which is sometimes called a giant comet. Chiron occasionally passes near Saturn, as Phoebe might have done before it was captured and became Saturn’s most distant known moon. Phoebe has no known atmosphere.
Phoebe was discovered in 1898 by American astronomer William Pickering. The moon is named after a character in Greek mythology who was the virgin goddess of the hunt and of the moon.
Izvor:Microsoft® Encarta® Encyclopedia
Dopuna: 22 Avg 2005 11:15
Atmosfera prestenova
Interesantni podaci stižu sa letelice Kasini. Po upravo objavljenim na sajtu ESA Science izlazi da Satrunovi prstenovi imaju sopstvenu atmosferu. U bliskom prolazu pored sistema Saturnovih prstenova instrumenti na letelici su oko njih zabeležili molekule kiseonika. Po sastavu vrlo sličnu atmosferu imaju i veliki Jupiterovi sateliti Evropa i Ganimed.
Inače, Saturnovi prestenovi se sastoje uglavnom od vodenog leda pomešanog sa malom količinom prašine i kamenja. Prstenovi su izuzetno tanki: mada imaju oko 250 000 kilometara u prečniku oni su svega 1,5 km debeli. Uprkos impresivnom izgledu ima vrlo malo materijala u prstenovima. Kada bi se prstenovi sabili u jedno telo ono ne bi bilo veće od 100 km u prečniku.
Poreklo prstena nije do kraja razjašnjeno. Naučnici su ranije smatrali da su prstenovi formirani u isto vreme kada i planete, spajanjem vrtložnih oblaka međuzvezdanog gasa pre četiri milijarde godina. Međutim, sada se čini da su oni mlađi, verovatno svega nekoliko stotina miliona godina.
Druga teorija sugeriše da je neka kometa proletela blizu Saturna i da se raspala usled Saturnovih plimnih sila. Moguće je i da je jedan od Saturnovih meseca u prošlosti pogodio neki asteroid te se razbio u komadiće koji sada čine prstenove.
U svakom slučaju, sistem prstenova nije stabilan i on mora da ee obnavlja tokom vremenom, možda lomljenjem većih satelita.
Misija Kasini – Hajgens je u julu proslavila prvu godinu orbitiranja oko Saturna. Kasini je ušao u orbitu oko Saturna 1. jula 2004. godine nakon šest godina puta, prevalivši više od tri milijarde kilometara. Inače ova mislija je projekat tri agencije: NASA, ESA i ASI (Italijanska svemirska agencija)
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