"Кассини" !

[Space Alien]

На Титане нашли два типа дюн[/size]

Астрономы обнаружили на Титане два типа дюн. Ученые надеются, что этот факт поможет в понимании погодных процессов на спутнике Сатурна. Статья ученых появилась в журнале Icarus, а ее краткое изложение приводится на сайте NASA.

По словам исследователей, тип дюн определяется преимущественно расположением - на севере находятся относительно жиденькие дюны, в то время как на юге они более "полные". Также тип дюн зависит от высоты региона - в низинах расстояния между гребнями много меньше, чем на возвышенностях.

 По мнению ученых, дюны на Титане состоят из частиц замерзших углеводородов. Линейные размеры частиц составляют порядка 0,1 сантиметра (как именно они образуются, ученым до сих пор неизвестно). Сами дюны достигают 100 метров в высоту, 1-2 километра в ширину и сотни километров в длину. Дюны располагаются в полоске между 30 градусами южной широты и 30 градусами северной.

 При этом, с точки зрения геологии, дюны являются вторым по распространенности ландшафтом на спутнике - они занимают 13 процентов площади Титана. На первом месте при этом находятся однородные (во всяком случае они представляются такими со спутника) равнины. По мнению ученых, разные свойства дюн могут объясняться неравномерным распределением углеводородных океанов на Титане.

 Титан - первое небесное тело, на котором обнаружен цикл наподобие земного круговорота воды в природе. Роль воды в нем выполняет метан. На Титане идут метановые дожди, есть озера жидкого метана и метановый туман. Все эти открытия были сделаны при помощи зонда "Кассини", данные которого использовались и в последней работе.

http://lenta.ru/news/2012/01/24/titan/

[Petrovich]

Советую посмотреть  :wink:



В полноэкранном режиме и в наушниках

[FarEcho]

Советую посмотреть  :wink:



В полноэкранном режиме и в наушниках

Фантастика! Петрович, спасибо за ссылку.

[testest]

Просто потрясающее видео!

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http://www.nasa.gov/multimedia/imagegallery/image_feature_2166.html

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Очередной фотошоп

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Cassini Top 10 Science Highlights -- 2011

Jan. 10, 2011

All throughout 2011, the Cassini spacecraft collected science at a torrid pace. Mission scientists dished up exciting results all year long, from scrutinizing a huge new storm on Saturn, to discovering salty particles in Enceladus' plumes, to studying Saturn's rings with radio science experiments.

Those Cassini scientists took a quick breath to compile their list of Top 10 Science Highlights. See what in 2011 stood out to them.

   1. The Great Northern Storm on Saturn
   2. Spring Rain on Titan
   3. Discovery of Salty Particles in Enceladus' Plume
   4. Ripples in the Rings: A Record of Past Impacts
   5. Reversal of North-south SKR Periodicities
   6. Discovery of the Auroral Footprint of Enceladus
   7. The Falling Haze Layer on Titan
   8. First SAR images of Enceladus
   9. Helene the Ice Queen
  10. Bursty F-ring Activity

http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20120110/

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Рея и Титан

http://www.nasa.gov/mission_pages/cassini/multimedia/pia14595.html

[X]

А тем временем на Титане нашли озеро в умеренных широтах южного полушария (примерно 40°ю.ш. 280°з.д.)
http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2766.pdf
Для сравнения, озеро Онтарио - 72°ю.ш. 183°з.д.

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Iapetus' peerless equatorial ridge

Whenever I want to find something new and different to write about, it's easy to find good ideas: just go to the website of a scientific journal and look at their current table of contents or papers in press. I checked out the Journal of Geophysical Research - Planets yesterday, and found a new paper in press by Dombard, Cheng, McKinnon, and Kay that claims to explain how Iapetus' equatorial ridge formed. Cool!

First, let me set the stage. Iapetus is bizarre in a lot of ways. It has a yin-yang surface of dark and bright material. It is unexpectedly flat pole-to-pole. It seems to have an excessive number of large impact basins. And it has a mountainous ridge, up to 20 kilometers tall, running exactly along its equator for well over three-quarters of its entire circumference. The existence of this ridge was revealed very early in the Cassini mission, in an encounter at the end of 2004.
   


Iapetus in color
This portrait of Iapetus is from Cassini's first encounter with the distant moon, on December 31, 2004, just before the Huygens descent. The flyby produced astonishing views of the moon, revealing its "belly band" of mountains as well as several previously undiscovered giant impact basins. The image is an approximately natural color mosaic taken at a distance of about 173,000 kilometers. Iapetus is 1470 kilometers across, the third-largest moon of Saturn after Titan and Rhea. Credit: NASA / JPL / SSI / color composite by Gordan Ugarkovic

But Cassini got much better views of the ridge later, during its one and only close flyby of Iapetus. Here's one of those views. You can really see how Iapetus' disk has a fairly smoothly round shape that's interrupted, right at the equator, by a long ridge that is essentially triangular in cross-section (though battered by impacts).



Iapetus' belly band
Iapetus has a mountain range exactly on its equator, measuring in some places 13 kilometers tall and 20 kilometers wide. This image was taken on September 10, 2007. Credit: NASA / JPL / SSI / Color composite by Emily Lakdawalla

The ridge is mostly seen on the dark, leading hemisphere, breaking up a bit on the trailing hemisphere. In this wonderful image sequence, Cassini sights across the limb as it flies past, and you can see the relatively bright white slopes of trailing hemisphere material come into view toward the end.



Cassini flies over Iapetus' equatorial ridge
As Cassini flew by Iapetus on September 10, 2007, it took 13 images of the equatorial ridge rising over the horizon. Most of the flight is over Iapetus' dark terrain, but at the very end of the animation, the white flanks of the Voyager mountains begin to appear. Credit: NASA / JPL / SSI / Animation by Emily Lakdawalla

Before Dombard and his coauthors can explain how the ridge formed, they begin by explaining what it is. Here are the salient features of Iapetus:

    * It is equatorially flattened by 4.5%, meaning that it's about 70 kilometers shorter pole-to-pole than it is wide across the equator. (For comparison, Earth's rotational flattening is only 0.3%.) This amount of equatorial flattening would make sense if Iapetus had a day only 16.5 hours long, but its days are actually 79 Earth days long.
    * The equatorial ridge is up to 20 kilometers high, 200 kilometers wide, and spans more than 75% of Iapetus' equator. If you do the math, you'll find it contains 0.1% of all of Iapetus' mass. This is both a little and a lot. All of Earth's oceanic crust amounts to 0.1% of Earth's mass.
    * It is perfectly straight and sits exactly on the equator.
    * It is heavily cratered, so appears ancient.
    * Iapetus doesn't have any other major geomorphic features other than the ridge and lots of craters.
    * The ridge is "supported by the lithosphere without an obvious flexural signal." Don't worry, I'll explain.

This last observation is really important, so I'll explain what it means. It has to do with something called isostasy. The planets and moons of the solar system all have solid outer crusts, which seem firm and unbending. In fact, though, solid worlds flex and bend all the time. On Earth, it's quite a common thing to have mountains appear suddenly (in terms of geologic time, that is). Mountains are heavy, and their weight exerts a very strong force on the rock beneath them. The rock of Earth's mantle flows out of the way of this pressure (flowing in the solid state, over geologic time), and the stiffer rock of Earth's crust bends downward beneath the load of the mountain. So, wherever you have big, suddenly created mountains, like volcanoes, there's usually a flexural moat around them, an area where the crust is depressed by the weight of the mountain. Of course, on Earth, such moats are rapidly filled by sediment, so they don't show up in topographic surveys, but the moats are very easy to see in gravity survey data.

Back to Iapetus: it's very surprising that a ridge so massive, so tall, and so narrow doesn't have a flexural trough running down each side of it. What that means is that the ridge formed at a time when Iapetus was so cold and stiff that the moon's crust has been strong enough to support the weight of that ridge without bending at all ever since the ridge formed. The first part of the paper explores exactly how cold Iapetus has needed to be since the ridge formed. The answer is very cold; specifically, it can have been putting out heat energy amounting to no more than 3 milliwatts per square meter since the ridge formed. This means that the ridge must have formed after all of Iapetus' primordial heat had pretty much dissipated. Current models for Iapetus' formation don't get this cold until about a billion years after the moon itself formed. The ridge formed relatively late in Iapetus' history.

So, any idea for how the ridge formed must explain the following things. The ridge:

    * formed late;
    * sits exactly on the equator;
    * is found only on the equator (that is, there are no other ridges on Iapetus); and
    * is found only on Iapetus.

Next, Dombard et al. go through the mechanisms that have been proposed so far to explain the ridge's formation. This part of the paper depends strongly on observations of other moons across the solar system, all of which are made of the same materials under the same laws of physics, but whose histories have diverged to show us how different conditions can affect the same materials.

Some people have suggested that the ridge resulted from Iapetus' despinning (going from a 16.5-hour to 79-day day). But these can't explain why a ridge formed only on the equator, and not also in other places, like the tectonic features found all over Europa and Ganymede (and predicted to be observed on Pluto and Charon). Another suggestion is that it formed by some kind of push up from below; but this would require Iapetus' stiff outer layer to be relatively thin, so there should be a flexural moat,and there isn't. Another idea is that Iapetus was spinning so fast when it formed that it was near its rotational stability limit, and the equator was raised because the centrifugal force nearly counteracted gravity. But a body as large as Iapetus wouldn't be able to maintain a circular equator under such conditions; it would become a football shape, like Haumea and possibly Varuna.

The only remaining possibility is that the ridge actually started out as a debris ring, which collapsed onto Iapetus. Now, I must admit that I, personally, have always been hostile to this idea, because it's counterintuitive to me that you can build an edifice by crashing stuff in to a moon. Dombard and his coauthors don't seem to regard this as a problem, and neither do Wing Ip or Hal Levison, who have both written papers on the topic.

However, Dombard et al. point out that if this really is how Iapetus' ridge formed, then it's hard to explain why we see the ridge only on Iapetus and not on any other moon in the solar system. How can Iapetus be so unique?

So here's the last part of the paper: they tell a fairy tale about how the ridge formed, then poke at the fairy tale to see how the story would be different on other moons. Here's the fairy tale: As Iapetus was forming, very far from Saturn, it either captured a 100-kilometer-diameter satellite or suffered a giant impact whose debris coalesced into a satellite of that size. At Iapetus' orbital distance from Saturn, impacts between Iapetus and anything in a similar orbit would be very slow, well under a kilometer per second, so it's actually very easy to imagine Iapetus capturing a body of this size.

Initially, Iapetus' moon would have had a very elliptical orbit, but tidal forces would circularize the orbit. Then, the orbit would evolve, either going closer to Iapetus or farther away from Iapetus with time. If it started out on a retrograde orbit; it would get closer; if on a prograde orbit, it would get farther. At this point in the paper they do a little physics and show that for pretty much any prograde orbit, the moon would eventually recede to about 9 times Iapetus' diameter away from the moon.

All this time, though, Saturn is exerting its own tidal forces on Iapetus, causing the moon to slow from its initially fast rotation to its present very slow rotation. This takes a while, possibly billlions of years. But once Iapetus is spinning so slowly, it acts as a brake on the little satellite's outward travel. The orbit starts to move closer and closer to Iapetus, rather rapidly.

Soon the satellite is so close to Iapetus that it's within the Roche limit, where Iapetus' gravity tears apart the satellite (which was most likely a rubble pile to begin with). A thick debris ring forms. Crashes among debris pieces make them smaller and also flatten the ring until, like Saturn's ring system, Iapetus' ring system is perfectly flat and perfectly aligned with its spin equator. Over time, the bits of moon de-orbit, one by one, striking Iapetus at very glancing impact angles and at very low speeds, under a hundred meters per second. These speeds are so low that they would build up rather than tear down the surface.



A putative ring at Rhea
This art shows Rhea, not Iapetus, but the idea is the same. Credit: NASA / JPL / JHUAPL

This is a nice story, a plausible one, backed up by physics, and it handily explains why the ridge is perfectly on the equator and only on the equator. Furthermore, since it must have happened after Iapetus had despun, it also would necessarily have happened at a time when Iapetus was stiff enough to support the ridge without any flexural moat. But it doesn't explain why this only seems to have happened on Iapetus. Why are there not fossil ring ridges on other moons?



Callisto doesn't have a ridge
Galileo captured this global view of Callisto on its 11th orbit of Jupiter, on November 5, 1997. The filters used for this image cover a broader range of the spectrum than human eyes can see. Galileo was 687,000 kilometers from Callisto when it took this image, at a phase angle of just 1°. Image scale is 6.9 kilometers per pixel. The image was reconstructed from the original data by Ted Stryk. About 10% of the right side of the disk was marred by data dropouts; Stryk reconstructed missing data from images taken through other filters. Credit: NASA / JPL / Ted Stryk



Does Oberon have a ridge?
Theory says it could, but if it does, it's not obvious in Voyager images. This is a "super-res" color view of the southern hemisphere of Oberon. Credit: NASA / JPL / Ted Stryk
   
Again, Dombard et al. turn to physics. First, they ask which moons in the solar system are far enough from their planet that they have a large enough gravitational zone of influence to capture and then hang on to a satellite for long enough for it to make a ring. Because of Iapetus' uniquely far orbital distance (it's 3.5 million kilometers from Saturn, compared to 1.2 million for Titan, and 1.9 million for Callisto from Juputer), Iapetus has a lot more space within its gravitational influence than any other outer solar system moon. It controls about ten times as much space as Callisto, Titan, and Oberon, the next three most gravitationally influential moons. Most moons couldn't capture or keep a satellite at all, because they're too close to their giant planets, and not influential enough. That gives Iapetus much better odds of having captured and held a satellite to make a ridge from.

Still, it does seem reasonable that Callisto, Titan, Oberon, and potentially Ganymede and Titania could have captured satellites and turned them into ridges. Why don't these moons have them? The answer has to do with time. Again, going back to physics, Dombard et al. show that on all these other moons, ridges certainly could have formed. But remember that ridge formation is triggered by the despinning of the moon, which happens faster for moons that are closer to their planets. In fact, any ridges would have formed at ages of a few tens to a few hundred millions of years for all moons except two. Just a few hundred million years into the formation of the solar system, there was still extremely heavy bombardment going on (whether you believe in the Late Heavy Bombardment or not), so any ridge that formed would most likely -- though not certainly -- have been eroded completely away by impacts.

The only two moons whose ridges would have formed comfortably after the violent early days of the solar system are Iapetus and Oberon. I chuckled when I read this, because it's very convenient that their paper makes a prediction like that for a moon that is very poorly observed and very likely to remain so for quite some time. (We did see Oberon with Voyager, of course, but Uranus and all its moons were tipped pole-on to the Sun at the time, meaning that their equators were mostly not imaged.)

To be fair, though, they also make many other predictions that can be tested with future work. We need to simulate impacts onto Iapetus to see if it really could have captured debris into a satellite. We need to test what happens when many small, grazing, impacts at low speeds really do build a ridge rather than create craters. We need to look at Iapetus' cratering record to see if the ridge really is much younger than the rest of the moon (though this would be complicated by the fact that the ridge was built by impacts so may retain an anomalously large number of craters).

I want to close this lengthy post by complimenting the authors of the paper on their prose. It's a theory-laden paper with (horrors!) equations, but the verbs are active, the adjectives colorful, the arguments easy to parse. The title of this blog entry is an exemplary phrase from the paper. I wish more scientists wrote like this!

http://www.planetary.org/blog/article/00003389/

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Титан и Прометей.

http://www.nasa.gov/multimedia/imagegallery/image_feature_2185.html

[Space Alien]

Титан и Прометей.

"Кассини" сфотографировал Титан и Прометей

"Кассини" сфотографировал Титан и Прометей на фоне Сатурна. Снимки и их описания появились на сайте проекта CICLOPS, который занимается анализом изображений, полученных зондом. Снимок (500 Кб) в высоком разрешении можно скачать тут.

Фотографии были сделаны еще 5 января 2012 года, однако опубликовали их только сейчас. На фото хорошо виден Сатурн, его кольца (а также тени, которые они отбрасывают на газовый гигант) и Титан. Прометей - маленькая белая точка над кольцами в правой части снимка.

 Снимок сделан с расстояния 685 тысяч километров от газового гиганта через инфракрасный фильтр. Масштаб изображения - 37 километров на один пиксель. Диаметр Титана составляет 5150 километров, диаметр Прометея - 86 километров.

http://lenta.ru/news/2012/02/29/fast/

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Cassini

Продолжительность 01:05:20

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Cassini Detects Hint of Fresh Air at Dione

NASA's Cassini spacecraft has "sniffed" molecular oxygen ions around Saturn's icy moon Dione for the first time, confirming the presence of a very tenuous atmosphere. The oxygen ions are quite sparse – one for every 0.67 cubic inches of space (one for every 11 cubic centimeters of space) or about 2,550 per cubic foot (90,000 per cubic meter) – show that Dione has an extremely thin neutral atmosphere.

At the Dione surface, this atmosphere would only be as dense as Earth's atmosphere 300 miles (480 kilometers) above the surface. The detection of this faint atmosphere, known as an exosphere, is described in a recent issue of the journal Geophysical Research Letters.

"We now know that Dione, in addition to Saturn's rings and the moon Rhea, is a source of oxygen molecules," said Robert Tokar, a Cassini team member based at Los Alamos National Laboratory, Los Alamos, N.M., and the lead author of the paper. "This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn't involve life."

Dione's oxygen appears to derive from either solar photons or energetic particles from space bombarding the moon's water ice surface and liberating oxygen molecules, Tokar said. But scientists will be looking for other processes, including geological ones, that could also explain the oxygen.

"Scientists weren't even sure Dione would be big enough to hang on to an exosphere, but this new research shows that Dione is even more interesting than we previously thought," said Amanda Hendrix, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., who was not directly involved in the study. "Scientists are now digging through Cassini data on Dione to look at this moon in more detail."

Several solid solar system bodies – including Earth, Venus, Mars and Saturn's largest moon Titan – have atmospheres. But they tend to be typically much denser than what has been found around Dione. However, Cassini scientists did detect a thin exosphere around Saturn's moon Rhea in 2010, very similar to Dione. The density of oxygen at the surfaces of Dione and Rhea is around 5 trillion times less dense than that at Earth's surface.

Tokar said scientists suspected molecular oxygen would exist at Dione because NASA's Hubble Space Telescope detected ozone. But they didn't know for sure until Cassini was able to measure ionized molecular oxygen on its second flyby of Dione on April 7, 2010 with the Cassini plasma spectrometer. On that flyby, the spacecraft flew within about 313 miles (503 kilometers) of the moon's surface.

Cassini scientists are also analyzing data from Cassini's ion and neutral mass spectrometer from a very close flyby on Dec. 12, 2011. The ion and neutral mass spectrometer made the detection of Rhea's thin atmosphere, so scientists will be able to compare Cassini data from the two moons and see if there are other molecules in Dione's exosphere.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena, Calif., 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 was designed, developed and assembled at JPL. The Cassini plasma spectrometer team and the ion and neutral mass spectrometer team are based at Southwest Research Institute, San Antonio.

For more information about the mission, visit: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .

http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20120302.html

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В атмосфере Дионы нашли кислород

Ученые обнаружили в экзосфере - верхнем слое атмосферы - спутника Сатурна Дионы кислород. Статья ученых появилась в журнале Geophysical Research Letters.

В рамках работы ученые использовали данные, собранные аппаратом "Кассини" в апреле 2010 года. Тогда зонд сближался со спутником и смог получить данные о его атмосфере. Количество кислорода крайне невелико - 0,01 - 0,09 иона кислорода на кубический сантиметр экзосферы.

По словам ученых, ионы кислорода образуются в результате бомбардировки поверхности спутника заряженными частицами, захваченными магнитным полем Сатурна. Помимо Дионы кислород был обнаружен на Рее, а также на юпитерианских спутниках Ганимед, Европа и Каллисто.

"Кассини", совместный проект ESA, NASA и Итальянского космического агентства, был запущен в 1997 году. Миссия зонда продлится до 2017 года. Диона - относительно крупный спутник. Его диаметр - 1123,4 километра. Состоит преимущественно из водяного льда и камня.

http://lenta.ru/news/2012/03/02/oxy/
http://www.agu.org/pubs/crossref/2012/2011GL050452.shtml

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Cassini Captures New Images of Icy Moon

These raw, unprocessed images of Saturn's second largest moon, Rhea, were taken on March 10, 2012, by NASA's Cassini spacecraft. This was a relatively distant flyby with a close-approach distance of 26,000 miles (42,000 kilometers), well suited for global geologic mapping.

During the flyby, Cassini captured these distinctive views of the moon's cratered surface, creating a 30-frame mosaic of Rhea's leading hemisphere and the side of the moon that faces away from Saturn. The observations included the large Mamaldi (300 miles, or 480 kilometers, across) and Tirawa (220 miles, or 360 kilometers, across) impact basins and the 29-kilometer (47-kilometer) ray crater Inktomi, one of the youngest surface features on Rhea (about 950 miles, or 1,530 kilometers, across).

http://www.nasa.gov/mission_pages/cassini/whycassini/rhea20120312.html

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"Кассини" сфотографировал Рею

Астрономы, работающие с "Кассини", опубликовали снимки сатурнианского спутника Рея. Фото и их описания доступны на сайте Лаборатории реактивного движения.

Все снимки были сделаны еще 10 марта 2012 года, в день очередного сближения зонда со спутником. В сообщении на сайте Лаборатории говорится, что аппарат прошел на достаточно большом расстоянии от Реи - примерно 26 тысяч километров. Подобные сближения идеально подходят для изучения глобальной геологической структуры спутника.

Диаметр Реи составляет 1,5 тысячи километров. Небесное тело является вторым после Титана по величине спутником Сатурна. Полный оборот вокруг газового гиганта она делает за 4,5 дня. Рея состоит преимущественно из водяного льда - по некоторым оценкам, его в коре более двух третей.

Зонд "Кассини" вместе с посадочным модулем "Гюйгенс" является совместным проектом NASA и Итальянского космического агентства (оно участвует в проекте самостоятельно, а не как часть Европейского космического агентства). Орбиты Сатурна аппараты достигли в 2004 году. Тогда же "Гюйгенс" совершил посадку на Титан. Текущая миссия "Кассини" должна завершиться в 2017 году.

http://lenta.ru/news/2012/03/13/rhea/

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Cassini Spies Wave Rattling Jet Stream on Jupiter
03.13.12

New movies of Jupiter are the first to catch an invisible wave shaking up one of the giant planet's jet streams, an interaction that also takes place in Earth's atmosphere and influences the weather. The movies, made from images taken by NASA's Cassini spacecraft when it flew by Jupiter in 2000, are part of an in-depth study conducted by a team of scientists and amateur astronomers led by Amy Simon-Miller at NASA's Goddard Space Flight Center in Greenbelt, Md., and published in the April 2012 issue of Icarus.



Following the path of one of Jupiter's jet streams, a line of V-shaped chevrons travels west to east just above Jupiter's Great Red Spot. Most of the planet is unfolded here in a single, flat map made on December 11 and 12, 2000, when NASA's Cassini spacecraft flew past Jupiter. At the left, the chevrons run into another storm called the South Equatorial Disturbance (SED). Credit: NASA/JPL/Space Science Institute

"This is the first time anyone has actually seen direct wave motion in one of Jupiter's jet streams," says Simon-Miller, the paper's lead author. "And by comparing this type of interaction in Earth's atmosphere to what happens on a planet as radically different as Jupiter, we can learn a lot about both planets."

Like Earth, Jupiter has several fast-moving jet streams that circle the globe. Earth's strongest and best known jet streams are those near the north and south poles; as these winds blow west to east, they take the scenic route, wandering north and south. What sets these jet streams on their meandering paths -- and sometimes makes them blast Florida and other warm places with frigid air -- are their encounters with slow-moving waves in Earth's atmosphere, called Rossby waves.

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New movies of Jupiter are the first to catch an invisible wave shaking up one of the giant planet's jet streams, an interaction that also takes place in Earth's atmosphere and influences the weather.

In contrast, Jupiter's jet streams "have always appeared to be straight and narrow," says co-author John Rogers, who is the Jupiter Section Director of the British Astronomical Association, London, U.K., and one of the amateur astronomers involved in this study.



The jet stream that circles Earth's north pole travels west to east. But when the jet stream interacts with a Rossby wave, as shown here, the winds can wander far north and south, bringing frigid air to normally mild southern states. Credit: NASA/GSFC

Rossby waves were identified on Jupiter about 20 years ago, in the northern hemisphere. Even so, the expected meandering winds could not be traced directly, and no evidence of them had been found in the southern hemisphere, which puzzled planetary scientists.

To get a more complete view, the team analyzed images taken by NASA's Voyager spacecraft, NASA's Hubble Space Telescope, and Cassini, as well as a decade's worth of observations made by amateur astronomers and compiled by the JUPOS project.

The movies zoom in on a single jet stream in Jupiter's southern hemisphere. A line of small, dark, V-shaped "chevrons" has formed along one edge of the jet stream and zips along west to east with the wind. Later, the well-ordered line starts to ripple, with each chevron moving up and down (north and south) in turn. And for the first time, it's clear that Jupiter's jet streams, like Earth's, wander off course.

"That's the signature of the Rossby wave," says David Choi, the postdoctoral fellow at NASA Goddard who strung together about a hundred Cassini images to make each time-lapse movie. "The chevrons in the fast-moving jet stream interact with the slower-moving Rossby wave, and that's when we see the chevrons oscillate."

The team's analysis also reveals that the chevrons are tied to a different type of wave in Jupiter's atmosphere, called a gravity inertia wave. Earth also has gravity inertia waves, and under proper conditions, these can be seen in repeating cloud patterns.

"A planet's atmosphere is a lot like the string of an instrument," says co-author Michael D. Allison of the NASA Goddard Institute for Space Studies in New York. "If you pluck the string, it can resonate at different frequencies, which we hear as different notes. In the same way, an atmosphere can resonate with different modes, which is why we find different kinds of waves."

Characterizing these waves should offer important clues to the layering of the deep atmosphere of Jupiter, which has so far been inaccessible to remote sensing, Allison adds.

Crucial to the study was the complementary information that the team was able to retrieve from the detailed spacecraft images and the more complete visual record provided by amateur astronomers. For example, the high resolution of the spacecraft images made it possible to establish the top speed of the jet stream's wind, and then the amateur astronomers involved in the study looked through the ground-based images to find variations in the wind speed.

The team also relied on images that amateur astronomers had been gathering of a large, transient storm called the South Equatorial Disturbance. This visual record dates back to 1999, when members of the community spotted the most recent recurrence of the storm just south of Jupiter's equator. Analysis of these images revealed the dynamics of this storm and its impact on the chevrons. The team now thinks this storm, together with the Great Red Spot, accounts for many of the differences noted between the jet streams and Rossby waves on the two sides of Jupiter's equator.

"We are just starting to investigate the long-term behavior of this alien atmosphere," says co-author Gianluigi Adamoli, an amateur astronomer in Italy. "Understanding the emerging analogies between Earth and Jupiter, as well as the obviously profound differences, helps us learn fundamentally what an atmosphere is and how it can behave."

http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20120313.html

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Enceladus, Saturn's Moon

Below a darkened Enceladus, a plume of water ice is backlit in this view of one of Saturn's most dramatic moons.

Dramatic plumes, both large and small, spray water ice from many locations along the moon's famed "tiger stripes" near the south pole of Enceladus. The tiger stripes are fissures that spray icy particles, water vapor and organic compounds.

The terrain seen here is on the leading hemisphere of Enceladus (313 miles, or 504 kilometers across). North is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Feb. 20, 2012. The view was acquired at a distance of approximately 83,000 miles (134,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 165 degrees. Image scale is 2,628 feet (801 meters) per pixel.

Image credit: NASA/JPL-Caltech/Space Science Institute

http://www.nasa.gov/multimedia/imagegallery/image_feature_2198.html

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Cassini Garners Top Honor From Air and Space Museum

PASADENA, Calif. – NASA's Cassini mission to Saturn, managed by the Jet Propulsion Laboratory, Pasadena, Calif., has received the top group honor from the Smithsonian's National Air and Space Museum – the Trophy for Current Achievement. Representatives for Cassini will receive the trophy on March 21 at a black-tie dinner in Washington, D.C.

"Here we are some 15 years since Cassini launched and it's amazing how well the spacecraft has operated," said Charles Elachi, director of JPL. "Thanks to the superb work of both the development team and the operations team, Cassini has been able to show us the beauty and diversity of the Saturn system and, beyond that, to study what is really a miniature solar system in its own right."

The trophies for current and lifetime achievement are the National Air and Space Museum's most prestigious awards. They recognize outstanding achievements in the fields of aerospace science, technology and their history.

"The National Air and Space Museum Trophy is among the most prestigious awards given by the Smithsonian, it recognizes significant aerospace accomplishments," said National Air and Space Museum Director Jack Dailey. "We are pleased to present it to the Cassini-Huygens Flight Team in the Current Achievement category."

The Cassini-Huygens mission, a cooperative project of NASA, the European Space Agency and the Italian Space Agency, launched in 1997. It performed a dramatic burn in June 2004 to slide into orbit around Saturn and, in December of that year, the spacecraft successfully released ESA's Huygens probe to pass down through the atmosphere of Saturn's largest moon Titan.

Mission highlights include discovering a plume of water ice and organic particles spraying from the icy moon Enceladus and watching signs of seasonal change from northern winter into northern spring, such as the evolution of a monster storm in Saturn's northern hemisphere. Cassini and Huygens have also revealed just how Earth-like Titan is, as the only body in the solar system other than Earth that has stable liquid on the surface. The mission has discovered two new rings around Saturn and four new moons.

The Cassini spacecraft has also been navigating the Saturn system for nearly eight years with accuracies often better than half a mile (kilometer) while 700 to 800 million miles (1.2 to 1.3 billion kilometers) away from Earth. Cassini has also flown within 16 miles (25 kilometers) of the surface of Enceladus and many times through the upper atmosphere of Titan

The project completed its original prime mission in 2008 and has been extended twice. It is now in its solstice mission, which will enable scientists to observe seasonal change in the Saturn system through the northern summer solstice.

"We are very proud of what Cassini has accomplished," said Robert Mitchell, Cassini program manager based at JPL. "But our workhorse spacecraft still has much work left to do. We can't wait to see what Saturn, its rings and photogenic moons will reveal to us next."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.

For more information about the Cassini-Huygens mission visit http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .

http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20120314.html