Cassini at Saturn, the Movie

Science becomes art! A unique and stunning compilation of images of the Saturn system from the Cassini mission. Created by Chris Abbas of Digital Kitchen, he says “The footage in this little film was captured by the hardworking men and women at NASA with the Cassini Imaging Science System. If you’re interested in learning more about Cassini and the on-going Cassini Solstice Mission, check it out at NASA’s Cassini website.”

Insanely Awesome Raw Cassini Images of Titan and Enceladus

Raw Cassini image of Titan and Enceladus backdropped by Saturn's rings. Image Credit: NASA/JPL/Space Science Institute

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An incredible set of images are beaming back from the Cassini spacecraft as it orbits Saturn, snapping away at the sights. The moons Titan and Enceladus snuggling up together in front of Saturn’s rings creates an amazing view, especially when they are all lined up together. These were taken on May 21, 2011. I’ve posted some of what I think are the most amazing, below, or you can see the whole set at the Cassini raw images page. When the Cassini imaging team gets a chance to process (and colorize) these, they’ll likely go down as some of the most representative images from the entire mission.


Titan snuggles up to Saturn and its rings. Image credit: NASA/JPL/Space Science Institute

Titan, Enceladus and an onside view of Saturn's rings. Credit: NASA/JPL/Space Science Institute

Hat tip to Stu Atkinson!

Studying Saturn’s Super Storm

Three views of Saturn's northern storm. ESO/University of Oxford/L. N. Fletcher/T. Barry

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First seen by amateur astronomers back in December, the powerful seasonal storm that has since bloomed into a planet-wrapping swath of churning clouds has gotten some scrutiny by Cassini and the European Southern Observatory’s Very Large Telescope array situated high in the Chilean desert.

The image above shows three views of Saturn acquired on January 19: one by amateur astronomer Trevor Barry taken in visible light and the next two by the VLT’s infrared VISIR instrument – one taken in wavelengths sensitive to lower atmospheric structures one sensitive to higher-altitude features. 

Cassini image showing dredged-up ammonia crystals in the storm. NASA/JPL/Univ. of Arizona.

While the storm band can be clearly distinguished in the visible-light image, it’s the infrared images that really intrigue scientists. Bright areas can be seen along the path of the storm, especially in the higher-altitude image, marking large areas of upwelling warmer air that have risen from deep within Saturn’s atmosphere.

Normally relatively stable, Saturn’s atmosphere exhibits powerful storms like this only when moving into its warmer summer season about every 29 years. This is only the sixth such storm documented since 1876, and the first to be studied both in thermal infrared and by orbiting spacecraft.

The initial vortex of the storm was about 5,000 km (3,000 miles) wide and took researchers and astronomers by surprise with its strength, size and scale.

“This disturbance in the northern hemisphere of Saturn has created a gigantic, violent and complex eruption of bright cloud material, which has spread to encircle the entire planet… nothing on Earth comes close to this powerful storm.”

– Leigh Fletcher, lead author and Cassini team scientist at the University of Oxford in the United Kingdom.

The origins of Saturn’s storm may be similar to those of a thunderstorm here on Earth; warm, moist air rises into the cooler atmosphere as a convective plume, generating thick clouds and turbulent winds. On Saturn this mass of warmer air punched through the stratosphere, interacting with the circulating winds and creating temperature variations that further affect atmospheric movement.

The temperature variations show up in the infrared images as bright “stratospheric beacons”. Such features have never been seen before, so researchers are not yet sure if they are commonly found in these kinds of seasonal storms.

“We were lucky to have an observing run scheduled for early in 2011, which ESO allowed us to bring forward so that we could observe the storm as soon as possible. It was another stroke of luck that Cassini’s CIRS instrument could also observe the storm at the same time, so we had imaging from VLT and spectroscopy of Cassini to compare. We are continuing to observe this once-in-a-generation event.”

– Leigh Fletcher

A separate analysis using Cassini’s visual and infrared mapping spectrometer confirmed the storm is very violent, dredging up larger atmospheric particles and churning up ammonia from deep in the atmosphere. Other Cassini scientists are studying the evolving storm and a more extensive picture will emerge soon.

Read the NASA article here, or the news release from ESO here.

 

The leading edge of Saturn's storm in visible RGB color from Cassini raw image data taken on February 25, 2011. (The scale size of Earth is at upper left.) NASA / JPL / Space Science Institute. Edited by J. Major.

Ride Along with Rhea

Animation made from raw Cassini image data acquired April 25, 2011

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Assembled from 29 raw images taken by the Cassini orbiter on Monday, April 25, this animation brings us along an orbital ride with Rhea as it crosses Saturn’s nighttime face, the planet’s shadow cast across the ringplane. Sister moons Dione and Tethys travel the opposite lane in the background, eventually appearing to sink into Saturn’s atmosphere.

Rhea's heavily cratered surface, imaged by Cassini on October 2010. NASA/JPL/SSI

The exposure varies slightly from frame to frame due to the fact that they are not all taken with the same color channel filter.

Rhea (1,528 kilometers, or 949 miles, wide), Dione (1,123 kilometers, or 698 miles wide) and Tethys (1,066 kilometers, or 662 miles wide) are all very similar in composition and appearance. The moons are composed mostly of water ice and rock, each covered in craters of all sizes and crisscrossed by gouges, scarps and chasms. All three are tidally locked with Saturn, showing the same face to their parent planet in the same way that the Moon does with Earth.

The Cassini spacecraft was 2,227,878 km (1,384,339 miles) from Rhea when the images were taken.

(The original images have not been validated or calibrated. Validated/calibrated images will be archived with the NASA Planetary Data System in 2012.)

Image credit: NASA / JPL / Space Science Institute. Animation by Jason Major.

Latest Saturnian Eye Candy from Cassini

Saturn is divided by its rings and the moons Tethys and Epimetheus. Credit: NASA/JPL/Space Science Institute

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Two moons and Saturn’s rings create a lopsided “divided by” symbol on the giant planet in one of the latest images released by the Cassini science team. The rings also cast shadows and darken the southern hemisphere of the planet. The moon Tethys (1,062 kilometers, or 660 miles across) sits above the rings, while the smaller moon Epimetheus (113 kilometers, or 70 miles across) hovers below. This image was taken by Cassini’s narrow-angle camera on March 8, 2011. See below for a few more recent looks at Saturn.

The moon Prometheus sits amid Saturn's rings. Credit: NASA/JPL/Space Science Institute
A dark Saturn with rings and shadows. Credit: NASA/JPL/Space Science Institute

Check out more images on the Cassini website. There are some brand new images in the “raw image” section, including some great looks at Titan. And look for more great images of Titan soon, as Cassini’s next close flyby of Saturn’s largest moon will be on May 8.

Enceladus and Saturn are Linked by Electromagnetic Currents

NASA's Cassini spacecraft has spotted a glowing patch of ultraviolet light near Saturn's north pole that marks the presence of an electrical circuit that connects Saturn with its moon Enceladus. Two images obtained by Cassini's ultraviolet imaging spectrograph on Aug. 26, 2008, separated by 80 minutes, showing how the ‘footprint’ moved according to changes in the position of Enceladus. Credit: NASA/JPL/University of Colorado/Central Arizona College

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The geysers and jets of Enceladus have just become more intriguing. A glowing patch of ultraviolet light near Saturn’s north pole appears to be evidence of a magnetic connection between the planet and the icy, geyser-spewing moon. Data from the Cassini spacecraft have revealed that the jets of gas and icy grains that emanate from the south pole of Enceladus become electrically charged and form an ionosphere, and the motion of Enceladus and its ionosphere through a magnetic bubble that surrounds Saturn acts like a dynamo, setting up a newly-discovered electrical current system that links the moon to the planet.

This video demonstrates the hiss-like radio noise generated by electrons moving along magnetic field lines from Enceladus to a glowing patch of ultraviolet light on Saturn.

Cassini’s Plasma Spectrometer’s electron spectrometer, (CAPS-ELS) has detected the beams of electrons that flow back and forth between Saturn and Enceladus. Magnetic field lines, invisible to the human eye but detectable by the fields and particles instruments on the spacecraft, arc from Saturn’s north polar region to south polar region. Enceladus resides in the arc of a set of the field lines and feeds charged particles into the Saturn atmosphere. The finding is part of a paper published in Nature.

From data Cassini collected in 2008, scientists saw a glowing patch of ultraviolet light emissions near Saturn’s north pole that marked the presence of a circuit between the two bodies, even though the moon is 240,000 kilometers (150,000 miles) away from the planet.

The patch occurs at the end of a magnetic field line connecting Saturn and its moon Enceladus. The area, known as an auroral footprint, is the spot where energetic electrons dive into the planet’s atmosphere, following magnetic field lines that arc between the planet’s north and south polar regions.

“The footprint discovery at Saturn is one of the most important fields and particle revelations from Cassini and ultimately may help us understand Saturn’s strange magnetic field,” said Marcia Burton, a Cassini fields and particles scientist at NASA’s Jet Propulsion Laboratory. “It gives us the first visual connection between Saturn and one of its moons.”

The auroral footprint measures approximately 1,200 kilometers (750 miles) by less than 400 kilometers (250 miles), covering an area comparable to California or Sweden. At its brightest, the footprint shone with an ultraviolet light intensity far less than Saturn’s polar auroral rings, but comparable to the faintest aurora visible at Earth without a telescope in the visible light spectrum. Scientists have not found a matching footprint at the southern end of the magnetic field line.

Scientists already knew that the giant planet Jupiter is linked to three of its moons by charged current systems set up by the satellites orbiting inside its giant magnetic bubble, the magnetosphere, and that these current systems form glowing spots in the planet’s upper atmosphere. The latest discovery at Enceladus shows that similar processes take place at the Saturnian system too.

“This now looks like a universal process — Jupiter’s moon Io is the most volcanic object in the solar system, and produces a bright spot in Jupiter’s aurora, “ said Dr. Andrew Coates from the University College in London, a co-author of the new paper. “Now, we see the same thing at Saturn — the variable and majestic water-rich Enceladus plumes, probably driven by cryovolcanism, cause electron beams which create a significant spot in Saturn’s aurora too.”

Paper: Wayne R. Pryor et al, “The auroral footprint of Enceladus on Saturn”, Nature, 472, 331–333, doi:10.1038/nature09928

Sources: University College, London, NASA

Is Titan Hiding an Ocean?

Titan holds yet more secrets, far beneath its haze...

 

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Saturn’s moon Titan just keeps throwing surprises at us. A multi-layered atmosphere thicker than our own? Check. A hydrologic cycle that relies on methane as the operating liquid? Check. Rivers, streams and lakes filled with this same liquid? Check, check and check. And now, scientists are suspecting that Titan may have yet another surprise: a subsurface ocean.

Observations of Titan’s rotation and orbit, carried out by researchers at the Royal Observatory of Belgium using Cassini data, point at an unusual rotational inertia; that is, its resistance to changes in its motion, also known as moment of inertia or angular mass. Basically Titan moves in a way that is not indicative of a solid body of its previously assumed density and mass. Rather, its motion – both around its own axis and in its tidally-locked orbit around Saturn – are more in line with an object that isn’t uniformly solid.

Titan's thick clouds hide its surface well. NASA / JPL / SSI / J.Major

According to the math, Titan may very well be filled with liquid!

Or, at least, have a liquid layer of considerable depth beneath its surface. How far below the surface, how deep and exactly what kind of liquid are all speculative at this point…it’s suggested that it may be a subsurface ocean of yet more methane. This would help answer the question of where Titan gets all of its methane in the first place; methane, – a.k.a. natural gas – is a compound that breaks down quickly in sunlight. In fact, the high-level haze that surrounds the moon like a wispy blue shell is made up of this broken-down methane. So if this stuff is raining down onto the surface in giant, frigid drops and filling streams and lakes, but is still being broken down by ultraviolet light from the Sun to enshroud the entire moon (Titan is BIG, remember…at 5,150 km – 3,200 miles – wide, it’s over a third the size of Earth!) then there has to be somewhere that this methane is coming from.

If these calculations are right, it may be coming from underground.

We propose a new Cassini state model for Titan in which we assume the presence of a liquid water ocean beneath an ice shell… with the new model, we find a closer agreement between the moment of inertia and the rotation state than for the solid case, strengthening the possibility that Titan has a subsurface ocean.

– Rose-Marie Baland et al.

Of course in order for this hypothesis to be proven many more numbers are going to have to be crunched and more data reviewed. And more possibilities considered, too; Titan’s orbital irregularities may in fact be the result of external forces, such as a close pass by a comet or other large body. Still, there’s something to be investigated here and you can bet there’ll be no shortage of attention on a problem as intriguing as this!

Titan may soon be joining the short list of moons speculated to possess subsurface oceans, alongside Jupiter’s Europa and Ganymede and sister Saturnian satellite Enceladus…and who knows how many others?

Read the article on MIT’s Physics arXiv Blog, or you can download the full report here.

Top image credit: NASA / JPL / SSI. (Edited by J. Major.)

New Studies: Planetary Rings Harbor Records of Past Smash-Ups

Saturn, imaged by Cassini on approach. Credit: CICLOPS

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Planetary rings are more than just astronomical marvels — they’re also a sort of archive, chronicling histories of impacts for decades.

A pair of studies were published online in Science today by two different teams that noticed odd characteristics in the rings of Saturn and Jupiter — and followed them to this promising conclusion. In the first, lead author Mark Showalter of the SETI Institute in Mountain View, Calif. and his team analyzed images of Jupiter’s rings observed in 1996 and 2000 by Galileo, and again in 2007 by Horizon, zeroing in on a pattern they labeled “corrugated,” like a tin roof. Around the same time, Matthew Hedman, from Cornell University in Ithaca, NY and his colleagues discovered similar ripple patterns in the rings of Saturn, from images taken by the Cassini spacecraft.

Image courtesy of Science/AAAS

The images above show how a vertical corrugation can be produced from an initially inclined ring. The top image shows a simple inclined ring (the central planet is omitted for clarity), while the lower two images show the same ring at two later times, where the ring particles’ wobbling orbits have sheared this inclined sheet into an increasingly tightly-wound spiral corrugation.

Carolyn Porco, a co-author on the Hedman-led study and director of the Cassini Imaging Central Laboratory for Operatons (CICLOPS), wrote in an email accompanying the release of the studies that “it has been known for some time that the solar system is filled with debris:  small rocky bits in the inner solar system and icy bits in the
outer solar system that routinely rain down on the planets and their rings and moons.  A couple hundred tons of such debris hits the Earth alone every day. Well, the origins of the spiral ripples in both ring systems have now been pinpointed to very recent impacts between clouds of cometary fragments and the rings.”

Showalter’s team describes a pair of superimposed ripple patterns that showed up in Galileo images in 1996 and again in 2000.

“These patterns behave as two independent spirals, each winding up at a rate defined by Jupiter’s gravity field,” they write. “The dominant pattern originated between July and October 1994, when the entire ring was tilted by ~2 km. We associate this with the ShoemakerLevy 9 impacts of July 1994. New Horizons images still show this pattern 13 years later and suggest that subsequent events may also have tilted the ring.”

Corrugation in Saturn's D-ring. Credit: NASA

Hedman and his team note that rippling had previously been observed in Saturn’s D ring; NASA released the above graphic to explain the phenomenon in 2006. “The C-ring corrugation seems to have been similarly generated, and indeed it was probably created by the same ring-tilting event that produced the D-ring’s corrugation,” they write.

That paper also compares the rate of impacts likely to visit each planet: “… Saturn should encounter debris clouds derived from comets disrupted by previous planetary encounters at a rate that is roughly 0.2 percent of Jupiter’s impact rate.”

They reason that if Jupiter sees impacts from 1-km-wide objects as often as once a decade, “the clouds of orbiting debris created by the disruption of a 1-km-wide comet should rain down on Saturn’s rings once every 5,000-10,000 years. The probability that debris from a previously disrupted comet would hit Saturn’s rings in the last 30 years would then be between roughly 1 percent and 0.1 percent, which is not very small. Such scenarios therefore provide a reasonable explanation for the origin of the observed corrugation in Saturn’s C ring.”

Taken together, the papers show that Saturn’s ring ripples were likely generated by a comet collision in 1983, while Jupiter’s ring ripples occurred after the impact of a comet the summer of 1994 — specifically, the impact of Comet Shoemaker-Levy 9 that left scars on Jupiter still visible today.

Showalter and his coauthors point out that impacts by comets and/or their dust clouds are common occurrences in planetary rings.

“On at least three occasions over the last few decades, these collisions have carried sufficient momentum to tilt a ring of Jupiter or Saturn off its axis by an observable distance. Once such a tilt is established, it can persist for decades, with the passage of time recorded in its ever-tightening spiral,” they write. “Within these subtle patterns, planetary rings chronicle their own battered histories.”

Both papers appear today at the Science Express website. See also the CICLOPS site.

Titan’s Spring Showers Bring Torrents of Methane, Maintain ‘Dry’ Gullies

NASA’s Cassini spacecraft chronicles the change of seasons as it captures clouds concentrated near the equator of Saturn’s largest moon, Titan, on 18 October 2010. Credit: NASA/JPL/Space Science Institute

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Titan’s skies dump methane rain on the bizarre moon a quarter of the year, which collects in northern methane lakes and maintains gullies and washes once presumed to have been sculpted in a wetter age.

Elizabeth Turtle from the Johns Hopkins University Applied Physics Laboratory (APL) is lead author on the new Science paper reporting that Cassini seems to have caught a storm in action last year: “We report the detection by Cassini’s Imaging Science Subsystem of a large low-latitude cloud system early in Titan’s northern spring and extensive surface changes,” write Turtle and her co-authors in the new paper, which appears today. “The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan’s low latitudes are carved by seasonal precipitation.”

While Saturn’s largest moon has methane lakes at high latitudes, its equatorial regions are mostly arid, with vast expanses of dunes. Researchers first observed dry, riverbed-like channels in these regions in Huygens probe images, but generally believed them to be remnants of a past wetter climate.

Turtle and her colleagues observed sudden decreases in the brightness of the surface near Titan’s equator after a cloud outburst. The authors consider several possible explanations for these changes, including wind storms and volcanism, but they conclude that rainfall from a large methane storm over the region is most likely responsible for the darkening they observed. The surface changes they noted after the storm spanned more than 500,000 square kilometers, about the size of California.

Simplified global atmospheric circulation and precipitation pattern on Titan and Earth. Most precipitation occurs at the intertropical convergence zone, or ITCZ, where air ascends as a result of convergence of surface winds from the northern and southern directions. Titan’s ITCZ was previously near the south pole (A) but is currently on its way to the north pole (B). The seasonal migration of the ITCZ on Earth is much smaller (C and D). Credit: P. Huey/Science © 2011 AAAS

In a related Perspectives piece, Tetsuya Tokan from the Universität zu Köln in Köln, Germany wrote that Titan’s precipitation climatology “is clearly different from that of Earth, and exotic climate zones unknown in Köppen’s classification may exist.” He was referring to a widely-used climate classification system coined by Wladimir Köppen in 1884.

Tokan writes that while Earth’s global circulation patterns concentrate precipitation in rainy belts along the equatorial regions, Titan’s “convergence zone” appears migrate north and south over time, distributing precipitation more equitably across the moon.

Source: “Rapid and Extensive Surface Changes Near Titan’s Equator: Evidence of April Showers,” by Elizabeth Turtle et al. and the related Perspectives piece, “Precipitation Climatology on Titan,” by Tetsuya Tokan. Both articles appear today in the journal Science.

Enceladus’ Internal Heat Much Higher Than Predicted

Dramatic plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Saturn's moon Enceladus. Credit: NASA/JPL/Space Science Institute

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The south polar region of Enceladus is turning out to be a veritable heat pump. The fissure- and geyser-laden region on this moon of Saturn is churning out internal heat-generated power of about 15.8 gigawatts, which is approximately 2.6 times the power output of all the hot springs in the Yellowstone region, or comparable to 20 coal-fueled power stations. This is more than an order of magnitude higher than scientists had predicted, according to Carly Howett, the lead author of a study published in the Journal of Geophysical Research on March 4. Just how that much power is being generated, however, is unknown.

“The mechanism capable of producing the much higher observed internal power remains a mystery and challenges the currently proposed models of long-term heat production,” said Howett.

2008 data from Cassini’s composite infrared spectrometer indicates a surprisingly high output of temperature from the south polar terrain on Enceladus, which makes it even more likely that liquid water exists below Enceladus’ surface, Howett said.

A 2007 study predicted the internal heat of Enceladus, if principally generated by tidal forces arising from the orbital resonance between Enceladus and another moon, Dione, could be no greater than 1.1 gigawatts averaged over the long term. Heating from natural radioactivity inside Enceladus would add another 0.3 gigawatts.

So these new readings come as a surprise.

This graphic, using data from NASA's Cassini spacecraft, shows how the south polar terrain of Saturn's moon Enceladus emits much more power than scientists had originally predicted. Images credit: NASA/JPL/SWRI/SSI

Recently, scientists studying ice particles ejected from the plumes discovered that some of the particles are salt-rich, and are probably frozen droplets from a saltwater ocean in contact with Enceladus’ mineral-rich rocky core. The presence of a subsurface ocean, or perhaps a south polar sea between the moon’s outer ice shell and its rocky interior would increase the efficiency of the tidal heating by allowing greater tidal distortions of the ice shell.

“The possibility of liquid water, a tidal energy source and the observation of organic (carbon-rich) chemicals in the plume of Enceladus make the satellite a site of strong astrobiological interest,” said Howett, who is a postdoctoral researcher at Southwest Research Institute in Boulder, Colorado.

A possible explanation of the high heat flow observed is that Enceladus’ orbital relationship to Saturn and Dione changes with time, allowing periods of more intensive tidal heating, separated by more quiescent periods. This means Cassini might be “lucky” enough to be seeing Enceladus when it’s unusually active.

The activity is centered on four roughly parallel linear trenches, 130 kilometers (80 miles) long and about 2 kilometers (1 mile) wide, informally known as the “tiger stripes.” These fissures eject great plumes of ice particles and water vapor continually into space, and have elevated temperatures due to heat leaking out of Enceladus’ interior.

Along one fissure, called Baghdad Sulcus, temperatures exceed 180 Kelvin ( – 92 C, -135 F), and may be higher than 200 Kelvin (- 73 C, -100 F). While chilly by Earth standards, peak temperatures, the temperatures are a cozy oasis compared to the numbing 50 Kelvin (-223 C, -370 F) of their surroundings.

Source: JPL