Cassini Focuses In On Two Moons

Image of Tethys and Titan taken in green visible light on July 14th 2011. Image Credit: NASA/JPL-Caltech/Space Science Institute

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In this new image from the Cassini Imaging Team Saturn’s moon Titan looks a little out of focus compared to the sharp, cratered surface of Tethys, seen in the foreground. But that’s only because Titan’s hazy atmosphere makes the moon look blurry. Titan’s current atmosphere is thought to resemble Earth’s early atmosphere, so we could be looking at an analog of early Earth.

And so, the Cassini mission is sharpening our understanding of Saturn and all its moons, but it might help us understand our own planet, as well.

At just over 1,000 kilometers in diameter, Tethys is believed to be almost entirely comprised of water ice, based on density estimates. Titan, at just over 5,000 kilometers in diameter is notable for being the second largest moon in our solar system, as well as having an atmosphere 1 1/2 times thicker than Earth. Titan is also known to have an active “liquid cycle” made up of various hydrocarbons, making Titan the second body in the solar system to have stable liquid on its surface.

The camera view is aimed at the Saturn-facing side of Titan and at the area between the trailing hemisphere and anti-Saturn side of Tethys. Not shown in frame is Saturn, which would be far to the left, from the perspective shown in the image.

The image was acquired with Cassini’s narrow-angle camera, in green visible light, on July 14, 2011. At a distance of roughly 3 million kilometers, the image scale for Titan is 19 kilometers per pixel. With Tethys at a distance of about 2 million kilometers, the image scale is roughly 11 kilometers per pixel.

If you’d like to learn more about the extended Cassini “solstice” mission, you can read more at: http://saturn.jpl.nasa.gov/mission/introduction/

Source: Cassini Solstice Mission Images

Cassini Captures a Menagerie of Moons

This Cassini raw image shows a portion of Saturn's rings along with several moons. How many can you find? Credit: NASA/JPL/Space Science Institute

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This stunning new Cassini image was captured on July 29, 2011, and shows a portion of Saturn’s rings along with several moons dotting the view. How many moons can you find, and can you name them?

See below for a color version of this image, put together by our own Jason Major!

Saturns moons and rings, in color. Credit: NASA / JPL / SSI. Edited by Jason Major. Click for larger version.

Jason shares on his Flickr page the process of how he edited the image. As Jason says, it’s a moon flash mob!

See the Cassini Solstice Mission raw images page for a larger view.

Hat tip to Stu Atkinson

Enceladus’ Salty Surprise

Enceladus' signature ice geysers in action. NASA / JPL / SSI

 

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Researchers on the Cassini mission team have identified large salt grains in the plumes emanating from Saturn’s icy satellite Enceladus, making an even stronger case for the existence of a salty liquid ocean beneath the moon’s frozen surface.

Cassini first discovered the jets of water ice particles in 2005; since then scientists have been trying to learn more about how they behave, what they are made of and – most importantly – where they are coming from. The running theory is that Enceladus has a liquid subsurface ocean of as-of-yet undetermined depth and volume, and pressure from the rock and ice layers above combined with heat from within force the water up through surface cracks near the moon’s south pole. When this water reaches the surface it instantly freezes, sending plumes of ice particles hundreds of miles into space.

Enceladus inside the E ring

Much of the ice ends up in orbit around Saturn, creating the hazy E ring in which Enceladus resides.

Although the discovery of the plumes initially came as a surprise, it’s the growing possibility of liquid water that’s really intriguing – especially that far out in the Solar System and on a little 504-km-wide moon barely the width of Arizona. What’s keeping Enceladus’ water from freezing as hard as rock? It could be tidal forces from Saturn, it could be internal heat from its core, a combination of both – or something else entirely… astronomers are still hard at work on this mystery.

Now, using data obtained from flybys in 2008 and 2009 during which Cassini flew directly through the plumes, researchers have found that the particles in the jets closest to the moon contain large sodium- and potassium-rich salt grains. This is the best evidence yet of the existence of liquid salt water inside Enceladus – a salty underground ocean.

“There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than salt water under Enceladus’s icy surface.”

– Frank Postberg, Cassini team scientist, University of Heidelberg, Germany

Looking down into a jetting "tiger stripe"

If there indeed is a reservoir of liquid water, it must be pretty extensive since the numerous plumes are constantly spraying water vapor at a rate of 200 kg (400 pounds) every second – and at several times the speed of sound! The plumes are ejected from points within long, deep fissures that slash across Enceladus’ south pole, dubbed “tiger stripes”.

Recently the tiger stripe region has also been found to be emanating a surprising amount of heat, even further supporting a liquid water interior – as well as an internal source of energy. And where there’s liquid water, heat energy and organic chemicals – all of which seem to exist on Enceladus – there’s also a case for the existence of life.

“This finding is a crucial new piece of evidence showing that environmental conditions favorable to the emergence of life can be sustained on icy bodies orbiting gas giant planets.”

– Nicolas Altobelli, ESA project scientist for Cassini

Enceladus has intrigued scientists for many years, and every time Cassini takes a closer look some new bit of information is revealed… we can only imagine what other secrets this little world may hold. Thankfully Cassini is going strong and more than happy to keep on investigating!

“Without an orbiter like Cassini to fly close to Saturn and its moons — to taste salt and feel the bombardment of ice grains — scientists would never have known how interesting these outer solar system worlds are.”

– Linda Spilker, Cassini project scientist at JPL

The findings were published in this week’s issue of the journal Nature.

Read more in the NASA press release here.

Image credits: NASA / JPL / Space Science Institute

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Jason Major is a graphic designer, photo enthusiast and space blogger. Visit his website Lights in the Dark and follow him on Twitter @JPMajor or on Facebook for the most up-to-date astronomy awesomeness!

Hello, Helene!

Color composite of Helene from June 18, 2011 flyby. NASA / JPL / SSI / J. Major

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On June 18, 2011, the Cassini spacecraft performed a flyby of Saturn’s moon Helene. Passing at a distance of 6,968 km (4,330 miles) it was Cassini’s second-closest flyby of the icy little moon.

The image above is a color composite made from raw images taken with Cassini’s red, green and blue visible light filters. There’s a bit of a blur because the moon shifted position in the frames slightly between images, but I think it captures some of the subtle color variations of lighting and surface composition very nicely!

3D anaglyph of Helene assembled by Patrick Rutherford.

At right is a 3D anaglyph view of Helene made by Patrick Rutherford from Cassini’s original raw images … if you have a pair of red/blue glasses, check it out!

Cassini passed from Helene’s night side to its sunlit side. This flyby will enable scientists to create a map of Helene so they can better understand the moon’s history and gully-like features seen on previous flybys.

(When Cassini acquired the images, it was oriented such that Helene’s north pole was facing downwards. I rotated the image above to reflect north as up.)

Helene orbits Saturn at the considerable distance of 234,505 miles (377,400 km). Irregularly-shaped, it measures 22 x 19 x 18.6 miles (36 x 32 x 30 km).

Helene is a “Trojan” moon of the much larger Dione – so called because it orbits Saturn within the path of Dione, 60º ahead of it. (Its little sister Trojan, 3-mile-wide Polydeuces, trails Dione at the rear 60º mark.) The Homeric term comes from the behavioral resemblance to the Trojan asteroids which orbit the Sun within Jupiter’s path…again, 60º in front and behind. These orbital positions are known as Lagrangian points (L4 and L5, respectively.)

Read more on the Cassini mission site here.

An irregular crescent: Cassini's flyby of Helene on June 18, 2011.

Images: NASA / JPL / Space Science Institute.

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.

A Cometary Case for Titan’s Atmosphere

Ancient comets may have created Titan's nitrogen-rich atmosphere

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Titan is a fascinating world to planetary scientists. Although it’s a moon of Saturn it boasts an opaque atmosphere ten times thicker than Earth’s and a hydrologic cycle similar to our own – except with frigid liquid methane as the key component instead of water. Titan has even been called a living model of early Earth, even insofar as containing large amounts of nitrogen in its atmosphere much like our own. Scientists have wondered at the source of Titan’s nitrogen-rich atmosphere, and now a team at the University of Tokyo has offered up an intriguing answer: it may have come from comets.

Traditional models have assumed that Titan’s atmosphere was created by volcanic activity or the effect of solar UV radiation. But these rely on Titan having been much warmer in the past than it is now…a scenario that Cassini mission scientists don’t think is the case.

New research suggests that comet impacts during a period called the Late Heavy Bombardment – a time nearly 4 billion years ago when collisions by large bodies such as comets and asteroids were occurring regularly among worlds in our solar system – may have generated Titan’s nitrogen atmosphere. By firing lasers into ammonia-and-water-ice material similar to what would have been found on primordial Titan, researchers saw that nitrogen was a typical result. Over the millennia these impacts could have created enough nitrogen to cover the moon in a dense haze, forming the thick atmosphere we see today.

“We propose that Titan’s nitrogen atmosphere formed after accretion, by the conversion from ammonia that was already present on Titan during the period of late heavy bombardment about four billion years ago.”

– Yasuhito Sekine et al., University of Tokyo, Japan

This model, if true, would also mean that the source of Titan’s nitrogen would be different than that of other outer worlds, like Pluto, and even inner planets like our own.

See the published results in the journal Nature, or read more on NewScientist.com.

Top image is a combination of a color-composite of Titan made from raw Cassini data taken on October 12, 2010 and a recolored infrared image of the comet Siding Spring, taken by NASA’s WISE observatory on January 10, 2010. The background stars were also taken by the Cassini orbiter. NASA / JPL / SSI and Caltech/UCLA. Edited by J. Major.

Note: the image at top is not scientifically accurate…the comet’s tail would be, based on the lighting of Titan, pointing more to the ten o’clock position as well as forward toward the viewer’s left shoulder. This would make it ‘look’ as if it were going the opposite direction though, away from Titan, and so I went with the more immediately decipherable version seen here. To see a more “realistic” version, click here.

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