Wobbling of a Saturn moon hints at what lies beneath

Using instruments aboard the Cassini spacecraft to measure the wobbles of Mimas, the closest of Saturn's regular moons, a Cornell University astronomer publishing in Science, Oct. 17, has inferred that this small moon's icy surface cloaks either a rugby ball-shaped rocky core or a sloshing sub-surface ocean.

"After carefully examining Mimas, we found it librates, that is to say, it subtly wobbles, around the moon's polar axis," Radwan Tajeddine, Cornell research associate in astronomy and lead author of the article.

"In physical terms, the back-and-forth wobble should produce about 3 kilometers of surface displacement."

"Instead we observed an unexpected 6 kilometers of surface displacement," he said.

"We're very excited about this measurement because it may indicate much about the satellite's insides."

"Nature is essentially allowing us to do the same thing that a child does when she shakes a wrapped gift in hopes of figuring out what's hidden inside," Tajeddine said.

The astronomy team used a technique called stereo-photogrammetry to interpret images taken by the Cassini Imaging Science Subsystem to measure the libration.

In this technique, astronomers employ Cassini photographs of Mimas taken at different times and from various vantage points to build precise 3-D computer models of the locations of hundreds of surface reference points.

From these, the researchers determined the moon's shape and were able to notice that the satellite didn't rotate smoothly but rocked back and forth a bit as well.

The amount of the to-and-fro motion indicates that Mimas' interior is not uniform. These wobbles can be produced if the moon contains a weirdly shaped, rocky core or if a sub-surface ocean exists beneath its icy shell.

Mimas is about 400 kilometers in diameter, and its possible internal global ocean is located under an icy crust ranging in thickness between 25 and 30 kilometers.

The moon itself is thought to have been formed either by the slow agglomeration of ring particles (a gradual buildup of matter) or direct growth within the primordial planetary gas nebula.

The odd-shaped core would favor gravitational flattening by nearby Saturn, Tajeddine said. The moon's relatively smooth and roughly spherical icy surface covers up whatever is underneath.

More information: Science. DOI: 10.1126/science.1255299

NASA Cassini Image: Saturn's moon Mimas in crescent profile

A thin sliver of Saturn's moon Mimas, is illuminated, the long shadows showing off its many craters, indicators of the moon's violent history.

Credit: NASA /JPL-Caltech /Space Science Institute

The most famous evidence of a collision on Mimas (246 miles, or 396 kilometers across) is the crater Herschel (Mimantean crater) that gives Mimas its Death Star-like appearance.

This view looks toward the anti-Saturn hemisphere of Mimas.

North on Mimas is up and rotated 40 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 20, 2013.

The view was acquired at a distance of approximately 100,000 miles (200,000 kilometers) from Mimas and at a Sun-Mimas-spacecraft, or phase, angle of 130 degrees. Image scale is 4,000 feet (1 kilometer) per pixel.

This image shows the crater Herschel (Mimantean crater) on Mimas in greater detail.

Credit: NASA

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI).

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.

Visit the Cassini imaging team homepage.

NASA's Cassini: Auroras over Saturn's north and south poles

Ultraviolet and infrared images from NASA's Cassini spacecraft and Hubble Space Telescope show active and quiet auroras at Saturn's north and south poles. 

Credit: NASA /JPL-Caltech /University of Colorado/Central Arizona College /ESA /University of Leicester /JPL-Caltech /University of Arizona /Lancaster University

NASA trained several pairs of eyes on Saturn as the planet put on a dancing light show at its poles.

While NASA's Hubble Space Telescope, orbiting around Earth, was able to observe the northern auroras in ultraviolet wavelengths, NASA's Cassini spacecraft, orbiting around Saturn, got complementary close-up views in infrared, visible-light and ultraviolet wavelengths.

Cassini could also see northern and southern parts of Saturn that don't face Earth.

The result is a kind of step-by-step choreography detailing how the auroras move, showing the complexity of these auroras and how scientists can connect an outburst from the Sun and its effect on the magnetic environment at Saturn.

Jonathan Nichols

"Saturn's auroras can be fickle—you may see fireworks, you may see nothing," said Jonathan Nichols of the University of Leicester in England, who led the work on the Hubble images.

"In 2013, we were treated to a veritable smorgasbord of dancing auroras, from steadily shining rings to super-fast bursts of light shooting across the pole."

The Hubble and Cassini images were focused on April and May of 2013.

Images from Cassini's ultraviolet imaging spectrometer (UVIS), obtained from an unusually close range of about six Saturn radii, provided a look at the changing patterns of faint emissions on scales of a few hundred miles (kilometers) and tied the changes in the auroras to the fluctuating wind of charged particles blowing off the Sun and flowing past Saturn.

"This is our best look yet at the rapidly changing patterns of auroral emission," said Wayne Pryor, a Cassini co-investigator at Central Arizona College in Coolidge, Ariz.

"Some bright spots come and go from image to image. Other bright features persist and rotate around the pole, but at a rate slower than Saturn's rotation."

Aikaterini Radioti 

The UVIS images, which are also being analyzed by team associate Aikaterini Radioti at the University of Liege, Belgium, also suggest that one way the bright auroral storms may be produced is by the formation of new connections between magnetic field lines.

That process causes storms in the magnetic bubble around Earth.

The movie also shows one persistent bright patch of the aurora rotating in lockstep with the orbital position of Saturn's moon Mimas.

While previous UVIS images had shown an intermittent auroral bright spot magnetically linked to the moon Enceladus, the new movie suggests another Saturn moon can influence the light show as well.

The new data also give scientists clues to a long-standing mystery about the atmospheres of giant outer planets.

More Information: NASA Cassini Saturn Aurora Campaign

NASA Cassini Image: Tiger Stripes of Enceladus

Pictured here is a high resolution Cassini image of Enceladus' Tiger Stripes, from a close flyby. 

Do underground oceans vent through the tiger stripes (in false-colour blue) on Saturn's moon Enceladus?

The long features, dubbed tiger stripes, are known to spew ice from the moon's icy interior into space, creating a cloud of fine ice particles over the moon's south pole and creating Saturn's mysterious E-ring.

Why Enceladus is active remains a mystery, as the neighbouring moon Mimas, approximately the same size, appears to be quite dead.

An analysis of dust captured by Cassini found evidence of sodium, as expected in a deep salty ocean.

Such research is particularly interesting since such an ocean would be a candidate to contain life.

Conversely however, recent Earth-based observations of ice ejected by Enceladus into Saturn's E-Ring showed no evidence of the expected sodium.

Image: Cassini Imaging Team, SSI, JPL, ESA, NASA

NASA Cassini: Saturn's Moons Janus and Mimas

Credit: NASA/JPL-Caltech/Space Science Institute

Janus is spotted over Saturn's north pole in this image while Mimas' shadow glides across Saturn.

Janus is the faint dot that appears just above Saturn's north pole. Mimas' shadow can be seen in the southern hemisphere of Saturn, south of the rings' shadow. 

This view looks toward the unilluminated side of the rings from about 25 degrees below the ringplane. 

The image was taken in visible light with the Cassini spacecraft wide-angle camera on Aug. 24, 2012.

The view was obtained at a distance of approximately 1.6 million miles (2.6 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 80 degrees.

Image scale is 94 miles (152 kilometers) per pixel. Janus has been brightened by a factor of 1.3 relative to Saturn to enhance its visibility.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

For more information about the Cassini-Huygens mission visit Saturn JPL . The Cassini imaging team homepage is at Ciclops.

Saturn's moons Mimas and Tethys bear 'Pac-Man' signatures

Saturn's moons Mimas (left) and Tethys are now both known to exhibit the curious temperature patterns.

Astronomers have seen that the temperature of Saturn's moon Tethys has hotter regions uncannily like the 1980s arcade game character Pac-Man.

A similar feature was spotted in 2010 on Mimas, another Saturnian moon.

A report in Icarus suggests the effect is due to high-energy electrons bombarding the sides of the moons that face their direction of orbital travel.

That compacts the surfaces to a hard, icy texture that does not heat or cool as rapidly as the unaffected surface.

Thermal images of both moons were obtained by the Cassini-Huygens mission, launched in 1997 to study the Saturn system in detail.

The temperatures seen by the spacecraft are distinctly chilly - the warmest parts of Tethys were at - 183C, but inside the "mouth" of the Pac-Man shape it was 15C cooler still.

At the time of the finding of the first Pac-Man shape on Mimas, scientists were unsure what might be the cause, theorising that differing surface textures probably played a role.

The existence of another such shape nearby has cemented the idea that fast-moving electrons are responsible.

"Finding a second Pac-Man in the Saturn system tells us that the processes creating these 'Pac-Men' are more widespread than previously thought," said Carly Howett, of the Southwest Research Institute in Texas and lead author of the study.

"The Saturn system - and even the Jupiter system - could turn out to be a veritable arcade of these characters," she said.

ESA Cassini Image: Scrambling Saturn’s B-ring

Clumpy particles in Saturn’s B-ring provide stark contrast to the delicately ordered ringlets seen in the rest of this view presented by the Cassini spacecraft.

Saturn’s B-ring is the largest and brightest of the gas giant’s rings, the outer portion of which is seen in the left side of this image.

The ring’s outside edge is influenced by meddling moon Mimas, which orbits the planet once for every two circuits the icy ring particles complete.

These periodic gravity perturbations are thought to compress the ring particles into clumps, while maintaining the ring’s well-defined outer edge.

Beyond the B-ring lies the Huygens gap, the widest dark void visible in this image, punctuated only by the bright Huygens ringlet.

Saturn's Moon Mimas

The 4800 km-wide Cassini Division separates the B-ring from the outermost A-ring, but itself is marked out with faint, concentric strands of ring material.

From Earth, the Cassini Division appears as a thin black gap in Saturn’s rings, but close-up views from spacecraft expose the delicate structures in fine detail.

This image was taken in visible light with the Cassini spacecraft narrow-angle camera on 10 July 2009 from a distance of 320 000 km from Saturn.

Cassini is a joint mission between ESA, NASA and ASI and has been in orbit around Saturn since 2004. It is now in its second extended mission phase, the Cassini Solstice Mission, which will continue until 2017.

NASA Cassini Image: Saturn's Moon Mimas - Cut-Off Rings

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

One of Saturn's moons, Mimas joins the planet's rings which appear truncated by the planet's shadow in this Cassini spacecraft image.

Saturn is off to the left, out of view here.

The inner rings are just visible there but the planet's shadow covers part of the rings across the middle of the image.

Mimas (246 miles, or 396 kilometers across) is closer to Cassini than the rings are here.

The bright speck above the rings is a star.

To increase visibility, the rings have been brightened by a factor of two relative to Mimas. This view looks toward the southern, un-illuminated side of the rings from just below the ring-plane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 21, 2011.

The view was obtained at a distance of approximately 1.7 million miles (2.7 million kilometers) from Mimas. Image scale is 10 miles (16 kilometers) per pixel on Mimas.



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.

For more information about the Cassini-Huygens mission visit www.nasa.gov/cassini and saturn.jpl.nasa.gov . The Cassini imaging team homepage is at ciclops.org

NASA Cassini Image: Saturn's Moons Mimas And Dione

Saturn's moon Mimas peeks out from behind the night side of the larger moon Dione in this Cassini image captured during the spacecraft's Dec. 12, 2011, flyby of Dione.

Dione is 698 miles, or 1,123 kilometers, across and its day side dominates the view on the right of the image. Smaller Mimas is on the left and measures 246 miles, or 396 kilometers, across.

Lit terrain seen here is on the Saturn-facing side of Mimas and in the area between the trailing hemisphere and anti-Saturn side of Dione. North on the moons is up and rotated 20 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera.

The view was obtained at a distance of approximately 58,000 miles (94,000 kilometers) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 42 degrees. Image scale is 1,833 feet (559 meters) per pixel on Dione.

The view was obtained at a distance of approximately 380,000 miles (611,000 kilometers) from Mimas and at a Sun-Mimas-spacecraft, or phase, angle of 41 degrees. Image scale is 2 miles (3 kilometers) per pixel on Mimas.

Cassini checking in on Saturn - The Big Picture

Cassini checking in on Saturn - The Big Picture

In this view captured by Cassini on its closest-ever flyby of Saturn's moon Mimas, large Herschel Crater - 130 km (80 mi) wide - dominates the scene.

Cassini came within about 9,500 km (5,900 mi) of Mimas on Feb. 13, 2010. (NASA/JPL/SSI)

A closer view of a feature on Enceladus called Baghdad Sulcus, one of four "tiger stripes" that cross Enceladus' south pole, seen on November 21, 2009.

Cassini was targeting the area to examine plume sources, the scale is approximately 30m (100ft) per pixel. (NASA/JPL)

Saturn's moon Rhea looms near its sibling moon Epimetheus in this Cassini image with the planet and its rings in the background. The two moons aren't actually close to each other.

The view was obtained at a distance of approximately 1.2 million km (746,000 mi) from Rhea and 1.6 million km (994,000 mi) from Epimetheus.

The image was taken on March 24, 2010. Image scale is 7 km (5 mi) per pixel on Rhea and 10 km (6 mi) per pixel on Epimetheus. (NASA/JPL/SSI)


Click on the picture or on this link to view more Cassini images

NASA Cassini image of Mimas: A Subtle Colour Difference in Herschel's Crater

A Subtle Difference
Image Credit: NASA/JPL/Space Science Institute

Subtle colour differences on Saturn's moon Mimas are apparent in this false-colour view of Herschel Crater captured by NASA's Cassini spacecraft during its closest-ever flyby of that moon.

The image shows terrain-dependent color variations, particularly the contrast between the bluish materials in and around Herschel Crater and the greenish cast on older, more heavily cratered terrain elsewhere.

The origin of the color differences is not yet understood, but may be caused by differences in the surface composition between the two terrains. False color images from Cassini's previous closest encounter, in 2005, also showed such variations.

The natural color of Mimas visible to the human eye may be a uniform gray or yellow color, but this mosaic has been contrast-enhanced and shows differences at other wavelengths of light.

During this flyby on Feb. 13, 2010, Cassini came within about 5,900 miles of Mimas and these images were obtained with Cassini's narrow-angle camera on that day at a distance of approximately 10,000 miles from Mimas.

The images were re-projected into an orthographic map projection. A black and white image, taken in visible light with the wide-angle camera, is used to fill in parts of the mosaic.

NASA Cassini Images: Saturn's moon Mimas with Pacman profile

NASA's Cassini spacecraft has made the most amazing discovery in the history of science: an image of Pac-Man on the Death Star.

This is the temperature map of Mimas, Saturn's icy moon. As you can see, it's basically a giant Pac-Man about to eat a power pellet. According to Cassini project scientist at NASA's Jet Propulsion Laboratory Linda Spilker, this is really weird:

Other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought it was. It has certainly given us some new puzzles.

NASA scientists at NASA Goddard Space Flight Center have some ideas. According to Dr. Mike Flasar, composite infrared spectrometer principal investigator at Goddard, the culprit may be the giant Herschel crater:

Even though we can't explain the observed pattern of surface temperatures on Mimas, the giant Herschel crater is a leading suspect. The energy of impact that created it several billion years ago has been estimated to be one-seventh of Mimas's own gravitational energy.

Anything much larger would likely have torn the moon apart. We really would like to see if there is also an anomalous temperature pattern on the other side of Herschel, which has not been observed so closely.

Cornell University's Paul Helfenstein—who's an associate in the Cassini imagine team—speculates that the pattern may be related to "silicate minerals or carbon-rich particles, possibly because of meteor dust falling onto the moon or impurities already embedded in surface ice.

As the sun's warming rays and the vacuum of space evaporate the brighter ice, the darker material is concentrated and left behind. Gravity pulls the dark material down the crater walls, exposing fresh ice underneath."

Cassini captures new close-up of Saturn's Moon Mimas

Cassini captured this image of Mimas' giant Herschel Crater, which measures about 140 kilometers (88 miles) wide, during its Feb. 13, 2010, flyby of the Death Star-like Saturnian moon. Image credit:

NASA/JPL/Space Science Institute.

Cassini Set to Do Retinal Scan of Saturnian Eyeball Mimas

NASA's Cassini spacecraft made its closest examination yet of Mimas, an eyeball-shaped moon of Saturn that has also been likened to the Death Star of "Star Wars."

The spacecraft returned the highest-resolution images yet of this battered satellite.

Mimas bears the mark of a violent, giant impact from the past - the 140-kilometer-wide (88-mile-wide) Herschel Crater - and scientists hope the encounter will help them explain why the moon was not blown to smithereens when the impact happened. They will also be trying to count smaller dings inside the basin of Herschel Crater so they can better estimate its age.

In addition, Cassini's composite infrared spectrometer was used to determine the thermal signature of the moon, and other instruments made measurements to learn more about the surface composition.

The Mimas flyby involved a significant amount of skill because the spacecraft passed through a dusty region to get there. Mission managers have planned for the Cassini spacecraft to lead with its high-gain antenna to provide a barrier of protection.

At closest approach, the spacecraft came within 9,500 kilometers (5,900 miles) of the moon. Cassini began taking images and measurements shortly after closest approach.

Mimas is an inner moon of Saturn that averages 396 kilometers (246 miles) in diameter. The diameter of Herschel Crater is about one-third that of the entire moon. The walls of the crater are about 5 kilometers (3 miles) high, and parts of the floor are approximately 10 kilometers (6 miles) deep.

Saturn's Moon Mimas snapped by Cassini

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).

This image is a narrow angle clear-filter image which was processed to enhance the contrast in brightness and sharpness of visible features.

Herschel crater, a 140-kilometer-wide (88-mile) impact feature with a prominent central peak, is visible in the upper right of this image.

This image was 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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Image Credit: NASA/JPL/Space Science Institute

The Waltzing Moons of Saturn

Casini Snaps: Eclipse darkens 'Death Star' Moon

The Cassini probe has caught its first glimpse of one Saturn moon eclipsing another, as the moon Enceladus passed silently in front of its neighbour Mimas.

Most of Saturn's moons orbit in the same plane as its rings, along the planet's equator. This arrangement creates frequent alignments, but eclipses are rare because the sun must illuminate this plane almost edge-on in order for moon shadows to be cast in the right direction.

For the first time in almost 15 years, Saturn has reached a spot in its orbit where such events can be seen. The planet's rings, which are tilted by some 27°, will be illuminated perfectly edge-on during Saturn's equinox on 11 August 2009.

The visible moon in this series of images is Mimas, which resembles the Death Star, the planet-destroying space station from the movie Star Wars. It is eclipsed by Enceladus, which spews giant plumes of ice and water vapour from its south pole.

Cassini snapped the eclipse on 13 May, when the probe was roughly 1.3 million kilometres from Mimas.