Saturn’s Moon Dione May Have Been Active Like Enceladus

Saturn's moon Dione, as seen by the Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute

From a JPL press release:

From a distance, most of the Saturnian moon Dione resembles a bland cueball. Thanks to close-up images of a 500-mile-long (800-kilometer-long) mountain on the moon from NASA’s Cassini spacecraft, scientists have found more evidence for the idea that Dione was likely active in the past. It could still be active now.

“A picture is emerging that suggests Dione could be a fossil of the wondrous activity Cassini discovered spraying from Saturn’s geyser moon Enceladus or perhaps a weaker copycat Enceladus,” said Bonnie Buratti of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who leads the Cassini science team that studies icy satellites. “There may turn out to be many more active worlds with water out there than we previously thought.”

Janiculum Dorsa, a mountain that appears as the long, raised scar in the middle of this Cassini image, is providing new evidence that the Saturnian moon Dione was recently active. Credit: NASA/JPL-Caltech/Space Science Institute.
Janiculum Dorsa, a mountain that appears as the long, raised scar in the middle of this Cassini image, is providing new evidence that the Saturnian moon Dione was recently active. Credit: NASA/JPL-Caltech/Space Science Institute.

Other bodies in the solar system thought to have a subsurface ocean – including Saturn’s moons Enceladus and Titan and Jupiter’s moon Europa – are among the most geologically active worlds in our solar system. They have been intriguing targets for geologists and scientists looking for the building blocks of life elsewhere in the solar system. The presence of a subsurface ocean at Dione would boost the astrobiological potential of this once-boring iceball.

Hints of Dione’s activity have recently come from Cassini, which has been exploring the Saturn system since 2004. The spacecraft’s magnetometer has detected a faint particle stream coming from the moon, and images showed evidence for a possible liquid or slushy layer under its rock-hard ice crust. Other Cassini images have also revealed ancient, inactive fractures at Dione similar to those seen at Enceladus that currently spray water ice and organic particles.

The mountain examined in the latest paper — published in March in the journal Icarus — is called Janiculum Dorsa and ranges in height from about 0.6 to 1.2 miles (1 to 2 kilometers). The moon’s crust appears to pucker under this mountain as much as about 0.3 mile (0.5 kilometer).

The topography of a mountain known as Janiculum Dorsa on the Saturnian moon Dione. Color denotes elevation, with red as the highest area and blue as the lowest. Credit: NASA/JPL-Caltech/Space Science Institute.
The topography of a mountain known as Janiculum Dorsa on the Saturnian moon Dione. Color denotes elevation, with red as the highest area and blue as the lowest. Credit: NASA/JPL-Caltech/Space Science Institute.

“The bending of the crust under Janiculum Dorsa suggests the icy crust was warm, and the best way to get that heat is if Dione had a subsurface ocean when the ridge formed,” said Noah Hammond, the paper’s lead author, who is based at Brown University, Providence, R.I.

Dione gets heated up by being stretched and squeezed as it gets closer to and farther from Saturn in its orbit. With an icy crust that can slide around independently of the moon’s core, the gravitational pulls of Saturn get exaggerated and create 10 times more heat, Hammond explained. Other possible explanations, such as a local hotspot or a wild orbit, seemed unlikely.

Scientists are still trying to figure out why Enceladus became so active while Dione just seems to have sputtered along. Perhaps the tidal forces were stronger on Enceladus, or maybe the larger fraction of rock in the core of Enceladus provided more radioactive heating from heavy elements. In any case, liquid subsurface oceans seem to be common on these once-boring icy satellites, fueling the hope that other icy worlds soon to be explored – like the dwarf planets Ceres and Pluto – could have oceans underneath their crusts. NASA’s Dawn and New Horizons missions reach those dwarf planets in 2015.

A Color View of Darling Dione

Color-composite of Dione made from raw Cassini images acquired on Dec. 23, 2012. (NASA/JPL/SSI. Composite by J. Major.)

Although made mostly of ice and rock, Saturn’s moon Dione (pronounced dee-oh-nee) does have some color to it, as seen in this color-composite made from raw images acquired by Cassini on December 23.

700 miles (1120 km) wide, Dione is covered pole-to-pole in craters and crisscrossed by long, bright regions of “wispy line” terrain — the reflective faces of sheer ice cliffs and scarps that are too steep for darker material drifting in from Saturn’s E ring to remain upon.

The composite  was assembled from raw images captured in red, green and blue visible light wavelengths by Cassini from a distance of 154,869 miles (249,238 km).

The view above looks at a region on Dione’s mid-northern hemisphere. The bright-walled crater in the center surrounded by warmer-hued terrain is named Creusa, and the long rift system next to it is Tibur Chasmata, which runs north-to-south. Dione’s north pole is to the upper left.

Dione’s heavily cratered areas are most common on its trailing hemisphere. Logically, a moon’s leading hemisphere should be the more heavily cratered, so it has been hypothesized that a relatively recent impact spun Dione around 180 degrees. The moon’s small size mean that even a modest-scale impact could have done the job.

Relative sizes of Earth, Moon and Dione (J. Major)

Dione orbits Saturn at a distance of 209,651 miles (377,400 km), closer than our Moon is to us.

See more images and news from the Cassini mission here. And for more on Dione, see some of my previous posts on Lights in the Dark.

Exploration at its Finest: Cassini Visits Dione

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After completing its most recent flyby of Enceladus, Cassini made a pass by Dione — its final visit of the icy moon for the next three years. Coming within  5,000 miles (8000 km) of Dione on May 2, Cassini captured some fantastic images of the moon’s heavily-cratered and frozen surface. Here’s just a few of the raw images that arrived back here on Earth earlier today:

Crescent-lit Dione, with some reflected light via Saturnshine
A nearly fully-lit Dione, with Saturn's rings in the background
Dione's extensively-cratered limb
Some of Dione's signature "wispy lines", bright icy faces of sheer cliffs now known to be tectonic in origin
A color-composite image of an ancient impact crater on the edge of Dione's Saturn-facing side - this could be from the impact that spun the moon 180 degrees. (NASA/JPL/SSI/J. Major)

698 miles (1123 km) in diameter, Dione orbits Saturn at about the same distance that the Moon orbits Earth. Its composition is two-thirds water ice, which at the incredibly cold temperatures found around Saturn behaves like rock does here on Earth.

 

Cassini won’t visit Dione so closely again until June 2015, after spending three years angled high out of the equatorial plane while it studies Saturn’s rings and polar regions.

As Carolyn Porco, Cassini Imaging Team Leader said today, “This is exploration at its finest. It won’t continue forever. So, enjoy it while it lasts!”

See more on the Cassini Imaging Central Laboratory for Operations (CICLOPS) site here.

Image credits: NASA/JPL/Space Science Institute 

 

Postcards From Saturn

Saturn's second-largest moon Rhea, in front of the rings and a blurred Epimetheus (or Janus) whizzing behind. Acquired March 29, 2012.

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Over the past few days NASA’s Cassini spacecraft has performed flybys of several of Saturn’s moons. From the ostentatious Enceladus with its icy geysers to the rugged relief of Rhea, the sharp peaks of Dione’s frigid craters and even diminutive Janus, Cassini has once again returned a stack of stunning views from the Saturnian system, nearly 815 million miles from home.

Check out some of the images, and wish you were there!

110-mile (177-km) -wide Janus in front of Saturn's night side.
A crescent-lit Enceladus shows off its jets. (South is up.)
Enceladus' fractured surface is some of the most reflective terrain in the Solar System.
Wide-angle view of Rhea, Saturn and Mimas
Crater peak on icy Dione

And here’s a color-composite of Janus I assembled from three raw images taken in ultraviolet, green and infrared color channels. The results were tweaked to make it a little more true-color as what we might see with our limited human vision:

Color composite of Janus in front of Saturn, made from raw images taken in UV, green and IR color filters. (NASA/JPL/SSI/J. Major)

“Though we’ve been in orbit around Saturn for nearly 8 years now, we still continue to image these moons for mapping purposes and, in the case of Enceladus, to learn as much as we can about its famous jets and the subterranean, organic-rich, salty, liquid water chamber from which we believe they erupt.”

– Carolyn Porco, Cassini Imaging Team leader

For more images from Cassini, check out JPL’s mission site and the CICLOPS imaging lab site here.

Image credits: NASA/JPL/Space Science Institute.

Two Moons In Passing

Animation of Tethys passing in front of Dione from Cassini's point of view. (CLICK TO PLAY)

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Saturn’s moon Tethys passes in front of its slightly larger sister Dione in this animation made from 25 raw images acquired by Cassini on March 14, 2012. Pretty cool! (Click the image to play.)

 

Tethys and Dione (NASA/JPL/SSI)

Tethys and Dione are similar in diameter, being 1,062 kilometers (660 miles) wide and 1,123 kilometers (698 miles) wide, respectively. Both are heavily cratered, ice-rich worlds.

In this view, Tethys’ enormous Odysseus crater can be seen on its northern hemisphere. 400 km (250 miles) across, Odysseus is two-fifths the diameter of Tethys itself, suggesting that it was created early in the moon’s history when it was still partially molten — or else the impact would have shattered the moon apart entirely.

The more extensively-cratered trailing side of Dione is visible here, its signature “wispy lines” rotated out of view. Since it makes sense that a moon’s leading face should be more heavily cratered, it’s thought that Dione has been spun around by an impact event in the distant past.

If you look closely, a slight rotation in Tethys can also be discerned from the first frame to the last.

Credit: NASA/JPL/SSI. Animation by Jason Major.

Colorful Holiday Treats from Saturn

The moons Titan and Dione are photographed with rings and Saturn in the background. Credit: NASA/JPL-Caltech/Space Science Institute

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“Hey! Look what our Santa at Saturn has sent our way!” said Carolyn Porco, the Cassini imaging team lead, in a post on Twitter. This wonderful collection of just-released colorful images from the Saturn system are a holiday gift from the Cassini and CICLOPS (Cassini Imaging Central Laboratory for Operations)team.

Above, Saturn’s third-largest moon, Dione, can be seen through the haze of the planet’s largest moon, Titan, in this view of the two posing before the planet and its rings from NASA’s Cassini spacecraft.

More treats below!

Saturn's moon Tethys, with its stark white icy surface, peeps out from behind the larger, hazy, colorful Titan in this view of the two moons obtained by NASA's Cassini spacecraft. Saturn's rings lie between the two. Credit: NASA/JPL-Caltech/Space Science Institute
These views from NASA's Cassini spacecraft look toward the south polar region of Saturn's largest moon, Titan, and show a depression within the moon's orange and blue haze layers near the south pole. Credit: NASA/JPL-Caltech/Space Science Institute
The colorful globe of Saturn's largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA's Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute
Saturn's largest moon, Titan, appears deceptively small paired here with Dione, Saturn's third-largest moon, in this view from NASA's Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute

To see more details and larger versions of these images, visit the CICLOPS website. (And thanks, Carolyn and team for the beautiful gifts!)

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.

Cassini Provides Stunning New Looks at Several Moons

Saturn's moon Helene. Credit: NASA//JPL/SSI, image enhanced by Stu Atkinson

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The Cassini spacecraft recently had a mini ‘grand tour’ of several of Saturn’s moons and just sent back some great images of Helene, Mimas, Enceladus and Dione. Above is an amazing view of the Trojan moon Helene, which is only 32 kilometers (20 miles across) and shares an orbit with Dione. Cassini came withing 28,000 km (17,398 miles) of Helene. Thanks to Stu Atkinson for an enhanced version of this raw Cassini image. See one of the original raw images of Helene here.

This image of Saturn's moon Enceladus was obtained by NASA's Cassini spacecraft on Jan. 31, 2011. It shows the famous jets erupting from the south polar terrain of Enceladus. Image credit: NASA/JPL/SSI

Cassini captured several images of the plumes spewing from Enceladus, and other closeup views of the moon’s terrain.

Closeup of Enceladus from approximately 78,015 kilometers away. Credit: NASA/JPL/SSI
This view shows the bright, icy Mimas in front of Saturn's delicate rings. Image credit: NASA/JPL/SSI

A crescent Dione was seen by Cassini on January 29, 2011 from approximately 767,922 kilometers away. Credit: NASA/JPL/SSI

See more of the latest raw images at the Cassini website.

Double Moon Illusion

Enceladus and Dione, as seen by Cassini. Credit: NASA/JPL/Space Science Institute

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We’ve all experienced the Moon Illusion, where our own full Moon looks bigger when seen on the Earth’s horizon. But how about this illusion where you can’t really tell which of these two moons of Saturn is actually bigger, or which is closer, as seen by the Cassini spacecraft? Here, Dione, top right, appears closer to the spacecraft because it is larger than the moon Enceladus, lower left. However, Enceladus was actually closer to Cassini when its visible light, narrow-angle camera took this image.

Dione (1,123 kilometers, or 698 miles, across) is more than twice the size of Enceladus (504 kilometers, or 313 miles, across). The two moons are contrasted with Enceladus’ bright, reflective trailing hemisphere, and Dione’s darker, micrometeor-dusted side, decorated with wispy lighter materials.

Cassini took this image on Dec. 1, 2010 from about 510,000 kilometers (317,000 miles) from Enceladus and approximately 830,000 kilometers (516,000 miles) from Dione. Image scale is 3 kilometers (2 miles) per pixel on Enceladus and 5 kilometers (3 miles) per pixel on Dione.

Source: CICLOPS, the Cassini imaging website

Stunning New Images From Cassini’s Close Flyby of Rhea

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Rhea, Saturn's rings and some sister moons. Credit: NASA/JPL/Space Science Institute

Jia-Rui C. Cook from the Cassini team sent out an alert that raw images from Cassini’s closest flyby of Saturn’s moon Rhea have begun streaming to Cassini’s raw image page, and they are well worth a look. At closest approach, Cassini came within about 69 kilometers (43 miles) of Rhea’s surface on Jan. 11. But there’s also some interesting group photos from within the Saturn System. One of the best is this image, above. How many moons can you find? I probably wouldn’t have seen them all but Emily Lakdawalla at the Planetary Blog spied five moons and the rings in this one wide-angle shot. The large moon is Rhea; above Rhea and just below the rings, is Dione; above and to the left of Rhea is Tethys. Then there are two tiny moons: squint hard to see Prometheus as tiny lump on the rings to the left of Dione, and Epimetheus is hovering between Tethys and Rhea. See some more, including closeups of Rhea and Saturn’s storm, below.

Continue reading “Stunning New Images From Cassini’s Close Flyby of Rhea”