Cassini Archives

April 14, 2014April 26, 2016 by Jason Major

Is Saturn Making a New Moon?

A 750-mile (1,200-km) -long feature spotted on Saturn's A ring by Cassini on April 15, 2013

Congratulations! It’s a baby… moon? A bright clump spotted orbiting Saturn at the outermost edge of its A ring may be a brand new moon in the process of being born, according to research recently published in the journal Icarus.

“We have not seen anything like this before,” said Carl Murray of Queen Mary University in London, lead author of the paper. “We may be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right.”

In images acquired with Cassini’s narrow-angle camera in 2013, a 1,200-kilometer-long, 10-kilometer-wide arc of icy material was observed traveling along the edge of the A ring. The arc is thought to be the result of gravitational perturbations caused by an as-yet unseen embedded object about a kilometer wide — possibly a miniature moon in the process of formation.

Cassini image of the 179-km-wide Janus from April 2010. Janus' gravity may have helped spur the formation of Peggy. (NASA/JPL-Caltech/SSI) — Cassini image of 179-km-wide Janus from April 2010. Janus’ gravity may have helped spur the formation of Peggy. (NASA/JPL-Caltech/SSI)

The half-mile-wide object has been unofficially named “Peggy,” after lead author Murray’s mother-in-law (whose 80th birthday it was on the day he was studying the Cassini NAC images.) Murray first announced the findings on Dec. 10, 2013 at the AGU 13 meeting in San Francisco.

According to the team’s paper, Peggy’s effects on the A ring has been visible to Cassini since May 2012.

Eventually Peggy may coalesce into a slightly larger moon and move outward, establishing its own orbital path around Saturn. This is how many of Saturn’s other moons are thought to have formed much further back in the planet’s history. Now, its rings having been depleted of moon-stuff, can only create tiny objects like Peggy.

“Witnessing the possible birth of a tiny moon is an exciting, unexpected event.”
– Linda Spilker, Cassini Project Scientist at JPL

While it is possible that the bright perturbation is the result of an object’s breakup rather than formation, researchers are still looking forward to finding out more about its evolution.

To find out more about the Cassini mission visit saturn.jpl.nasa.gov and www.nasa.gov/cassini. The Cassini imaging team’s website is at ciclops.org.

April 8, 2014December 23, 2015 by Jason Major

Cassini Sees a “Zen Garden” on Titan

Radar image of rows of dunes on Titan. Credit: NASA/JPL-Caltech

Looking like the flowing designs carved by a Zen gardener’s rake, long parallel dunes of hydrocarbon sand stretch across the surface of Saturn’s moon Titan. The image above, acquired by Cassini in July 2013, reveals these intriguing and remarkably Earthlike landforms in unprecedented detail via radar, which can easily pierce through Titan’s thick clouds.

I’m feeling a little more enlightened already.

Although it piles into dunes like sand does here, Titan’s sand is not the same as what you’d find on a beach here on Earth. According to an ESA “Space in Images” article:

While our sand is composed of silicates, the ‘sand’ of these alien dunes is formed from grains of organic materials about the same size as particles of our beach sand. The small size and smoothness of these grains means that the flowing lines carved into the dunes show up as dark to the human eye.

Titan's surface is almost completely hidden from view by its thick orange "smog" (NASA/JPL-Caltech/SSI. Composite by J. Major) — Titan’s surface is almost completely hidden from view by its thick orange “smog” (NASA/JPL-Caltech/SSI. Composite by J. Major)

Radar imaging, although capable of seeing through Titan’s opaque orange atmosphere, doesn’t capture visible-light images. Instead it’s sensitive to the varying textures of a landscape as they reflect microwaves; the smoother an object or an area is the darker it appears to radar, while irregular, rugged terrain shows up radar-bright.

The pixelated “seam” cutting horizontally across the center is the result of image artifacting.

Learn more about Cassini’s RADAR instrument here, and read more about this image on the ESA site here.

April 3, 2014December 23, 2015 by Nancy Atkinson

Cassini Spacecraft Confirms Subsurface Ocean on Enceladus

Jets of icy particles bursting from Saturn's moon Enceladus are shown in this Cassini image taken on November 2005. Credit: NASA/ESA/ASI.

Ever since the Cassini spacecraft first spied water vapor and ice spewing from fractures in Enceladus’ frozen surface in 2005, scientists have hypothesized that a large reservoir of water lies beneath that icy surface, possibly fueling the plumes. Now, gravity measurements gathered by Cassini have confirmed that this enticing moon of Saturn does in fact harbor a large subsurface ocean near its south pole.

“For the first time, we have used a geophysical method to determine the internal structure of Enceladus, and the data suggest that indeed there is a large, possibly regional ocean about 50 kilometers below the surface of the south pole,” says David Stevenson from Caltech, a coauthor on a paper on the finding, published in the current issue of the journal Science. “This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole.”

Artist’s impression of the possible interior of Enceladus based on Cassini’s gravity investigation. The data suggest an ice outer shell and a low-density, rocky core with a regional water ocean sandwiched between at high southern latitudes. Cassini images were used to depict the surface geology in this artwork. The mission discovered plumes of ice and water vapour jetting from fractures – nicknamed ‘tiger stripes’ – at the moon’s south pole in 2005. Credit: NASA/JPL-Caltech.

On three separate flybys in 2010 and 2012, the spacecraft passed within 100 km of Enceladus, twice over the southern hemisphere and once over the northern hemisphere.

During the flybys, the gravitational tug altered a spacecraft’s flight path ever so slightly, changing its velocity by just 0.2–0.3 millimeters per second.

As small as these deviations were, they were detectable in the spacecraft’s radio signals as they were beamed back to Earth, providing a measurement of how the gravity of Enceladus varied along the spacecraft’s orbit. These measurements could then be used to infer the distribution of mass inside the moon.

For example, a higher-than-average gravity ‘anomaly’ might suggest the presence of a mountain, while a lower-than-average reading implies a mass deficit.

On Enceladus, the scientists measured a negative mass anomaly at the surface of the south pole, accompanied by a positive one some 30-40 km below.

“By analyzing the spacecraft’s motion in this way, and taking into account the topography of the moon we see with Cassini’s cameras, we are given a window into the internal structure of Enceladus,” said lead author Luciano Iess.

“This is really the only way to learn about internal structure from remote sensing,” Stevenson added.

The only way to get more precise measurements would be to put seismometers on Enceladus’s surface. And that’s not going to happen anytime soon.

Stevenson said the key feature in the gravity data was the negative mass anomaly at Enceladus’s south pole. This happens when there is less mass in a particular location than would be expected in the case of a uniform spherical body. Since there is a known depression in the surface of Enceladus’s south pole, the scientists expected to find a negative mass anomaly. However, the anomaly was quite a bit smaller than would be predicted by the depression alone.

“The perturbations in the spacecraft’s motion can be most simply explained by the moon having an asymmetric internal structure, such that an ice shell overlies liquid water at a depth of around 30–40 km in the southern hemisphere,” Iess said.

While the gravity data cannot rule out a global ocean, a regional sea extending from the south pole to 50 degrees S latitude is most consistent with the moon’s topography and high local temperatures observed around the fractures – called ‘tiger stripes’ at Enceladus south pole.

Many have said Enceladus is one of the best places in the Solar System to look for life. Noted scientist Carolyn Porco and Chris McKay have a recent paper out titled, “Follow the Plume: The Habitability of Enceladus,” where they say that since analysis of the plume by the Cassini mission indicates that the “steady plume derives from a subsurface liquid water reservoir that contains organic carbon, biologically available nitrogen, redox energy sources, and inorganic salts” that samples from the plume jetting out into space are accessible with a low-cost flyby mission. “No other world has such well-studied indications of habitable conditions.”

These latest findings by Cassini make a mission to Enceladus even more enticing.

Paper in Science (paywall) “The Gravity Field and Interior Structure of Enceladus.”

Sources: ESA, Caltech

March 18, 2014December 23, 2015 by Jason Major

Surf’s Up on Titan! Cassini May Have Spotted Waves in Titan’s Seas

Cassini VIMS image of specular reflections in one of Titan's lakes from a flyby on July 24, 2012 (NASA/JPL-Caltech/SSI/Jason W. Barnes et al.)

It’s no surprise that Titan’s north polar region is covered with vast lakes and seas of liquid methane — these have been imaged many times by Cassini during its ten years in orbit around Saturn. What is surprising though is just how incredibly smooth the surfaces of these lakes have been found to be.

One would think that such large expanses of surface liquid — some of Titan’s seas are as big the Great Lakes — would exhibit at least a little surface action on a world with an atmosphere as dense as Titan’s. But repeated radar imaging has shown their surfaces to be “as smooth as the paint on a car.” Over the past several years scientists have puzzled over this anomaly but now they may have truly seen the light — that is, reflected light from what could actually be waves on Titan!

Seasonal winds may be finally kicking up waves in Titan's lakes. (Illustration © Ron Miller.) — Seasonal winds may be finally kicking up waves in Titan’s lakes. (Illustration © Ron Miller.)

Using data acquired during flybys of Titan in 2012 and 2013, planetary scientist Jason Barnes from the University of Idaho and a team of researchers from several other institutions including JPL, Cornell, and MIT, have identified what might be waves in the surface of Punga Mare, one of Titan’s biggest lakes.

For a sense of scale, Lake Victoria, the largest lake in Africa, could fit lengthwise across Titan’s 380-km (236-mile) -wide Punga Mare.

Variations in specular highlights in four pixels observed in the surface of Punga Mare by Cassini’s VIMS (Visible and Infrared Mapping Spectrometer) have been interpreted by the team as being the result of waves — or, perhaps more accurately, ripples, seeing as that they are estimated to be a mere 2 centimeters in height.

Still, based on what’s been observed thus far on Titan, that’s downright choppy.

If the Cassini observations interpreted by Barnes et al. are indicative of waves in Punga Mare, they could also explain previous specular variations seen in other bodies of liquid, like the smaller Kivu Lacus (top image).

Map of Titan's northern "Land o' Lakes" made from Cassini radar imaging passes (NASA/JPL/USGS) — Map of Titan’s northern “Land o’ Lakes” made from Cassini high-resolution radar imaging (NASA/JPL/USGS)

“If correct this discovery represents the first sea-surface waves known outside of Earth.”

– Jason W. Barnes et al.

Then again, wave action isn’t the only possible answer. Similar varied specular highlights could also be caused by a wet surface — like a methane mud flat. Further observations will be needed to rule out other possibilities and obtain a more accurate “surf forecast” for Titan.

The findings were presented by Jason Barnes at the 45th Lunar and Planetary Science Conference in Houston on March 17, 2014. Read the team’s abstract here, and read more in this article by Alexandra Witze on Nature News.

March 14, 2014December 23, 2015 by Jason Major

Watch Two Dark Moons Sneak Into Cassini’s Shots

Raw image of Saturn with two moons acquired by Cassini on March 11, 2014 (NASA/JPL-Caltech/SSI)

On March 11, NASA’s Cassini spacecraft was acquiring some images of Saturn’s back-lit limb when two of its moons decided to make an entrance. Like stage hands in a darkened theatre the moons quickly passed across the scene, moving between Saturn and the spacecraft and, because of exposure time and spacecraft motion, getting a bit blurred in the process.

In the image above the silhouette of one moon can be seen at bottom right — Mimas, perhaps — while another’s crescent can be made out at upper left… possibly Enceladus. Very cool!

Watch an animation of the moons below:

Two of Saturn's moons drift into the scene on March 11, 2014 (NASA/JPL-Caltech/SSI. Animation by Jason Major.) — Two of Saturn’s moons drift into the scene on March 11, 2014 (NASA/JPL-Caltech/SSI. Animation by Jason Major.)

While I admit I’m not 100% sure which moons these are, based on their apparent shapes, positions, and relative sizes I’d make my guess that these are 318-mile (511-km) -wide Enceladus and the 246-mile (395-km) -wide Mimas.

Possible location of icy spray, if this is Enceladus — Possible location of icy spray, if Enceladus is in fact this moon’s real name

Cassini was 843,762 miles (1,357,903 km) from Saturn when the images were acquired. And, if the larger moon at left is Enceladus, I’m thinking south in these images is up based on the barely-perceptible presence of a lighter area along its top edge that could be icy spray from its southern geysers. (See enlarged detail at right.)

Saturn, of course, is on the right. A small segment of the bright arc of its backlit limb is what’s running diagonally down across the image.

These images have not yet been calibrated or cataloged by NASA or the Cassini team.

See the latest raw images from Cassini on JPL’s mission page here.

*I say “dark moons” but actually Enceladus and Mimas are pretty bright, both being composed of a lot of ice. Enceladus is actually the most reflective world in the Solar System!

February 11, 2014December 23, 2015 by Elizabeth Howell

Music Video From Saturn Shows Off Dazzling Aurora Light Show

An aurora around Saturn's north pole in 2013. Credit: NASA/ESA/University of Leicester and NASA/JPL-Caltech/University of Arizona/Lancaster University

Above is the latest rave-like video from NASA. While the images are reminiscent of what could play during an awesome Friday night party, what you’re actually seeing is a timelapse of auroras on Saturn. These pictures are pretty to look at, but they also tell us more about how the sun’s belches of stuff influence the magnetic field around the ringed gas giant.

“Saturn’s auroras can be fickle — you may see fireworks, you may see nothing,” stated Jonathan Nichols of the University of Leicester in England, who led the work on the Hubble Space Telescope images shown in the video. “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 light show was captured by both the Saturn-orbiting Cassini spacecraft and the Earth-orbiting Hubble. Cassini managed to nab its images from three Saturn distances away (which is apparently an unusually close vantage point.) This location “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,” NASA stated.

Here are a few things scientists are learning (or hoping to learn soon) from the light show:

How auroras are formed. The Cassini images suggest that as magnetic field lines forge new links, this is where the storms are centered. This process happens on Earth, so it would make sense for it to happen elsewhere. Researchers also found that some of the auroras stick close to the orbital position of Mimas, suggesting that the moon may be influencing some of the storms (a process already known to happen with Enceladus).
The nature of Saturn’s atmosphere. While the answers are still forthcoming, scientists are examining why the top of Saturn’s atmosphere (and other gas giants) are warmer than would be expected given how far they are from the sun. “By looking at these long sequences of images taken by different instruments, we can discover where the aurora heats the atmosphere as the particles dive into it and how long the cooking occurs,” stated Sarah Badman, a Cassini visual and infrared mapping spectrometer team associate at Lancaster University, England.
What color the auroras are. Red on the bottom, and purple on the top, depending on how Saturn’s hydrogen is excited and what light it emits. (For reference, Earth’s is green on bottom and red at top due to excitement of nitrogen and oxygen).
Where charged particles around Saturn go. More data from the W.M. Keck Observatory and NASA’s Infrared Telescope Facility (both in Hawaii) could show “how particles are ionized in Saturn’s upper atmosphere,” NASA stated. Better yet, scientists can compare that information to the stuff gathered from outside of Earth’s atmosphere by Hubble and Cassini. This will allow them to see what distortions the ground-based observatories experienced due to Earth’s atmosphere, and improve the accuracy of the observations.

Not bad work for a single music video, isn’t it? For more information on auroras on Saturn, check out these past Universe Today stories:

Source: NASA Jet Propulsion Laboratory

February 11, 2014December 23, 2015 by Elizabeth Howell

Saturn’s Ring Shows A Twist In Cassini’s Glimpse Of Planet

Saturn's F ring appears distorted in this October 2013 picture from the Cassini spacecraft. The twisting may be because the F ring is crashing repeatedly into a "single small object", NASA stated. Credit: NASA/JPL-Caltech/Space Science Institute

What’s up with this distortion? This picture from the Cassini spacecraft shows some kind of twist happening in the F ring of Saturn. Scientists in fact have seen other strange shapes in this delicate ring, indicating that something is disturbing it from time to time.

“Saturn’s F ring often appears to do things other rings don’t. In this Cassini spacecraft image, a strand of ring appears to separate from the core of the ring as if pulled apart by mysterious forces. Some ring scientists believe that this feature may be due to repeated collisions between the F ring and a single small object,” NASA stated this month.

There’s a debate in the scientific community about where the rings arose in the first place. “It’s been going back and forth for ages and it still goes back and forth. Are they old, or have they been there a long period of time? Are they new? I don’t know what to think, to be quite honest. I’m not being wishy-washy, I just don’t know what to think anymore,” Kevin Grazier, a planetary scientist with the Cassini mission for over 15 years, told Universe Today in December.

While this picture dates from October, you can check out Cassini images as they come in to NASA’s raw image database. Even in unprocessed form, the planet and its rings look beautiful — as you can clearly see in samples below.

The bulk of Saturn looms to the side of this shot of Saturn's rings taken in February 2014 by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute — The bulk of Saturn looms to the side of this shot of Saturn’s rings taken in February 2014 by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

The variety of Saturn's rings is visible in this raw shot from the Cassini spacecraft taken in February 2014. Credit: NASA/JPL/Space Science Institute — The variety of Saturn’s rings is visible in this raw shot from the Cassini spacecraft taken in February 2014. Credit: NASA/JPL/Space Science Institute

Although Saturn's rings look solid and substantial in images such as this, they are made up of many tiny, icy objects collecting as thin as 6.2 miles (10 kilometers) deep. Image taken by the Cassini spacecraft in February 2014. Credit: NASA/JPL/Space Science Institute — Although Saturn’s rings look solid and substantial in images such as this, they are made up of many tiny, icy objects collecting as thin as 6.2 miles (10 kilometers) deep. Image taken by the Cassini spacecraft in February 2014. Credit: NASA/JPL/Space Science Institute

February 9, 2014December 23, 2015 by Jason Major

Prometheus Practices Its Pull

Shepherd moon Prometheus hovers just inside the reflective F ring

Lit by eerie, reflected light from Saturn’s F ring (and a casting a faint shadow through a haze of icy “mist”) Saturn’s moon Prometheus can be seen in the raw image above, captured by Cassini’s narrow-angle camera on Feb. 5 from a distance of 667,596 miles (1,074,392 km). It’s also receiving some light reflected off Saturn, which is off frame at the top (where the outermost edge of the A ring and the Keeler gap can be seen.)

As the potato-shaped Prometheus approaches the ring it yanks fine, icy material in towards itself, temporarily stretching the bright particles into long streamers and gaps and even kicking up bright clumps in the ring. It’s a visual demonstration of gravity at work! Watch an animation of this below, made from images acquired just before and after the one above:

Streamers and clumps created by the passing Prometheus on Feb. 5, 2014. (NASA/JPL/SSI. Animation by Jason Major.)

At its longest Prometheus is about 92 miles (148 km) across, but only 42 miles (68 km) in width. It circles Saturn in a wave-shaped, scalloping orbit once every 14.7 hours.

Raw images: NASA/JPL-Caltech/Space Science Institute.

January 23, 2014December 23, 2015 by David Dickinson

Watch: An Amazing, Mesmerizing Full Rotation of Jupiter

Jupiter as imaged by Michael Phillips on July 25th, 2009... note the impact scar discovered by Anthony Wesley to the lower left. — Jupiter as imaged by Michael Phillips on July 25th, 2009.

Jupiter is a happening place in the solar system. While bashful Mars only puts on a good show once every two year opposition period, and inner worlds such as Mercury and Venus yield no surface details to backyard observers at all, the cloud tops of Jupiter display a wealth of changing detail in even modest backyard telescopes.

And this month is a great time to start observing Jupiter, as the largest planet in our solar system just passed opposition on January 5^th. Recently, veteran astrophotographer Michael Phillips amazed us here at Universe Today once again with a stunning time-lapse sequence of Jupiter and its moons Ganymede and Io. Now, he’s outdone himself with a new full rotation compilation of the gas giant planet.

The capture is simply mesmerizing to sit and watch. At 9.9 hours, Jupiter has the fastest rotational period of any planet in our solar system. In fact, with Jupiter currently visible low to the east at sunset, it’s possible to follow it through one rotation in the span of a single long January winter night.

We caught up with Michael recently and asked him about this amazing capture. The sequence was actually accomplished over the span of five successive evenings. This made it challenging to stitch together using a sophisticated program known as WINJupos.

“While this is possible on a long winter night when it is darker longer, I typically find it easier to do over multiple nights than one long sleepless night,” Michael told Universe Today. “If you wait too many days between observations, the features will change significantly, and then two nights will not match up clearly. The seams that result from using multiple nights are tricky to stick together. I created multiple non-overlapping seams and tried to blend them out against one another as layers in my image editing software. The result is smoother, but not quite the same as a single observation.”

A 14” f/4.5 Newtonian reflecting telescope was used for the captures. “Similar weather conditions and camera settings help quite a bit to make the multiple nights’ segments match up better,” Michael noted. “Keeping the same settings, using the same location away from my house in the corner of the yard (to reduce local atmospheric turbulence) night after night gives consistent results after removing the variability of the weather.”

Planetary photography also requires special considerations prior to imaging, such as getting Jupiter high enough in the sky and at specific longitudes to get full coverage in the rotation sequence.

“I try to consider the local weather patterns and atmospheric stability (seeing), but in reality, I pushed myself to get out as much and often as I could,” Michael told Universe Today. “Typically, I try to wait until Jupiter is at the highest in the sky, as the result is looking through less atmosphere and thus more stable conditions. Sometimes, the planets jiggle around and you just want to scream ‘SIT STILL!’ Basically around the time of opposition I go out as often as it’s clear, as those are opportunities that you don’t get back again until next year.”

Jupiter reaches opposition just over once every 13 months, moving roughly one constellation eastward each time. 2013 was an “oppositionless” year for Jupiter, which won’t occur again until 2025. Michael also notes that from his observing location at 35 degrees north latitude, Jupiter currently peaks at an altitude of 77 degrees above the horizon when it transits the local meridian. “I wasn’t going to squander it waiting for perfect conditions!”

In fact, Jupiter is currently in a region in the astronomical constellation of Gemini that will be occupied by the Sun in just over five months time during the June Solstice. Currently at a declination of around 22 degrees 45’ north, Jupiter won’t appear this high in the northern sky near opposition again until 2026.

It’s also amazing to consider the kind of results that backyard observers like Michael Phillips are now routinely accomplishing. It’s an interesting exercise to compare Michael’s capture side-by-side with a sequence captured by NASA’s New Horizons spacecraft during its 2006 flyby of Jupiter:

Both sequences capture a wealth of detail, including the enormous Great Red Spot, the Northern and Southern Equatorial Belts, and numerous white spots and smaller swirls and eddies in the Jovian atmosphere.

To date, six spacecraft (Pioneer 10 and 11, Voyagers 1 and 2, New Horizons and Cassini) have made flybys of Jupiter, and one, Galileo, orbited the planet until its demise in 2003. Juno is the next in this legacy, and will be inserted into orbit around Jupiter in July 2016.

Now is the time to get out and observe and image Jupiter and its moons, as it moves higher into the sky on successive evenings towards eastern quadrature on April 1st, 2014.

Congrats to Michael Phillips on an amazing sequence!

January 12, 2014December 23, 2015 by Jason Major

A Distant View of Janus, One of Saturn’s ‘Dancing Moons’

Cassini narrow-angle camera image of Janus from Sept. 10, 2013 (NASA/JPL-Caltech/SSI)

One of 62 moons discovered thus far orbiting giant Saturn, Janus is a 111-mile (179-km) -wide pockmarked potato composed of rock and ice rubble. The image above shows Janus as seen with Cassini’s narrow-angle camera on September 10, 2013, from a distance of 621,000 miles (1 million km), floating against the blackness of space.

Despite its apparent isolation in the image above, though, Janus isn’t alone. It shares its orbit around Saturn with its slightly smaller sister moon Epimetheus, and they regularly catch up to each other — and even switch places.

Janus and Epimetheus: Saturn's "dancing moons" (NASA/JPL/SSI) — Janus and Epimetheus: Saturn’s “dancing moons” (NASA/JPL/SSI)

Janus and Epimetheus travel in nearly the same track, about 94,100 miles (151,500 km) out from Saturn. They occasionally pass each other, their gravity causing them to switch speeds and positions as they do; Janus goes faster and higher one time, slower and lower the next – but the two never come within more than about 6,200 miles of each other.

The two moons switch positions roughly every four years.

This scenario is referred to in astrophysics as a 1:1 resonance. Astronomers were initially confused when the moons were discovered in 1966 as it wasn’t known at the time that there were actually two separate moons in a single orbit. (This wasn’t confirmed until Voyager 1’s visit to Saturn in 1980.) It’s been suggested that Janus and Epimetheus will eventually come to orbit a single Lagrangian point around Saturn instead of trading places… in about another 20 million years.

The view above looks toward the Saturn-facing side of Janus. Covered in both dark and light colored material, Janus’ surface is thought to be coated with a layer of fine dust that slides down its steeper slopes, revealing the brighter ice beneath.

Cassini image of Janus from April 2010 (NASA/JPL-Caltech/SSI)

Want to see more images of Janus? Click here.

Source: Cassini Solstice Mission release