The Galilean Moons, named in honor of Galileo Galileo, who first observed them in 1610, are a fascinating collection of satellites. For decades, scientists have been immensely fascinated by the three icy companions – Europa, Ganymede, and Callisto – which have oceans in their interiors that possibly support life. But Io has also been a focal point of interest lately, owing to the volcanic activity on its surface and lava plumes reaching 300 to 500 km (186 to 310 mi) into space. Since 2016, NASA’s Juno probe has provided stunning images of Io as it continues to orbit its main science objective, Jupiter.
The latest was acquired by the Juno probe’s main camera (JunoCam) on July 31st, 2023, at 05:03 AM UTC (01:03 AM EDT; July 30th, 10:03 PM PDT) and showed Prometheus spewing out lava. This active volcano is located within a 28-km (17-mi) -wide volcanic pit named Prometheus Patera on the hemisphere facing away from Jupiter. Prometheus is known for its regular eruptions, hence its nickname in the astrogeological community, “Old Faithful of Io.” A processed image of the eruption was shared by the NASA Planetary Science Division via Twitter (see below).
A map of Saturn's F ring from 2006 shows one of the few bright, extended clumps (indicated by a green box) seen during six years of observation by Cassini. Image credit: NASA/JPL-Caltech/SSI
Nothing stands still. Everything evolves. So why shouldn’t Saturn’s kookie, clumpy F ring put on a new face from time to time?
A recent NASA-funded study compared the F ring’s appearance in six years of observations by the Cassini mission to its appearance during the Saturn flybys of NASA’s Voyager mission, 30 years earlier.
A kink in part of Saturn’s F ring. While the ring is held together by the shepherd moons Prometheus and Pandora, which orbit just inside and outside the ring, embedded moonlets are believed responsible for the kinks and clumps. The rings is several hundred kilometers wide. Credit: NASA
While the F ring has always displayed clumps of icy matter, the study team found that the number of bright clumps has nose-dived since the Voyager space probes saw them routinely during their brief flybys 30 years ago. Cassini spied only two of the features during a six-year period.
Scientists have long suspected that moonlets up to 3 miles (5 km) wide hiding in the F ring are responsible for its uneven texture. Kinks and knots appear and disappear within months compared to the years of observation needed changes in many of the other rings.
Saturn’s F ring is extremely narrow compared to the historic A, B and C rings. It measures just a few hundred kilometers across. Credit: NASA/Cassini
“Saturn’s F ring looks fundamentally different from the time of Voyager to the Cassini era,” said Robert French of the SETI Institute in Mountain View, California, who led the study along with SETI Principal Investigator Mark Showalter. “It makes for an irresistible mystery for us to investigate.”
A 2007 artist impression of small boulder-like chunks of ice that comprise Saturn’s rings. The largest are about 10-12 feet across.They clump together to form elongated, curved aggregates, continually forming and dispersing. Credit: NASA/JPL/Univ. of Colorado
Because the moonlets lie close to the ring and cross through it every orbit, the research team hypothesizes that the clumps are created when they crash into and pulverize smaller ring particles during each pass. They suspect that the decline in the number of exceptionally bright kinks and the clumps echoes a decline in the number of moonlets available to do the job.
So what happened between Voyager and Cassini? Blame it on Prometheus. The F ring circles Saturn at a delicate point called the Roche Limit. Any moons orbiting closer than the limit would be torn apart by Saturn’s gravitational force.
The culprit? Prometheus measures 74 miles (119 km) across and orbits the inner edge of Saturn’s F ring. Credit: NASA
“Material at this distance from Saturn can’t decide whether it wants to remain as a ring or coalesce to form a moon,” said French. “Prometheus orbits just inside the F ring, and adds to the pandemonium by stirring up the ring particles, sometimes leading to the creation of moonlets, and sometimes leading to their destruction.”
Every 17 years the orbit of Prometheus aligns with the orbit of the F ring in a way that enhances its gravitational influence. The researchers think the alignment spurs the creation of lots of extra moonlets which then go crashing into the ring, creating bright clumps of material as they smash themselves to bits against other ring material.
Sounds like a terrifying version of carnival bumper cars. In this scenario, the number of moonlets would gradually drop off until another favorable Prometheus alignment.
The Voyagers encounters with Saturn occurred a few years after the 1975 alignment between Prometheus and the F ring, and Cassini was present for the 2009 alignment. Assuming Prometheus has been “working” to build new moons since 2009, we should see the F ring light up once again with bright clumps in the next couple years.
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.
Saturn, its rings and three moons are visible in this image from Cassini. Credit: NASA/ESA
It’s a good thing NASA labeled the moons in this image of Saturn, because they are pretty hard to see. But they are there, keeping each other company in this Cassini spacecraft image of Saturn’s night side. And as the Cassini team says, it seems fitting that they should do so since in Greek mythology, their namesakes were brothers.
In Greek mythology these three were all sons of Iapetus (another of Saturn’s moons), and supposedly Prometheus and Epimetheus were tasked with creating humans. Prometheus was a pretty good sort, and gave gifts to humans like fire; Epimetheus gave humans evil – not so good. And famously, Atlas ended up having the weight of the world on his shoulders.
But in science, Prometheus the moon is about 86 kilometers across (53 miles) and is located just inside the F ring in this image, while Epimetheus is about 113 kilometers across (70 miles) and is farther from the rings, due right of Prometheus in this image. Atlas is the tiny guy (30 kilometers across (19 miles) and can be just barely seen between the A and F rings almost right below Epimetheus,
This view looks toward the unilluminated side of the rings from about 30 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Sept. 19, 2012.
The view was obtained at a distance of approximately 2.2 million kilometers (1.4 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 96 degrees. Image scale is 128 kilometers (80 miles) per pixel. Epimetheus has been brightened by a factor of 1.5 and Atlas’ brightness has been enhanced by a factor of 3 relative to the rings and Prometheus to improve visibility.
Raw wide-angle Cassini image of Saturn’s rings (NASA/JPL/SSI)
Recently I posted an image of two of Saturn’s shepherd moons, Pandora and Prometheus, captured by Cassini in a face-off across the spindly F ring. Now here’s a much wider-angle view of the gas giant’s rings, seen by Cassini two days later on December 20, and the same two moons can still be seen staring each other down… two tiny points of light visible across the wavering line of the F ring at lower center.
This is just one raw image in a series of 56 that Cassini captured on the 20th, and I’ve combined them together to make a GIF animation — click below to watch:
Animation of Saturn’s rings made from raw images acquired by Cassini on Dec. 20, 2012 (NASA/JPL/SSI. Animation by J. Major)
In the animation you can see Pandora and Prometheus promenade around Saturn (detail at right) as well as a “spoke” of light material moving within the inner dark edge of the A ring. Also many clumps are visible in the thin F ring — caused by embedded moonlets and the gravitational influence of the shepherd moons.
Saturn’s enormous shadow engulfs the entire ring system at the top of the scene.
Cassini was moving relative to Saturn while these images were captured so some background stars make brief appearances, as well as a couple of pixel flares and a cosmic ray hit. These are common in Cassini images.
See more news and images from the Cassini mission here.
Raw Cassini image acquired on Dec. 18, 2012 (NASA/JPL/SSI)
Two of Saturn’s shepherd moons face off across the icy strand of the F ring in this image, acquired by the Cassini spacecraft on December 18, 2012.
In the left corner is Pandora, external shepherd of the ropy ring, and in the right is Prometheus, whose gravity is responsible for the subtle tug on the wispy ring material. (Please don’t blame the moon for any recent unsatisfying sci-fi films of the same name. There’s no relation, we promise.)
Similar in size (Pandora is 110 x 88 x 62 km, Prometheus 148 x 100 x 68 km) both moons are porous, icy, potato-shaped bodies covered in craters — although Prometheus’ surface is somewhat smoother in appearance than Pandora’s, perhaps due to the gradual buildup of infalling material from the F ring.
Check out some much closer images of these two moons below, acquired during earlier flybys:
Here’s Pandora, as seen by Cassini on September 5, 2005:
False-color image of Pandora (NASA/JPL/SSI)
…and here’s Prometheus, seen during a close pass in 2010 and color-calibrated by Gordan Ugarkovic:
Prometheus casting a shadow through F ring haze (NASA/JPL/SSI/Gordan Ugarvovic)
The external edge of the A ring with the thin Keeler gap and the wider Encke gap can be seen at the right of the top image. Both of these gaps also harbor their own shepherd moons — Daphnis and Pan, respectively.
These moons keep their gaps clear, as well as maintain the crisp edge shapes of the nearby rings — hence the term “shepherd.”
The space shuttle Enterprise’s final journey to a museum.
NASA’s new plans for commercializing space
A quick review of the new movie Prometheus
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It may be one of the best images from Cassini yet this year! Cloud-covered Titan and tiny Prometheus (can you see it just above the rings on the right?) are literally dwarfed by their parent Saturn in an image captured on Jan. 5, 2012.
Prometheus’ pinpoint shadow can also be seen on Saturn’s cloud tops, just inside the thin, outermost F ring shadow at bottom left.
The two moons themselves couldn’t be more different; Titan, 3,200 miles (5,150 km) wide, is wrapped in a nitrogen and methane atmosphere ten times thicker than Earth’s and is covered with vast plains of dark hydrocarbon dunes and crisscrossed by rivers of liquid methane.
Prometheus imaged by Cassini in Dec. 2009.
Prometheus, on the other hand, is a potato-shaped shepherd moon 92 miles long and 53 miles wide (148 x 53 km) that orbits Saturn just inside the narrow, ropy F ring. While it doesn’t have an atmosphere, it does create some impressive effects on the icy material in the ring!
Another moon, Pandora, casts its shadow onto Saturn just outside the F ring shadow at bottom center. 50 miles (80 km) wide, Pandora shepherds the outer edge of the F ring but is itself not visible in this image. Watch an animation here.
This image was featured on the Cassini Imaging Central Laboratory for Operations (CICLOPS) website on Feb. 28, 2012. The view looks toward the southern, unilluminated side of the rings from about 1 degree below the ringplane.
Image credit: NASA / JPL / Space Science Institute.
Saturn's moon Prometheus creates streamer channels in the planet's rings. Credit: NASA/JPL/Space Science Institute
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I’ve frequently said the Cassini spacecraft is an artist, so when Carolyn Porco, the mission’s imaging team lead, mentioned on Twitter that Saturn’s moon Prometheus is akin to Michelangelo, I had to take a look. Wow, this gorgeous image is suitable for framing! Visible in the perturbed, thin F ring, is the potato-shaped Prometheus, and having performed the perturbing, it continues in its orbit. Click the image for the super-huge version.
Prometheus (148 kilometers, 92 miles across) periodically creates streamer-channels in the F ring, and the moon’s handiwork can be seen as the dark channels. Here’s a movie made from Cassini images showing this process:
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 1, 2010. A star is also visible through the rings near the center right of the image.
There are also some additional features in the F ring, courtesy of Prometheus.
This Cassini image shows icy particles in Saturn’s F ring clumping into giant snowballs as the moon Prometheus makes multiple swings by the ring. Scientists say that the gravitational pull of the moon sloshes ring material around, creating wake channels that trigger the formation of objects as large as 20 kilometers (12 miles) in diameter.
“Scientists have never seen objects actually form before,” said Carl Murray, a Cassini imaging team member based at Queen Mary, University of London. “We now have direct evidence of that process and the rowdy dance between the moons and bits of space debris.”
Rhea poses with Saturn's rings; Janus and Prometheus are off in the distance. Credit: NASA/JPL/Space Science Institute. Click for larger version
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Oh, wow — what a gorgeous image! Just the latest from our resident artist in space, the Cassini spacecraft. Rhea, saturn’s second largest moon sits in front of the rings, joined by two smaller moons in the background. Rhea (1528 kilometers, 949 miles across) is in the center foreground. Janus (179 kilometers, 111 miles across) can be seen beyond the rings on the right of the image. Prometheus (86 kilometers, 53 miles across) is visible orbiting between the main rings and the thin F ring on the left of the image. Lit terrain seen on Rhea is on the area between that moon’s trailing hemisphere and anti-Saturn side. This view looks toward the northern, sunlit side of the rings from just above the ringplane.
If you like contrast images, there’s a great one below.
Saturn's rings contrast with the blackness of space. Credit: NASA/JPL/Space Science Institute
This image is a beautiful contrast between dark and light. Atlas can be seen just above the center of this Cassini spacecraft image as the moon orbits in the Roche Division between Saturn’s A ring and thin F ring.
