Saturn’s Moon Pan Stuns in Cassini’s Spectacular Close-Up Pictures
published during a waxing gibbous moon.
03/09/2017

 

Pan is one of at least 62 moons around Saturn, but right now it’s the unmistakable star. On March 7, 2017, the Cassini spacecraft photographed the moon from just 24,572 kilometers away, taking striking photographs in unprecedented detail.

These photographs are real images of a real moon in our solar system, not three-dimensional models or a computer reconstruction with vertical exaggeration. “We’ve never seen these kinds of details in the skirt that goes around the equator,” explained Paul Helfenstein, a planetary scientist at Cornell University. “We’ve never seen the kind of surface details that we’ve seen here.”

Researchers think the equatorial ridge — the skirt running in a band around the moon — is formed by material scooped out of the ring that slammed into the surface. “Pan is orbiting Saturn so that the equator corresponds to the orientation of the rings,” explained Helfenstein. “If it’s sucking up material from the rings, that material is going to preferentially deposit on the equator.”

Although he hasn’t yet had time to fully analyze the image, Helfenstein estimates the ridge pushes the equator out about 8 kilometers beyond the moon’s mean radius of 14.1 kilometers. The ridge is knife-edge sharp in some places and widens out to approximately 4.5 kilometers thick in the blockier regions.

Moon Pan

Cassini captures the closest-ever images of Saturn’s moon Pan from just 24,572 kilometers away. Credit: NASA/JPL-Caltech/Space Science Institute

“When you see something in greater detail than you’ve seen before, it’s both familiar and astonishing,” said Matthew Tiscareno, a planetary scientist at the SETI Institute who specializes in ring dynamics. “The basic contours are what we expected, but the particular form that it takes with these very steep and sharp shapes is just spectacular and astounding.” The sharp topography betrays another secret: the material crashing into Pan to build this ridge must be sticky and cohesive. “In one place it looks kind of like a blade or fin, in another place it has a squared-off, blocky appearance,” Tiscareno continued. “It has some resistance to rolling downhill and making a gentle slope.”

The sharp topography reveals another secret: the material crashing into Pan building this ridge must be sticky and cohesive. “In one place it looks kind of like a blade or fin, in another place it has a squared-off, blocky appearance,” Tiscareno continued. “It has some resistance to rolling downhill and making a gentle slope.”

Pan is light for its size: with a density of just 420 kilograms per meter, it’s less dense than a snowball. That low density makes it particularly vulnerable to fracturing in the tidal stress of Saturn’s enormous gravitational grasp. While the researchers anticipated fractures on the rounded central core, seeing them on the ridge for the first time was unexpected. “We don’t know where to start,” exclaimed Helfenstein. “This stuff is so cool!”

“Hey, guess what! I discovered a moon of Saturn today.”

Pan was discovered in 1990 by Mark Showalter, a planetary scientist now at the SETI Institute. He’d been tracking developing theories, then realized he had all the data he needed to exhaustively search for the moon. “I literally woke up in the morning with this idea in my head,” he remembered. “I said to my spouse ‘I’m going to discover a moon of Saturn today.’” He pored over existing Voyager 2 data. “By 2 o’clock that afternoon I had called them up and said ‘Hey, guess what! I discovered a moon of Saturn today.’” He laughed at the memory. “That’s not the kind of thing that happens very often in one’s scientific career, but it was a great, great moment for me.”

Moon Pan

Cassini captures the closest-ever images of Saturn’s moon Pan from just 24,572 kilometers away. Credit: NASA/JPL-Caltech/Space Science Institute

Pan is Saturn’s innermost moon at 134,000 kilometers from the center of the planet and makes a complete orbit every 13.8 hours. During this rapid dance around Saturn, it holds open the 325-kilometer wide Encke Gap in Saturn’s A-ring. “Pan is big enough that is able to push the ring so far out of the way that the gap remains all the way around the ring’s circumference,” explains Tiscareno. This capacity to hold open a gap marks Pan as a shepherd moon and made naming it “almost a no-brainer” for Showalter.

“When we found Pan, the thinking was, ‘Well, what’s the association with shepherds?’” Showalter recalled.  “Pan was the patron god of shepherds in ancient Rome.”

Helfenstein is part of the Cassini imaging team that planned the maneuvers which produced these spectacular photographs. “We had to wait towards the end of the mission for the Cassini spacecraft orbits to bring us to a configuration that would allow us to obtain these images,” he said. The Cassini spacecraft is on its farewell tour of Saturn, collecting its final observations of Saturn’s moons in ever-more-daring maneuvers before its grand finale that will take it plunging into the depths of the gas giant in September 2017.

While this is the final planned observation of Pan, this isn’t the last moon we’ll see with such unusual topography. Atlas also has a prominent equatorial ridge and is on Cassini’s docket for closer inspection on April 12, 2017. The spacecraft will be swooping closer to that moon, bumping the resolution up even higher than in Pan’s approximately 150 meters per pixel masterpieces.

“It’s really cool when something goes from a pixel to a real object where you can see craters and ridge systems,” Showalter said. “It’s just beautiful and gratifying.” This is the second time Showalter has had this experience. He was on the team that discovered Pluto’s smallest moons, Styx and Kerberos, in Hubble images, then saw them evolve into detailed worlds in New Horizons photography. “For something to go from a dot to a world is always exciting!”