Cassini gave us a good look a Saturn's moon Pan last week . . . and what a strange world it is.
Who ordered that? The universe served up a piece of astro-pareidolia last week, when humanity got its first closeup look at Saturn's tiny moon Pan. Appropriately named after the half-man, half-goat satyr from Greek mythology, Pan is nestled in the Encke (pronounced EN-key) Gap within Saturn's A ring. NASA's Cassini spacecraft flew just 15,268 miles past the moonlet (closer than the distance to the geosynchronous satellites from Earth) on March 7th.
“Nearing its end, Cassini delights again,” says Carolyn Porco (Space Science Institute) on Twitter. “Here is 35-km Pan in mind-blowing detail with its unmistakable accretionary equatorial bulge.”
Mark Showalter (then at Stanford University) discovered Pan on July 16, 1990. Showalter and colleagues first inferred the tiny moon's presence by the waves it kicked up in the wake of its passage through the Encke Gap. After accurately predicting the moon's orbit, Pan was found in 11 images taken by Voyager 2 during its August 1981 flyby.
The moon orbits Saturn every 13.8 hours from an average distance of 134,000 kilometers (80,150 miles), equivalent to about one-third the Earth-Moon distance, and just 73,000 miles from the Saturnian cloud tops), the 34x31x21-kilometer moon carves out the Encke Gap in Saturn's outermost bright A Ring. On Earth, Pan would just barely fit inside Tampa Bay. The moon has an albedo (or reflectance) of 50%, equivalent to dirty snow.
Unlike the narrow 35-kilometer-wide Keeler gap occupied by the tiny moonlet Daphnis, the wider 325-kilometer Encke Gap also hosts a tenuous ringlet that Pan braids and modifies. While the Daphnis is slightly inclined to the plane of Saturn's rings by 0.0036 degrees and kicks up vertical waves in its wake, the orbit of Pan is nearly flat with an inclination of only 0.0001 degrees, and it induces spiral density waves in the ring plane.
Brave Little Moon
For a while now, scientists have had a hunch that there's something askew about Pan and Atlas, based on distant views obtained by Cassini in 2007.
How did Pan get its strange shape? The leading idea is that the flange of ice around its equatorial bulge is ring material swept up and collected by the moon as it cruises through the Encke Gap.
“Pan got its distinctive “skirt” because of the last stages of its formation (continued even in slow motion today) in which it accreted material from the rings it's embedded within,” says Porco. “During the last stages the rings were very thin and so the material falling onto Pan at this time came down on its equator and built the ridge you see.”
The skirt of ice towers several kilometers above the surface. One can only wonder what sort of alien sky an observer standing next to it would see, with glorious Saturn and the edge-on rings filling the view beyond the frozen wall. Is it rock hard, or soft as newly fallen snow? Or is pasta-shaped Pan just filled with cheesy goodness all the way through?
The release of the images also sparked a flurry of commentary across social media last week. Space fans saw in the moon's bizarre shape anything from a filled pasta, to a space cabbage, to an empanada, perhaps reflecting a true “hunger” out there for space exploration.
“My first impression when I saw the first image? That it was an artist's depiction of what Pan might look like,” says Porco. “It looked so alien and 'well executed,' so to speak, that I didn't think it was real. But the thrill of discovery is why we do this. Cassini, once again, delivered us a wonderful gift!”
Cassini's distant images of Saturn's moon Atlas indicate it's probably similar to Pan. We'll get a good look at that moonlet next month, when Cassini flies just 13,000 kilometers past Atlas on April 12th.
Launched on October 15, 1997, Cassini arrived at Saturn on July 1, 2004. Cassini is currently finishing up a series of twenty ring-grazing orbits, swooping in through the ring plane of Saturn once every seven days. Next month, Cassini prepares for the climax, a series of Grand Finale orbits that will end with the demise of the spacecraft on September 15, 2017, at 8:07 AM EDT (12:07 Universal Time), when it burns up in Saturn's atmosphere.
Strange new worlds such as Pan remind us that there are still bizarre, unexplored corners of the solar system. Cassini promises to give us some dramatic new views of Saturn and its enigmatic moons, right up to the very end.
Read more papers by the Cassini imaging team on Saturn's “saucer-shaped" moons: Saturn's Small Inner Satellites: Clues to Their Origins and The Equatorial Ridges of Pan and Atlas: Terminal Accretionary Ornaments?