Pan, the ravioli-shaped moon in the Encke Gap
Pan is a small moon of Saturn, about 28 km across, that orbits inside the Encke Gap, a 325 km wide lane in the outer A ring. It is the innermost known moon of Saturn with an official name, and close-up Cassini images taken in 2017 revealed its striking shape: a rounded core girdled by a thin, sharp equatorial ridge that makes Pan look like a ravioli, an empanada or a walnut.
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A moon found by its fingerprints
Pan is a rare example of a moon predicted before it was seen. Voyager images of the early 1980s showed wavy, scalloped edges along the Encke Gap. In the mid-1980s, Jeffrey Cuzzi and Jeffrey Scargle argued that such patterns were the wakes of a small moon orbiting within the gap, and estimated where it should be. Mark Showalter used those predictions to search the Voyager 2 archive, and in 1990 he found the moon in images taken nine years earlier, in 1981.
The name refers to the Greek god of shepherds and flocks, a fitting choice for a moon that "herds" ring particles. Pan is the textbook example of what astronomers call a shepherd moon.
How Pan keeps the Encke Gap open
The Encke Gap lies about 133,600 km from Saturn's center, some 3,000 km inside the outer edge of the A ring. It is the widest gap within the A ring itself (the much broader Cassini Division lies between the A and B rings), and is described with the other ring gaps.
Each time ring particles pass close to Pan, they receive a small gravitational kick. Over many passes these kicks push the particles away from Pan's orbit, clearing a lane. Particles just inside Pan's orbit move faster and overtake it; particles just outside move slower and are overtaken. In both cases the encounter leaves a ripple in the particles' paths, and these ripples form the wavy wakes seen on both edges of the gap, trailing downstream from the moon.
The gap is not entirely empty. Cassini found several narrow, kinked ringlets inside it, one of which shares Pan's orbit. These dusty ringlets change over time and show clumps and kinks, partly shaped by Pan's gravity.
A ravioli in orbit
On 7 March 2017, during its Ring-Grazing orbits that preceded the Grand Finale, Cassini flew within about 25,000 km of Pan, the closest it ever came. The images show a central body measuring roughly 34 × 31 × 21 km, with a ridge running around the equator.
Earlier, more distant views from 2005 and 2007 had already shown a pointed equatorial outline, but the 2017 images made the structure plain. The ridge is smooth, almost featureless, unlike the more textured central body. The leading explanation is that Pan has swept up fine ring particles over a long time. Because Pan's orbit lies almost exactly in the ring plane, material falls onto it at the equator and piles up there, building a skirt. Simulations published in 2017 and 2018 show that this kind of accretion can produce ridges similar to those observed on Pan and on Atlas.
Pan's very low density, about 0.4 g/cm³, means its interior must be porous, possibly a loose aggregate of icy particles. Its weak gravity and position deep inside Saturn's tidal zone suggest that material on parts of the ridge is barely bound to the moon.
Pan among the ring moons
Pan belongs to a family of small, low-density moons that live in or right next to the rings: Daphnis in the narrower Keeler Gap, Atlas just beyond the A ring, and Prometheus and Pandora on either side of the F ring. Several of them show equatorial ridges or smooth surfaces, a sign that ring material is still being added to them. Studying these moons helps scientists understand how ring particles and moonlets exchange material, and possibly how such moons formed from the rings in the first place, a question tied to the debate over the age of the rings.
FAQ
Why does Pan look like a ravioli?
Pan has a thin equatorial ridge made of fine material it has accumulated from Saturn's rings. Because it orbits in the ring plane, particles land mainly on its equator, building a rim around a rounder core.
How big is Pan?
Pan measures about 34 × 31 × 21 km, for a mean diameter of about 28 km. It is small enough that its gravity is extremely weak.
Where is Pan located?
Pan orbits inside the Encke Gap in Saturn's outer A ring, about 133,600 km from the center of the planet, completing an orbit in about 13.8 hours.
Who discovered Pan?
Mark Showalter discovered Pan in 1990 by searching Voyager 2 images from 1981, guided by predictions based on the wavy edges of the Encke Gap.
Sources
- NASA Science: Saturn's moons
- NASA: Cassini mission
- Showalter, M. R., "Visual detection of 1981S13, Saturn's eighteenth satellite, and its role in the Encke gap", Nature 351 (1991)
- Buratti, B. J. et al., "Close Cassini flybys of Saturn's ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus", Science 364 (2019)