Just when you thought a moon was a more-or-less spherical body, we have the Saturn moon Pan. What in the
world universe would create this?
These two images from NASA’s Cassini spacecraft show how the spacecraft’s perspective changed as it passed within 15,300 miles (24,600 kilometers) of Saturn’s moon Pan on March 7, 2017. This was Cassini’s closest-ever encounter with Pan, improving the level of detail seen on the little moon by a factor of eight over previous observations.
The views show the northern and southern hemispheres of Pan, at left and right, respectively. Both views look toward Pan’s trailing side, which is the side opposite the moon’s direction of motion as it orbits Saturn.
Cassini imaging scientists think that Pan formed within Saturn’s rings, with ring material accreting onto it and forming the rounded shape of its central mass, when the outer part of the ring system was quite young and the ring system was vertically thicker. Thus, Pan probably has a core of icy material that is denser than the softer mantle around it.
The distinctive, thin ridge around Pan’s equator is thought to have come after the moon formed and had cleared the gap in the rings in which it resides today. At that point the ring was as thin as it is today, yet there was still ring material accreting onto Pan. However, at the tail end of the process, that material was raining down on the moon solely in (or close to) its equatorial region. Thus, the infalling material formed a tall, narrow ridge of material. On a larger, more massive body, this ridge would not be so tall (relative to the body) because gravity would cause it to flatten out. But Pan’s gravity is so feeble that the ring material simply settles onto Pan and builds up. Other dynamical forces keep the ridge from growing indefinitely.
These views are also presented in stereo (3-D) in PIA21435. The images are presented here at their original size.
The views were acquired by the Cassini narrow-angle camera at distances of 15,275 miles or 24,583 kilometers (left view) and 23,199 miles or 37,335 kilometers (right view). Image scale is 482 feet or 147 meters per pixel (left view) and about 735 feet or 224 meters per pixel (right view).
Image: NASA/JPL-Caltech/Space Science Institute