The Cassini spacecraft, which has been orbiting the planet Saturn since 2004, first found Pac-Man in thermal images of Saturn's moon Mimas back in 2010. Now after taking similar images of the moon Tethys, it looks like the spacecraft has found a second moon resembling the ubiquitous video game character.
"Finding a second Pac-Man in the Saturn system tells us that the processes creating these Pac-Men are more widespread than previously thought," said Carly Howett, a planetary scientist with the Southwest Research Institute, who is the lead author of the paper reporting Cassini's finds. "The Saturn system — and even the Jupiter system — could turn out to be a veritable arcade of these characters."
The Pac-Man-like heat pattern seen from these moons is thought to be caused as the moon's leading-side — the side facing in the direction of the moon's motion as it orbits around the planet — interacts with high-energy electrons in Saturn's magnetosphere, hardening the surface ice there. This area of hardened ice is harder to heat, and is therefore roughly 15 degrees C cooler during the day than the rest of the moon, showing up as the darker 'mouth' of Pac-Man on infrared imagery.
"Studies at infrared wavelengths give us a tremendous amount of information about the processes that shape planets and moons," said Mike Flasar, from NASA's Goddard Space Flight Center, who is the principle investigator of Cassini's Composite Infrared Spectrometer (CIRS). "A result like this underscores just how powerful these observations are."
These new images of Tethys also reveal how dominant this interaction with high-energy electrons is, because Tethys maintains this Pac-Man thermal signature even as new snow and ice rain down on it from plumes of snow and ice from Enceladus, the moon orbiting Saturn between Mimas and Tethys.
Another unusual feature of this Pac-Man signature of Tethys (unlike that of Mimas) is that you can see it very subtly in visible light images as well. This was first noticed in imagery sent back by Voyager 1 in 1980, as the hardened ice shows up slightly darker than the rest of the surface.