A similar feature was spotted in 2010 on Mimas, another Saturnian moon.
A report in Icarus suggests the effect is due to high-energy electrons bombarding the sides of the moons that face their direction of orbital travel.
That compacts the surfaces to a hard, icy texture that does not heat or cool as rapidly as the unaffected surface.
Thermal images of both moons were obtained by the Cassini-Huygens mission, launched in 1997 to study the Saturn system in detail.
The temperatures seen by the spacecraft are distinctly chilly – the warmest parts of Tethys were at – 183C, but inside the “mouth” of the Pac-Man shape it was 15C cooler still.
At the time of the finding of the first Pac-Man shape on Mimas, scientists were unsure what might be the cause, theorising that differing surface textures probably played a role.
The existence of another such shape nearby has cemented the idea that fast-moving electrons are responsible.
“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, of the Southwest Research Institute in Texas and lead author of the study.
“The Saturn system – and even the Jupiter system – could turn out to be a veritable arcade of these characters,” she said.