Although Tethys and Janus both orbit Saturn and are both made of more or less the same materials, they are very different worlds.
Although both worlds are probably geologically dead today, Tethys (660 miles or 1062 kilometers across) is large enough to be spherical and to have varied geology like chasms and smooth plains (most likely dating back to a time when it was warmer and could be more geologically active), as well as the expected impact craters. Janus (111 miles or 179 kilometers across), on the other hand, is irregularly shaped and has so far shown little geology other than impact craters.
This view looks toward the sunlit side of the rings from about 1 degree above the ringplane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Nov. 23, 2015.
The view was acquired at a distance of approximately 28,000 miles (44,000 kilometers) from Tethys and at a Sun-Tethys-spacecraft, or phase, angle of 54 degrees. Image scale is 2 miles (3 kilometers) per pixel.
The Cassini Solstice Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.