Although Tethys and Janus both Orbit Saturn and both made of similar materials, they are very Different Worlds

This view looks toward the sunlit side of the rings from about 1 degree above the ring plane. 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 1.8 miles (3 kilometers) per pixel. Image Credit: NASA/JPL-Caltech/Space Science

Their contrasts are related, in large part, to their sizes. Tethys (660 miles across) is large enough to be spherical and to have varied geology, like chasms and smooth plains, along with some puzzling arc-shaped features (see PIA19637 http://photojournal.jpl.nasa.gov/catalog/PIA19637) . Much smaller Janus (111 miles across) is irregularly shaped and has (so far) shown few signs of geologic activity apart from impact craters.

This view looks toward the sunlit side of the rings from about 1 degree above the ring plane. 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 1.8 miles (3 kilometers) per pixel. http://www.nasa.gov/image-feature/jpl/pia18356/different-worlds