Seyugi III is the smallest planet in the Seyugi system, and is a cratered rocky planet with three-quarters of its surface covered by ice.
Seyugi III’s low density indicates that it is composed of a rocky core taking up less than one-third of the planet's mass, with the rest composed of water-ice. The temperature on Seyugi III is -174 degrees in direct sunlight and between -200 and -220 degrees in the shade. The planet is heavily cratered with bright wispy markings. Its surface can be divided into two geologically different areas based on crater density. The first area contains craters which are larger than 40 kilometers in diameter, and the second area, in parts of the polar and equatorial regions, has craters under 40 kilometers in diameter. This suggests that a major resurfacing event occurred some time during its formation. On the trailing hemisphere, there is a network of bright swaths on a dark background and few visible craters. It is thought that these bright swaths may be material ejected from ice volcanoes early in Seyugi III's history when it was still liquid inside.
The western hemisphere of the planet is dominated by a huge impact crater called Ophesus, whose 500 km diameter is nearly two-thirds of that of Seyugi III itself. The crater is now quite flat (or more precisely, it conforms to the planet's spherical shape), like the craters on Peledor, without the high ring mountains and central peaks commonly seen on other cratered worlds. This is most likely due to the slumping of Seyugi III's weak icy crust over geologic time.
The second major feature seen on Seyugi III is a huge valley called Jeshu Chasma, 100 km wide and 3 to 5 km deep. It runs 1500 km long, and is thought to have been formed as Seyugi III's internal liquid water solidified, causing the planet to expand while cracking the surface to accommodate the extra volume within. Earlier craters from before Seyugi III solidified were probably all erased by geological activity before then. There is another theory about the formation of Jeshu Chasma: when the impact that caused the great crater Odphesus occured, the shockwave traveled through Seyugi III and fractured the icy, brittle surface on the other side.