Tethys's most impressive feature is Ithaca Chasma, a giant crack several kilometres deep that extends along three-quarters of the moon's circumference and accounts for 5–10 percent of its surface. Because the ridges around the feature are heavily cratered, scientists have theorized that the chasm was produced early in the moon's geologic history, when the water that composes its interior froze and expanded. A second notable feature is the crater Odysseus, which measures 400 km (250 miles) across and has a large central peak. The density of impact craters on Tethys is high, suggesting that the surface is ancient. Nevertheless, the surface is highly reflective, especially on Tethys's leading face, which is not typical of geologically old surfaces. Planetary scientists suspect that this distribution of surface brightness is affected by the deposition of micrometre-sized ice particles from Saturn's E ring, in which Tethys is well-embedded. Cited as evidence is the observation that many of the craters on Tethys have bright floors, whereas the craters on Saturn's moon Hyperion, which orbits relatively far from Tethys and the E ring, tend to have dark floors.