Scattered Disc - Composition

Composition

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane. Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane.

Astronomers originally supposed that the entire trans-Neptunian population would show a similar red surface colour, as they were believed to have originated in the same region and subjected to the same physical processes. Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into complex organic molecules by energy from the Sun. This would absorb blue light, creating a reddish hue. Most classical objects display this colour, but scattered objects do not; instead, they present a white or greyish appearance.

One explanation is the exposure of whiter subsurface layers by impacts; another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets. Mike Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from the Sun, its atmosphere of methane is frozen over its entire surface, creating an inches-thick layer of bright white ice. Pluto, conversely, being closer to the Sun, would be warm enough that methane would freeze only onto cooler, high-albedo regions, leaving low-albedo tholin-covered regions bare of ice.