Two types of surface in the outer solar system

Kuiper Belt Objects display a tremendous variety of surface colours, from slightly bluer than the Sun to extremely red. This surface variety is unparalleled in the solar system, and probably reflects a range of initial compositions because these objects have been exposed to the same environment for the last 4 Gyr or so. The different initial compositions probably also imply formation at different distances from the Sun. If KBOs had all formed at the same distance from the Sun and hence at similar temperatures, we should expect them to have the same composition.

When we combine the colour information with albedo data measured using the Herschel Space Observatory, we find that KBOs form two groups, one with redder surfaces (steeper spectral slopes) and brighter surfaces (higher albedo), and another with more neutral and darker surfaces.

Two surfaces in the Kuiper Belt
Distribution of Kuiper Belt Objects in a colour-albedo diagram. Two groups are apparent – see next Figure.

What is most interesting is that KBOs in dynamical classes believed to have formed beyond Neptune (cold Classical KBOs, detached KBOs, KBO in outer resonances) all have surfaces in the bright-red group. Dynamical classes including objects that were scattered into the Kuiper belt region (hot Classicals, Scattered Disc, etc) appear in both surface types.

Two types of surface in the outer solar system.
Same as the previous Figure, but highlighting the two main surface types: Bright Red and Dark Neutral surfaces.

The current orbits of the objects with bright-red and dark-neutral surfaces are not clearly distinct, so the different surfaces are not caused by ongoing effects. The two surfaces must originate early and so they are an indication that the composition of objects formed beyond Neptune may have been different.

Orbits of Bright Red and Dark Neutral KBOs.
The orbits of Bright Red and Dark Neutral KBOs are not clearly distinct.

See also these pages and papers:

This work was partly funded by the Royal Society through a Newton Fellowship.