Origin of the carbon-dioxide shoulder bimodality

Identify the physical origin of the bimodal behavior in the 2.7 μm carbon-dioxide shoulder absorption among organic-rich trans-Neptunian objects.

Background

The linear model reproduces the 4.2 μm carbon-dioxide feature reasonably well but does not account for the broader diversity of the 2.7 μm shoulder feature. The observed shoulder displays bimodality with optical color rather than the nearly linear trend predicted by the model.

The paper leaves the cause of this bimodality unresolved and speculates that object size may be an unrecognized contributing factor. Determining its origin could clarify the compositional and evolutionary relationship between the organic-rich subclasses.

References

It remains to be seen what that source of that bimodality is, though we speculate that size may play an as yet unrecognized factor in the organic-rich objects.

— Linear Continuum Modelling to Explain The Majority of Bulk Features of Kuiper Belt Object Spectra  (2608.23926 - Fraser et al., 25 Aug 2026) in Section 4, Discussion