Editing Trans-Neptunian object (section)

==={{Anchor|Spectra}} Spectral type ===
{{See also|Asteroid spectral types}}

Among TNOs, as among [[Centaur (minor planet)|centaurs]], there is a wide range of colors from blue-grey (neutral) to very red, but unlike the centaurs, bimodally grouped into grey and red centaurs, the distribution for TNOs appears to be uniform.<ref name="Peixinho 2003"/> The wide range of spectra differ in reflectivity in visible red and near infrared. Neutral objects present a flat spectrum, reflecting as much red and infrared as visible spectrum.<ref name="Barucci ACM2005">A. Barucci ''Trans Neptunian Objects’ surface properties'', [[IAU]] Symposium No. 229, Asteroids, Comets, Meteors, Aug 2005, Rio de Janeiro</ref> Very red objects present a steep slope, reflecting much more in red and infrared.
A recent attempt at classification (common with centaurs) uses the total of four classes from '''BB''' (blue, or neutral color, average B−V {{=}} 0.70, V−R {{=}} 0.39, e.g. [[90482 Orcus|Orcus]]) to '''RR''' (very red, B−V {{=}} 1.08, V−R {{=}} 0.71, e.g. [[90377 Sedna|Sedna]]) with '''BR''' and '''IR''' as intermediate classes. BR (intermediate blue-red) and IR (moderately red) differ mostly in the infrared [[Infrared astronomy#Astronomers' infrared spectrum|bands I, J and H]].

Typical models of the surface include water ice, [[amorphous carbon]], [[silicate]]s and organic macromolecules, named [[tholin]]s, created by intense radiation. Four major tholins are used to fit the reddening slope:
* Titan tholin, believed to be produced from a mixture of 90% N<sub>2</sub> (nitrogen) and 10% {{CH4}} (methane)
* Triton tholin, as above but with very low (0.1%) methane content
* (ethane) Ice tholin I, believed to be produced from a mixture of 86% {{H2O}} and 14% C<sub>2</sub>H<sub>6</sub> ([[ethane]])
* (methanol) Ice tholin II, 80% H<sub>2</sub>O, 16% CH<sub>3</sub>OH ([[methanol]]) and 3% {{CO2}}
As an illustration of the two extreme classes BB and RR, the following compositions have been suggested
* for Sedna (RR very red): 24% Triton tholin, 7% carbon, 10% N<sub>2</sub>, 26% methanol, and 33% methane
* for Orcus (BB, grey/blue): 85% amorphous carbon, +4% Titan tholin, and 11% H<sub>2</sub>O ice