Are Makemake and Eris Sputnik Planets?

Makemake and Eris have high albedos (Sicardy et al. 2011; Ortiz et al. 2012) and show strong spectral absorption by CH₄ ice (Licandro et al. 2006; Brown et al. 2007; Dumas et al. 2007). Energetic space radiation breaks C-H bonds in CH₄ producing fragments that recombine into dark, red macromolecular materials (tholins, e.g., Johnson et al. 1987; Thompson et al. 1987; Strazzulla et al. 1991). This fact, coupled with Pluto's strong CH₄ ice absorption bands and high albedo led Stern (1988) to pose the question "why is Pluto bright?". New Horizons has confirmed that Pluto refreshes its surface via seasonal volatile transport (e.g., Stern et al. 2015). However, one part of Pluto refreshes itself in a different way, too. This is the informally named Sputnik Planitia, a vast plain of volatile ice partly filling a probable impact basin. The ice is thick enough to act as a barrier to internal radiogenic heat flow, which drives convective overturning on 10⁵ to 10⁶ year timescales (e.g., McKinnon et al. 2016; Trowbridge et al 2016). Vigorous convection in Sputnik mixes radiolytic products from the surface down into the bulk of the ice, diluting it, and thus maintaining the high albedo of the surface.We propose that the surfaces of Eris and Makemake are similarly refreshed by convection in deep volatile ice deposits, perhaps covering the majority of their surfaces, unlike Pluto's Sputnik, which only covers a small fraction. The local fluxes of energetic radiation dictate production rates for tholin. Assuming steady-state production over the age of the solar system and mixing into the volatile ice, the colors and albedos of the bodies can be used to estimate the thickness of the volatile ice into which the tholin has been diluted through convective mixing. Likewise, for plausible radiogenic internal heat production, lower limits can be set on the thickness of the ice, to support convective mixing. We don't know the rheological properties of mixed N₂+CH₄ ice, let alone what happens when plausible additional contaminants, such as CO, Ar, C₂H₂, C₂H₄, C₂H₆, etc. are added, but bounding cases for N₂-dominated and CH₄-dominated ice compositions can be calculated.