Comparative KBOlogy: The Dynamic Surfaces of Pluto and Triton

Pluto, the largest Kuiper Belt dwarf planet, and Triton, Neptune's largest satellite and a former Kuiper Belt Object [1], are the largest known objects with rock/ice compositions found at or beyond Neptune's orbit. The similarities between Pluto and Triton provide a backdrop for the differences and a window into physical processes at work on icy surfaces.

Seasonal transitions on Pluto and Triton are moving in opposite directions, with surface composition changes observed on both over 10+ years. Pluto is approaching N. hemisphere summer in 2029 while Triton entered S. hemisphere summer in 2000. Volatile N₂ and CH₄ band depths are decreasing on Pluto [2] and increasing on Triton [3]. This makes sense at first glance, but the reasons for these changes may be different.

Increasing volatiles on Triton could indicate ongoing deposition or regions of past deposition being revealed in the N. hemisphere. Continued spectral observations of Triton over the coming years will provide clues to evaluate ongoing changes: A decrease in H₂O band area would point to deposition, even if volatile band areas remain constant due to volatile transport. It remains to be seen if volatiles will be completely removed from the N. hemisphere of Pluto over the next decade, mimicking the current state of Triton's S. hemisphere [3]. Yearly observations of Pluto are critical in the coming years, as models predict the complete loss of volatiles from the N. hemisphere by 2030 [4]; "matched pairs" of spectra made at comparable sub-observer latitudes and longitudes will make yearly comparisons possible.

The same strategy is not as critical for Triton due to the lack of a Sputnik Planitia-like feature that dominates the spectrum (evidence of a depleted volatile inventory compared to Pluto) and because non-volatile H₂O absorption is constant with longitude [3]. The difference in volatile inventory between Pluto and Triton, a result of different formation histories [1,5], should play a key role in the magnitude of the band depth changes with time. These differences will be discussed in the context of spacecraft and other ground-based observations.

References.

[1] Agnor & Hamilton 2006. Nature 441, 192-194. [2] Grundy et al. 2014. Icarus 235, 220-224. [3] Holler et al. 2016. Icarus 267, 255-266. [4] Bertrand & Forget 2016. Nature 540, 86-89. [5] Canup 2011. AJ 141, 35.