A Multidisciplinary Investigation of the Origins of Pluto’s Dark Surface Materials

The flyby of Pluto by the New Horizons mission unveiled a world with surprisingly diverse surface compositions and colors as well as an extensive atmospheric haze. In the enhanced color images returned by the Multispectral Visible Imaging Camera (MVIC), Pluto’s surface appears covered with a range of brown to yellow- and red-brown hues. The main ices identified on Pluto’s surface (N₂, CH₄, CO, H₂O, CH₃OH) are all colorless at visible wavelengths in their pure form. The observed surface colors therefore indicate the presence of one or more colored components mixed in or superimposed on the ices.

In Pluto’s atmosphere, the CH₄ mixing ratio has been observed to vary with altitude (from 0.3% <350 km to 5% at ~700 km in 2015) and is predicted to vary from 0.01% to 5% over annual orastronomical timescales. The CO atmospheric mixing ratio (0.05% in 2015) is not expected to vary much over time. Haze particles resulting from the photolysis and radiolysis of gaseous N₂, CH₄, and CO in Pluto’s atmosphere are expected to settle and accumulate onto the surface and could thus darken the surface and contaminate the ices.

Here we present the results of an interdisciplinary investigation combining experimental, modeling and observation research efforts to assess the contribution of Pluto’s atmospheric haze particles to the dark materials present on various regions across Pluto’s surface, in order to reach a better understanding of the processes that result in the surprising diversity of colors and spectral features observed by New Horizons. We focused our study on three main regions of interest that are covered with dark materials of very different colors and spectral features: Lowell Regio (yellow), Sputnik Planitia (pale orange), and Cthulhu Macula (red).