Testing tholins as analogs of the dark reddish material covering the Cthulhu region

Abstract : Pluto's fly-by by the New Horizons spacecraft in July 2015 has revealed a dark reddish equatorial region (informally named Cthulhu). The non-icy material constituting the terrains of this region may have been formed from the sedimentation of aerosols [1,2]. Here, we investigate this hypothesis through the interpretation of the data collected by the visible imager (MVIC) and the near-infrared spectrometer (LEISA). Analogs (tholins) of Pluto's aerosols were synthesized in conditions that mimic Pluto's high atmosphere, and their optical properties were determined and used in Hapke models. We show that some of these tholins fit the reflectance level fairly well in the near-infrared, but don't match the shape of the red visible slope. In addition, several tholin bands are absent in LEISA observations, which might be due to a highly porous crust (formed from ice/tholin sublimation or microgravity), or to GCR irradiation. The first is our preferred interpretation.