Abstract
Pluto’s fly-by by the New Horizons spacecraft in July 2015 has revealed a dark reddish equatorial region, named Cthulhu, covered by a dark, non-icy material whose origin and composition have yet to be determined. It has been suggested that this material could form from the sedimentation of photochemical aerosols, originating from dissociation and ionisation processes in Pluto’s high atmosphere (similarly to aerosols forming Titan’s haze). This hypothesis is here further investigated by comparing New Horizons spectra collected both in the visible and the near-infrared to laboratory reflectance measurements of analogues of Pluto’s aerosols (Pluto tholins). These aerosols were synthesised in conditions mimicking Pluto’s atmosphere, and their optical and reflectance properties were determined, before being used in Hapke models. In particular, the single scattering albedo and phase function of Pluto tholins were retrieved through Hapke model inversion, performed from laboratory reflectance spectra collected under various geometries. From reconstructed reflectance spectra and direct comparison with New Horizons data, some of these tholins are shown to reproduce the photometric level (i.e. reflectance continuum) reasonably well in the near-infrared. Nevertheless, a misfit of the red visible slope still remains and tholins absorption bands present in the modelled spectra are absent in those collected by the New Horizons instruments. Several hypotheses are considered to explain the absence of these absorption features in LEISA data, namely high porosity effects or GCR irradiation. The formation of highly porous structures, which is currently our preferred scenario, could be promoted by either sublimation of ices initially mixed with the aerosols, or gentle deposition under Pluto’s weak gravity.
Highlights
Pluto is the largest of the Trans-Neptunian objects (TNOs)
The size and shape of Pluto tholins were first characterised from Scanning Electron Microscopy (SEM) observations
The SEM analysis was performed on the two types of Pluto tholins (5%CH4 and 1%CH4, respectively), with the JEOL JSM-840 A SEM at University Paris VI and the ZEISS Ultra55 FEG-SEM at CMTC INP Grenoble operating at 10 kV
Summary
Pluto is the largest of the Trans-Neptunian objects (TNOs). Before the flyby performed by the New Horizons’ spacecraft in July, 2015, knowledge of Pluto’s surface composition was limited to groundbased and Hubble Space Telescope (HST), disc-integrated observations. Before the New Horizons mission, CH4 (Cruikshank et al, 1976), N2 and CO ices (Owen et al, 1993) were identified from ground-based measurements, while the presence of water ice was expected but not firmly confirmed (Grundy and Buie, 2002). The spectrophotometric data collected during Pluto’s flyby confirmed the presence of CH4, N2, CO and H2O-ice on the surface (Grundy et al, 2016; Schmitt et al, 2017; Protopapa et al, 2017), but they showed possible evidence for methanol CH3OH ice (Cook et al, 2019)
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