Laboratory investigation of Pluto’s surface organic matter: a surface photochemical origin?

When the New Horizons spacecraft flew by Pluto, the Ralph instrument revealed the presence of a dark-red refractory organic material spreading mostly along the equator, on a region named Cthulhu. To date, two major hypotheses have been proposed to explain the origin of this organic material: irradiation of the volatile ices constituting Pluto’s surface, or sedimentation of atmospheric photochemical aerosols onto the surface. In order to test the hypothesis of a surface origin of Pluto’s dark organic material, we simulated in the laboratory the ultraviolet photolysis of Pluto surface ice analogues (mixture of molecular nitrogen, methane and carbon monoxide in relevant mixing ratios). We investigated the spectral evolution of Pluto ice analogues in the UV-Visible and mid-infrared wavelength ranges during the photolysis process. After warming-up the photolyzed ice analogues to ambient temperature, we retrieved a yellowish refractory organic material, also called tholins, for which we determined the optical constants. In this presentation, we will discuss the importance of radical chemistry in the formation of the refractory organic material recovered at ambient temperature. We will also present the optical constants determined for the synthesized tholins, and compare them to those of tholins produced from radiolysis of Pluto’s atmospheric gas analogues using a plasma discharge as the energy source. These optical constants can be used in future modeling studies of Pluto’s or other relevant trans-Neptunian objects’ surfaces.