Chemical composition of Pluto aerosol analogues

Abstract

Photochemical aerosols were observed in Pluto atmosphere during the New Horizons flyby on July 14th, 2015, as several thin haze layers extending at >350 km of altitude. This flyby has raised numerous questions on the aerosols formation processes and their impact on Pluto radiative transfer and climate. In order to gain a better understanding, we synthesized Pluto aerosol analogues in a room-temperature dusty plasma experiment and inferred their chemical composition from infrared spectroscopy, elemental composition analysis and very high-resolution mass spectrometry (ESI+/Orbitrap device). Three types of samples were synthesized at 0.9 ± 0.1 mbar, called P400, P600 and PCO-free. The samples P400 and P600 were produced from gas mixtures mimicking Pluto atmosphere at around 400 and 600 km of altitude, respectively, in order to determine if CH4 mixing ratio has an influence on the chemical composition of the aerosols. The sample PCO-free was produced from a gas mixture similar to the one forming the sample P400, but without carbon monoxide, in order to identify the impact of CO.Our study shows that the molecules constituting samples P400 and P600 are very rich in nitrogen atoms (up to 45% in mass of N elements) and, compared to the molecules constituting the PCO-free sample, a significant incorporation of oxygen atoms was detected. Moreover, our results on the variation of CH4 mixing ratio demonstrate that different ratios lead to different reactivity between N2, CH4 and CO. In particular, more nitrogen and oxygen atoms are detected in the bulk composition of the analogues P400. Due to the presence of nitrogenated and oxygenated molecules in the analogues of Pluto aerosols, we suggest that these aerosols will have an impact on Pluto radiative transfer, and thus on climate, that will differ from predictions based on the optical constants of Titan aerosol analogues.