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Organic Molecules Exist on Pluto’s Surface
New Evidences Found Using Synchrotron Light Source
Fig. 1 Ultraviolet (UV) absorption spectra of electron-bombarded CH4/N2 matrix samples during deposition at (A) 10 K, (B) 20 K, (C) 33 K, (D) 44 K. (E) The inverted HST-COS Pluto spectrum (Stern et al. 2012).

An astrochemistry team of the NSRRC led by Dr. Yu-Jong Wu, found a new evidence of the existence of organic matter on Pluto’s surface. They presents a study of electron irradiated CH4:N2 ices. IR spectra show formation of N3 and CH3 radicals at low temperatures while only nitrile species were observed at higher deposition temperatures. This was attributed to radical recombination enabled by diffusion at the higher temperatures. UV absorption spectra of electron irradiated samples at 44K show a feature that compares well with the Pluto spectrum observed by Stern et al 2012 using Hubble telescope. This along with the IR based identifications implies that 2-cyano ketenimine exists on Pluto's surface. This paper entitled “Infrared and Ultravilet Spectra of Methane Diluted in Solid Nitrogen and Irradiated with Electrons during Deposition at Various Temperatures” has been published in The Astrophysical Journal Supplement Series in June, 2016.

Dr. S. A. Stern of the Southwest Research Institute USA, reported the ultraviolet reflectance spectra of Pluto using the Hubble Space Telescope in 2012. An absorption feature between 210 and 240 nm with an absorption maximum near 222 nm was found in the 95º longitude of Pluto. Because most nitriles and/or large hydrocarbons have UV absorptions in this spectral region, they suggested that complex molecules may exist on the surface of Pluto. Wu, interested by the Hubble finding, designed a laboratory that simulated the space environment of the dwarf planet and bombarded gaseous molecules — mostly nitrogen with a small amount of methane, which is similar to Pluto’s atmospheric and icy surface composition — with a high-energy electron beam, which was used to simulate cosmic rays.

However, the team failed to recombine those molecules to form a more complex organic molecule in the first three years of experiments. Until the New Horizons flyby to Pluto, data collected during the flyby inspired Wu to redesign his experiment. They changed to apply electron bombardment of the gas sample before solidification on the cold target. The temperature on Pluto’s surface ranges from 33 K to 55 K, which means that laboratory simulations of chemical reactions at 10 K might be improper. As demonstrated by the present work, the products that are form from of matrix samples deposited at various temperatures are clearly different. Radical species, such as CH3 and N3, can be preserved completely in solid N2 at lower temperatures, though they disappear to form more stable species at higher temperatures. This laboratory observation might also suggest that the N2 ice on Pluto’s surface is not rigid enough to preserve reactive species, but that reactive species diffuse and combine to form large, complex, and stable species. Therefore, our earlier simulated UV spectra of ices did not reproduce an absorption band similar to that observed on Pluto’s surface.

Figure 1(E) depicts a UV reflectance spectrum at the 95º longitude of Pluto recorded by HST (Stern et al. 2012); this spectrum is inverted for ease of comparison with laboratory UV absorption spectra. As compared with Figs. 1(A)–(C), our results suggest that the absorption of CH3 and N3 might have a small contribution to the band shoulder of Pluto’s band near 222 nm. Although laboratory results indicate that the amounts of CH3 and N3 decrease with increasing temperature, the environment on Pluto exists in a dynamical equilibrium between its atmosphere and surface. Therefore, the amounts of CH3 and N3 may reach a steady state. Compared with Fig. 1(D), the HST feature is almost identical to our UV absorption spectrum recorded at 44 K in terms of band center, shape, and width. This result therefore further confirms that complex nitrile species exist on Pluto’s surface.