| dc.description.abstract | To understand the evolutionary process from the early solar system and find its relevant evidences, astrophysicists have been dedicated to investigating Kuiper Belt Objects (KBOs) of Trans-Neptunian Objects (TNOs) in recent decades. Through astronomical observation, some astrophysicists discovered that the surfaces of KBOs have different colors and properties. Temperature differences might be caused by its distance from the sun; thus, further analysis of the evolution between KBOs and solar system is required. In this research, we choose Pluto to investigate the relevant photochemical mechanism on surfaces of KBOs. Unlike previous studies in using the high energy particles as the energy sources, we use vacuum ultra-violet (VUV) and extreme ultra-violet (EUV) as two different light sources to discuss the effect of photon energies on photolysis of Pluto.
We used (1) a microwave discharged hydrogen-flow lamp (MDHL) as the VUV light source, and (2) the High Flux beamline at National Synchrotron Radiation Research Center (NSRRC) as the EUV light source. We chose N2:CH4:CO (10:1:1) ice mixtures as the simulated surface material of Pluto, and used different energy distributions of light sources for studying the photolysis on it. Fourier Transform Infrared spectroscopy (FTIR) and Quadrupole Mass Spectrometry (QMS) were employed to analyze the formation mechanism and production yield of the products simultaneously. The results showed that the C-O bearing photoproducts as CO2, HCO, H2CO and CH3CHO and N-C bearing photoproducts such as CH2N2 and HCN could be detected under both VUV and EUV irradiation. However, photoproducts of N3, HNCO and OCN- were only observed in EUV irradiation experiment.
Based on this study, the column density of C-O bearing photoproducts (CO2, H2CO and CH3CHO) are only 0.1-0.6 % compared to that of CO molecules. This might suggest why these absorption spectral features in Pluto are hardly to be found. Especially under the light condition containing about 80% Ly-α photon, the production quantities of CO2 and HCO are relatively less. However, the production quantities of H2CO and CH3CHO are not much different under different irradiation light conditions. In addition, we could distinguish clearly the N-C bearing photoproducts between CH2N2 and HCN. Under the VUV irradiation, the production quantities of CH2N2 are directly correlated to the consumption of CH4 while the production quantities of HCN are produced easily by the interaction of CH2N2 with CH2 radicals rather through than the CH2N2 decomposition. Under the EUV irradiation, HCN could be produced by the combination of CH and N. The N atoms tend to produce the molecules with CN bearing, such as HNCO with OCN- in EUV irradiation; therefore, the production quantities of HCN and CH2N2 are much less in EUV irradiation.
In this study, the process of OCN- thermal desorption is in agreement with those reported in Moore and Hudson (2001) and Po-yu Huang (2010) and it supports that OCN- is produced in photolysis. As shown in IR spectra of warm-up process, some complex molecules can be in the substrate identified after either VUV or EUV irradiations. This study could be important for coming astronomical exploration in the future, such as the New Horizon spacecraft in 2015 or provide worthwhile reference for the formation study of complex molecules in laboratory experiments. | en_US |