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Project Progress

Ammonia or ammonia hydrate has been reported to be present in the surfaces of some outer solar system icy bodies, such as, Saturn’s satellite Enceladus, Uranus’s satellite Miranda, Pluto’s satellite Charon, and Kuiper Belt Object (50000) Quaoar. Solid ammonia has two major infrared bands at near-infrared region, one centered at ~2.2 μm wavelength, the other centered at ~2.0 μm. The identification of ammonia or ammonia hydrate on those icy bodies is mostly based on the 2.2μm band.

We conducted a systematic study of the near-IR and mid-IR spectra of ammonia-water ices at various NH₃/H₂O ratios. The differences between the spectra of amorphous and crystalline ammonia-water ices were also investigated. The 2.0-μm ammonia band shifts from 2.006±0.003 μm (4985±5 cm-1) to 1.993±0.003 μm (5018±5 cm^-1) and the 2.2-μm ammonia band shifts from 2.229±0.003 μm (4486±5 cm^-1) to 2.208±0.003 μm (4528±5 cm-1) when the percentage of ammonia decreases from 100% to 1%. These results are important for comparison with astronomical observations as well as to estimate the concentration of ammonia in the outer solar system ices.

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We traced the formation of hydroxylamine (NH₂OH) in electron irradiated ammonia-water ices at temperatures between 10 K and 50 K. In addition, the synthesis of molecular hydrogen (H₂), molecular nitrogen (N₂), molecular oxygen (O₂), hydrazine (N₂H₄), and hydrogen peroxide (H₂O₂) was also observed. These newly formed species were trapped inside the ices and were released into the gas phase during the warm up phase. Quantitatively spoken, the production rates of the newly formed species at 10 K are higher compared to 50 K. In addition, the experiments provided compelling evidence that crystalline ammonia-water ice samples can be partially converted to amorphous ices during the electron irradiation; similar to the chemical process, the irradiation-induced amorphization of the ices is faster at 10 K than at 50 K.

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