NASA Astrobiology Institute (NAI)

1. Chemistry of the NH3/H2O system

   Project Investigators: David Jewitt, Ralf Kaiser, Weijun Zheng

   Summary

   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. We conducted a systematic study of the near-IR and mid-IR spectra of ammonia-water ices at various NH₃/H₂O ratios. These results are important for estimate the concentration of ammonia in the outer solar system ices.

   Astrobiology Roadmap Objectives:

   • Objective 2.2: Outer Solar System exploration

   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 cm¹) and the 2.2μm 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.

   
   

   Shift of the ammonia 2.0-μm and 2.3-μm band due to the addition of water into the ice mixture. (a) 2.0-μm band, (b) 2.2-μm band.

   
   

   Near-IR spectra of water-ammonia ice mixtures of different ratios. The spectra were taken at 10 K. The spectra with 10%, 5%, 2.5% and 1% ammonia had been annealed to 130 K. Those with 50%, 90%, 95% and 99% ammonia had been annealed to 84 K.

   
   

   Mid-IR spectra of water-ammonia ice mixtures of different ratios. The spectra were taken at 10 K. The spectra with 10%, 5%, 2.5% and 1% ammonia had been annealed to 130 K. Those with 50%, 90%, 95% and 99% ammonia had been annealed to 84 K.

   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.

   
   

   Mid-IR spectra of ammonia-water ice (10% ammonia) before and after irradiation.

   
   

   Absorption features of the new products after ammonia and water sublimed.

   
   

   The temporal evolution of the ion currents during the warming up of cubic crystalline ammonia. (a) no irradiation, (b) irradiated at 10K, (c) irradiated at 50K.

Publications

Zheng, W., Jewitt, D. & Kaiser, R.I.  (2007).  Infrared Spectra of Ammonia-Water Ices.  Astrophysical Journal Supplement Series:submitted.

Zheng, W., Jewitt, D. & Kaiser, R.I.  (2007).  Formation of Hydroxylamine in Electron Irradiated Ammonia-Water Ices.  Astrophysical Journal:submitted.

Zheng, W., Jewitt, D., Osamura, Y. & Kaiser, R.I.  (2008).  Formation of Nitrogen and hydrogen-Bearing Molecules in Solid Ammonia and Implications to Solar System and Interstellar Ices.  Astrophysical Journal, 674:1242.