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Project Progress

Ammonia has been found in the atmospheres of Jupiter, Saturn, Uranus, and Neptune. In comets and interstellar ices, ammonia is present at around 1% abundance relative to water ice. Ammonia is a potentially important chemical component in the solar system both as a repository of nitrogen in primitive objects such as the nuclei of comets and as an agent by which ice convection can be enhanced in the deep interiors of ice-rich bodies. The spectra obtained in our experiments might be useful for comparison with astronomical observations to estimate the concentration of ammonia in outer solar system ices.

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 central wavelength is a function of the ammonia/water ratio. It shifts from 2.006 ± 0.003 μm (4985 ± 5 cm−1) to 1.993 ± 0.003 μm (5018 ± 5 cm−1) as the percentage of ammonia decreases from 100% to 1%. The 2.2 μm ammonia band center shifts from 2.229 ± 0.003 μm (4486 ± 5 cm−1) to 2.208 ± 0.003 μm (4528 ± 5 cm−1) over the same range. Temperature-dependent shifts of those bands are below the uncertainty of the measurement, and therefore are not detectable. These results are important for comparison with astronomical observations as well as for estimating the concentration of ammonia in outer solar system ices.