Near-infrared laboratory spectra of H2O trapped in N2, CH4, and CO: hints for trans-Neptunian objects' observations
Dipartimento di Fisica e Astronomia, Universit di Catania,
via Santa Sofia 64, 95123 Catania, Italy e-mail: firstname.lastname@example.org
2 INAF – Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy e-mail: email@example.com
Accepted: 16 November 2009
Context. Recent mid-infrared spectroscopic observations of Pluto and Triton suggest a wide distribution of H2O ice into surface regions containing more volatile species such as N2, CH4, and CO. This disagrees with the common idea that because of their typical surface temperature, water should not be involved in volatile transport processes, standing easily segregated from the more volatile species.
Aims. We analyse infrared H2O band profiles when water is diluted in solid matrices dominated by methane, carbon monoxide, and/or molecular nitrogen. We also show the results of temperature-related diffusion processes of solid N2 into H2O ice deposited at different temperatures. Finally, we analyse ion irradiation effects for some of the mixtures considered.
Methods. Solid samples were analysed by infrared (1.0–16.0 μm) transmission and reflectance spectroscopy at different temperatures (15–150 K), before and after ion irradiation with 200 keV protons.
Results. When water is highly diluted in solid matrices, the profile of its near-infrared bands is strongly modified. Two narrow bands appear around 1.89 μm and 1.39 μm instead of the well known pure water ice bands at 2 μm and 1.5 μm, respectively. Furthermore, the peak position and width of the 1.89 and 1.39 μm bands depend on the initial mixture water is embedded in. The intensity of these bands decreases after ion irradiation.
Conclusions. Since the mixtures considered closely resemble the surface composition of trans-Neptunian objects, experiments here discussed show that the bands around 1.89 μm and 1.39 μm are appropriate to investigating the presence of diluted water regions on their surface. In fact, the irradiation dose required for a significant decrease in their intensity would be accumulated on a timescale larger than the timescale for resurfacing processes on the surfaces of trans-Neptunian objects. Results shown here will contribute in a strong way to the interpretation of New Horizons near-infrared observations.
Key words: Kuiper belt: general / methods: laboratory / techniques: spectroscopic / infrared: planetary systems / line: profiles / molecular processes
© ESO, 2010