Near IR Spectra of CH₄ in H₂O

Infrared (IR) spectra have demonstrated that solid CH₄ is present on a number of outer Solar System objects, including Triton, Pluto, where it is thought to be frozen into N₂ ice (Quirico et al. 1999; Grundy and Buie 2001), the Kuiper Belt Objects Quaoar (Brown 2003 and personal communication), 90377 Sedna (Barucci et al. 2005), UB313 (Trujillo et al. 2005; Brown, Trujillo and Rabinowitz 2005), and FY9 (Barkume, Brown, and Schaller 2005), and CH₄ is known to be present in number of comets (Gibb et al. 2003).

Since H₂O is nearly ubiquitous in the outer Solar System (Roush 2001) CH₄ on icy planetesimals is likely to come into contact with H₂O ice, potentially changing its spectral properties. Since mathematical addition of spectra of pure materials is not equivalent to the spectra of actual mixtures, fitting CH₄ profiles in spectra of outer Solar System bodies will require lab spectra of solid CH₄ intimately mixed with H₂O at relevant temperatures.

Near-IR spectra of CH₄ as a pure solid and mixed in N₂ have been measured in the lab at various temperatures and concentrations (Quirico and Schmitt 1997, Grundy, Schmitt and Quirico 2002). Absolute absorption intensities (A values) for near-IR absorptions of pure solid CH4 were published recently (Gerakines et al. 2005), and real and imaginary indices of refraction (ns and ks) have been determined for both pure methane (Khare et al. 1989, Pearl et al. 1991) and pure H₂O, albeit for just the hexagonal phase (Grundy and Schmitt 1998). Despite this wealth of beautiful data, there is a lack of near-IR spectra of CH₄ intimately mixed with solid H₂O, as it is likely it will be found on some icy bodies in the outer Solar System.

Since an interaction with H₂O on a molecular level has been shown to cause significant changes in the position and profile of CH₄ absorptions in the mid-IR (e.g., Hudgins et al. 1993), the presence of H₂O could change peaks in the near-IR as well. This would complicate the interpretation of reflection spectra of outer Solar System bodies. In this paper we present near IR spectra of H₂O-CH₄ ice mixtures at various concentrations and temperatures from 15 to 150 K, and document how peaks shift and broaden, both as a result of interactions with H₂O and as a result of changes in temperature. It is to be hoped that this data will be used, in conjunction with observations, to constrain the state of solid CH₄ on the surface of outer Solar System bodies.

Figure 1 (below). Figure 1, The 1.25-19.2 micron (8000-520cm^-1) IR spectrum of an H₂O/CH₄= 20 ice mixture at 15 K. The inset shows a magnified view of absorptions too weak to be seen in the upper trace. The absorptions at ~1.5, 1.89, ~2, ~3, 4.5, ~6 and 13.3 micron (6700, 5300, 5100, 3250, 2200, 1600, and 750 cm^-1) are all caused by solid H₂O. The other sharper features are caused by CH₄.

Figure 2 (below). Figure 2, A comparison of the 2.3-2.4 micron (4350-4150 cm^-1) IR spectra of pure methane and three H₂O-CH₄ mixtures at 15 K. The methane absorptions shift to shorter wavelength (higher frequency) and become broader when CH₄ is diluted in H₂O. In addition the relative peak areas also change.

Figure 3 (below)The 2.3-2.4 micron (4360-4150 cm^-1) IR spectra of an H₂O/CH₄= 20 ice mixture at 15 K (top), during warming up to 150 K (middle), and recooling back to 30 K (bottom). The temperature-dependant shifts in the position, width, and relative intensity of CH₄ absorptions are reversible on recooling (see figures 4-6), despite the fact that the phase changes of the water ice are irreversible. The absolute intensity never entirely recovers, presumably due to sublimation of CH₄.

Figure 4 (below) Peak position vs. temperature for the 2.32 and 2.38 micron (4300 and 4200 cm^-1 (triangles and circles respectively) methane absorptions of an H₂O/CH₄ = 20 ice. From left to right vapor deposited at 15 K, warmed to 150 K, and then recooled to 30 K. The peak positions increase in frequency with increasing temperature, but return to essentially the original values on recooling. The empty symbols represent the positions of the same peaks vapor deposited at 74 K.

Figure 5 (below), Peak widths vs. temperature for the 2.32 and 2.38 micron (4300 and 4200 cm^-1; triangles and circles respectively) methane absorptions of an H₂O/CH₄ = 20 ice. From left to right vapor deposited at 15 K, warmed to 150 K, and then recooled to 30 K. The peak widths increase with increasing temperature, but return to essentially the original values on recooling. The empty symbols represent the widths of the same peaks vapor deposited at 74 K.

Figure 6 (below) Ratios of areas of 2.38/2.32 micron (4200/4300 cm^-1) methane absorptions vs. temperature for an H₂O/CH₄ = 20 ice vapor deposited at 15 K, warmed to 150 K, and then recooled to 30 K. Like the position and width, the ratio of areas seems to be reversible on re-cooling.

Figure 7 (below). The 1.658-1.680 micron (6030-5950 cm^-1) IR spectrum of an H₂O/CH₄ = 4 ice showing changes in the CH₄ 2v₃ absorption with temperature. For comparison this feature was reported to be present in the spectrum of UB313 at 1.670 microns (5988 cm^-1) at the 2005 DPS meeting by Trujillo et al. Abstract # 52.06.

Figure 8 (below). The 1.715-1.732 micron (5830-5775 cm^-1) IR spectrum of an H₂O/CH₄ = 4 ice showing changes in the CH₄ v₂+v₃+v₄ absorption with temperature. For comparison this feature was reported to be present in the spectrum of UB313 at 1.723 microns (5804 cm^-1) at the 2005 DPS meeting by Trujillo et al. Abstract # 52.06.

All but the last two of these figures are from a paper that is now in press at Icarus. Look for them in the journal in late 2005.