N2 Ice Research Articles

Sticking of CO to crystalline and amorphous ice surfaces.

We present results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of crystalline ice Ih and to the surface of amorphous ice Ia. The calculations were performed for normal incidence at a surface temperature Ts = 90 K for ice Ia, and at Ts = 90 and 150 K for ice Ih. For both surfaces, the sticking probability can be fitted to a simple exponentially decaying function of the incidence energy, Ei: Ps = 1.0e(-Ei(kJ/mol)/90(kJ/mol)) at Ts = 90 K. The energy transfer from the impinging molecule to the crystalline and the amorphous surface is found to be quite efficient, in agreement with the results of molecular beam experiments on the scattering of the similar molecule, N2, from crystalline and amorphous ice. However, the energy transfer is less efficient for amorphous than for crystalline ice. Our calculations predict that the sticking probability decreases with Ts for CO scattering from crystalline ice, as the energy transfer from the impinging molecule to the warmer surfaces becomes less efficient. At high Ei (up to 193 kJ/mol), no surface penetration occurs in the case of crystalline ice. However, for CO colliding with the amorphous surface, a penetrating trajectory was observed to occur into a large water pore. The molecular dynamics calculations predict that the average potential energy of CO adsorbed to ice Ih is -10.1 +/- 0.2 and -8.4 +/- 0.2 kJ/mol for CO adsorbed to ice Ia. These values are in agreement with previous experimental and theoretical data. The distribution of the potential energy of CO adsorbed to ice Ia was found to be wider (with a standard deviation sigma of 2.4 kJ/mol) than that of CO interacting with ice Ih (sigma = 2.0 kJ/mol). In collisions with ice Ia, the CO molecules scatter at larger angles and over a wider distribution of angles than in collisions with ice Ih.

0.8–2.5 Micron Reflectance Spectroscopy of Pluto

ABSTRACTNear‐infrared (0.8–2.5 μm) spectrophotometry, acquired on 2002 July 18.25, is presented for the Pluto‐Charon system. The sub‐Earth longitude and latitude of Pluto at the time of the observations were 80° and 28 .°5, with a phase angle of 1 .°25. The data span wavelength segments that are typically covered by different instruments and includes the seldom‐observed 1–1.3 μm region. The geometric albedo is compared to previous observations that were close to this longitude, and no significant differences are seen. Wavelength shifts, relative to laboratory values for pure methane ice and methane immersed in N2 ice, are reported for a number of methane features in Pluto’s spectrum.

The Abundances of Solid N2and Gaseous CO2in Interstellar Dense Molecular Clouds

We present 2338-2322 per centimeter (4.277-4.307 micrometer) infrared spectra of a number of N2-containing mixed molecular ices and demonstrate that the strength of the infrared forbidden band due to the N=N stretch near 2328 per centimeter (4.295 micrometer) is extremely sensitive to the composition of the ice. The strength of the 2328 per centimeter N2 fundamental is significantly enhanced relative to that of pure N2 ice when NH3, H2O, or CO2 are present, but is largely unaffected by the presence of CO, CH4 or O2. We use the laboratory data in coil junction with ISO data that probes several lines-of-sight through dense molecular clouds to place limits on the abundance of interstellar solid phase N2 and the composition of the ices. Deriving upper limits is complicated by the presence of overlapping absorptions due to CO2 gas in the clouds and, in some cases, to photospheric CO in the background star. These upper limits are just beginning to be low enough to constrain interstellar grain models and the composition of possible N2-bearing interstellar ices. We outline the search criteria that will need to be met if solid interstellar N2 is to be detected in the future. We also discuss some of the implications of the presence of warm CO2 gas along the lines-of-sight to embedded protostars and demonstrate that its presence may help resolve certain puzzles associated with the previously derived gas/solid CO2 ratios and the relative abundances of polar and nonpolar ices towards these objects. Finally, we briefly comment on the possible implications of these results for the interpretation of N2 detections on outer solar system bodies.

Ethane on Pluto?

The article “Telling Pluto and its partner apart” by Mark Sincell (News of the Week, 30 July, p. 649), apparently adapted from a Web site (1), reports the detection of frozen ethane on Pluto from spectra obtained at the new Subaru telescope on Mauna Kea, Hawaii. We are great admirers of Subaru and are delighted to see it coming into action, but this “Web site publication” suffers from the lack of peer review. Since 1991, we have routinely used the 3.8-meter U.K. Infrared Telescope, also on Mauna Kea, to obtain spectra of Pluto (2). Since 1995, the Cooled Grating Spectrometer at this telescope has been producing spectra that are higher in signal precision than the spectrum shown on the Subaru Web site. For example, CO and N2 ice absorption bands are well defined in our spectra, but in the Subaru spectrum they are barely distinguishable. In addition, the posted spectrum must be shifted by about +0.0125 micrometers to bring the wavelengths of the absorption bands into accord with their correct positions. An illustration of our more recent data is shown in a 1998 review paper (3), and the spectra have been presented at various conferences around the world. In a paper now in press (4), we model the Pluto spectrum with frozen mixtures of nitrogen, methane, carbon monoxide, and water. We have specifically searched for ethane, which is predicted from photochemical calculations (5, 6), by systematically adding it to our best models. We found no discernable contribution of C2H6 at the wavelengths noted. The real spectral features shown in the Subaru spectrum presented by Nakamura and his colleagues are completely modeled with the mixture noted above, and the two features attributed to ethane ice are in fact due to methane ice. We have successfully modeled these same features in the spectrum of Triton as well.

Zonally averaged thermal balance and stability models for nitrogen polar caps on Triton

We have analyzed Voyager four color imaging data of Triton to calculate the bolometric hemispheric as a function of latitude and longitude. Zonal averages of these data have been incorporated into a thermal balance model involving insolation, reradiatation and latent heat of sublimation of N2 ice for the surface. The current average bolometric albedo of Triton's polar is 0.8, implying an effective temperature of 34.2K and a surface pressure of N2 of 1.6 μb for unit emissivity. This pressure is an order of magnitude lower than the surface pressure of 18 μb inferred from Voyager data [Broadfoot et al., 1989; Conrath et al., 1989], a discrepancy that can be reconciled if the emissivity of the N2 on Triton's surface is 0.66. The model predicts that Triton's surface north of 15° N latitude is experiencing deposition of N2 frosts, as are the bright portions of the south polar cap near the equator. This result explains why the south cap covers the entire southern hemisphere of Triton. Initial results from a seasonal model are also described briefly, although they do not agree with the Voyager observations.

Triton: Scattering models and surface/atmosphere constraints

Modeling of Triton's spectrum indicates a bright scattering layer of optical depth τ ≃ 3 overlying an optically deep layer of CH4 with high absorption and little scattering. UV absorption in the spectrum indicates τ ≃ 0.3 of red‐yellow haze, although some color may also arise from complex organics partially visible on the surface. An analysis of this and other (spectro) photometric evidence indicates that Triton most likely has a bright surface, which was partially visible in 1977–1980. Geometric albedo p = 0.62−0.12+0.18, radius r = 1480 ± 180km, and temperature T = 48 ± 6 K. With scattering optical depths of 0.3–3 and ∼ 1–10mb of N2, a Mars‐like atmospheric density and surface visibility pertain. Imaging with the 0.62 µm CH4 filter of the Voyager 2 wide angle camera could show ∼20% contrast between the average surface and clean exposures of CH4 ice (which is not limited to the polar caps). Low far‐infrared atmospheric opacity will in principle allow the detection of thermal gradients in the surface caused by optically transmitting but infrared opaque CH4 and N2 ice.

Grain metamorphism in polar nitrogen ice on Triton

We calculate the rate of nitrogen grain growth on putative N2‐rich polar caps on Triton. For most plausible assumptions of independent variables, mean grain sizes in polar N2 are meter‐scale. Triton's polar caps should constitute the definitive solar‐system test bed for the process of ice grain metamorphism. Interpretation of data already in hand may require long path length through condensed N2, possibly due to grain growth. Upcoming Voyager data may clarify the situation, although possible complications in detecting a glaze of N2 ice exist.