Condensation of Nitrogen: Implications for Pluto and Triton

Abstract

We have observed in the laboratory that nitrogen condenses initially as a transparent layer if the deposition rate is less than about 3 μm/hr. To correlate our laboratory results with the Voyager observations of Triton and ground-based observations of Pluto, we have calculated the nitrogen deposition rates on these bodies. The maximum deposition rate for Pluto (with 40K icy surface temperature) is calculated to be about 1.65 μm/hr and for Triton is ≈1.38 μm/hr at 38 K. This implies that nitrogen on these bodies may preferentially condense as a transparent layer. Voyager 2 observations in August 1989 revealed that Triton's northern hemisphere had an overall lower albedo than the southern hemisphere. This was surprising since at that time deposition of fresh N2frost should have taken place over most of Triton's northern hemisphere (subsolar point was 45°S). In contrast, the subliming southern cap was seen as a bright feature. Our new approach to this problem is based on the combination of the energy balance calculations and the laboratory results. We explain the darker northern hemisphere by the condensation of an initially transparent layer on a dark substrate, which might be dark organics produced by UV photolysis of CH4ice. The bright southern cap can be a result of N2shattering due to the earlier passage of the cubic–hexagonal phase transition fronts. If there is a similar puzzling north/south albedo asymmetry on Pluto, as indicated by two independent sets of observations (M. W. Buie, D. J. Tholen, and K. Horne, 1992,Icarus97, 211–227; E. F. Young and R. P. Binzel, 1993,Icarus102, 134–149), our results offer an explanation for it. We also infer an original grain size of N2ice on Triton and, possibly, on Pluto. The role of impurities is discussed.