Chemical processes in Triton's atmosphere and surface

Abstract

Liquid solutions of N 2 containing up to one-third CH 4 can exist on Triton's surface in regions T > 62.5°K. More generally, subsurface oceans of N 2 solution are expected to be stable beneath overlying, thermally insulating, less dense layers of the abundant light hydrocarbon products of radiochemical synthesis: C 2H 6, C 3H 8, and C 4H 10. Cosmic rays are the main source of energy, capable of producing synthesis of organic compounds from N 2CH 4 solutions on the surface. For baseline Triton models with R = 2500 km, ϱ = 2.1 g cm −3, and T s = 65 or 55°K, respectively, 4 × 10 −3 or 7 × 10 −3 erg cm −2 sec −1 (49 or 87% of the total incident flux) is deposited within a few meters below the surface. Using yields from laboratory experiments, we estimate the quantities of products produced: over 4.5 billion years, the cosmic ray flux alone produces 2 to 4 m of organic product, about half of which is C 2H 6. For ocean depths <250 m, C 2H 6 will reach its saturation limit and form a surface “slick.” For ocean depths <10 km, all of the other products also oversaturate and exsolve, adding to the surface slick and/or to a denser bottom sediment. Products produced from solid N 2CH 4 mixtures will accumulate as evaporite deposits because of the rapid volatile transport (of N 2 and CH 4) over Triton's surface. The complex, reddish organic solid found in laboratory simulations is probably the source of Triton's reddish color. Estimated yields over 4.5 billion years (for 7 × 10 −3 erg cm −2 sec −1) are 190 (C 2H 6), 58 (NH 3), 17 (HCN), 3.5 (HN 3), 2.5 (C 4H 10), 0.35 (CH 3CN), and 0.14 (C 2H 5N 3) g cm −2. More basic laboratory work on the low-temperature, low-pressure solvent properties and phase equilibria of N 2-hydrocarbon systems is clearly needed.