On the origin of Titan's atmosphere

The present atmosphere of Titan exhibits evidence of extensive evolution, i n the form of rapid photochemical destruction of methane and a large fracti onation of the nitrogen and oxygen isotopes. Attempts to recover the initia l inventory of volatiles lead toward a model in which nitrogen was original ly supplied as NH3, essentially unmodified from its relative abundance in t he outer solar nebula. Titan's atmospheric methane, in contrast, appears to have been formed from carbon and other carbon compounds, either by gas pha se reactions in the subnebula or by accretional heating during the formatio n of Titan. These conclusions can be tested by further studies of abundance s and isotope ratios in Titan's atmosphere, augmented by studies of comets. The possible similarity of carbon and nitrogen inventories on Titan to tho se on the inner planets makes this investigation particularly intriguing. ( C) 2000 Elsevier Science Ltd. Al rights reserved.