Replacement and late formation of atmospheric N2 on undifferentiated Titan by impacts

Abstract

Saturn’s moon, Titan, has remarkable surface features—a massive N2 atmosphere and hydrological cycle of CH4—that are often compared with that of Earth^1^. However, the origin and evolution of Titan’s atmosphere remains largely unknown. The proposed formation mechanisms for Titan’s N2 require a prolonged, warm proto-atmosphere during accretion^2-4^. These mechanisms accordingly would not have worked efficiently if Titan stayed cold, as indicated by the incompletely differentiated interior observed by Cassini^5^. Because formation of a massive secondary atmosphere on a planetary body would associate with a major differentiation of its sold body during accretion^6–8^, the presence of such an atmosphere on undifferentiated cold Titan poses a serious dilemma on our view of how planetary bodies develop atmospheres. Here we propose a new mechanism for the post-accretion formation of Titan’s N2 to resolve this problem: conversion and replenishment of N2 from NH3 contained in Titan by impacts during the late heavy bombardment (LHB)^9^. Our results show that Titan, regardless of its thermal history, would acquire sufficient N2 to account for the current atmosphere during the LHB and that most of the pre-LHB atmosphere would have replaced by impact-induced N2. This is the first scenario capable of generating a N2-rich and nearly primordial Ar-free atmosphere on undifferentiated cold Titan. We also suggest that Titan’s N2 was delivered from a different source in the solar nebula compared with Earth and that the origins of N2 on Titan and Triton are fundamentally different with that of N2 on Pluto.