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
