Saturn's moon Titan has attracted much attention because of its massive nitrogen atmosphere, but the origin of this atmosphere is largely unknown. Massive secondary atmospheres on planets and satellites usually form only after a substantial differentiation of the body's interior and chemical reactions during accretion, yet Titan's interior has been found to be incompletely differentiated. Here we propose that Titan's nitrogen atmosphere formed after accretion, by the conversion from ammonia that was already present on Titan during the period of late heavy bombardment about four billion years ago. Our laser-gun experiments show that ammonia ice converts to N_2 very efficiently during impacts. Numerical calculations based on our experimental results indicate that Titan would acquire sufficient N_2 to sustain the current atmosphere and that most of the atmosphere present before the late heavy bombardment would have been replaced by impact-induced N _2. Our scenario is capable of generating a N_2-rich atmosphere with little primordial Ar on undifferentiated Titan. If this mechanism generated Titan's atmosphere, its N_2 was derived from a source in the solar nebula different from that for Earth, and the origins of N_2 on Titan and Triton may be fundamentally different from the origin of N_2 on Pluto.
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