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



Saturn’s moon Titan has attracted much attention because of its massive nitrogen atmosphere1, 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 accretion2,3,4,5,6,7, yet Titan’s interior has been found to be incompletely differentiated8. 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 ago9. Our laser-gun experiments show that ammonia ice converts to N2 very efficiently during impacts. Numerical calculations based on our experimental results indicate that Titan would acquire sufficient N2 to sustain the current atmosphere and that most of the atmosphere present before the late heavy bombardment would have been replaced by impact-induced N2. Our scenario is capable of generating a N2-rich atmosphere with little primordial Ar on undifferentiated Titan. If this mechanism generated Titan’s atmosphere, its N2 was derived from a source in the solar nebula different from that for Earth, and the origins of N2 on Titan and Triton may be fundamentally different from the origin of N2 on Pluto.

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Figure 1: Efficiency of impact-induced N2 production from NH3–H2O ice for different NH3 content (red: 50%; blue: 10%).
Figure 2: Evolution of Titan’s N2 inventory during the LHB.
Figure 3: The concentration of NH3, nNH3, on Titan required to accumulate 1.5 bar of N2 for various surface temperatures.

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This study was supported by Grant in Aid from the Japan Society for the Promotion of Science and the Mitsubishi Foundation. Y.S. thanks S. Fukuzaki for help with the experiments and K. Kuramoto, K. Hamano, M. Arakawa, S. Watanabe, V. Shuvalov, C. P. McKay, S. K. Atreya and D. F. Strobel for helpful discussions.

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Y.S. designed the ice target system for the experiments, performed the experiments, modelled Titan’s N2 inventory, and wrote the manuscript. H.G. performed the SPH simulations. T.K. designed the laser-gun system. All the authors discussed and contributed intellectually to the interpretation of the results.

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Correspondence to Yasuhito Sekine.

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The authors declare no competing financial interests.

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Sekine, Y., Genda, H., Sugita, S. et al. Replacement and late formation of atmospheric N2 on undifferentiated Titan by impacts. Nature Geosci 4, 359–362 (2011).

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