Atmospheric replacement and late formation of N2 on undifferentiated Titan during the Late Heavy Bombardment

Details Atmospheric replacement and late formation of N2 on undifferentiated Titan during the Late Heavy Bombardment Atmospheric replacement and late formation of N2 on undifferentiated Titan during the Late Heavy Bombardment

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p14a..06s&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P14A-06

Physics

[5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6281] Planetary Sciences: Solar System Objects / Titan

Scientific paper

Saturn’s largest moon, Titan, has a massive N2 atmosphere, often compared with that of Earth; however, the origin and evolution of Titan’s atmosphere remains largely unknown. Although several mechanisms (UV photolysis(1), atmospheric shock heating(2) and endogenic(3)) have been proposed for forming Titan’s N2 atmosphere from primordial NH3, all these mechanisms require a prolonged, warm proto-atmosphere during Titan’s accretion. These mechanisms accordingly would not have worked efficiently, if Titan has stayed cold, as indicated by the incompletely differentiated interior observed by Cassini(4). Because formation of a massive secondary atmosphere on a planetary body is suggested to associate with a major differentiation of its sold body during accretion, the presence of such an atmosphere on undifferentiated Titan is enigmatic and has challenged our view of how planetary bodies develop atmospheres. Here we propose a new mechanism for the post-accretion formation of atmospheric N2 on Titan to resolve this problem: conversion and replenishment of N2 from NH3 contained in Titan by cometary impacts during the late heavy bombardment (LHB)(5). Recently, we developed a new chemically clean technique to accelerate projectiles using a high-energy laser pulse (the laser gun)(6). Using this laser gun, we investigate chemistry in impact-induced vapor cloud by direct measurements of gas species formed by impact experiments. Our experiments and calculations show that Titan would acquire sufficient N2 to account for the current atmosphere during the LHB and that most of the pre-existing 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 Titan. We also suggest that Titan’s N2 was delivered from a different source in the solar nebula compared with Earth, based on the comparison of isotopic composition of N2(7). Reference: (1) Atreya et al. 1979, Science 201, 611. (2) McKay et al. 1988, Nature 332, 520. (3) Glein et al. 2009, Icarus 204, 637. (4) Iess et al. 2010, Science 327, 1367. (5) Gomes et al. 2005, Nature 435, 466. (6) Fukuzaki et al. 2010, Icarus, in press. (7) Niemann et al. 2005, Nature 438, 779.

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