Cometary Nitrogen-Noble gases and the Origin of the Oceans: Waiting for ROSINA-Rosetta Data

Wednesday, 17 December 2014
Bernard Marty1, Ursina Calmonte2, Kathrin Altwegg2, Hans R Balsiger2, Akiva Bar-Nun3, Jean-Jacques Berthelier4, Andre Michel Bieler5, Frederik Dhooghe6, Björn Fiethe7, Urs A. Mall8, Olivier Mousis9, Tobias C Owen10 and Martin Rubin2, (1)CRPG-CNRS,Université de Lorraine, Vandoeuvre les Nancy, France, (2)University of Bern, Bern, Switzerland, (3)Tel Aviv University, Tel Aviv, Israel, (4)LATMOS Laboratoire Atmosphères, Milieux, Observations Spatiales, Paris Cedex 05, France, (5)University of Michigan, Ann Arbor, MI, United States, (6)Belgian Institute for Space Aeronomy, Brussels, Belgium, (7)Technische Universitat Braunschweig, Braunschweig, Germany, (8)Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, (9)University of Franche-Comté, Besançon, France, (10)Univ Hawaii, Honolulu, HI, United States
The origin of the terrestrial oceans and atmosphere is still a matter of intensive debate. For a while high D/H ratios in comets compared to both meteorites and Earth’s oceans (which share approximately the same range of values) has been taken as evidence for an asteroidal origin of the oceans. This possibility is in line with the N isotope composition of meteorites encompassing the terrestrial atmosphere value. Recently, Earth-like D/H ratios have been reported for two comets, thus re-vitalizing the possibility of cometary contribution to terrestrial volatiles. Nitrogen in cometary CN, HCN, and NH2 (the N-bearing molecules that can be analyzed remotely) is enriched by a factor of approx. 80% in 15N compared to terrestrial N. The question is whether or not these N-bearing molecules are representative of bulk N in comets. If nitrogen is trapped as N2 in cometary ices, its isotope composition could reflect that of the protosolar nebula, which was depleted by 40% relative to Earth. A cometary cocktail consisting of 1/3 reduced, 15N-rich N and 2/3 of nebula-like N2 would make the terrestrial N isotope composition. Another strong constraint will arise from the analysis of cometary noble gases which, if trapped at sufficiently low temperature in ice, could account for the elevated noble gas/H,C ratio of the terrestrial surface reservoirs. Hopefully, the ROSINA instrument on board of the Rosetta spacecraft which will analyze volatile elements degassed by comet 67/P Churyumov-Gerasimenko, which should permit to shed light on this fundamental issue.