2:45 PM - 3:00 PM
[PPS24-10] Adsorption experiments of ammonia and clay minerals to understand nitrogen isotopic fractionation in molecular clouds
Keywords:nitrogen isotopic fractionation, adsorption, ammonia, molecular clouds
In this study, we focused on adsorption process of ammonia on grain surface of interstellar dusts as a potential mechanism for the extreme 15N-enrichment and its high-heterogeneity found in extraterrestrial materials. Ammonia is a primitive nitrogen-containing compound and also one of major molecules in molecular clouds. Since ammonia is a highly reactive chemical, it is a precursor for nitrogen-involving organic molecules. The adsorption of ammonia on grain surface would be the first step for the formation of more complicated organic molecules. In order to examine the isotopic fractionation of nitrogen through adsorption of ammonia on grain surface, we performed experiments using ammonia gas and several adsorbents. For the experiments, six clay minerals (montmorillonite, saponite, dickite, kaolinite, pyrophyllite, and halloysite) were selected as the adsorbents. They were kept at 110℃ prior to the experiments to minimize adsorbed water. The each clay mineral was enclosed into a vacuumed glass vial and then ammonia gas (27‰, SI science) was introduced. A few days later, the glass vial was opened and the nitrogen isotopic composition of the adsorbed ammonia was determined by nanoEA/IRMS . The results showed a relationship between δ15N values and the adsorbed ratio, which is explained by Rayleigh fractionation model. The adsorbents with low adsorption ratio have higher δ15N values compared to initial ammonia gas. The difference in the degree of 15N-enrichment and adsorption property among clay minerals was also observed. These results imply that the adsorption of ammonia on grain surface should be considered as one of potential scenarios for 15N-enrichment.
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