title: Adsorption of NH3 on grain surface in molecular clouds as a possible mechanism for 15N-enrichment in extraterrestrial organic matters

authors: H. Sugahara, Y. Takano, N. O. Ogawa, Y. Chikaraishi, and N. Ohkouchi

Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), JAPAN


abstract:      Stable nitrogen isotopic compositions vary greatly in the solar system. The bulk solar system value, which is inferred from the Sun and Jupiter, is ?|400??[1, 2]. In contrast, the primitive solar system materials such as chondrites, comets, and interplanetary dust particles (IDPs) show various degrees of 15N-enrichment up to +1500?? (normalized as vs. Air) by their bulk analysis [3, 4]. In addition, anomalously high 15N-enrichments, which reach as high as +5000??, have been frequently found within a single material and they are called hot spots [e.g., 5]. These 15N-enrichments are considered to be originated in cold interstellar environments. However, only a few models can explain the considerable 15N-enrichments in molecular clouds [e.g., 6] and likely isotopic discrimination processes have never been well understood. 
     In this study, we focused on adsorption of ammonia on grain surface of interstellar dusts as a potential mechanism for the extreme 15N-enrichment and its high-heterogeneity in molecular clouds. Ammonia is a primitive nitrogen-containing compound and abundant in molecular clouds. It is considered to be a precursor for nitrogen-involving organic molecules due to its high reactivity. The adsorption of ammonia on grain surface would be the first step for grain surface chemistry to synthesize 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), silica gel, and graphite were selected as the adsorbents. They were kept at 110??C prior to the experiments to minimize adsorbed water. The each adsorbent was encosed into vacuumed glass vial and then 1 atm of 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 [7]. The result showed that the adsorbed ammonia had 15N-enrichment than initial ammonia gas and the highest value observed in the experiments was 92??. There was a decreasing trend in ??15N value with the increase of adsorption ratio, which is explained by Rayleigh fractionation model. The adsorption ratios and the ??15N value differ among adsorbents and graphite did not show any adsorptive capacity. Our findings propose that the adsorption of ammonia on grain surface should be considered as one of potential mechanisms to explain ??15N diversity in the primitive solar materials.

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