16:15 〜 16:30
[MIS19-09] Understanding the origins of complex organics in Enceladus’ ocean
キーワード：Enceladus, hydrothermal chemistry, organic synthesis
In-situ analyses of Enceladus’ plumes by the Cassini spacecraft have revealed that this icy moon has an alkaline subsurface ocean and hydrothermal activities [1]. Enceladus’ ocean also contains simple molecules, such as NH3, HCN, H2CO, and PO4, that can be synthesized into complex organics [2, 3]. In fact, some plume particles are enriched in complex organics, such as unsaturated hydrocarbon chains, aromatics, O-bearing moieties, and amine group organic compounds [4, 5]. These observations raise a question of whether these complex organics are generated via chemical evolution within Enceladus or originated from primordial organics in icy planetesimals that formed Enceladus [4].
In this study, we perform a series of organic synthesis experiments that simulate Enceladus’ hydrothermal reactions in the rocky core and freeze-thaw cycles in the icy crust. Analyses of the samples were conducted using a high-performance liquid chromatograph, nuclear magnetic resonance spectroscopy, and an ion chromatograph to understand the reaction processes of organic synthesis. Some of the solution samples were analyzed with a laser-induced liquid beam ion desorption (LILBID) mass spectrometer, which is a laboratory analog system to simulate an impact-induced ionization mass spectra of plume particles obtained with the Cassini Cosmic Dust Analyzer (CDA).
Our results indicate the abundant formation of amino acids, amines, and carboxyl compounds from solutions with H2CO, HCN, and NH3 under alkaline hydrothermal conditions, as reported previously [6]. However, high abundances of aromatic compounds and unsaturated hydrocarbon chain organics are not formed under hydrothermal and freeze-thaw conditions similar to Enceladus. Additionally, no clear phosphorylation is observed in our experiments. The mass spectra pattern of our samples taken with LILBID are characterized by the predominant peaks of amino acids and amines without aromatics and unsaturated hydrocarbons, which are distinct from those of plume organics obtained with CDA. Our results suggest that organics in Enceladus’ plumes would not be primarily the product of chemical evolution within the moon, but that at least some of them may have originated in icy planetesimals or those partly altered within the moon. Alternatively, there might be a geophysical process in Enceladus that can concentrate particular compounds in the plume sources from a range of organics formed by hydrothermal reactions.
References:
[1] Hsu et al. (2015) Nature; [2] Waite et al. (2017) Science; [3] Postberg et al. (2023) Nature; [4] Postberg et al. (2018) Nature; [5] Khawaja et al. (2019) MNRAS; [6] Kebukawa et al. (2017) Sci. Adv.
In this study, we perform a series of organic synthesis experiments that simulate Enceladus’ hydrothermal reactions in the rocky core and freeze-thaw cycles in the icy crust. Analyses of the samples were conducted using a high-performance liquid chromatograph, nuclear magnetic resonance spectroscopy, and an ion chromatograph to understand the reaction processes of organic synthesis. Some of the solution samples were analyzed with a laser-induced liquid beam ion desorption (LILBID) mass spectrometer, which is a laboratory analog system to simulate an impact-induced ionization mass spectra of plume particles obtained with the Cassini Cosmic Dust Analyzer (CDA).
Our results indicate the abundant formation of amino acids, amines, and carboxyl compounds from solutions with H2CO, HCN, and NH3 under alkaline hydrothermal conditions, as reported previously [6]. However, high abundances of aromatic compounds and unsaturated hydrocarbon chain organics are not formed under hydrothermal and freeze-thaw conditions similar to Enceladus. Additionally, no clear phosphorylation is observed in our experiments. The mass spectra pattern of our samples taken with LILBID are characterized by the predominant peaks of amino acids and amines without aromatics and unsaturated hydrocarbons, which are distinct from those of plume organics obtained with CDA. Our results suggest that organics in Enceladus’ plumes would not be primarily the product of chemical evolution within the moon, but that at least some of them may have originated in icy planetesimals or those partly altered within the moon. Alternatively, there might be a geophysical process in Enceladus that can concentrate particular compounds in the plume sources from a range of organics formed by hydrothermal reactions.
References:
[1] Hsu et al. (2015) Nature; [2] Waite et al. (2017) Science; [3] Postberg et al. (2023) Nature; [4] Postberg et al. (2018) Nature; [5] Khawaja et al. (2019) MNRAS; [6] Kebukawa et al. (2017) Sci. Adv.