10:45 AM - 12:15 PM
[PCG20-P05] Formation of Local Cage Structures relevant to Methane Clathrate Hydrate under Interstellar Conditions
Keywords:Amorphous water, Methane hydrate, Thermal desorption
Clathrate hydrates are inclusion compounds in which a guest molecule is trapped in a hydrate (water cage) structure. Among hydrate classes, methane hydrates are well-studied for planetary science and theoretically proposed to exist in the conditions of 10-4 Pa·50 K relevant to cometary nuclei based on thermodynamic equilibrium [1]. In laboratory experiments, Ghosh et al. reported the formation of methane hydrate under cryogenic and ultrahigh vacuum conditions of 10-8 Pa·30 K relevant to interstellar clouds, using infrared spectroscopy (IR) and temperature programmed desorption mass-spectrometry (TPD) in 2019 [2]. However, the IR spectra of methane hydrate reported in ref. [2] are different from those of methane hydrate formed under high-pressure conditions in previous study [3]. In addition, Ghosh et al. interpreted the desorption of methane at around 140 K (T2) as a result of the dissociation of methane hydrates, because T2 is much higher than the sublimation temperature of pure solid methane (T1 = 35-50 K). However, it is difficult to distinguish the dissociation of methane hydrates from “molecular volcano” [4], that is, the release of trapped gas in concert with the crystallization of amorphous water. Therefore, no consensus has been reached for the formation of methane hydrate under interstellar conditions [5]. Diffraction experiments under cryogenic and ultrahigh vacuum conditions are critical to clearly identify the formation of methane hydrates [6, 7].
In this study, we performed in situ analysis of the structural change for CH4-H2O (1:1) mixed ice prepared by vapor-deposition at 10 K by heating using reflection high-energy electron diffraction (RHEED), IR, and TPD methods, in order to elucidate the formation of methane hydrate under cryogenic and ultrahigh vacuum conditions. As a result, methane desorption was observed not only at 35 K indexed as T1 but also at 140 K indexed as T2. Moreover, RHEED diffraction patterns of the mixed ice at 40-130 K were different from those recorded for pure H2O ice or solid methane, and they are relevant to sI clathrate hydrates (CH4·5.75H2O). On the other hand, the present IR spectra do not show any clear evidence for the formation of clathrate hydrates.This implies that hydrate structures were built on the surface of the mixed ice locally.
Our experimental results suggest that hydrate-like structures relevant to methane hydrates can be produced locally on the surface of icy grain mantles in interstellar clouds. Using neutron diffraction method, Yamamuro and Kikuchi previously reported the formation of amorphous clathrate hydrates by vapor-deposition of a xenon/water mixed gas onto the substrate at 102 Pa·10 K and confirmed the formation of sI clathrate hydrates by heating the amorphous clathrate hydrates [8]. This previous study supports our present experimental results for the methane hydrate formation. Further details of our experiments will be presented in the session.
[1] Luspay-Kuti A., et al., 2016, Sci. Adv., 2 : e1501781
[2] Ghosh J., et al., 2019, PNAS, vol. 116, no. 5, 1526
[3] Dartois E., & Deboffle D., 2008, A&A, 490, L19
[4] Alan May R., et al., 2013, JCP, 138, 104501, Alan May R., et al., 2013, JCP, 138, 104502
[5] Choukroun M., 2019, PNAS, vol. 116, no. 29, 14407
[6] Kouchi A., 1990, Journal of Crystal Growth, 99, 1220
[7] Bauer R. P. C., 2021, JPCC, 125, 26892
[8] Yamamuro O., Kikuchi T., 2009, High Pressure Science and Technology, Vol. 19, No. 3, 201
In this study, we performed in situ analysis of the structural change for CH4-H2O (1:1) mixed ice prepared by vapor-deposition at 10 K by heating using reflection high-energy electron diffraction (RHEED), IR, and TPD methods, in order to elucidate the formation of methane hydrate under cryogenic and ultrahigh vacuum conditions. As a result, methane desorption was observed not only at 35 K indexed as T1 but also at 140 K indexed as T2. Moreover, RHEED diffraction patterns of the mixed ice at 40-130 K were different from those recorded for pure H2O ice or solid methane, and they are relevant to sI clathrate hydrates (CH4·5.75H2O). On the other hand, the present IR spectra do not show any clear evidence for the formation of clathrate hydrates.This implies that hydrate structures were built on the surface of the mixed ice locally.
Our experimental results suggest that hydrate-like structures relevant to methane hydrates can be produced locally on the surface of icy grain mantles in interstellar clouds. Using neutron diffraction method, Yamamuro and Kikuchi previously reported the formation of amorphous clathrate hydrates by vapor-deposition of a xenon/water mixed gas onto the substrate at 102 Pa·10 K and confirmed the formation of sI clathrate hydrates by heating the amorphous clathrate hydrates [8]. This previous study supports our present experimental results for the methane hydrate formation. Further details of our experiments will be presented in the session.
[1] Luspay-Kuti A., et al., 2016, Sci. Adv., 2 : e1501781
[2] Ghosh J., et al., 2019, PNAS, vol. 116, no. 5, 1526
[3] Dartois E., & Deboffle D., 2008, A&A, 490, L19
[4] Alan May R., et al., 2013, JCP, 138, 104501, Alan May R., et al., 2013, JCP, 138, 104502
[5] Choukroun M., 2019, PNAS, vol. 116, no. 29, 14407
[6] Kouchi A., 1990, Journal of Crystal Growth, 99, 1220
[7] Bauer R. P. C., 2021, JPCC, 125, 26892
[8] Yamamuro O., Kikuchi T., 2009, High Pressure Science and Technology, Vol. 19, No. 3, 201