2:45 PM - 3:00 PM
[PPS07-15] Feasibility Study of Measuring the Hydrogen Temperature and the D/H Ratio in a Cometary Coma Using Hydrogen Imager onboard Comet Interceptor
Keywords:Comets, Comet Interceptor, D/H Ratio, atmosphere, Ultraviolet rays
Long period comets are focused on as especially pristine comets. Hydrogen atoms in comet atmospheres (comae), which are generated through photodissociation of water molecules sublimated from surface layers, are clues to grasp activities. The deuterium to hydrogen (D/H) ratio is one of the indicators for identifying candidate water sources for the early Earth.
The development of the Comet Interceptor (CI) spacecraft, which will flyby a long period comet, is underway. The Hydrogen Imager (HI) onboard CI including a band-pass filter and gas filters (absorption cells) measure Ly-α radiance distributions. The absorption cell is a small glass cell filled with hydrogen (or deuterium) molecules. It can selectively absorb only hydrogen (or deuterium) Ly-α when the voltage is applied to filaments in it. However, as its absorption rate depends on hydrogen temperatures of targets, observational constraints on hydrogen temperatures are necessary for improving measurement accuracy. Besides, Ly-α radiance is no longer proportional to hydrogen column densities in the vicinity of nuclei, where multiple scattering and self-absorption of Ly-α by hydrogen atoms are dominant. Therefore, optical measurement of hydrogen and deuterium number densities requires conversion of observed Ly-α radiance to hydrogen column density by numerical calculations using a radiative transfer model.
In this study, estimation accuracy of hydrogen temperatures in a coma is calculated based on evaluation experiments of filament temperature dependence of cell absorption rates. Consequently, it was found that hydrogen temperature can be determined with an error of approximately 1,000 K. Subsequently, Ly-α radiance is calculated as a function of hydrogen column densities along the line of sight of CI/HI using radiative transfer model, which was constructed based on UV spectroscopic observations of long period comets by the Hisaki satellite. As a result, D/H ratios can be measured with relative errors less than 60%, which means that we can distinguish high-D/H-ratio comets such as 67P/Churyumov-Gerasimenko and low-D/H-ratio comets such as 103P/Hartley 2. Restriction of hydrogen temperature with 1,000 K accuracy will realize measurement with relative errors of approximately 40%, with which high-, mid-, and low-D/H-ratio comets can be distinguished by CI/HI.
In this presentation, we introduce quantitative feasibility studies on measuring hydrogen temperatures and D/H ratios in comae of long period comets using gas filters installed in the Hydrogen Imager onboard Comet Interceptor spacecraft.
The development of the Comet Interceptor (CI) spacecraft, which will flyby a long period comet, is underway. The Hydrogen Imager (HI) onboard CI including a band-pass filter and gas filters (absorption cells) measure Ly-α radiance distributions. The absorption cell is a small glass cell filled with hydrogen (or deuterium) molecules. It can selectively absorb only hydrogen (or deuterium) Ly-α when the voltage is applied to filaments in it. However, as its absorption rate depends on hydrogen temperatures of targets, observational constraints on hydrogen temperatures are necessary for improving measurement accuracy. Besides, Ly-α radiance is no longer proportional to hydrogen column densities in the vicinity of nuclei, where multiple scattering and self-absorption of Ly-α by hydrogen atoms are dominant. Therefore, optical measurement of hydrogen and deuterium number densities requires conversion of observed Ly-α radiance to hydrogen column density by numerical calculations using a radiative transfer model.
In this study, estimation accuracy of hydrogen temperatures in a coma is calculated based on evaluation experiments of filament temperature dependence of cell absorption rates. Consequently, it was found that hydrogen temperature can be determined with an error of approximately 1,000 K. Subsequently, Ly-α radiance is calculated as a function of hydrogen column densities along the line of sight of CI/HI using radiative transfer model, which was constructed based on UV spectroscopic observations of long period comets by the Hisaki satellite. As a result, D/H ratios can be measured with relative errors less than 60%, which means that we can distinguish high-D/H-ratio comets such as 67P/Churyumov-Gerasimenko and low-D/H-ratio comets such as 103P/Hartley 2. Restriction of hydrogen temperature with 1,000 K accuracy will realize measurement with relative errors of approximately 40%, with which high-, mid-, and low-D/H-ratio comets can be distinguished by CI/HI.
In this presentation, we introduce quantitative feasibility studies on measuring hydrogen temperatures and D/H ratios in comae of long period comets using gas filters installed in the Hydrogen Imager onboard Comet Interceptor spacecraft.