3:30 PM - 5:00 PM
[PPS05-P12] A study on the vertical thermal structures of the Martian atmosphere in the radio occultation measurements with Full Spectrum Inversion
Keywords:Radio occultation, Mars, Polar night, Carbon dioxide, Atmospheric principal condensation
Mars has a climate cold enough to cause condensation or supersaturation of carbon dioxide (CO2), which is the major constituent in the Martian atmosphere. The condensation of the major constituent has not been observed on Earth and is unique to Mars. In the polar night on Mars, CO2 snow falls and accumulates on the surface, or CO2 directly condenses on the ground surface to the polar cap. CO2 sublimates again from the polar cap after the polar night ends. The total amount of the Martian atmosphere changes by 20-30% as the condensation and sublimation repeats in the polar night region.
The study of the atmospheric CO2 supersaturation and condensation needs the observations of high-precision temperature with high vertical resolution. We focused on radio occultation technique, which provides atmospheric thermal structure with high vertical resolution. Radio occultation measurements utilize the radio waves emitted from a spacecraft orbiting a planet which should be observed to a receiving station on Earth to obtain the vertical profiles of the atmospheric temperature and pressure of the planet. The traditional radio occultation technique was based on the Geometric Optics (GO) method, in which the vertical resolution was limited to 500m to 1km because of multipath effects and Fresnel zone limitations. In this study, we adopted a new method, the Ful Spectrum Inversion (FSI) method, which is one of the radio holography methods. While the GO method maps one frequency to one time (i.e., frequency is a function of time), the FSI method maps one time to one frequency (i.e., time is a function of frequency). Therefore, the relationship between frequency and time can be correctly described even when multipath effects occur. In addition, the FSI method performs the Fourier transform of all received signals at once, whereas the GO method performs the Fourier transform by dividing the time series of the received signal into short segments, achieving a high vertical resolution (160m).
In the present study, we reprocessed the frequency times series obtained in the Mars Global Surveyor (MGS) Radio Science (RS) mission (1998-2007) with the FSI method to obtain the vertical profiles of temperature and pressure. We also applied the GO method to the same dataset for comparison. We obtained 18 profiles in the polar night region (70-80°S) in the winter season (Ls=110~115°) . The rederived profiles of temperature showed that the amplitudes of temperature fluctuations were larger in the FSI method than in the GO method. The number of events where the temperature lapse rate exceeded the adiabatic lapse rate of the dry CO2 atmosphere increased by about 80% in the temperature profiles derived with the FSI method. This suggests that the FSI method detects neutral layers which were overlooked by the GO method.
The study of the atmospheric CO2 supersaturation and condensation needs the observations of high-precision temperature with high vertical resolution. We focused on radio occultation technique, which provides atmospheric thermal structure with high vertical resolution. Radio occultation measurements utilize the radio waves emitted from a spacecraft orbiting a planet which should be observed to a receiving station on Earth to obtain the vertical profiles of the atmospheric temperature and pressure of the planet. The traditional radio occultation technique was based on the Geometric Optics (GO) method, in which the vertical resolution was limited to 500m to 1km because of multipath effects and Fresnel zone limitations. In this study, we adopted a new method, the Ful Spectrum Inversion (FSI) method, which is one of the radio holography methods. While the GO method maps one frequency to one time (i.e., frequency is a function of time), the FSI method maps one time to one frequency (i.e., time is a function of frequency). Therefore, the relationship between frequency and time can be correctly described even when multipath effects occur. In addition, the FSI method performs the Fourier transform of all received signals at once, whereas the GO method performs the Fourier transform by dividing the time series of the received signal into short segments, achieving a high vertical resolution (160m).
In the present study, we reprocessed the frequency times series obtained in the Mars Global Surveyor (MGS) Radio Science (RS) mission (1998-2007) with the FSI method to obtain the vertical profiles of temperature and pressure. We also applied the GO method to the same dataset for comparison. We obtained 18 profiles in the polar night region (70-80°S) in the winter season (Ls=110~115°) . The rederived profiles of temperature showed that the amplitudes of temperature fluctuations were larger in the FSI method than in the GO method. The number of events where the temperature lapse rate exceeded the adiabatic lapse rate of the dry CO2 atmosphere increased by about 80% in the temperature profiles derived with the FSI method. This suggests that the FSI method detects neutral layers which were overlooked by the GO method.