[P-PS05] 火星と火星衛星

[PPS05-02] Preliminary study on thermal emission removal for the MIRS/MMX imaging infrared spectrometer data

*Gael David¹、Antonella Maria Barucci¹、Merlin - Frédéric¹、Leyrat - Cédric¹、Delbo - Marco²、Poggiali - Giovanni¹、Fornasier - Sonia¹、Beck - Pierre³ (1.LESIA-Observatoire de Paris, Université PSL, CNRS, Université Paris, Cité Sorbonne Université, Meudon, France、2.Laboratoire Lagrange, Observatoire de la Côte d’Azur, Université Côte d’Azur, CNRS, Nice, France、3.Institut de Planétologie et d’Astrophysique de Grenoble, OSUG/CNRS, 122 rue de la piscine, F-38000 Grenoble, France)

キーワード：MIRS instrument, Infrared spectroscopy, Calibration, Thermal emission

The Martian Moon eXploration (MMX) mission is scheduled to launch to the Martian system in 2024 [1]. Among its scientific payload, it will carry the MIRS instrument (2), an infrared imaging spectrometer dedicated to the study of the surface of Mars, but mostly of its two satellites: Phobos and Deimos. The mineralogical information provided by MIRS will be particularly fruitful in order to characterize the surface composition of the two moons. The spectral range covered by MIRS (0.9-3.6 μm) is well suited for several mineral phases of geological interest, allowing their detection and assessment of their abundance. However, at long wavelengths (beyond ˜2.5 μm), the signal collected by the instrument observing Phobos and Deimos will be a combination of reflected sunlight and thermal emission from the observed surfaces. This thermal emission strongly modifies the continuum of the spectra and the width of the absorption bands. The thermal tails of spectra are mainly controlled by the surface temperatures and emissivities. For airless bodies, like Phobos and Deimos, the surface temperatures can be highly fluctuating (from ˜130 up to ˜300 K), due to diurnal and seasonal illumination variations. The way it varies is also controlled by thermal inertia and surface roughness. In this study, in preparation for the correction of future MIRS data, two simple methods of thermal emission correction are tested to evaluate their potential and limitations. The first method is to retrieve temperature and emissivity from IR spectra and remove the thermal emission, and is based on the work of Clark et al. [3]. This approach was originally developed to correct lunar M³ observations from the Chandrayaan-1 spacecraft and was used in their calibration pipeline. While this method has been shown to be effective in correcting the reflectance data of the Moon out to about 3.3 μm (M³ maximum wavelength), it is unclear whether this approach is effective in correcting further into the infrared, such as the MIRS range. The second approach tested in this study was specifically developed to correct IR observations in a larger range of wavelengths (i.e., up to 4.3 μm). Both approaches are iterative methods and use the same assumption based on the linear relationship of reflectance data in the near-infrared range where the thermal contribution is negligible to estimate the thermal corrected reflectance in the mid-infrared region. From the reflectance data with and without thermal correction, a temperature can be derived thanks to blackbody Planck functions. The method of Clark et al. performs several iterations to adjust the temperature, and the wavelength dependent emissivity is determined by using Kirchhoff’s law. With the second method, after an initial temperature prediction, a constant emissivity with wavelength is obtained by adjusting the blackbody to the original data in the mid-infrared region with a scaling factor. Finally, representative synthetic infrared spectra of Phobos and Deimos with different surface temperatures and emissivities will be generated by means of a thermophysical model [4] to test the two thermal correction models and evaluate their robustness.
Acknowledgments: This research was carried out with financial support from the Centre National d’Etudes Spatiales (CNES) in France. MMX is under developed and built by JAXA, with contributions from CNES, DLR, NASA and ESA.
References: [1] Kuramoto, K., et al., (2021), Martian moons exploration MMX: sample return mission to Phobos elucidating formation processes of habitable planets. EPS. [2] Barucci, M. A., et al., (2021), MIRS: an imaging spectrometer of the MMX mission. EPS. [3] Clark, R. N., et al., (2011), Thermal removal from near-infrared imaging spectroscopy data of the moon. JGR, vol 16, EOOG16. [4] Delbo, M., et al., (2007), Thermal inertia of near-earth asteroids and implications for the magnitude of the Yarkovsky effect. Icarus.