9:30 AM - 9:45 AM
[PAE18-03] Empirical transit spectra of Mars
Keywords:Exoplanet, Transit spectroscopy, Mars
Observations for Earth-like planets have garnered significant interest, and both current and future telescopes are designed to support this effort. For example, the James Webb Space Telescope (JWST) has the capability to investigate the atmospheres of terrestrial exoplanets around nearby low-mass stars, such as those in the TRAPPIST-1 system, by analyzing their transit and emission spectra. Observations of TRAPPIST-1b and 1c have suggested that their atmospheres are thin. Further observations of the remaining planets are planned for the near future.
Retrieving atmospheric information from transit spectra poses several challenges, primarily due to their relatively low signal-to-noise ratio, limited spectral resolution, and the fact that the measurements represent an integration over the atmospheric transmittance at different locations along the day-night terminator. This is a significant coarse-graining of likely complex three-dimensional atmospheric profiles of real planets, and it is invaluable to examine the spectral features of well-characterized planetary atmospheres. Thanks to recent planetary exploration programs, spectroscopic data from the terrestrial planets of our solar system - such as Earth, Venus, Mars, and Titan - are available for comparative studies.
In this study, we focus on Mars, which has a thin CO2-rich atmosphere and is covered with dust. Mars is the most extensively studied planet, second only to Earth, in terms of spectroscopic observations. The European Space Agency's ExoMars Trace Gas Orbiter (TGO) is a dedicated spacecraft designed for solar occultation measurements, a technique analogous to transit observations of exoplanets. TGO carries two spectrometers used for these measurements: Nadir and Occultation for Mars Discovery (NOMAD) and the Atmospheric Chemistry Suite (ACS).
In this study, we utilize data from the NOMAD instrument's Solar Occultation (SO) channel, which operates at wavelengths of 2.3-4.3 micron with a relatively high spectral resolution (R=17,000), to reproduce the transit spectra as if Mars were an exoplanet. Within the SO channel, the Acousto-Optical Tunable Filter (AOTF) functions as a passband filter, selecting spectral intervals corresponding to specific diffraction orders of the echelle grating, which has a free spectral range of approximately 22 cm-1. The AOTF allows for instantaneous changes in observing diffraction orders, and under nominal operations, the SO channel typically measures five or six different orders. On certain occasions, NOMAD-SO performs a specialized "full-scan" mode, covering all diffraction orders between 2.3 and 4.3 micron. In this study, we analyze 29 observations obtained using this full-scan mode, spanning different locations and seasons on Mars. We average the transmittance over each diffraction order to generate empirical transit spectra at low spectral resolution.
Our analysis reveals two distinctive spectral features in the generated empirical transit spectra: CO2 absorption at 2.7-2.8 micron and water ice cloud signatures around 3.1 micron, together with the continuum/slope due to dust. We find that the atmosphere below 30 km are largely opaque, which substantially weakens spectral features. Detecting water ice cloud features is particularly intriguing, given the low abundance of water in Mars' atmosphere. This suggests that such spectral signatures could serve as key indicators of, somewhat counterintuitively, a dry and cold atmosphere, similar to that of Mars. Additionally, we find that the amplitudes of these spectral features vary with the Martian seasons: CO2 absorption features are slightly weaker, while water ice cloud features are more pronounced when the atmosphere contains larger amount of dust. This suggests that atmospheric dust abundance plays a critical role in defining the altitude sensitivity of transit spectra, and the temporal variability of these spectral signatures is a characteristic feature of Mars' dusty atmosphere. In the presentation, we will also discuss the potential for detecting these features in the atmospheres of TRAPPIST-1 planets if they exhibit Mars-like atmospheric conditions.
Retrieving atmospheric information from transit spectra poses several challenges, primarily due to their relatively low signal-to-noise ratio, limited spectral resolution, and the fact that the measurements represent an integration over the atmospheric transmittance at different locations along the day-night terminator. This is a significant coarse-graining of likely complex three-dimensional atmospheric profiles of real planets, and it is invaluable to examine the spectral features of well-characterized planetary atmospheres. Thanks to recent planetary exploration programs, spectroscopic data from the terrestrial planets of our solar system - such as Earth, Venus, Mars, and Titan - are available for comparative studies.
In this study, we focus on Mars, which has a thin CO2-rich atmosphere and is covered with dust. Mars is the most extensively studied planet, second only to Earth, in terms of spectroscopic observations. The European Space Agency's ExoMars Trace Gas Orbiter (TGO) is a dedicated spacecraft designed for solar occultation measurements, a technique analogous to transit observations of exoplanets. TGO carries two spectrometers used for these measurements: Nadir and Occultation for Mars Discovery (NOMAD) and the Atmospheric Chemistry Suite (ACS).
In this study, we utilize data from the NOMAD instrument's Solar Occultation (SO) channel, which operates at wavelengths of 2.3-4.3 micron with a relatively high spectral resolution (R=17,000), to reproduce the transit spectra as if Mars were an exoplanet. Within the SO channel, the Acousto-Optical Tunable Filter (AOTF) functions as a passband filter, selecting spectral intervals corresponding to specific diffraction orders of the echelle grating, which has a free spectral range of approximately 22 cm-1. The AOTF allows for instantaneous changes in observing diffraction orders, and under nominal operations, the SO channel typically measures five or six different orders. On certain occasions, NOMAD-SO performs a specialized "full-scan" mode, covering all diffraction orders between 2.3 and 4.3 micron. In this study, we analyze 29 observations obtained using this full-scan mode, spanning different locations and seasons on Mars. We average the transmittance over each diffraction order to generate empirical transit spectra at low spectral resolution.
Our analysis reveals two distinctive spectral features in the generated empirical transit spectra: CO2 absorption at 2.7-2.8 micron and water ice cloud signatures around 3.1 micron, together with the continuum/slope due to dust. We find that the atmosphere below 30 km are largely opaque, which substantially weakens spectral features. Detecting water ice cloud features is particularly intriguing, given the low abundance of water in Mars' atmosphere. This suggests that such spectral signatures could serve as key indicators of, somewhat counterintuitively, a dry and cold atmosphere, similar to that of Mars. Additionally, we find that the amplitudes of these spectral features vary with the Martian seasons: CO2 absorption features are slightly weaker, while water ice cloud features are more pronounced when the atmosphere contains larger amount of dust. This suggests that atmospheric dust abundance plays a critical role in defining the altitude sensitivity of transit spectra, and the temporal variability of these spectral signatures is a characteristic feature of Mars' dusty atmosphere. In the presentation, we will also discuss the potential for detecting these features in the atmospheres of TRAPPIST-1 planets if they exhibit Mars-like atmospheric conditions.