Anatomy of Empirical Transit Spectra of Mars based on TGO/NOMAD

Abstract

Transit spectroscopy is a powerful tool for probing atmospheric structures of exoplanets. Accurately accounting for the effects of aerosols is key to reconstructing atmospheric properties from transit spectra, yet this remains a significant challenge. To advance this effort, it is invaluable to examine the spectral features of well-characterized planetary atmospheres. Here, we synthesize empirical transit spectra of Mars across different seasons based on data from the NOMAD's Solar Occultation channel onboard ExoMars/TGO, which operates at wavelengths of 0.2-0.65 and 2-4 micron. In the generated empirical transit spectra, the atmosphere below 25 km is found to be largely opaque due to the presence of micron-sized dust and water ice clouds, both of which substantially weaken spectral features. The spectra exhibit CO2 absorption features at 2.7-2.8 micron and signatures of sub-micron-sized mesospheric water ice clouds around 3.1 micron, accompanied by a continuum slope. The amplitudes of these spectral features are found to vary with the Martian seasons, where the dust storms weaken the CO2 signatures and strengthen the water ice features, which serve as potential indicators of a dusty planet like Mars. If TRAPPIST-1f possessed a Mars-like atmospheric structure, both CO2 and water ice features would be detectable at a noise level of 3 ppm, a level likely beyond current observational capabilities. Nevertheless, the 3.1 micron feature produced by sub-micron-sized mesospheric water ice clouds offers a novel avenue for characterizing the atmospheres of habitable-zone exoplanets.

Figures (10)

• Figure 1: An example of spectra obtained by NOMAD/SO in full-scan mode. The spectra were taken between 19.86 km and 22.43 km above the surface of Mars. The red curves indicate the central 120 pixels of each spectrum, which are averaged to produce empirical spectra with low spectral resolution. The averaged transmittances for each diffraction order are shown as orange dots. The inset at the lower right shows a zoomed-in view of the 3.0–3.2 region.
• Figure 2: (a,b) An example of the transmittance spectra taken with the full-scan mode of the SO channel of TGO/NOMAD. (c,d) An example of the transmittance spectra taken with the full-scan mode of the UVIS channel.
• Figure 3: Illustration of the relationship between the Sun and Mars throughout a Martian year. The solar longitude (Ls) is the angle between the Sun and Mars, measured from the position of the Northern hemisphere’s spring equinox, defined as Ls = 0$^{\circ }$. Thus, Ls = 90$^{\circ }$ corresponds to the northern summer solstice, Ls = 180$^{\circ }$ marks the northern autumn equinox, and Ls = 270$^{\circ }$ corresponds to the northern winter solstice.
• Figure 4: Empirical transit spectra generated from the TGO/NOMAD Solar occultation measurements taken at (a) northern summer (Ls=30--150$^{\circ }$), (b) southern summer (Ls=210--340$^{\circ }$), and (c) equinox (Ls=340--30$^{\circ }$, 150--210$^{\circ }$). The color variation represents the latitudes of the measurements. The results obtained from the UVIS and infrared SO channels are shown in the left and right panels, respectively.
• Figure 5: Empirical transit spectra of Mars derived from TGO/NOMAD measurements are shown for the northern summer period (blue curve), equinox period (black curve), and southern summer period (red curve). The right y-axis indicates the transit depth for Mars (black axis), scaled to represent the case of TRAPPIST-1f.