10:45 AM - 11:00 AM
[PPS09-07] Vertical distribution of dust on Mars inferred from the Nadir observations of two CO2 bands at 2.01 μm and 2.77 μm by OMEGA/MEx: Preliminary results
★Invited Papers
Keywords:Mars, Dust, Meteorological
Martian dust plays a crucial role in climate and meteorology through horizontal and vertical transport in the atmosphere. Previous nadir observations in the ultraviolet, visible, and infrared wavelengths provided insights into the dust horizontal distribution (e.g., Smith et al., 2001; Montabone et al., 2015; Battalio and Wang, 2021; Leseigneur and Vincendon, 2023). Previous solar occultation and limb observations provided valuable information on dust vertical distribution, but uncertainties persist regarding its spatial variability and transport mechanisms. Conventional limb and solar occultation observations can retrieve dust altitude information directly, but their spatial and temporal coverage is limited. This study aims to obtain new insights into the dust vertical distribution by applying the spectral synergy method to nadir observations.
The spectral synergy method estimates the altitude of atmospheric constituents by leveraging differences in absorption characteristics at multiple wavelengths (Pan et al., 1995; Pan et al., 1998; Edwards et al., 2009). This method applied in Earth remote sensing for retrieving the vertical distributions of CO2 (Christi & Stephens, 2004), O3 (Landgraf & Hasekamp, 2007), and CH4 (Razavi et al., 2009) and is recently used to analyze water vapor in the Martian atmosphere (Knutsen et al., 2022). However, no studies have applied this approach to Martian dust yet. This study evaluates the feasibility and challenges of using the spectral synergy method to estimate the vertical distribution of Martian dust.
We analyzed nadir observation data acquired by the OMEGA near-infrared imaging spectrometer onboard Mars Express during Martian Years (MY) 27-29. Specifically, we compared dust optical depth (DOD) retrieved at two different wavelengths: a 2.77 μm CO2 absorption saturated band (Kazama et al., in prep.) and a 2.01 μm CO2 absorption band (Leseigneur and Vincendon, 2023). The 2.77 μm band is saturated under typical Martian atmospheric conditions (>400 Pa) and is primarily sensitive to dust at altitudes of 20-30 km. In contrast, the 2.01 μm band, with weaker CO2 absorption, primarily reflects dust at altitudes of 10-20 km. By combining these two wavelengths, we attempt to infer the dust vertical distribution from nadir observations.
The ratio of DODs at 2.01 μm and 2.77 μm (Ratio = 2.77 μm / 2.01 μm) is expected to be around 1.12, according to typical extinction coefficient ratios and a well-mixed condition assumption. However, considering retrieval uncertainties due to temperature, atmospheric pressure, dust optical parameters, and instrument factors, this ratio may range between 1 and 5 in well-mixed conditions. If the ratio exceeds 5, it suggests that dust is more strongly detected at 2.77 μm than at 2.01 μm, indicating a higher concentration of dust at high altitudes. Conversely, a ratio below 1 suggests that dust is predominantly confined to lower altitudes and thus weakly detectable at 2.77 μm.
We applied this method to ~4,000 observations from MY27-29, including the MY28 global dust storm. Our preliminary analysis revealed seasonal variations in the two-wavelength ratio. Specifically, during the low-dust seasons (Ls = 0°-180°), observations with ratios exceeding 5 became more frequent, suggesting that dust was more strongly detected at 2.77 μm compared to 2.01 μm. This result implies that during these clear seasons, dust may be concentrated at high altitudes, possibly forming detached dust layers. In contrast, during high-dust seasons (Ls = 180°-360°), most observations fell within the expected range of 1-5, consistent with a well-mixed vertical dust distribution.
Compared to limb and solar occultation observations, this method provides broader spatiotemporal coverage; however, significant challenges remain in quantitatively retrieving precise vertical dust distributions. In this presentation, we will discuss the details and limitations of this method, interpret the obtained results, and outline future works.
The spectral synergy method estimates the altitude of atmospheric constituents by leveraging differences in absorption characteristics at multiple wavelengths (Pan et al., 1995; Pan et al., 1998; Edwards et al., 2009). This method applied in Earth remote sensing for retrieving the vertical distributions of CO2 (Christi & Stephens, 2004), O3 (Landgraf & Hasekamp, 2007), and CH4 (Razavi et al., 2009) and is recently used to analyze water vapor in the Martian atmosphere (Knutsen et al., 2022). However, no studies have applied this approach to Martian dust yet. This study evaluates the feasibility and challenges of using the spectral synergy method to estimate the vertical distribution of Martian dust.
We analyzed nadir observation data acquired by the OMEGA near-infrared imaging spectrometer onboard Mars Express during Martian Years (MY) 27-29. Specifically, we compared dust optical depth (DOD) retrieved at two different wavelengths: a 2.77 μm CO2 absorption saturated band (Kazama et al., in prep.) and a 2.01 μm CO2 absorption band (Leseigneur and Vincendon, 2023). The 2.77 μm band is saturated under typical Martian atmospheric conditions (>400 Pa) and is primarily sensitive to dust at altitudes of 20-30 km. In contrast, the 2.01 μm band, with weaker CO2 absorption, primarily reflects dust at altitudes of 10-20 km. By combining these two wavelengths, we attempt to infer the dust vertical distribution from nadir observations.
The ratio of DODs at 2.01 μm and 2.77 μm (Ratio = 2.77 μm / 2.01 μm) is expected to be around 1.12, according to typical extinction coefficient ratios and a well-mixed condition assumption. However, considering retrieval uncertainties due to temperature, atmospheric pressure, dust optical parameters, and instrument factors, this ratio may range between 1 and 5 in well-mixed conditions. If the ratio exceeds 5, it suggests that dust is more strongly detected at 2.77 μm than at 2.01 μm, indicating a higher concentration of dust at high altitudes. Conversely, a ratio below 1 suggests that dust is predominantly confined to lower altitudes and thus weakly detectable at 2.77 μm.
We applied this method to ~4,000 observations from MY27-29, including the MY28 global dust storm. Our preliminary analysis revealed seasonal variations in the two-wavelength ratio. Specifically, during the low-dust seasons (Ls = 0°-180°), observations with ratios exceeding 5 became more frequent, suggesting that dust was more strongly detected at 2.77 μm compared to 2.01 μm. This result implies that during these clear seasons, dust may be concentrated at high altitudes, possibly forming detached dust layers. In contrast, during high-dust seasons (Ls = 180°-360°), most observations fell within the expected range of 1-5, consistent with a well-mixed vertical dust distribution.
Compared to limb and solar occultation observations, this method provides broader spatiotemporal coverage; however, significant challenges remain in quantitatively retrieving precise vertical dust distributions. In this presentation, we will discuss the details and limitations of this method, interpret the obtained results, and outline future works.