Retrieving atmospheric dust opacity on Mars by imaging spectroscopy at large angles

Abstract

We propose a new method to retrieve the optical depth of Martian aerosols (AOD) from OMEGA and CRISM hyperspectral imagery at a reference wavelength of 1 µm. Our method works even if the underlying surface is completely made of minerals, corresponding to a low contrast between surface and atmospheric dust, while being observed at a fixed geometry. Minimizing the effect of the surface reflectance properties on the AOD retrieval is the second principal asset of our method. The method is based on the parametrization of the radiative coupling between particles and gas determining, with local altimetry, acquisition geometry, and the meteorological situation, the absorption band depth of gaseous CO 2 . Because the last three factors can be predicted to some extent, we can define a new parameter β that expresses specifically the strength of the gas-aerosols coupling while directly depending on the AOD. Combining estimations of β and top of the atmosphere radiance values extracted from the observed spectra within the CO 2 gas band at 2 µm, we evaluate the AOD and the surface reflectance by radiative transfer inversion. One should note that practically β can be estimated for a large variety of mineral or icy surfaces with the exception of CO 2 ice when its 2 µm solid band is not sufficiently saturated. Validation of the proposed method shows that it is reliable if two conditions are fulfilled:(i) the observation conditions provide large incidence or/and emergence angles(ii) the aerosol are vertically well mixed in the atmosphere. Experiments conducted on OMEGA nadir looking observations as well as CRISM multi-angular acquisitions with incidence angles higher than 65°in the first case and 33°in the second case produce very satisfactory results. Finally in a companion paper the method is applied to monitoring atmospheric dust spring activity at high southern latitudes on Mars using OMEGA.

Evolution of south seasonal cap during Martian spring: Insights from high-resolution observations by HiRISE and CRISM on Mars Reconnaissance Orbiter

“…However, an alternative possibility is that the amount of aerosols in the atmosphere over the pole increases as a result of the progressive retreat of the seasonal cap that leaves behind bare soil and dust susceptible to be lifted and transported by winds. It should be possible to distinguish between these hypotheses in the future by directly retrieving local values of atmospheric opacity from the targeted CRISM observations themselves as recently undertaken by Douté et al [2010].…”

Ice state evolution during spring in Richardson crater, Mars

“…The atmospheric gas contribution was corrected using ab initio line by line radiative transfer calculations (Douté, 2014), based on general circulation models (GCM) predictions (Forget et al, 1999;Forget et al, 2006). The mineral aerosols atmospheric contribution was corrected using aerosols optical thickness (AOT) estimations by Wolff et al, 2009 and the method developed by Douté et al, 2013, that take into account the radiative coupling between gas and aerosols. No correction method has proven 100% efficient to remove the aerosols contribution for CO 2 ice covered areas (Vincendon et al, 2007(Vincendon et al, , 2008Smith, 2009;Wolff et al, 2009;Ceamanos et al, 2013;Douté et al, 2013;Douté, 2014).…”

“…The mineral aerosols atmospheric contribution was corrected using aerosols optical thickness (AOT) estimations by Wolff et al, 2009 and the method developed by Douté et al, 2013, that take into account the radiative coupling between gas and aerosols. No correction method has proven 100% efficient to remove the aerosols contribution for CO 2 ice covered areas (Vincendon et al, 2007(Vincendon et al, , 2008Smith, 2009;Wolff et al, 2009;Ceamanos et al, 2013;Douté et al, 2013;Douté, 2014). Therefore, we expect correction errors, that have to be taken into account in the inversions.…”

Monitoring atmospheric dust spring activity at high southern latitudes on Mars using OMEGA

“…The OMEGA instrument has acquired a comprehensive set of observations in the near-infrared (0.93-5.1 microns) at the high southern latitudes of Mars from mid-winter solstice (L S =110°, December 2004, MY 27) to the end of the recession (L S =320°, November 2005). We systematically process a subset of these observations from L S =220°to 280°by performing the inversion of radiative transfer schemes described in Douté et al (2013) and Vincendon et al (2008) on every top of the atmosphere reflectance spectra forming the OMEGA images. As a result, we obtained a series of maps depicting the distribution of aerosol optical depth (AOD) noted τ k0 aer .…”

“…Experimental estimation of factor β for a given pixel means evaluating the intensity of the 2 µm absorption band of gaseous CO 2 . Practically this is only possible for surfaces spectrally dominated by minerals or water ice, even though the procedure can be extended for spectra showing saturated 2 µm CO 2 ice absorption band but with some remaining radiance coming from the surface such as in the outer part of the seasonal cap (Douté et al, 2013). For that purpose we first need the observed spectrum and, secondly, the corresponding transmission spectrum T k gaz (h, lat, long) ψ(ν) computed ab-initio.…”