Millefeuille: The layering of the Martian atmosphere observed in forward scattering geometry

We report on observations of the atmosphere above the limb of Mars in forward scattering geometry (~174° phase angle) using the Colour and Stereo Surface Imaging System (CaSSIS) aboard the European Space Agency's ExoMars Trace Gas Orbiter spacecraft. These unique observations provide high-resolution (18 meters per pixel) measurements of light scattered by aerosols in the atmosphere. The observations are characterized by subkilometer-scale layering, ±20% variations in color of the individual layers, and high radiance (<i>I</i>/<i>F</i> > 1). Strong spatial (latitudinal) variability is also evident, suggesting highly inhomogeneous absorption and atmospheric heating. Models of the radiance, while likely to be degenerate, indicate good agreement with lower-resolution results inferred from other techniques. Mesospheric aerosol layers (strongly blue in CaSSIS color ratios) are also evident. The observed decrease in the CaSSIS RED/BLU color ratio with altitude is indicative of a decrease in particle radius.