In our work, all aerosol observations are accompanied by measurements of atmospheric pressure and temperature from the same solar occultation session, which are used for calculations of saturation ratio S. Surprisingly, supersaturation S > 1 was detected in only 5 out of 19 cases of CO2 cloud layers (26 %); extremely low values S < 0.1 were found in 9 out of 19 cases (47 %) (see Table 2). Figure 9 shows two examples of T, P and S profiles retrieved independently from NIR and MIR data. Similarity between the two instruments is evident. The former example (#5401, egress), as was discussed above, featured two CO2 clouds layers, both having S < 0.1. The latter one, ingress occultation #14501, had three layers with CO2 particles, and only at the altitudes of the topmost one S > 1 was found. Such behavior when only one out three CO2 layers had S > 1 was observed in three solar occultation observations.
Discovery of CO2 clouds existing at extremely low values of S, even with a supersaturated layer at highest altitudes, is unexpected, and requires a comprehensive microphysical modeling coupled with a GCM to account for evolution of temperature profiles for interpretation of this phenomenon. A simple explanation, that particles in the supersaturated region are condensing, growing, and gravitationally falling in the subsaturated region where they are detected, is quantitatively inadequate. For example, in case of #14501 occultation (bottom of Figure 9), an estimation of carbon dioxide ice particles lifetime using measured P and T profiles (following the “linearized model” from Listowski et al., 2013) results in ≪ 1 s timescale at altitudes <70 km where above freezing temperatures were observed. It is several orders of magnitudes lower compared to the time needed for a particle at a sedimentation speed (calculated using expression 20.4 from Jacobson, 2005) to reach the altitude of the lowest CO2 layer, which means that a particle under the observed conditions should sublimate long before reaching those altitudes.

[Figure: Figure 9. Examples of pressure (panels a, d), temperature (panels b, e) and saturation ratio (panels c, f) profiles retrieved from NIR (red) and MIR (blue) data from egress occultation #5401 (top) and ingress occultation #14501 (bottom). Bold lines and filled areas correspond to best fits and 1−σ uncertainty, respectively. Altitudes where CO2 clouds were detected are marked by horizontal orange bars.]

Among the 1663 unique ACS solar occultation observations analyzed in this work, S > 1 was observed in 28 measurements (< 2 %). As was pointed out before, CO2 clouds were detected only in five of them, while in other cases there were no aerosol layers at the altitudes of saturation ratio maximum. A similar pattern was observed by Jiang et al. (2019) from the IUVS data: out of seven occultations of 𝜏-Sco star that detected supersaturation only one featured aerosol layer. These results raise the questions about the source and microphysical properties of condensation nuclei in the Martian mesosphere.
Thus, we confirm that observed S > 1 condition alone is not enough to make a confident conclusion about CO2 clouds presence and may lead to a substantial overestimation of CO2 clouds occurrence. On the other hand, from our observations it follows that the S > 1 condition coinciding with an aerosol layer always corresponds to a CO2 cloud detection and can be used as a proxy for an unambiguous CO2 identification, but will lead to numerous overlooked CO2 clouds.