A Long‐Term Study of Mars Mesospheric Clouds Seen at Twilight Based on Mars Express VMC Images

Abstract

We present the first systematic study of clouds observed during twilight on Mars. We analyze images obtained by the Visual Monitoring Camera on Mars Express between 2007 and 2020. Using an automated retrieval algorithm, we found 407 cases of clouds observed at twilight, in which the geometry of the observations allows to derive the minimum altitude, revealing that many of these clouds are in the mesosphere (above 40 km and up to 90 km). The majority of these mesospheric clouds were detected in mid‐latitudes at local autumn and winter, a new trend only hinted at by previous studies. In particular, we find a massive concentration of clouds in the southern mid‐latitudes between Terra Cimmeria and Aonia, a region where high altitude events have been previously observed. We propose that there is an unknown mechanism in these regions that enhances the probability to host high altitude clouds around the southern winter solstice.

Twilight Mesospheric Clouds in Jezero as Observed by MEDA Radiation and Dust Sensor (RDS)

“…(2023) at the same location and time does not show a minimum at the same period, nor do the water ice column opacities ( τ ice−column ) derived from orbiter observations. This indicates that the reduced cloud activity observed in this work is occurring at altitudes above 30–35 km but not below, and that, as a result of the small opacities given in Figure 5 with respect to τ ice−column , the decrease in cloud activity at Ls ∼ 70° is not evident in such measurements. Less cloud activity (above 30–35 km) during the aphelion was also found in previous years (e.g., Hernández‐Bernal et al., 2021) over the equator and the southern belt around 45°S, which might indicate this is a seasonal feature. Based on previous analyses and the predictions by the MCD model, we infer these variations in the cloud activity might be due to a decrease in the concentration of the cloud nuclei (dust particles) or in the water vapor mixing ratio in the mesosphere. The cloudiest time over the Perseverance site for the analyzed period is between Ls = 120°–150° for which we also find the greatest cloud opacities (maximum value 0.04 ± 0.01 weighted over the RDS Top sensors FOV).…”

“…Less cloud activity (above 30–35 km) during the aphelion was also found in previous years (e.g., Hernández‐Bernal et al., 2021) over the equator and the southern belt around 45°S, which might indicate this is a seasonal feature. Based on previous analyses and the predictions by the MCD model, we infer these variations in the cloud activity might be due to a decrease in the concentration of the cloud nuclei (dust particles) or in the water vapor mixing ratio in the mesosphere.…”

“…Hernández‐Bernal et al. (2021) investigated the formation of clouds in the mesosphere using images from the Mars Express VMC camera at twilight, and found a decrease in the cloud activity around the same period. However, this particular seasonal variation does not appear to be present in orbiter‐derived records of water ice column opacity ( τ ice−column ).…”

“…We also see a decrease in the number of cloud and haze detections in the period between Ls ∼ 50° and 114°, as well as a decrease in the cloud altitude and opacity, which confirms the conclusions derived in previous section from the correlation slope analysis; within the ACB season and assuming the cloudy season started in Jezero at Ls ∼ 39° or before, the RDS twilight observations point to a discontinuity in the cloud activity above ∼30 km around aphelion (Ls ∼ 70°). In general, observations made by the Visual Monitoring Camera (VMC) camera aboard Mars Express along MYs 29 to 35 show a low presence of twilight clouds at the latitude of Jezero crater compared to latitudes farther south (Hernández‐Bernal et al., 2021). A comparison of both methods suggests that with RDS we easily detect morning and evening clouds or hazes of low optical thickness while from orbit we detect clouds with much higher optical thicknesses.…”