Martian Upper Tropospheric Twilight Clouds: First-time observation from India's First Mars Orbiter Mission (MOM)

MOM completed nearly 8 years in the Mars Orbit (2014–2022). During this period Mars Colour Camera (MCC) captured images of different types of clouds formed in the Martian Atmosphere. In present work, we mainly concentrate on the clouds appearing in the morning and evening terminator or in short seen at twilight. We considered 15 cases of clouds observed at twilight, in which the geometry of the observations allows us to derive the minimum altitude of the clouds, revealing that many of these clouds are in the upper troposphere (above 25 km and up to 45 km). The majority of these upper tropospheric clouds were detected in mid-latitudes (30N - 60N) during the Martian autumn and early winter season (Ls 20 to 90 deg). In this mid-latitude region, we also reported a detached layer of dust and water ice cloud in our previous work (Kalita et al., 2021a; 2021b). We propose a plausible mechanism that enhances the probability of the formation of a high-altitude cloud through temperature variation. Our selection process is manual and based on the contrast enhancement through GIMP. Image geometry confirms the incidence angle in the range of 95 to 105 degrees for evening clouds and 75 to 85 degrees during morning and further, using the angle values we estimated the height of the twilight clouds. Further verification with MCS data concretizes our finding regarding upper tropospheric cloud formation.