Martian CO2 cloud formation as observed by MCS and radio occultation

The existence of carbon dioxide (CO₂) clouds in the Martian atmosphere necessitates extremely low temperatures for formation and was initially observed during polar night at low altitudes. Later observations revealed similar clouds at higher altitudes near the equator, especially during spring and summer [1]. Further evidence has shown their occurrence at northern mid-latitudes and in the southern hemisphere during late autumn. Unlike water vapour clouds, which form from a minor atmospheric component, CO₂ clouds are composed of a major atmospheric constituent. The polar CO₂ clouds are convective in nature. Data from multiple missions indicate that the temperature profiles in the polar regions often align with the CO₂ saturation curve up to 30 km, implying that CO₂ condensation helps regulate these temperatures. Significant cloud opacity between 0 and 25 km altitude also supports the presence of CO₂ clouds.

Figure 1: Formation of CO₂ clouds in the Martian atmosphere [2].

Data from the Pathfinder mission indicate that CO₂ exceeded saturation levels during equatorial descent phases at altitudes near 80 km, implying that CO₂ cloud formation in equatorial regions may occur at significantly higher altitudes compared to polar regions [3]. The genesis of these high-altitude equatorial CO₂ clouds is modulated by conditions in the Martian mesosphere. Notably, mesospheric temperatures can drop well below the CO₂ condensation threshold, particularly near aphelion, when diurnal atmospheric tides promote additional cooling conducive to cloud formation. Furthermore, high-altitude CO₂ cloud formations were detected at solar longitudes between 264° and 330°, located above 90 km in altitude [4]. These clouds exhibit limited horizontal extent, spanning approximately 500 to 700 km.

This study investigates Martian CO₂ cloud formations and their duration during the Northern Hemisphere winter and dust season. For this, we use the open-access data from the Mars Climate Sounder (MCS) on board the Mars Reconnaissance Orbiter (MRO) as well as Mars Express (MEX) and Mars Atmosphere and Volatile EvolutioN (MA VEN) radio occultation (RO) to detect clouds in the atmosphere. We also explore the inter-annual variations to see the impact of dust storms on CO₂ cloud formation.

Figure 2: Examples of MCS temperature profiles (blue) with the CO₂ saturation curve [5].

References:

[1] Määttänen A. et al. (2010), Icarus, 209(2) :452–469.

[2] Mars Climate Modelling Centre. GCM overview: Lecture, November 2021.

[3] Schofield J. T. et al. (1997), Science, 278(5344) :1752–1758.

[4] Jiang F. Y. et al. (2019), GRL, 46(14) :7962–7971.

[5] Mathilde V. (2024), Master Thesis, Université Catholique de Louvain, Belgium.