They are called polar and mesospheric CO2 clouds. The latter were first identified by Clancy & Sandor (1998) based on the Pathfinder descent images showing presence of blue detached aerosol layers above 70 km (Smith et al., 1997) and observed temperatures below the CO2 condensation level (Schofield et al., 1997).
The first long-term mapping of mesospheric CO2 clouds covering Martian years (MY) 24-26 (we use numbering of MY proposed by Clancy et al. [2000]) at local time 13-14 h was made by Thermal Emission Spectrometer (TES) and Mars Orbiter Camera (MOC) onboard Mars Global Surveyor (MGS) (Clancy et al., 2007). They were detected at 60–80 km at equator (15°N– 15°S) mostly at solar longitudes (LS) 20–40° and 140–160°. Spatial and temporal distributions of observed mesospheric CO2 clouds suggested influence of thermal tides that was supported by climate modeling (González-Galindo et al., 2011; Määttänen et al., 2010; Montmessin et al., 2007), although additional cooling by gravity waves appears to be necessary to reach the temperatures required for CO2 condensation (Spiga et al., 2012; Yiğit et al., 2015). Spatially-resolved nadir images of these clouds obtained by the Thermal Emission Imaging System (THEMIS) instrument onboard the Mars Odyssey spacecraft showed the same altitudinal, seasonal and spatial distribution (McConnochie et al., 2010).
The first unambiguous identification of CO2 aerosols in mesospheric clouds that did not require temperature information was made by the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) near infrared imaging spectrometer onboard the Mars Express spacecraft (Montmessin et al., 2007). In the presence of carbon dioxide clouds, OMEGA spectra demonstrated one or two distinct peaks inside a saturated CO2 gaseous absorption band centered at 4.3 μm. Similar spectral pattern was also observed by the Planetary Fourier Spectrometer (PFS) onboard Mars Express (Aoki et al., 2018). Another spectroscopic identification of mesospheric CO2 clouds was made from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data onboard the Mars Reconnaissance Orbiter (MRO) limb spectra in 0.4–4 μm range (Vincendon et al., 2011; Clancy et al., 2019).
During the night, mesospheric CO2 clouds were detected from stellar occultation observations in the UV by the SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM UV) instrument onboard Mars Express (Montmessin et al., 2006) and by the Imaging UltraViolet Spectrograph (IUVS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft (Jiang et al., 2019). They were located at altitudes 80–110 km. In both works identification of carbon dioxide aerosol was based on simultaneously retrieved temperature profiles.

Recently, analysis of solar occultation observations performed by Nadir and Occultation for MArs Discovery (NOMAD) instrument onboard the Trace Gas Orbiter (TGO) resulted in discovery of CO2 clouds at morning and evening terminators of Mars (Liuzzi et al., 2021). It was based both on spectroscopic and temperature identifications of aerosol type. In most cases, their seasonal and spatial distribution is consistent with previous observations of mesospheric CO2 clouds detected at dayside.
In this work, we present observations of mesospheric CO2 clouds based on solar occultation measurements by the Atmospheric Chemistry Suite (ACS) onboard TGO. To distinguish CO2 aerosol particles from dust and water ice particles, we study spectra in the 2.7−2.8 μm region that contains a characteristic carbon dioxide absorption band. This is the first time that the described aerosol spectral feature of CO2 ice is directly observed on Mars. In addition, measured in parallel ACS temperature profiles are used to calculate CO2 saturation ratio within locations of carbon dioxide clouds, which is crucial for understanding condensation processes on Mars and modeling of clouds’ formation.