First Detection and Thermal Characterization of Terminator CO₂Ice Clouds With ExoMars/NOMAD

Abstract

CO 2 ice clouds are unique features of Mars' atmosphere, occurring often around aphelion and in polar nights. Conditions for CO 2 condensation were first observed during the Pathfinder descent (Schofield et al., 1997) around 80 km, and CO 2 ice cloud formation has been discussed in multiple works. Clancy and Sandor (1998) and Magalhães et al. (1999) showed that CO 2 can condense in supersaturated air pockets formed by constructive interference of thermal tides and/or gravity waves. González-Galindo et al. (2011) determined that mesospheric CO 2 ice clouds form in temperature minima due to atmospheric thermal tides, even where the temperature is not low enough to cause condensation. Wherever gravity waves do

Troposphere-to-mesosphere microphysics of carbon dioxide ice clouds in a Mars Global Climate Model

“…Figures 16 and 17 show that the modeled mass mixing ratios at altitudes above 10 Pa remain below 10 *7 . Recent observations of CO 2 ice clouds by the NOMAD instrument in solar occultation (Liuzzi et al, 2021) revealed mass mixing ratios between 10 *5 and 5 10 *4 , two orders of magnitude or more higher than the values obtained by our model.…”

“…Figure 1 shows this as a function of latitude and solar longitude for the two simulations (MPCO2, top; MPCO2+H2OCN: bottom). The figure includes also a comparison to mesospheric CO 2 ice cloud observations (black dots) from several instruments (Clancy et al, 2007;Montmessin et al, 2006Montmessin et al, , 2007Määttänen et al, 2010;McConnochie et al, 2010;Scholten et al, 2010;Vincendon et al, 2011;Aoki et al, 2018;Clancy et al, 2019;Jiang et al, 2019;Liuzzi et al, 2021) and to the area of observed CO 2 supersaturation in the polar regions (black solid line) derived from MCS temperature profiles (Hu et al (2012). The two panels in Figure 1 clearly demonstrate that the predicted polar winter cloud distributions are overall very similar in the two simulations, and the largest di erences appear around the equator where the mesospheric clouds form.…”

“…reported observations of lower altitude (60 km) CO 2 ice clouds during the night (3 LT), best fitted with far larger particle sizes (7 m e ective radius) than during the day. AlsoLiuzzi et al (2021) reported larger crystals at dawn than at dusk. There is no indication of a large-particle nighttime cloud population in our model results,…”

“…The first datasets pointed to large, up to r ef f ˘1 m e ective radii near the equator during the day (Montmessin et al, 2007;Määttänen et al, 2010), and smaller, submicrometer-sized (r ef f ˘100 nm) particles during the night in tropical and midlatitudes (Montmessin et al, 2006). More recent work (Clancy et al, 2019;Liuzzi et al, 2021) reported an increase in particle size during the night, however at much lower altitudes (45-60 km) than the submicron crystals observed at but Figure 18 does show a small trend of increasing particle sizes during the night, although the particle sizes are around 100 nm even at their maximum.…”

“…Some observations have allowed for extracting information on particle sizes in the mesospheric CO 2 ice clouds (see, e.g., Montmessin et al, 2006Montmessin et al, , 2007Määttänen et al, 2010;Clancy et al, 2019;Liuzzi et al, 2021). The first datasets pointed to large, up to r ef f ˘1 m e ective radii near the equator during the day (Montmessin et al, 2007;Määttänen et al, 2010), and smaller, submicrometer-sized (r ef f ˘100 nm) particles during the night in tropical and midlatitudes (Montmessin et al, 2006).…”