Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS‐MIR Channel Onboard the Trace Gas Orbiter

Abstract

The Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) European Space Agency‐Roscosmos mission began science operations in March 2018. ACS Mid‐InfraRed (MIR) channel notably provides solar occultation observations of the Martian atmosphere in the 2.3‐ to 4.2‐ normalμ m spectral range. Here, we use these observations to characterize water ice clouds before and during the MY 34 Global Dust Storm (GDS). We developed a method to detect water ice clouds with mean particle size ≤ 2 normalμ m and applied it to observations gathered between Ls=165∘ and Ls=243∘. We observe a shift in water ice cloud maximum altitudes from about 60 km before the GDS to above 90 km during the storm. These very high altitude, small‐sized ( reff≤0.3.3emnormalμ m) water ice clouds are more frequent during MY 34 compared to non‐GDS years at the same season. Particle size frequently decreases with altitude, both locally within a given profile and globally in the whole data set. We observe that the maximum altitude at which a given size is observed can increase during the GDS by several tens of kilometers for certain sizes. We notably notice some large water ice particles ( reff≥1.5.3emnormalμ m) at surprisingly high altitudes during the GDS (50–70 km). These results suggest that GDS can significantly impact the formation and properties of high‐altitude water ice clouds as compared to the usual perihelion dust activity.

Clouds in the Martian Atmosphere

“…So far, the most commonly reported values for water ice clouds particle radii range from 1 to 5 μm (Chassefière et al, 1992;Clancy et al, 2003;Fedorova et al, 2020;Guzewich & Smith, 2019;Luizzi et al, 2020;Madeleine et al, 2012;Olsen et al, 2019;Stcherbinine et al, 2020). It is to be noted that the reported values are in good agreement with the seminal predictions of Michelangeli et al (1993), who theoretically bracketed the excursion of water ice crystal particle radii based on the first application of a detailed microphysical model to Martian atmosphere conditions.…”

“…Since TES, other instruments have contributed to the climatology of water ice clouds on Mars, such as ACS and NOMAD on the Trace Gas Orbiter (Fedorova et al, 2020;Liuzzi et al, 2020;Stcherbinine et al, 2020), THEMIS (Smith, 2019) on Mars Odyssey, MCS (McCleese et al, 2010), MARCI (Clancy et al, 2016;Wolff et al, 2019) on MRO, SPICAM (Montmessin et al, 2017;Willame et al, 2017), OMEGA (Madeleine et al, 2012;Olsen et al, 2019), andPFS (Giuranna et al, 2019) on the Mars Express mission, as well as in situ information delivered by the Phoenix (Whiteway et al, 2009) and the MSL (Kloos et al, 2016;McConnochie et al, 2018) missions.…”

“…As such, it is nearly independent of the particle shape and provides robust access to the particle properties. Since the Phobos mission, the characterization of cloud profiles has been documented from the UV to the thermal infrared (Benson et al, 2010(Benson et al, , 2011Fedorova et al, 2009Fedorova et al, , 2020Guzewich & Smith, 2019;Liuzzi et al, 2020;Määttänen et al, 2013;Maltagliati et al, 2013;McCleese et al, 2010;Rannou et al, 2006;Smith et al, 2013;Stcherbinine et al, 2020).…”

Properties of Water Ice and Dust Particles in the Atmosphere of Mars During the 2018 Global Dust Storm as Inferred From the Atmospheric Chemistry Suite

“…During the GDS peak, we observed an intense and abrupt appearance of high altitude (50–100 km) DLs consisting of submicron (0.1–0.8 μm) water ice particles. The high‐altitude ice clouds were also observed with the ACS MIR channel (Stcherbinine et al., 2020). During the 2001 GDS (MY 25), TES/MGS also observed small ( r eff ≤ 1 μm) water ice particles at 60–80 km (Clancy et al., 2010).…”

Studies of the 2018/Mars Year 34 Planet‐Encircling Dust Storm

“…The Atmospheric Chemistry Suite also onboard TGO similarly saw water ice clouds at exceptionally high altitudes, greater than 90 km during the storm, with water ice particle effective radii (∼1.5 μm) typical of much lower altitudes (Stcherbinine et al., 2020). As in the case of Curiosity (Section 3), these large particle effective radii at high altitudes imply that strong upward motion was likely occurring within the storm relative to seasonally typical conditions.…”