Anni Määttänen | Searcharxiv

arXiv2022

Impact of the coagulation of dust particles on Mars during the 2018 global dust storm

Coagulation of particles occurs when two particles collide and stick together. In the Martian atmosphere, dust coagulation would increase the effective particle size, as small particles accrete to larger particles. Murphy et al. (1990) showed that Brownian coagulation of dust in the Martian atmosphere was not significant, due to the low dust particle mixing ratios, while Montmessin et al. (2002) and Fedorova et al. (2014) showed that it mostly involves particle radii smaller than 0.1 um. However, the effects of coagulation have never been explored in 3D, during a global dust storm, i.e. in presence of larger numbers of small particles. Here we revisit this issue by using the NASA Ames Mars Global Climate Model (MGCM) to investigate the temporal and spatial changes in dust particle sizes during the 2018 global storm due to coagulation and the overall impact of these processes on Mars' climate. Our parameterization for coagulation includes the effect of Brownian motion, Brownian diffusion enhancement, and gravitational collection. We show that Brownian motion and Brownian diffusion enhancement dominate gravitational collection. The impact of coagulation is significant during the storm, with coagulation rates increased by a factor of 10 compared to non-storm conditions. The effective particle radius can be increased by a factor of 2 due to coagulation, leading to a 20K colder atmosphere above 30 km altitude. Overall, our parameterization improves the representation of the decay phase of the storm relative to observations. The process remains significant outside the storm period if large numbers of submicron-sized particles are involved. It may be possible, in GCMs, to lift larger amounts of submicron-sized particles from the surface without excess dust buildup in the atmosphere, thus improving the agreement with some of the observations without diverging from the observed column opacities.

astro-ph.EP↗

arXiv2012

Rocket dust storms and detached dust layers in the Martian atmosphere

Airborne dust is the main climatic agent in the Martian environment. Local dust storms play a key role in the dust cycle; yet their life cycle is poorly known. Here we use mesoscale modeling that includes the transport of radiatively active dust to predict the evolution of a local dust storm monitored by OMEGA on board Mars Express. We show that the evolution of this dust storm is governed by deep convective motions. The supply of convective energy is provided by the absorption of incoming sunlight by dust particles, rather than by latent heating as in moist convection on Earth. We propose to use the terminology "rocket dust storm", or conio-cumulonimbus, to describe those storms in which rapid and efficient vertical transport takes place, injecting dust particles at high altitudes in the Martian troposphere (30 to 50 km). Combined to horizontal transport by large-scale winds, rocket dust storms produce detached layers of dust reminiscent of those observed with Mars Global Surveyor and Mars Reconnaissance Orbiter. Since nighttime sedimentation is less efficient than daytime convective transport, and the detached dust layers can convect during the daytime, these layers can be stable for several days. The peak activity of rocket dust storms is expected in low-latitude regions at clear seasons (late northern winter to late northern summer), which accounts for the high-altitude tropical dust maxima unveiled by Mars Climate Sounder. Dust-driven deep convection have strong implications for the Martian dust cycle, thermal structure, atmospheric dynamics, cloud microphysics, chemistry, and robotic and human exploration.

astro-ph.EP↗