ALMA observations of the 2018 global dust storm on Mars

The most striking phenomenon on Mars is a planet-encircling storm, "global dust storm". Once it starts, optical depth at visible wavelengthreaches values 10-25 times larger compared to non-storm conditions. The suspendeddust-particles block sunlight from reaching the surface and absorb the solar radiation, which directly leads to heating the atmosphere. In contrast, surface temperature decreases due to the shield of dust. Such a drastic change in the thermal structure induces significant modulation in the dynamics as well as in the compositions. However, due to the opaque dust covering the planet, the detail of atmospheric state inside the dust storm is still not well understood. Sub-millimeter heterodyne spectroscopy by ALMA is one of the best and effective remote-sensing techniques for investigating the Martian atmosphere deep inside global dust storms. This is because the dust opacity becomes almost transparent at these wavelengths.

Starting from the middle of June 2018, a global dust storm has occurred on Mars for the first time in the past 11 years. Taking this unique opportunity, we observed Mars with ALMA on 30 June 2018 and 16 July 2018 that correspond to active periods of the global dust storm. We used both Band 5 (183 GHz) and 6 (220 and 230 GHz) on 30 June, and only Band 6 on 16 July. Band 6 measurements allow us to investigate maps of temperature vertical profiles and wind fields from the CO lines. Band 5 measurements enable us to investigate maps of water vapor vertical profiles and H₂O₂ abundances. The data calibration has been already performed and the data analysis is currently under elaboration. In the presentation, preliminary results from the ALMA observations will be discussed.