The largest of dust storms on Mars are global dust events (GDEs) that affect essentially every aspect of the Martian atmosphere but do not occur in every Mars year. The Mars Climate Sounder globally observed the most recent GDE in 2018 throughout its lifecycle. The event started shortly after the southern spring equinox, with the first Mars Climate Sounder signature detected on 2 June 2018 (Ls = 186.2°). It grew into a mature global event by the end of June (Ls~198°), with drastic changes to the atmospheric temperature, dust, and water ice profiles. It decayed over several months, reaching background conditions around southern summer solstice in October 2018. The 2018 GDE was very similar to the 2001 event, including the seasonal development and temperatures. GDEs appear to be distinct from even the largest regional dust storms. GDEs have 50-Pa zonal tropical temperatures >220 K while no regional dust storm exceeds 205 K. Plain Language Summary Mars has global dust storms where dust fills the atmosphere and blankets the entire planet. These are rare, but one occurred in 2018. It started in early June and lasted through mid-October. The Mars Climate Sounder, an instrument in orbit around Mars, measured the global changes in the atmospheric temperature as well as the amount of dust in the atmosphere during the storm. These observations significantly improved our understanding of global dust storms on Mars. The one in 2018 was very similar to a global storm that occurred in 2001. Based on studying these storms, it appears that global dust storms are fundamentally different from the smaller, regional storms. Global dust storms grow much faster than their smaller cousins, and they have a larger impact on the atmosphere.

We present for the first time an assimilation of Thermal Emission Spectrometer (TES) water vapour column data into a Mars global climate model (MGCM). We discuss the seasonal cycle of water vapour, the processes responsible for the observed water vapour distribution, and the cross-hemispheric water transport. The assimilation scheme is shown to be robust in producing consistent reanalyses, and the global water vapour column error is reduced to around 2-4 pr-µm depending on season. Wave activity is shown to play an important role in the water vapour distribution, with topographically steered flows around the Hellas and Argyre basins acting to increase transport in these regions in all seasons. At high northern latitudes, zonal wavenumber 1 and 2 stationary waves during northern summer are responsible for spreading the sublimed water vapour away from the pole. Transport by the zonal wavenumber 2 waves occurs primarily to the west of Tharsis and Arabia Terra and, combined with the effects of western boundary currents, this leads to peak water vapour column abundances here as observed by numerous spacecraft. A net transport of water to the northern hemisphere over the course of one Mars year is calculated, primarily because of the large northwards flux of water vapour which occurs during the local dust storm around L S = 240• -260• . Finally, outlying frost deposits that surround the north polar cap are shown to be important in creating the peak water vapour column abundances observed during northern summer.

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A vigorous regional dust storm substantially altered both the global atmospheric thermal structure and the magnitude and spatial distribution of dust loading within the Mars atmosphere between 1 and 9 June 2018. We examine the development and decay of this storm in latitude, longitude, altitude, and time, employing observations by the Mars Climate Sounder on board the Mars Reconnaissance Orbiter. Dust layer top altitudes rose from seasonal-normal values of~40 to~70 km. Dust lofting to high altitudes was localized between 0°and~60°W longitude, and between 60°N and 60°S latitude. Intensification of paired meridional overturning circulation cells within the study area is confirmed by strong nighttime dynamical heating in higher latitudes of both hemispheres. By the end of this episode, significant dust loading was present at altitudes greater than 50 km above all longitudes on Mars, and other dust lifting centers had been activated.Plain Language Summary Why do some Martian dust storms, in some Mars years, expand to become global in scale, while the vast majority do not? We use infrared observations of the Mars atmosphere by the Mars Climate Sounder, on board the Mars Reconnaissance Orbiter spacecraft, to study this question. We examine the earliest days of the global dust storm of 2018, from 29 May through 9 June 2018. One key difference between regional and global dust storms is the altitude, within the atmosphere, to which dust is lofted during the storms. We show that dust was carried to much higher altitudes than normal during the first week of June, and that this major pulse of high-altitude dust lofting was localized between about 0°and 60°W longitude. These longitudes include the "Acidalia storm track." The circulation pattern, within this corridor, looks very much like a textbook Hadley circulation, where warm air rises rapidly above the equatorial latitudes, flowing both northward and southward, to later sink back toward the surface in middle latitudes. This circulation strengthened considerably during the earliest days of the global storm. These observations have important implications bearing on the mechanisms of Martian global dust storm occurrence.