Major dust storms are the main cause of Martian climate variability due to the impact of atmospheric dust on heating in the thin atmosphere. To date, the largest storms – often referred to as global dust storms (GDS) – have all occurred in the second half of the Mars year but otherwise show large variability in onset location and timing, as well as how they evolve. The mechanisms responsible for their onset, growth, and decay are not well understood, and efforts to improve our knowledge through observation are hampered by their rarity: they typically occur only once or twice per Earth decade.
However, there was an opportunity to gain new insights in 2018 when the Mars Year (MY) 34 storm, which became global that June, was one of the most comprehensively observed GDS ever, with five spacecraft in Mars orbit and two operating on the Martian surface at its onset.
A special collection in JGR: Planets includes papers that reflect the increase in understanding of GDS that was enabled by analyzing these datasets and interpreting them via the use of atmospheric models.
How did the MY34 global storm compare to other recent global storms?
Smith  and Montabone et al.  use thermal mapping from Mars’s orbit to show that the MY34 GDS began shortly after southern spring equinox and consisted of two intense periods of increasing dust load (at Ls~187 and 196°), with a lull in between, followed by a slow decay of the storm thereafter.
The MY34 storm was one of the earliest GDS ever observed on Mars, with only the MY25 (2001) GDS also starting shortly after equinox; other GDS, including the only other recent one in MY28 (2007), typically occurred much closer to southern summer solstice, when solar forcing is more intense. However, the two equinoctial GDS did not evolve identically.
Smith  show that, despite its two-stage growth pattern, the MY34 GDS was shorter than that in MY25 (and in MY28), while Wolkenberg et al.  show that the maximum temperature and dust opacity coincided during the MY25 GDS, whereas the temperature maximum preceded that of dust opacity during the MY34 (and MY28) GDS.
What new things did we learn about Martian dust storms?
Using orbital data, Kleinboehl et al.  show strong diurnal variations in mid-to-low latitude dust during the GDS, with similar dust amounts found 5 to 10 kilometers higher in the late afternoon than overnight. In certain regions, Heavens et al.  show that dusty deep convection, analogous to moist convection in Earth’s clouds, occurred episodically during the early stages of both the MY28 and MY34 GDS, pluming dust up to over 70 kilometers.
Major storms also significantly and rapidly increased atmospheric water vapor at up to 100 kilometers in altitude during the MY34 GDS, according to Aoki et al.  and Heavens et al. . Girazian et al.  show that the peak altitude of the ionosphere and its variability both increase during GDS, suggesting enhancement of dynamical processes coupling the lower and upper atmosphere.
Meanwhile, at the surface inside Gale Crater, Viudez-Moreiras et al.  show that the daily maximum UV radiation decreased by 90 percent in just 10 sols as the opacity increased by a factor of approximately 8, leading to greatly decreased daily temperature ranges.
What big questions did the MY34 storm help us answer, and what do we still not know?
A major remaining unknown is what causes onset and growth of some Mars dust storms but not others. Shirley et al.  discuss a possible link between the occurrence of GDS and the changes in the orbital motion of Mars; this effect may have influenced the onset of the MY 34 GDS.
Bertrand et al.  use maps of dust column abundance produced by Montabone et al.  to infer where dust was lifted during the MY34 GDS, then model the end point of dust lifted from various regions. They find that, while Tharsis contributed little dust early on, once deposition occurred here it became a major source that was key to the storm growing. This suggests complex feedbacks between surface dust cover, wind stress, and lifting that may be crucial to understanding the onset of dust storms.
Finally, Gillespie et al.  and Bertrand et al.  both investigate the role of transport in the expansion of the MY34 GDS to become global and show that rapid eastward transport, driven in part by the storm itself, may have been critical.
Understanding the onset, evolution, and impact of global dust storms will enable us to correctly model past climates and atmospheric escape, and may even allow us to predict future storms. However, more data on how dust is lifted and the feedbacks responsible for storm growth on Mars are likely needed first. With human exploration of Mars planned for the near future, predicting the Martian climate and dust cycle is a very timely problem.
—Anni Määttänen ([email protected]; 0000-0002-7326-8492), Editor, JGR: Planets, and LATMOS/IPSL – Centre national de la recherche scientifique, France; and Claire Newman ( 0000-0001-9990-8817), Associate Editor, JGR: Planets, and Aeolis Research, Arizona, USA