The Onset and Growth of the 2018 Martian Global Dust Storm.

We analyze the onset and initial expansion of the 2018 Martian Global Dust Storm (GDS 2018) using ground‐based images in the visual range. This is the first case of a confirmed GDS initiating in the Northern Hemisphere. A dusty area extending about 1.4×105 km2 and centered at latitude +31.7°±1.8° and west longitude 18°±5°W in Acidalia Planitia was captured on 30 and 31 May 2018 (Ls = 184.9°). From 1 to 8 June, daily image series showed the storm expanding southward along the Acidalia corridor with velocities of 5 m/s and simultaneously progressing eastward and westward with horizontal velocities ranging from 5 to 40 m/s. By 8 June the dust reached latitude ‐55° and later penetrated in the South polar region, whereas in the North the dust progression stopped at latitude approximately +46°. We compare the onset and expansion stage of this GDS with the previous confirmed storms. Key Points: May‐June 2018 ground‐based images show the onset and early evolution of a Martian Global Dust Storm (GDS)The outbreak took place at location (North hemisphere) and time (solar longitude 184.9°) unusual for most GDSsThe expansion involved horizontal velocities in all directions in the range 5‐40 m/s [ABSTRACT FROM AUTHOR]