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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]

A northern ocean of Mars is still debated and, if it existed, it may have accompanied valley networks and/or outflow channels, which may have led to the emplacement of a large amount of water to the northern lowlands during the Noachian and/or Hesperian times. However, it is unclear how and under what conditions (submarine or subaerial) geologic features such as mounds and giant polygons formed in the northern lowlands. The densely-distributed mounds in Chryse and Acidalia Planitia, >1000 km-wide basins of the northern plains, were suggested to be ancient mud volcanoes formed in an aqueous setting, which is controversial (i.e., mud vs. igneous and submarine vs. subaerial). However, these mounds have not been quantitatively well characterized, particularly with respect to their detailed topography. Here we generated forty digital elevation models (DEMs) with resolution of up to 1 m/pixel from High Resolution Imaging Science Experiment (HiRISE) stereo image pairs, and we accurately measured the morphometric parameters of ~1300 mounds within the southern part of the Acidalia basin. Their heights and diameters resulted in good accordance with those of mud and igneous volcanoes in submarine/subaerial settings on Earth. Maximum depths of their source reservoirs vary from ~30 to ~450 m for a subaqueous setting and from ~110 to ~860 m for a subaerial setting, both of which are consistent with fluid expulsion from the ~100–4500 m-thick flood deposits (Vastitas Borealis Formation, VBF). On the basis of the morphometric values, we estimated rheological properties of materials forming the mounds and found them consistent with a mud flow origin, which does not rule out an igneous origin. The conditions of possible submarine mud or igneous volcanoes may have harbored less hazardous environments for past life on Mars than those on an ocean-free surface. [ABSTRACT FROM AUTHOR]