The 2018 opposition of Mars & the global dust storm: Part II

2022 December 8

Richard McKim

A report of the Mars Section. Director: R. J. McKim.

In Part I, we described the evolution and behaviour of the global dust storm and its complex interaction with the Martian surface. In Part II, we discuss the lesser dust storms observed, white cloud activity and the behaviour of the polar regions. Several minor dust storms at high latitude were associated with the recession of the S. polar cap (SPC), while a large Regional event in 2019 January stretched from Hellas to Claritas, having originated in southern Chryse. The Equatorial Cloud Band and the Syrtis Blue Cloud showed standard seasonal behaviour, until all diurnal and orographic clouds were interrupted for months by the global storm. As described fully here, the early spring recession of the SPC was accelerated by the deposition of dust upon the cap, but its recession was otherwise extremely close to that in 2003, a year with no global storm. The seasonal separation of Novus Mons at Ls = 237° was marginally early. The SPC was observed until Ls = 345°, and the NPC first appeared at Ls = 346°, in 2019.

Dust storms

Part I described and illustrated the 2018 global storm.⁴¹ The Director’s drawings in Figure 13 portray that event and later phenomena. Here, we review the lesser dust storms: see Figures 14–15.

2017 November – 2018 May

On 2017 Nov 1–3, Foster’s image (disc diameter (D) = 3.9ʺ) showed the NPC apparently divided N–S by a rift at CM = 135–155°. As the Rima Borealis rift (which Foster would only resolve days later) runs E–W at that CM, it was surely a dust streak, and the season would have favoured it, though the evidence depends upon small red-filter images. On 2017 Dec 1 and 2018 Jan 12 (D = 4–5ʺ), Foster imaged further dust streaks upon the NPC at right angles to the detached Olympia. These latter events were observed by the MRO; its weekly weather reports referred explicitly to Olympia.⁹

In 2018 March, a small Regional storm – shown in Figure 14A – began near Solis Lacus. MRO images of Mar 16, 17 & 21 show a small dust cloud NW of that marking, and dust expanded into Valles Marineris for some days from Mar 22.^9,37 Valimberti’s Mar 21 image and Kumamori’s of the following day confirmed a small cloud NE of Solis Lacus, weakly brighter in red light. On Mar 29, Valimberti saw a bright nucleus at the E. end of Valles Marineris and others in S. Chryse (Figure 14A). Adachi, Iwamasa, Kumamori and MacNeill also followed the event that covered E. Valles Marineris during Mar 23–31; by Mar 28 a specific bright cloud lay over W. Margaritifer Sinus, and there were brighter dust clouds over Chryse. From Mar 29, Margaritifer began to recover, but bright dust continued to be visible in Chryse–Xanthe and, less obviously, in E. Valles Marineris for several days. (The Apr 2–8 MRO bulletin mentions dust at E. Marineris.) MacNeill’s Apr 4 image showed Margaritifer still a little faint, with a large, bright dusty area to its NW. Upon Foster’s Apr 14–16 images, Margaritifer was normal, but Chryse alone was bright and yellowish, its static nature suggesting fallout. Peach on Apr 27 showed only E. Chryse bright.

MRO data for Apr 5 showed a very small dust cloud in Amazonis,^9,42 NW of Olympus Mons. Images by Morales and Peach actually confirmed the cloud (brighter in red light), but without MRO data it might have been overlooked.

Iwamasa’s RGB image of Apr 20 showed the W. part of Mare Sirenum (which had been normal, Apr 17–18) faint, while red and IR images emphasised a number of small dust clouds obscuring it, and others on Apr 21 confirmed obscuration. Images during April 20–25 showed a brighter dust cloud over central Zephyria at –7°, 186°, adjacent to this activity (indicated in Figure 14B). Mare Sirenum looked normal to Yunoki and others on Apr 22, when further dust activity to the SW at Electris–Eridania was seen (indicated in Figure 14B). A bright dust cloud recorded by Justice on Apr 25 at –24°, 180° in S. Zephyria at W. Mare Sirenum / E. Mare Cimmerium was the most conspicuous feature (Figure 14B). MRO images showed the latter began on Apr 24 (when we had no record of that longitude),⁴³ reached a maximum next day and disappeared after Apr 27 (as we confirmed).

Foster’s images of Apr 23 suggested a tiny dust cloud in W. Hellas, at the time of an SPH incursion in its south part. The dust disappeared next day; a resurgence on Apr 29 is described below.

On 2018 Apr 30, under CM = 198°, Foster detected a minute projection on the evening terminator at +30°, 153°, over N. Amazonis, bright in red light. MRO verified it was a local storm, long axis ca. 520km. By May 2 it had dissipated.

From late April onwards, we saw three separate events at the N. edge of the SPC. To quote the May 7–13 MRO bulletin: ‘Large local-scale dust storms occurred along much of the seasonal south polar ice cap edge’.⁹

The first was detected on Apr 29 by Carvalho. A storm in S. Hellas, it was reobserved by Peach on May 5 when it overlapped the SPC edge. Next day, Peach imaged dust in S. and W. Hellas. W. Hellas continued bright and dusty, slightly overlapping the cap, on May 10–12; it faded on May 14–17 and disappeared on May 18. See Figure 14C. The second event occurred during May 6–13, when Australasian observers recorded a local dust storm over Argyre that extended onto the edge of the SPC. It seemed at maximum development on May 12 (Figure 14C). The third storm, which reached Regional status, occurred during May 12–14 with light orange streamers and patches of dust at high S. latitude, over W. Phaethontis and Electris, that partly overlapped the cap and ran Sp.–Nf. from λ ~ 150–200°, breaking up into small bright patches over SW Mare Sirenum to SW Electris on the last date. (This somewhat recalls a dust storm south of Mare Sirenum in 2016 June.¹) See Figure 14C.

The global storm, 2018 May – August

See Part I.⁴¹

Further dust activity, 2018 September – October

Peach’s high-resolution Sep 17 Chilescope image revealed a minute storm at the NW tip of the rapidly subliming westernmost component of Novus Mons, as mentioned in an MRO bulletin.⁹ See Figure 17I. An equally fine Sep 8 image (Figure 17H) had not shown the cloud. It would appear to have been the result of a ‘cascade’, where rapid sublimation at a sharply pointed outlier lifts dust. Such phenomena were more common during 2003, when there was no global storm.⁴⁵

A dust storm at high southern latitude occurred during Oct 1–5 and was followed by observers in the USA.¹³ A small elliptical cloud – bright and strongly yellowish – was seen to the south of Mare Sirenum, displaying boundary changes from night to night. Brightest in red and green light, the storm was also obvious in blue: at relatively high latitude, it may have possessed a water ice component. Shown here in Figure 14D is a collage of images by Hood and Walker. From Oct 2–4, its centre moved from –41°, 178° to –42°, 156°, at 20km h^–1 to the east.

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