2024 September 7

Mars Opposition Blog 7 September 2024

Richard McKim

As you may have read from the alerts and follow-ups that I posted upon the BAA Forum, we have recently had a major Regional dust storm. I will cover this in some preliminary notes below which extend up to and including September 4. But first, to follow up upon the previous happenings….

The local dust storm described in the previous blog post lasted only a few days, but in late July and early August there was a slight resurgence of activity when a local dust cloud appeared in southern Chryse. This too lasted only briefly. On Aug 3 there was virtually no trace of it on the morning side to the Japan observers, while on the same date J.Warell (Sweden) caught the now inactive region at the evening terminator, proving that no activity had spread to the west.

It was to be in a similar location that more significant activity began on Aug 18, when images by Clyde Foster from 05:10 UT onwards showed very bright dust activity along eastern and central Valles Marineris, with two bright cores overlapping the southern deserts of Chryse/Xanthe. At Ls = 313.5 degrees, it had commenced at a point fractionally beyond the last moment for an encircling storm to have developed, but a large Regional event could not be ruled out at that stage, and indeed turned out to be the case. The eastern bright core was centred at -8.7, 034.4 degrees according to images at 00:23 UT taken by the MARCI instrument aboard the Mars Reconnaissance Orbiter (MRO), as provided by A.Sanchez-Lavega. (This location is close to Eos or Pyrrhae Region on the classic albedo maps.) Eric Sussenbach caught the western end of the event at 10:04 UT.

Next day, Aug 19, the storm expanded to the north over Chryse-Xanthe, and to the south over Mare Erythraeum. At this time I suggested that secondary dust cores would probably arise, perhaps over some or all of Meridiani Sinus, Margaritifer Sinus and Argyre, as they have done in similar events in the past.

By Aug 20 the storm had definitely reached Regional status (its long axis extending over 2000 km, or 34 angular degrees). Foster’s images (from Namibia) and Nick Haigh’s (UK) show that it had reached and faded E. Meridiani Sinus on the eastern front, while Peter Tickner’s full daylight IR image (from the UK) timed at 08.45 UT showed dust surrounding a still dark Aurorae Sinus to the west, and still expanding south west, with apparently one discrete cloud near Solis Lacus. Dust continued to occupy Chryse and Xanthe, with Mare Acidalium to the north largely unaffected. Although UK observers were now losing the emergence site over the morning limb at dawn, the eastern end would remain in view.

Further secondary cores now emerged, and I cannot mention all of them here. The dust core located near Solis Lacus became large and extremely bright on Aug 22 according to images by Gary Walker, Frank Melillo and others. Gregory Terrance, using a 50 cm aperture, showed that this new core was in fact highly condensed. However, little further development on the western side occurred. On Aug 21, Haigh caught a bright dust core between Meridiani Sinus and Margaritifer Sinus which lasted a few days, while to the south, dust was invading western Noachis.

A new (or resonant) core formed in Hellas: it is shown on the Aug 19 and 21 MARCI maps that have been forwarded by colleagues in the USA. Dust remained within the basin. On Aug 25 however, Haigh and Peach showed a bright yellow cloud in the NW corner of the basin spilling out of it into Iapigia, which was confirmed by Foster next day. The core was small and bright in early September, but by then dust was confined to the basin. An image by Walker dated Sep 4 shows this aspect. Furthermore, a new dark patch had developed adjacent to it on the west, as often happens as a result of events here. (Larger events cause a broadening and darkening of all of Mare Serpenis-Yaonis Fretum, bordering Hellas.)

Dust had covered Mare Erythraeum and Argyre and environs in the form of several parallel E-W belts. Within days these bright areas would become static, indicating that fallout was occurring. Sussenbach’s images on Aug 23 and 24 show daily changes.

On Aug 25 I posted the following at the Forum: “As of today this event seems to be ending. The eastern secondary core at Meridiani Sinus and more prominent western ones at or near Solis L. did not last long; for a time there was a bit of atmospheric dust raised in Hellas. The area around Argyre is still bright, but its lack of daily movement suggests it is now dust fallout from the storm. This area is larger than Argyre, as was the case with the similar event in 2022. Compared with the 2020 and 2022 regional storms at similar seasonal dates, the present one was less energetic and less extensive.”

There were a few final flourishes. Mike Hood and Walker on Aug 29 recorded a new bright dust cloud in SE Chryse more or less following the line of Oxia Palus-Indus. It did not develop, and more recent images show that the emergence site has returned to normal, in spite of the strong activity that had taken place there, while the Oxia Palus-Indus dark feature and other halftone streaks crossing Chryse are well seen again.

Several Japanese observers were very active as usual, but could hardly watch the storm. However, their work showed that the evening orographic cloud at Arsia Mons remained bright throughout its course, showing that dust did not significantly warm the atmosphere beyond the confines of the event. Great storms warm the entire atmosphere, causing all white crystal clouds to disappear.

In the north, although the polar hood presently persists (and should continue to do so for a long time) we have witnessed the first signs of the ground cap. Around Aug 9 the polar region had become brighter and sharply defined in red images (and to a lesser extent in IR images), at Ls = 309 degrees, though the larger overlying hood was visible in blue light. Makoto Adachi and myself consider that the ground cap was captured on this date by S.Ito and S.Watanabe, while Adachi may have seen it visually on Aug 7. (In 2012 BAA data showed the ground cap was first sighted at Ls = 312 degrees.) It is usual for the polar region to be changeable at this Ls, and indeed the hood is often more prevalent than the ground cap in the longitude of Mare Acidalium. We did not see the hood recede during the current Regional dust storm, confirming that dust was obviously not penetrating very far north of the equator.

Posted here are several daily compilations showing different stages of the Regional dust storm. This is in no way intended to be a complete account. I would like to thank all those who responded to the Forum posts and to personal email alerts. We now have over 36 observers sending or posting data, and the list of them on the front page has been updated.

Front page teaser image – Eastern End of Dust Storm, Clyde Foster. Western End of Dust Storm, Eric Sussenbach.

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